Talks

Thanks to Chandra's high resolution capabilities, we have enjoyed unique resolved X-ray images of galaxies of all morphological types, allowing for population studies of X-ray binaries (XRBs), hot gas, and supernovae (SNe) in a variety of environments. XRBs probe the demographics of compact objects, close binaries, and massive stars, as well as the physics of accretion onto compact objects. Hot gas and SNe provide important tracers of stellar feedback in galaxy interstellar mediums (ISMs). As multiwavelength data sets on galaxies have vastly expanded over the last 25 years, Chandra data have been central to linking our understanding these X-ray emitting components to the properties of their host galaxies. I will discuss a number of galaxy studies and the insights that have been established over the years. I will further highlight how Chandra's archival and forthcoming PHANGS Legacy survey data will continue to be used to make headway in several highly active areas of astrophysics, complementing our understanding of stellar evolution models, ISM ionization in low-metallicity and high-redshift galaxies, formation pathways of other binary-related objects (e.g., GRBs, SNe, gravitational wave sources, etc.), and heating of the intergalactic medium during the epoch of heating at z > 8.

Ring Galaxies (RiGs) provide a rare window into distinct phases of galaxy evolution. It is theorized that RiGs form through a nearly head-on collision with an intruder galaxy, creating gravitational perturbations that trigger density waves. These waves propagate through the disc of the target galaxy, generating an expanding ring of gas and stars. Such interactions are often marked by elevated star formation rates, ranging from ~0.1 to 20 solar masses per year, suggesting that the collision stimulates intense star formation. While theoretical models can reproduce the morphology of RiGs, they lack comparison with the actual properties of their stellar populations X-ray observations have proven invaluable in studying the recently formed stellar populations within these galaxies, offering key insights into their energetic properties linked with the end products of recent star formation (like early type stars). The high resolution of the Chandra X-ray Observatory, in particular, has been crucial for resolving individual sources in the ring and analyzing their properties in detail. Early in the mission, Chandra observed the iconic Cartwheel galaxy, the prototypical RiG, yielding groundbreaking discoveries –such as the identification of the galaxy with the largest number of Ultra Luminous X-ray sources (ULXs), and one of the few Hyper Luminous sources– as well as spectacular images. In addition, Chandra has observed six more “bright and famous” RiGs, two smaller distant ones, and will observe seven additional RiGs in the coming year. X-ray data have been also crucial in driving multiwavelength follow-up studies, allowing for a more comprehensive understanding of the complex dynamics and star formation processes in RiGs. Without X-ray observations, many of these discoveries would not have been possible. We will present a review of the findings from these X-ray observations (via Chandra and XMM-Newton) and their extensive multiwavelength follow up. Highlights include the detection of ULXs, X-ray luminosity functions, and age and metallicity studies aimed at following the stellar population created in the wake of the shock wave. Although RiGs represent only a small fraction of galaxies (estimated to comprise 0.02–0.2% of all spiral galaxies), they represent a striking and dynamic environment, often appearing quite spectacular!

Cosmic dust is a small (1-2%) but significant component of the Galactic interstellar medium (ISM), appearing at every stage of stellar evolution—from evolved stars and supernovae to protoplanetary disks. Dust plays a crucial role in the evolution of our Galaxy: it cools interstellar clouds, enabling star formation; provides surfaces for chemical reactions; and is thought to be the primary building block of new planets. However, the exact chemical composition, size, and lattice structure, i.e. (non)-crystallinity, of ISM dust remain active areas of study. High-resolution X-ray spectroscopy is a powerful tool for examining the properties of interstellar dust. The X-ray band is sensitive to absorption from n=1 (K edge) and n=2 (L edge) electrons of the most abundant interstellar metals: C, N, O, Ne, Si, Mg, and Fe. In particular, X-ray absorption fine structures (XAFS) are spectroscopic features observed near the photoelectric absorption edges of solid material (dust). The shape of these features provides detailed information about the dust's chemical composition, size, morphology, and lattice structure. To study XAFS, we need up-to-date dust models. In this talk, I will discuss our collaborative effort to build a comprehensive X-ray dust extinction model based on laboratory experiments. I will demonstrate the latest laboratory experiments on XAFS from astrophysical dust samples at the O K and Fe L photoabsorption edges (Psaradaki et al. 2020, 2021). Furthermore, I will present recent findings (Psaradaki et al. 2023, 2024) on dust mineralogy in the diffuse regions of our Galaxy, utilizing new dust models and high-resolution X-ray spectra from the Chandra and XMM-Newton satellites. By analyzing the O K and Fe L edges for a sample of background sources along the Galactic plane, we discovered that Mg-rich amorphous pyroxene dust (Mg0.75Fe0.25SiO3) and metallic iron constitute the bulk of the dust chemistry in the diffuse ISM. This provides the most comprehensive view of dust mineralogy in the diffuse regions of the ISM through the X-ray energy band to date.

More than 7000 exoplanets have been discovered in the Milky Way. The methods used to find them rely on high precision measurements that are not possible for planetary systems in other galaxies. Chandra, on the other hand, was able to make a reliable detection of a short-duration transit to zero flux, for an X-ray binary (XRB) in the Whirlpool galaxy, M51. The transiting object, M51-ULS-1b is roughly the size of Saturn and is in a wide orbit. A comprehensive analysis of all possible models finds that only the planet model comfortably fits all of the data available ​from Chandra, XMM-Newton, and HST. The discovery was possible because of the coincidence that the effective radii of X-ray emitting regions in XRBs are comparable to planetary radii. A single transit can be used to determine the size of the transistor and its orbital speed at the time of transit. The discovery of M51-ULS-1b established the feasibility of using Chandra light curves to discover planets and other short-duration phenomena. We have continued our search through Chandra data for planets and other phenomena in the Milky Way, its globular clusters, and in other galaxies. We report on the program and some of its results, including possible X-ray triples. We also describe our plans for a publicly available catalog of Chandra-discovered short-duration events. We hope to engage a broader segment of the Chandra community in this type of ​e​xciting investigation.

The discovery of the first exoplanets precedes the Chandra launch by only a few years. At that time, nobody expected such low-mass objects to become targets for X-ray observations. However, the past two decades have shown that many exciting physical processes in star-planet systems can be probed by Chandra and other X-ray observatories. I will review cornerstone results in the areas of exoplanet atmospheres, star-planet interactions, exoplanetary space weather, and the evolution of planets around different types of host stars, which would have been impossible to achieve without Chandra.

