• Einstein
• Fellowship

Young AGN outburst running over older X-ray cavities

Although the energetic feedback from active galactic nuclei (AGN) is believed to have a profound effect on the evolution of galaxies, details of the AGN heating remain elusive and highly debated. In this presentation, I will discuss an efficient way to transfer the AGN energy to the outskirts of galaxies, which allows to reheat the gas and offset the cooling in a large volume. Based on X-ray and radio data, we studied NGC193, a nearby lenticular galaxy residing in a poor