We report resolution of a halo of soft X-ray emission surrounding the ZAMS G8.5V star HD 61005 by Chandra ACIS-S. Located only ~36 pc distant, HD 61005 is young (100 +50/-50 Myr), x-ray bright, observed with nearly edge-on geometry, and surrounded by Local Interstellar Medium material ~103 times denser than in the Sun’s environs. HD 61005 is known to harbor large amounts of circumstellar dust in a mm-sized particle dense ecliptic plane + extended “wing like structures” full of micron sized particles. While the dust wings are only a small fraction of the total sensible dust mass, they are evidence for a strong LISM-dust disk interaction. All these properties aided our ability to resolve the system's ~200 au (5.5”) wide astrosphere, a first for a main sequence G-star. The observed x-ray emission morphology is roughly spherical, as expected for an astrosphere dominated by strong stellar wind outflow. The Chandra spectrum of HD 61005 is a combination of a hard stellar coronal emission (T ~ 8 MK) at Lx ~6 x1029 erg/sec, plus a softer extended halo contribution at Lx ~1 x1029 erg/sec dominated by charge exchange (CXE) lines like OVIII and NeIX. Since the Chandra CXE x-ray morphology does not track the planar dust morphology but does extend out roughly to where the base of the dust wings begins, we present a toy model of x-ray emission produced by CXE SW-LISM interactions, tracing out the edges of an astrosphere beyond which micron-sized dust is deflected under LISM ram pressure.

Massive stars, i.e., spectral types O and B, represent a small fraction of the stars in the Galaxy yet have an outsized impact on their surrounding environment. This is due to not only their eventual supernovae but also their stellar winds, which are millions of times stronger than the Sun’s. Both are dependent on the star’s mass-loss rate, making it a key parameter for star formation and galactic feedback modelling. High spectral resolution X-ray observations with Chandra have become essential for constraining mass-loss rates and investigating the winds as the X-rays are generated within the winds through shocks between wind clumps. I will discuss the history of massive stars as surprising X-ray bright objects, how 25 years of Chandra observations have advanced our understanding of the physics governing the winds, and how future observatories will answer questions Chandra revealed.

The EWOCS (Extended Westerlund 1 and 2 Open Clusters Survey) project has the objective of extending our knowledge of star and planets formation and early evolution in starburst environments, from the study of the Galactic supermassive star clusters Westerlund 1 and 2. With a mass in excess of 10^4 solar masses, the very few supermassive star clusters known in the Milky Way represent the most accessible examples of starburst regions, which are very rare in our Galaxy today, but common in galaxies experiencing epochs of intense star formation. These regions are characterized by very high stellar density, and they are dominated by a rich and compact ensemble of massive stars that produce an environment dominated by energetic radiation and particles. With a distance of 3.87 kpc and 4.5 kpc, respectively, the Westerlund 1 and 2 clusters are the closest supermassive star clusters to the Sun, and thus the best targets to study star formation and evolution in the most energetic star forming environment known, and how such supermassive clusters can assemble and evolve. In this talk, I will present the status and the first results of the EWOCS project, which is mainly based on a 1Msec Chandra/ACIS-I Large Project and two JWST GO programs.

Understanding the high-energy environments of host stars is crucial for characterizing the atmospheres of terrestrial exoplanets. We present a comprehensive analysis of seven nearby benchmark exoplanetary systems with confirmed rocky exoplanets. By combining newly acquired data from Chandra and eROSITA monitoring with archival data from XMM-Newton, we investigate the X-ray flux behavior of these systems over varying timescales and estimate the resulting photoevaporation mass loss of their exoplanets. Our study employs a Bayesian framework for inferring the multi-temperature coronal plasmas. Integrating with the VPLanet modeling package, we assess the environmental context shaped by XUV flux and predict the atmospheric conditions and evolution. This presentation will highlight the diverse high-energy characteristics of currently known rocky planet host stars and what this means for the atmospheres of our neighbors.

The combination of the South Pole Telescope and the Chandra X-ray Observatory have allowed us to study the evolution of the hot intracluster medium over the past 10 billion years. In this talk I will summarize ongoing work to study the ICM in the most distant clusters with Chandra, including (i) the evolution of the AGN feedback / cooling balance, (ii) the evolution of cool cores and cooling flows, (iii) the evolution of the ICM metallicity, and (iv), the evolution of the cluster dynamical state. These studies require the exquisite angular resolution of Chandra, which can detect structure in the ICM at the epoch of cluster formation.

Chandra observations of galaxy clusters have played an important role in helping to consolidate the standard model of cosmology. Early Chandra measurements of the baryonic mass fraction in clusters enabled precise constraints to be placed on the mean matter density of the Universe, while the extension of these measurements to higher redshifts enabled direct confirmation of the discovery from supernovae studies that the expansion of our Universe is accelerating. Chandra observations transformed cosmological studies with cluster counts, enabling the first detection of the effects of dark energy in slowing the growth of cosmic structure. Chandra has also provided key insights into the nature of dark matter, facilitating stringent tests of the cold dark matter paradigm, and has enabled novel tests of gravity, neutrino properties and inflation.

Over the last years, the synergy between multi-wavelength facilities has revolutionized the understanding of the complex interplay between cluster-central jetted AGN and their surrounding hot gas halo. Chandra has played a key role in revealing how this interplay involves heating and cooling processes that are regulated by the central AGN. In this talk, I will focus on the strong cool core galaxy cluster RBS797: first observed in the early 2000s and then revisited in 2020 with a Chandra Large Program observation (400ks), this system offers a perfect opportunity to investigate extreme AGN feedback in action. The Chandra data revealed that, within its central 150 kpc, RBS 797 hosts four perpendicular and equidistant X-ray cavities, as well as three nested shock fronts, all generated by the central AGN over successive epochs of jet activity. Thanks to the combination of the X-ray Chandra data with optical HST imaging, and JVLA, LOFAR, EVN, VLBA and eMerlin radio data, over the last few years our team has provided new insights into the formation of shocks and X-ray cavities, heating and cooling balance, repetitive feedback, jet reorientation and precession mechanisms. These results highlight that only high resolution Chandra observations can connect the complex structure of hot cluster atmospheres with the propagation of energetic jets from radio galaxies.

I will highlight how Chandra's spectroscopic and imaging capabilities have allowed us to observationally probe dark matter parameter space across twenty orders of magnitude in particle mass. First, I will describe direct and indirect detection searches for three leading dark matter candidates: the WIMP, the sterile neutrino, and the axion. Second, I will outline the growing interest in probing light and ultralight dark matter with high-energy astrophysics. Third, I will focus on the sweet spot for dark matter searches with current X-ray observatories (dark matter mass $\leq$ keV). The bulk of my talk will highlight how 25 years of Chandra have contributed to our understanding of the possible composition of dark matter. I will finish by reflecting on the prospect of reassessing the dark matter interpretation of the 3.5 keV line in view of the Chandra Legacy Program and ongoing XRISM observations.

We present new mid-infrared IFU observations of the Phoenix Cluster (SPT-CLJ2344-4243) with JWST. We use this data to map out multiple gas phases, including warm molecular gas (H2), warm ionized gas at 10^4 K (traced by [Ne II]), and coronal gas at 10^5.5 K (traced by [Ne VI]). For the first time in any galaxy cluster, we detect extended coronal emission on scales >10 kpc. These new data represent more than an order of magnitude improvement in spatial resolution compared to previous observations of the coronal gas phase using O VI 1032,1038 A with HST-COS, and they are in a wavelength regime with almost no extinction. By combining these new data with deep Chandra observations, we find that the intermediate-temperature (10^5.5K) gas is cospatial with the minimum entropy of the hot ICM, indicating that the cooling time is at a minimum, and is trailing in the turbulent wake of a buoyantly rising X-ray bubble. The bubble may be inducing cooling by uplifting low-entropy gas from the core and/or driving in-situ formation of coronal gas through turbulence. We estimate the cooling rate from the MIR lines to be between 5,000-23,000 Msun/yr and propose that this represents a short-lived condensation of the hot atmosphere. These results highlight the critical role that Chandra’s high angular resolution plays in multiwavelength analyses, with all of the relevant physics (bubbles, low-entropy peak, central quasar) happening within the inner 1-2” of the Phoenix cluster.

The Perseus Cluster's intracluster medium displays spiral-shaped X-ray surface brightness discontinuities, commonly referred to as cold fronts. These structures act as invaluable indicators of the intricate physical processes at play within the hot gas and the merger history of galaxy clusters. Our presentation will focus on recent observations in Perseus, where cold fronts extend to significant radii, posing questions about the merger events responsible for such structures. Trying to unveil the merger scenarios that generate and sustain cold fronts capable of propagating to large radii, we will present simulations of binary cluster mergers using the AREPO magnetohydrodynamics code exploring various initial conditions. The results will shed light on the nuanced aspects of cold fronts, including their propagation time, distribution, and constraints on subcluster position and trajectory — a particularly crucial aspect given the challenges in identifying the subcluster in sloshing cold front clusters.

Chandra has transformed our understanding of the cosmic cycle of elements, from their creation in stellar cores and explosions to the chemical composition of interstellar dust, which seeds star formation and planetary disks. When it comes to studying exoplanets, a holistic view of habitability, examining factors beyond the traditional "habitable zone,” now includes the impact of stellar activity on exoplanetary atmospheres and surface radiation. While X-ray emission decreases as stars age, key questions remain about the variability and occurance rates of extreme events in Sun-like and low-mass stars — prime candidates in the search for potentially life-bearing planets. This talk will highlight Chandra’s contributions to exoplanet science and its lasting influence on next-generation searches for habitable worlds.

With the advent of large-scale photometric monitoring and exoplanet search programs, the number of stars with known magnetic activity cycles (like the 11-year solar cycle) has grown rapidly. In parallel, advances in theoretical modeling are leading to a revolution in our understanding of stellar structure and magnetic field generation in late-type stars. A key diagnostic of magnetic activity is X-ray emission, which complements perspectives on stellar cycles provided by more common photospheric and chromospheric monitoring. Of the hundreds of stars now known to have activity cycles, only about 50 have well studied CaII H&K chromospheric emission, and only ten (including the Sun) have published X-ray cycles. Although the X-ray sample is very small, some important insights have emerged, such as the correlation of cycle amplitude and Rossby number (dependent on rotation). Over the past fifteen years, we have conducted a program that nearly doubles the number of stars with X-ray cycle monitoring; here we present the latest results obtained from observations by Chandra, XMM, and Swift.

Using Chandra ACIS-S, from 2014 to 2015 we conducted low-resolution x-ray imaging spectrophotometry of the Pluto system in support of the New Horizons flyby. Over 174 ksec of observation, we detected 8 x-ray photons in the 0.31 – 0.60 keV range within an 11 x 11 pixel² region (covering ~121,000 x 121,000 km² or ~100 R_Pluto x 100 R_Pluto) at Pluto, with no photons detected in the 0.60 – 1.0 keV range. After accounting for background, we observed a net signal of 6.8 counts with a statistical noise level of 1.2 counts, indicating a > 99.95% confidence detection of Pluto in this energy range. The detected photons are aligned with the 90% flux aperture co-moving with Pluto, do not match the spectral shape of the background, and are not confused with any background sources. These photons imply an average X-ray power from Pluto of 200 +200/-100 MW, consistent with the range of X-ray emissions from known solar system sources such as auroral precipitation, solar X-ray scattering, and charge exchange (CXE) between solar wind ions and atmospheric neutrals.Auroral effects are ruled out as a source as Pluto lacks a known magnetic field, and the New Horizons Alice UV spectrometer observed no airglow. Although nano-scale atmospheric haze could enhance solar x-ray scattering, the energy signature of the detected photons does not correspond to the solar spectrum, and estimates of Pluto’s scattered x-ray emission are 2 to 3 orders of magnitude lower than the observed rate of 3.9 ± 0.7 x 10⁻⁵ cps. CXE from solar wind carbon, nitrogen, and oxygen ions could account for the observed emission, and the 6 x 10²⁵ neutral gas escape rate from Pluto (as per Gladstone et al. 2016) supports the ~3.0 +3.0/-1.5 x 10²⁴ X-ray photons/s emission rate required. Using solar wind proton data from the Solar Wind Around Pluto (SWAP) instrument, we estimate that the minor ion flux in an 11 x 11 pixel² region around Pluto is 40 +40/-20 times lower than needed to support the observed emission rate, suggesting that the solar wind must be significantly focused and enhanced within 60,000 km of Pluto. Confirmatory measurements by our team using XMM in 2017, during the last solar minimum, did not yield statistically significant results. New x-ray observations during the strong 2024 – 2025 solar maximum of the Pluto system would be highly beneficial.

Stellar X-ray and UV radiation can significantly affect the survival, composition, and long-term evolution of the atmospheres of planets in or near their host star’s habitable zone (HZ). Especially interesting are planetary systems in the solar neighborhood that may host temperate and potentially habitable surface conditions, which may be analyzed by future ground and space-based direct-imaging surveys for signatures of habitability and life. To advance our understanding of the radiation environment in these systems, we have leveraged ∼3 Msec of Chandra and XMM-Newton observations in order to measure three fundamental stellar properties at X-ray energies for 57 nearby FGKM stellar systems: the shape of the stellar X-ray spectrum, the luminosity, and the timescales over which the stars vary (e.g., due to flares). These systems possess HZs that will be directly imageable to next-generation telescopes such as the Habitable Worlds Observatory and ground-based Extremely Large Telescopes (ELTs). We identify 29 stellar systems with LX/Lbol ratios similar to (or less than) that of the Sun; any potential planets in the habitable zones of these stars therefore reside in present day X-ray radiation environments similar to (or less hostile than) modern Earth, though a broader set of these targets could host habitable planets.

The most powerful stellar flares driven by magnetic energy in single stars occur during the early pre-main sequence (PMS) phase. We review the global properties of thousands of PMS flares captured through observations of over 40 Galactic star-forming regions by the Chandra X-ray Observatory. These properties are compared with those of contemporary solar flares, examining aspects such as stellar magnetic dynamos, surface magnetic field configurations and strengths, as well as flare-related plasma temperatures, durations, energetics, coronal extents, and occurrence rates. Overall, PMS flares represent enormously scaled-up solar flares. Understanding these extreme events provides valuable insights into the early evolution of stars and impact of energetic flare radiation on surrounding environments, including protoplanetary disks and young planets.

Chandra's high spatial resolution and sensitivity has revolutionized our understanding of supernovae, supernova remnants (SNRs), and the hot interstellar medium around massive stars. I will review some of the major advances that Chandra has enabled related to SNRs and to massive star-forming regions in the Milky Way and nearby galaxies.

Over its 25-year life span Chandra X-ray Observatory (CXO) has performed deep observations of about 20 pulsar wind nebulae (PWNe). We will present the highlights of these observations with a focus on the connection between small- and large-scale structures of PWNe. The latter include ram-pressure-confined parsec-long pulsar tails, faint large-scale extensions of pulsar jets swept back by the ram pressure due to the pulsar motion, enigmatic collimated outflows strongly misaligned with the pulsar motion direction, and faint asymmetric diffuse emission components extending for several parsecs from the pulsar. We will also discuss the variability observed for several PWNe, thanks to multiple CXO observations over a long time span, and connection to the radio and TeV counterparts.

Planetary nebulae (PNe) are fleetingly brief but highly revealing episodes that mark the termination of the lives of intermediate-mass (1-8 solar mass) stars. Beginning in Cycle 1, Chandra has revealed X-ray-emitting "hot bubbles" and collimated outflows, as well as central star X-ray sources, within dozens of PNe. Individually and collectively, these results have provided unique insight into energetic stellar wind collisions and PN progenitor star systems. I will describe the progress in our understanding of PNe engendered by Chandra X-ray imaging throughout its lifetime. I first summarize the statistics and trends of diffuse and point-like PN X-ray sources yielded by the multi-Cycle Chandra Planetary Nebula Survey, and then present first results from joint Cycle 25 Chandra/JWST studies of nebula-shaping shocks in the rapidly evolving, point-symmetric PNe NGC 2440 and NGC 6537.

Chandra's arc-second-level spatial resolution, combined with its superb spectral resolution, has led to several new advances in understanding the X-ray evolution of young supernovae (SNe). In this talk we will present examples of Chandra X-ray spectroscopy of SNe in which the author was involved, and that have led to a significant increase in our understanding of young SNe, including but not limited to: (1) SN 2012ca, a type Ia-CSM, which was the first (and till now the only) Type Ia SN to be detected in X-rays. This SN appears to be evolving in an extremely high-density environment, atypical of Type Ia SNe. (2) SN 1993J, perhaps the best observed X-ray SN in the northern hemisphere. (3) SN 2005kd, one of the brightest Type IIn SNe. (4) SN 1986J, a SN that is still detectable in X-rays more than 4 decades after explosion, and appears to be expanding in a complex multi-component medium. For the latter three, we will discuss their temporal and spectral evolution, and synergies with multi-wavelength data that have contributed to enhance our understanding of these SNe. In the process we will illustrate why Chandra was often the only mission, existing or planned, that was able to carry out these X-ray observations, and how young SN research will be severely impacted without Chandra's arc-second spatial resolution.

Kepler's supernova remnant (SNR), the remnant of the famous historical Type Ia supernova (SN 1604), provides an excellent opportunity to study thermonuclear stellar explosions and their evolution in a modified environment. Based on our recent Chandra Large Program of deep HETG observations of Kepler's SNR, we perform the most comprehensive measurements of radial velocities to date for numerous small metal-rich ejecta features. We also measure proper motions of these ejecta features using the wealth of archival Chandra imaging data taken at several epochs between 2000 and 2022. These high-resolution X-ray spectral and imaging data sets, combining the total of ~600 ks HETG and ~1 Ms ACIS observations, represent an unprecedented opportunity to reveal the 3-D structure of the clumpy SN ejecta in Kepler's SNR. Our preliminary results show radial velocities for dozens of ejecta knots ranging from -3000 to +7000 km s⁻¹, which aligns statistically with Millard et al. (2020), despite their study using a much smaller sample size and significantly shorter exposure times than ours. We aim to expand our sample size of small ejecta knots by an order of magnitude over that by Millard et al, with the goal of constructing an extensive 3-D map of clumpy ejecta features throughout the entire Kepler's SNR.

Since its official "First Light" image of Cassiopeia A, the Chandra X-ray Observatory has revolutionized our understanding of the cosmos through its exquisite angular resolution. Its sub-arcsecond view of the X-ray sky has made powerful contributions to almost every area of astrophysics. I will give an overview of some of the most exciting discoveries of the past two and a half decades that were uniquely enabled by Chandra's angular resolution and focus on some areas in which I have been closely involved, including gravitationally lensed quasars and close binary systems in globular clusters.

Supernova 1987A (SN 1987A) continues to be a cornerstone for our understanding of core-collapse supernovae and their aftermath. While previous analysis of the Chandra observations have already provided valuable insights into the evolution of this iconic supernova remnant, the spatial resolution has still been limited by the Point Spread Function (PSF).  In this contribution, we present a uniform re-analysis of almost 100 observations of archival SN1987A data using “Jolideco”: a new joint likelihood deconvolution method which allows us to resolve image structure on a sub-PSF scale.  As a result we obtain the highest X-ray resolution images available to date and are able to trace the evolution of the smallest resolved features of the remnant over a timeline of 25 years.Using a high exposure observation sequence from 2007 to 2010 we can even resolve the X-ray emission from the inner ejecta (“keyhole”) as well as parts from a secondary ring of SN1987A for the first time. Both features correspond to structures seen in images of HST and JWST. By combining subsequent observations in a weighted likelihood scheme, we can also reconstruct and visualize the evolution of the SNR with combined high time and spatial resolution. We clearly confirm the change in the east-west asymmetry and the beginning of the fading of the remnant. Furthermore we share insights from studying systematic uncertainties of the PSF and discuss the agreement between ACIS grating and non-grating observations in image analysis.

The high-energy universe reveals some of the most energetic and extreme sources, such as active galactic nuclei (AGN). However, our ability to fully understand the complex origins of the high energy emission and potentially discover exotic AGN systems is severely constrained by the limited angular resolution and astrometry of current X-ray telescopes. Gravitational lensing presents a unique opportunity to significantly enhance the performance of the Chandra X-ray telescope, boosting its spatial resolution from kiloparsec- to parsec-scales. In this talk, I will discuss how gravitational lensing can be leveraged to enhance the spatial resolution of the Chandra telescope. I will review recent results from AGN offset and binary AGN searches, focusing on sources such as B0712+472, B1608+656, and B2016+112, where just 24 X-ray photons revealed a double source at X-ray wavelengths. Additionally, I will explore the potential of using both new and archival Chandra X-ray data to pave the way for detecting offset and binary AGN at small separations. With upcoming large-scale surveys by the Square Kilometre Array (SKA), Euclid, and the Vera C. Rubin Observatory expected to discover ∼10^5 strong lenses, there is a tremendous potential to further our understanding of AGN evolution and identify supermassive black hole binary candidates—key targets for gravitational wave detection with Pulsar Timing Arrays (PTA).

The raw instrument (ACIS) resolution of Chandra is 0.5". But the HRMA PSF is narrower with 20% of the power lying within0.25", a factor ~2 smaller. This resolution can be recovered ecause Chandra continuously moves its pointing position on the sky (called "dither'.) This allows the use of the sharp central peak of the Chandra PSF to reconstruct ~¼ arcsecond resolution images. This capability was planned for by Leon van Speybroeck and is a continuous version of the "drizzle" technique used with HST. That extra resolution has allowed us to resolve the faint X-ray structures around a two dozen nearby AGNs. With this enhanced resolution we can match X-ray with HST, ALMA, JWST imaging. We learn about AGN feedback from the direct effects of radiation, fast-moving AGN outflows and relativistic jets on scales from ~30 pc to a few kiloparsecs. Diagnostic soft X-ray emission lines pick out jet/ISM interactions and the photoionized ISM, while the high energy continuum and Fe-K lines pick out interactions with molecular clouds in the central ~kiloparsec. A microcalorimeter IFU on a Lynx-like mission would gives us kinematics and better diagnostics to see which feedback mechanism dominates in which physical cases.

As radio jets evolve, they significantly impact their environment. Chandra observations reveal their large-scale effects through cavities, bubbles, and shock signatures in the hot medium of X-ray clusters. On galaxy scales, jets can generate turbulence, influence star formation, and drive outflows, actively contributing to feedback processes. These smaller-scale feedback signatures are best studied in compact radio sources (less than a few kpc in size) where the relatively young radio structures are fully embedded within the galaxy's environment. Chandra observations of such compact radio sources offer an unprecedented X-ray perspective on the energetics and structures associated with these evolving radio jets. In this contribution, we present the results of a deep Chandra observation of PKS 0023-26, a powerful radio source a few kiloparsecs in size, which has the potential to significantly affect the interstellar medium of its host galaxy. Our study provides the highest-resolution X-ray image to date, allowing us to directly trace associations of multi-band structures on sub-arcsec scales. Specifically, X-rays reveal the properties of the hot medium, which may be heated by the jet, while ALMA sub-mm observations offer detailed insights into the ionization state, distribution, and velocity of the molecular gas. Morphology of molecular gas at various excitation levels relative to the X-ray structure suggests the presence of a cocoon formed by a shock associated with the evolution of the radio source. Additionally, as determined by the kinematics of the molecular gas, the impact of the jet is the strongest in the sub-kiloparsec region. At larger distances, the molecular gas is less impacted by the jet, but the velocity structure is aligned with the radio lobes consistent with the presence of a cocoon affecting pre-existing molecular clouds. This supports the results of numerical simulations indicating that the impact of the AGN on the ISM is not limited to outflows.

Many classes of X-ray source emit across a broad fraction of the electromagnetic spectrum, from radio through gamma-rays. In the inner region of our Milky Way Galaxy, in the jets of nearby active galactic nuclei, in the cores of globular clusters, and in other galaxies, there are classes of sources that require sub-arcsecond angular resolution in the X-rays and in other bands to be understood. I will discuss these classes of sources, and how current and upcoming optical, infrared and radio facilities are vital, along with facilities like Chandra, in order to understand the physics of these exciting objects.

Although supermassive black holes (BHs) are widely observed in both the nearby and distant universe, their origins remain debated, with two viable formation scenarios. In the light seed model, BHs form from the collapse of massive stars, whereas in the heavy seed model, they originate from the collapse of pristine, massive gas clouds. Detecting BHs in the high-redshift universe offers the most effective way to probe the origins of the first BHs. In this presentation, I will focus on the unique synergy between Chandra and JWST, which allows the detection and characterization of BHs in the early universe (z~10). Based on ultra-deep Chandra X-ray data, we identified an X-ray-luminous, massive BH in UHZ1, a JWST-identified, gravitationally-lensed galaxy at z=10.1. Interestingly, the mass of this BH, 10^7-10^8 Msun, is comparable to the stellar mass of its host galaxy. This unusually high BH mass, combined with the exceedingly large BH-to-galaxy stellar mass ratio merely 500 million years after the Big Bang, suggests that the BH in UHZ1 likely originates from heavy seeds. Finally, I will discuss prospects for detecting additional high-redshift BHs in other fields, and highlight upcoming observations with Chandra and JWST.

JWST has revealed a population of low-luminosity AGN at $z>4$ in compact, red hosts (the "Little Red Dots'', or LRDs), which are largely undetected in X-rays. We investigate this phenomenon using GRRMHD simulations of super-Eddington accretion onto a SMBH with $\Mblack=10^7\Msun$ at $z\sim6$, representing the median population; the SEDs that we obtain are intrinsically X-ray weak. The highest levels of X-ray weakness occur in SMBHs accreting at mildly super-Eddington rates ($1.4<\fedd<4$) with zero spin, viewed at angles $>30^\circ$ from the pole. X-ray bolometric corrections in the observed $2-10$ keV band reach $\sim10^4$ at $z=6$, $\sim5$ times higher than the highest constraint from X-ray stacking. Most SEDs are extraordinarily steep and soft in the X-rays (median photon index $\Gamma=3.1$, mode of $\Gamma=4.4$). SEDs strong in the X-rays have harder spectra with a high-energy bump when viewed near the hot ($>10^8$ K) and highly-relativistic jet, whereas X-ray weak SEDs lack this feature. Viewing a SMBH within $10^\circ$ of its pole, where beaming enhances the X-ray emission, has a $\sim1.5\%$ probability, matching the LRD X-ray detection rate with Chandra. Next-generation observatories like AXIS will detect X-ray weak LRDs at $z\sim6$ from any viewing angle. Although many SMBHs in the LRDs are already estimated to accrete at super-Eddington rates, our model explains 50% of their population by requiring that their masses are overestimated by a mere factor of $\sim3$. In summary, we suggest that LRDs host slowly spinning SMBHs accreting at mildly super-Eddington rates, with large covering factors and broad emission lines enhanced by strong winds, providing a self-consistent explanation for their X-ray weakness and complementing other models.

Efficient gas accretion through mergers can trigger luminous Active Galactic Nuclei (AGN), but hide them behind heavy obscuration reaching the Compton-thick regime. Most photons created near the super-massive black hole remain trapped, while the hard X-ray photons can escape and reach our telescopes. The superb X-ray focusing ability of Chandra's mirrors and the high throughput of Chandra/ACIS reveal signatures of Compton-thick black hole accretion. Through ray-tracing simulations varying the assumed AGN torus geometry we learn that the Fe fluorescent line and the Compton hump are geometry tracers. At z~0.5-3, with these signatures redshifted into the sensitive Chandra energy window, Chandra is still holding the record for largest samples of Compton-thick AGN with the deep and wide Chandra surveys and their extensive multi-wavelength legacy data. Based on these, I will review constraints of the Compton-thick accretion history over cosmic time, and present self-consistent survey analyses varying the assumed obscurer geometry. This successfully predicts the cosmic X-ray background at harder energies, and indicates that the Compton-thick fraction is robustly known to ~1/3. I will close with an outlook on resolving the recent discrepancy with JWST's newly discovered populations at z>6 with Athena.

Galaxy evolution is driven largely by the baryon cycle of material flowing into and out of galactic atmospheres. Only in galaxy clusters, where galactic atmospheres get hot enough to glow in X-rays, can this entire baryon cycle be observed -- from the rapid cooling of the intracluster medium (ICM) that contributes to the level of star formation in the central galaxy, to the eventual feeding and triggering of feedback from the largest supermassive black holes (SMBHs) residing in them. Such cooling and subsequent feedback from these active galactic nuclei (AGN) is the primary driver of the baryon cycle in the most massive, brightest cluster galaxies (BCGs), and until recently, has only been observable in the most nearby or brightest galaxy clusters. In this talk, we build on these previous works by assembling a large (N = 95) sample of BCGs from Sunyaev-Zel’dovich detected South Pole Telescope clusters, spanning a large redshift range (0.3 < z < 1.7) corresponding to 10 Gyr in evolution. This unique dataset is complete with multi-wavelength follow up from Chandra, providing great legacy value for this one-of-a-kind observatory, as well as observations from Magellan, ATCA, and others. This comprehensive coverage allows us to robustly study the evolution of BCG growth channels, the effectiveness of AGN feedback over time, and the conditions for ICM cooling as a function of redshift. In this talk, we will focus in particular on whether the observed ICM entropy threshold below which multiphase cooling triggers star formation and AGN feedback at low redshifts persists to higher redshifts and has evolved with time.

Abstract Pending

In the binary system SS 433, oppositely directed, precessing jets produce X-ray line emission from highly ionized, metal-laden plasma moving at 0.26c from the compact object. Due to precession of the jet direction, the Doppler shifts of the emission lines vary over a range of over 80,000 km/s (both blue- and red-shifted) during the precession period of 162 days. Over the course of 25 years of Chandra operations, there have has been just over one mega-second of observations of this target. About 80% of these observations have been obtained with the HETGS in 20-150 ks portions that show many X-ray emission lines and the rest involves imaging of the arcsecond-scale extended emission discovered in the first Chandra observation of the binary. We will summarize what we have learned from the HETGS observations about the jets' contents, their sizes, their shapes, the jet launching process and evolution from a continuous stream into discrete emission features (as observed in the radio and optical bands), and their impact on the local environment. In addition, we will present results from a recent study of the connection between the X-ray intensity (using Swift) and the extended X-ray flux (using the Chandra HRC) as a test of the hypothesis that SS 433 is an ultra-luminous X-ray source (ULX) that is oriented about 80 degrees to the line of sight.

The Galactic center (GC) is home to the largest known concentration of exotic X-ray sources ever identified in our Galaxy, including low-mass X-ray binaries (LMXBs) and cataclysmic variables (CVs), as well as thousands of faint X-ray sources – detected by Chandra – whose nature is unclear. Despite their significance for our understanding of galaxy evolution, the rate of gravitational wave events, and other astrophysical and cosmological topics, the positive identification of most GC X-ray sources has to date remained elusive. Utilizing data from two new groundbreaking surveys that were carried out with the XMM-Newton and Chandra X-ray observatories, in conjunction with near-infrared surveys, we are conducting a comprehensive population study into the characteristics and distributions of the X-ray sources in the GC and the surrounding areas. We investigate the nature of the various source populations, as well as their spatial and luminosity distributions and how they vary in distinct regions, like the Central Molecular Zone (CMZ) and the Nuclear Stellar Disk (NSD). This study will improve our understanding of the compact object populations in the GC.

The accretion flows in low-mass X-ray binaries (LMXBs) are commonly accompanied by jet outflows or disk winds. Dedicated studies of LMXBs with luminosities up to ~20% Eddington suggest that these types of outflows are to a large extent mutually exclusive suggesting accretion disk winds as a possible jet suppression mechanism (e.g. Ponti et al. 2012). However, in a more recent study of near- and super-Eddington LMXBs, Homan et al. (2016) found four sources that show evidence that, at high luminosities, jets and winds might coexist. The presence of jets in these sources is deduced from radio observations, while disk winds are inferred from high-resolution X-ray spectra. The lack of strictly simultaneous observations prevented them from robustly concluding that, in these systems, jet and disk winds are launched at the same time. To explore this scenario further, we present a dedicated multi-wavelength multi-instrument (JVLA, Chandra/HETG, Nicer) campaign of the near-Eddington neutron-star LMXB GX 13+1. The simultaneous radio and (high-resolution) X-ray observations show the presence of a persistent ionised wind and a highly variable jet outflow. This not only verifies the coexistence of the two distinct outflows, but also presents an exclusive opportunity to study the connection between the wind, the jet, and the X-ray continuum components. Based on the results of our multi-wavelength campaign, I will present a new view of the interplay between jets and disk winds at near-Eddington luminosities, providing the implications on the outflow launching mechanisms, and comparing it with the relation observed between accretion disk wind and jet in other bright LMXBs (e.g. the microquasar GRS 1915+105).

An important legacy of Chandra will be the grating spectra of novae in outburst, more than 50 exposures of 19 novae - two of them observed in two different recurrent nova outbursts - and of 8 supersoft X-ray sources that appear to be steadily burning hydrogen in shell. The LETG gives us a window on the shell surface phenomena with spectra of the surface of these extremely hot white dwarfs with their atmospheres and outflows. Both LETG and HETG probe the powerful shocks after a nova outburst, that, initially, even accelerate particles. We have observed and analyzed a wide variety of phenomena, including charge exchange and using these luminous sources as lamp-posts to study the ISM in certain directions.

The era of data-driven discovery is producing a wave of new science in high energy astrophysics, and the Chandra Source Catalog (CSC) is an effective tool that enables it. The treasure trove found in CSC datasets has propelled population studies, the search for high energy transients, and the characterization of accretion in luminous X-ray systems. But the CSC has also become a valuable tool in machine learning studies, as it provides an exquisite training set that relates the basic units of X-ray data -single photon detections from a source- to astrophysically relevant measurables such as the spectral parameters of acreeting binaries or the timescales of exotic cosmic explosions. In this talk, I will present an overview of how the X-ray community is using machine learning in combination with the CSC to produce new representations of X-ray datasets that improve of fundamental tasks of X-ray astronomy, such as the classification of sources, the inference of physical parameters, and the discovery of anomalies of astrophysical relevance. I will also provide a perspective of how the CSC -and Chandra data in general- represent a legacy dataset as we enter the era of foundation AI.

We leverage JWST data from COSMOS-Web in order to provide updated measurements on the autopower spectrum of the now resolved Cosmic Infrared Background (CIB) and the cross-power spectrum of the CIB and unresolved Cosmic X-ray Background observed by Chandra at z > 6. IR sources are masked below the Spitzer limit and and cross-correlated with the [0.5-2] keV CXB. We find that on scales between 5 − 3000′′ component in the CXB-CIB cross-power spectrum consistent with previous measurements with a signal-to-noise ratio (S/N ) of 5.15 and 3.87, for fluctuations between z = 0 – 13 and z = 6 – 13, respectively. These results indicate that there are X-ray populations among galaxies, resolved by JWST, including star-forming galaxies (SFGs) and active galactic nuclei (AGNs). We fit the CXB-CIB cross-power spectra to recover the X-ray number counts as a function of redshift which our consistent with both SFGs and AGNs. Further modelling of the CXB-CIB cross-power signal will be implemented to better constrain the contributions from both populations with respect to redshift. Finally we find the by including JWST sources Chandra was able to completely identify the nature of the unresolved galactic CXB: mission accomplished!

During its 25 years of observations, Chandra has uncovered more than 400,000 X-ray sources. Most of these sources were observed serendipitously, thus their nature remains largely unknown. Typically, the X-ray data alone is insufficient to reliably classify these sources, particularly for the faint sources whose population dominates the Chandra Source Catalog, so additional multi-wavelength data must be used. We have developed a multi-wavelength machine-learning classification pipeline (MUWCLASS), that can quickly classify a large number of CXO sources, taking into account multi-wavelength (e.g., optical, NIR, IR) information. Here we will discuss the results of several recent studies carried out using this pipeline in various environments, including unidentified Fermi-LAT and TeV sources. We will also discuss upgrades we are making to the pipeline to expand its capabilities to new datasets with larger positional uncertainties (e.g., XMM-Newton, eROSITA).

By combining Chandra’s sub-arcsecond on-axis spatial resolution and low instrumental background with consistent data processing, the Chandra Source Catalog (CSC) provides carefully-curated, high-quality, and uniformly calibrated and analyzed tabulated positional, spatial, photometric, spectral, and temporal source properties. Multiple types of source- and field-based FITS format science-ready X-ray data products are provided that can be used as a basis for further research, significantly simplifying followup analysis of scientifically meaningful source samples. CSC Release 2.1 was released to the community in April 2024, and provides properties for 407,806 unique X-ray sources on the sky, extracted from more than 1.3 million individual observation detections identified in 15,533 Chandra ACIS and HRC-I imaging observations released publicly prior to the end of 2021. The compact detection sensitivity limit for most observations is ∼5 photons over most of the field of view, achieved by co-adding observations and using an optimized source detection approach. A Bayesian X-ray aperture photometry code produces robust fluxes even in crowded fields and for low count sources. CSC 2.1 is tied to the Gaia-CRF3 astrometric reference frame for the best sky positions for catalog sources. We describe the content and characteristics of the catalog, discuss the updates that significantly enhance the scientific utility of CSC 2.1, and demonstrate how the catalog content can be immediately and effectively utilized for scientific investigations. This work has been supported by NASA under contract NAS 8-03060 to the Smithsonian Astrophysical Observatory for operation of the Chandra X-ray Center.

The non-linear X-ray to UV luminosity relation in quasars can be used to derive their distances, and to build a Hubble diagram up to z~7. I will present a series of observational results, based on Chandra and XMM-Newton X-ray observations, strongly supporting the redshift stability of the relation and its intrinsic precision. In particular, I will show that (1) the slope of the relation does not evolve with redshift; (2) the spectral properties of the quasars in our sample are the same at all redshifts; (3) the dispersion of the relation becomes much smaller if precise spectroscopic flux measurements are used; (4) the Hubble diagram of quasars and that of supernovae are in full agreement in the common redshift range; (5) the residual intrinsic dispersion is fully explained by quasar variability and disk inclination effects. I conclude that the X-ray to UV non-linear relation is due to a universal, redshift-independent physical process, and provides reliable distance measurements.

I will discuss some of the exciting transient and time domain science enabled by Chandra’s sensitivity and high spatial resolution. I’ll start by outlining the physics we derive from long-term X-ray and multi-wavelength monitoring of the neutron star merger, GW170817. Then I’ll discuss the ongoing LIGO-Virgo-KAGRA O4 campaign in which some 2-10 NS-NS and NS-BH mergers are anticipated (median localization ~100 sq.deg). I’ll also briefly review key results from studies of supernovae, gamma-ray bursts, fast radio bursts, and tidal disruption events, all enabled by Chandra. I’ll wrap up with a wider discussion of explosive transients and prospects for studying them with existing, planned, and proposed X-ray facilities.

Quasi-periodic Eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs), undergoing instabilities or interacting with a stellar object in a close orbit. It has been suggested that this disk could be created when the SMBH disrupts a passing star, implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs, and two observed TDEs have exhibited X-ray flares consistent with individual eruptions. TDEs and QPEs also occur preferentially in similar galaxies. However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. I will report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 hours from AT2019qiz, a nearby and extensively studied optically-selected TDE. Our work establishes the connection between TDEs and QPEs. We detect and model the X-ray, ultraviolet and optical emission from the accretion disk, and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs. I will discuss the critical role of Chandra in this discovery and implications for future studies of QPEs in the imminent era of the Rubin observatory.

Due to galactic mergers, massive black hole binaries are thought to reside in the cores of numerous galaxies. As the massive black holes migrate inwards, they will eventually emit gravitational waves, which are expected to be detected by LISA. A critical component to understanding where and how black holes merge and how they shape galactic evolution is host galaxy identification, which relies on electromagnetic (EM) observations. In my talk, I will show novel tell-tale EM signatures that would provide strong evidence for a black hole binary before or during a merger. I will argue that when the binary is close to merger, the accretion is disrupted, turning the binary X-ray dark. I will argue that the upcoming time-domain surveys and X-ray mission such as Chandra might be able to observe these signatures, and that they could be crucial for LISA source identification.

As of today, we know of hundreds of supermassive black holes that exist during the Epoch of Reionization—and the more we find, the denser the puzzle on accretion physics and early Universe conditions becomes. However, fundamental insights into the accretion of supermassive black holes come from X-ray observations, and, so far, very few X-ray photons have been detected for a handful of these sources. With enough X-ray luminosity to be detected in a 160 second scan by SRG/eROSITA, the quasar CFHQS J142952+544717 (z = 6.19) is, by far, the X-ray brightest object known in the Epoch of Reionization. With an X-ray luminosity in excess of 10^46 erg/s and a billion solar mass black hole lurking at its center, it is radio loud, yet it does not show clear signs of beaming. Follow-up XMM and Chandra observations unveiled several details of the system, though they cannot conclusively pinpoint the nature of the source. By coincidence, 110 days after the Chandra pointing, we obtained NuSTAR data of J1429. And if we thought we knew something about the system so far, we were mistaken: the source 3-7 keV flux varied by a factor of 2.6 in ~15 days (rest-frame)! This brightening is the quickest X-ray variability ever seen in the Epoch of Reionization. In this talk, I will present our combined analysis of J1429 with Chandra and NuSTAR, highlighting the possible scenarios that could produce such rapid variability.

Over the last two and a half decades, the Chandra X-ray Observatory has revolutionized our understanding of relativistic jets from super-massive black holes. Starting from its very first observation of the quasar PKS 0637-752 which lead to the very unexpected detection of X-rays from the resolved, kpc-scale jet, through the more recent discovery that these jets are variable on months to years timescales, the very high angular resolution of Chandra is essential to characterize and map these jets in fine detail, from their origin at the central black hole engine to scales well beyond the host galaxy. I will highlight the major discoveries and how these impacted theoretical models, and will discuss some of the persistently open questions, which become particularly apparent when we look at the whole multi-wavelength picture. Finally I will conclude with some predictions for what we may find in the future with new and more sensitive observatories such as AXIS.

AGN don’t keep all their energy to themselves; they launch various winds that carry mass and energy outward, potentially influencing the evolution of the entire galaxy. In this presentation, I will focus on a prime example of AGN outflow: the warm absorbers in the archetypical Seyfert 1 galaxy NGC 3783, known for its deep absorption lines and broad range of ionization states. Using the 1 Ms deep Chandra data observed with HETGS 20 years ago, we developed a time-evolving, non-equilibrium photoionization model of NGC 3783’s absorbers. This new model provides stringent constraints on the densities and distances of the wind components, many of which appear to be marginally escaping the supermassive black hole's grip. But the story doesn't end there! In the summer of 2024, a large observing campaign involving seven space missions, including Chandra/HETGS, was conducted simultaneously on NGC 3783. XRISM is playing a central role in this campaign. I will share key findings from this groundbreaking effort, which represents no doubt a major leap forward in constructing a comprehensive picture of AGN winds.

Changing-look quasars (CLQs) exhibit significant variations in continuum emission, accompanied by the disappearance or appearance of broad emission lines on timescales as short as months. Changing-look phenomena have been attributed to tidal disruption events, significant changes in intrinsic absorption or in accretion rate, but all these hypotheses suffer theoretical or empirical challenges. X-ray observations, both in bright and faint states, are crucial for constraining the SMBH accretion rate and intrinsic absorption. We are searching for real-time detection of CLQ transitions among some thirty thousand quasars that are being monitored spectroscopically by the SDSS-V Black Hole Mapper program. Our Chandra ToO program (allocated up to 8 ToOs in Chandra Cycle 24 and 25) for newly-discovered CLQs with archival Chandra or XMM-Newton X-ray observations is characterizing CLQ changes in X-ray luminosity, slope, and intrinsic absorption. The X-ray flux generally changes along with the optical variability. Combining X-ray, optical, and VLA radio observations, we test models with promising analogies to X-ray binary accretion variability.

Since its first light, the Chandra X-ray observatory has played a fundamental role in the study of the high-energy emission produced by extra-galactic relativistic jets. Indeed, thanks to its unmatched angular resolution in the X-rays, Chandra established that the majority of the kilo-parsec scale jets detected in the radio band have a high-energy counterpart. In this talk I will present the multi-wavelength analysis of the most distant kpc-scale jet currently known, powered by a quasar at z=6.1. This work is based on deep X-ray observations with Chandra (370 ksec) as well as dedicated VLA+eMERLIN observations covering the observed 1-10 GHz frequency range at a resolution of 0.1-1.5 arcsec. Given the very high redshift and luminosity (L[2-10keV]~6e44 erg/sec) of the jet, this source represents a unique laboratory where we can constrain the properties of kpc-scale jets in the early Universe. In particular, I will show how the study of this extreme system, through SED modelling and morphology analysis, can be used to put very strong constraints on the, still debated, origin of the high-energy emission produced by kpc-scale jets and their evolution across cosmic time.

Mechanical feedback from central supermassive black holes regulates star formation in galaxy clusters by offsetting gas cooling losses. AGN feedback carves X-ray cavities in the ICM, heating the gas and preventing excessive star formation. In cosmological simulations like IllustrisTNG, this process is modeled through kinetic energy injection from SMBHs. Consequently, simulated clusters display highly perturbed core regions with features like shock fronts and X-ray cavities. Here, we conduct a pioneering quantitative and comparative analysis of X-ray cavities in a cosmological simulation with Chandra observations. We select a volume-limited sample of galaxy cluster from the Chandra Archive and a matching sample of cluster analogs in the TNG-Cluster simulation, the newest addition to the IllustrisTNG suite. Using the high-imaging capabilities of ACIS, we identify X-ray cavities and measure their size and energetics in both real and simulated clusters, employing mock Chandra images for TNG-Cluster. Our goal is to determine if the kinetic feedback model accurately reproduces the observational and energetic properties of X-ray cavities through a direct, apple-to-apple comparison. Our results suggest that TNG-Cluster cavities align broadly with the properties of observed ones, showing similar detection rates, sizes, and energetics. Despite the simple prescription for AGN feedback in the simulation, TNG-Cluster produces realistic and diverse X-ray cavities comparable to those detected in Chandra-imaged clusters. Our comparative work opens new avenues for studying the role of X-ray cavities in the overall feedback cycle of galaxy clusters. We show that detailed comparisons of such small-scale structures in clusters can help refine and validate feedback models for the next generation of cosmological simulations.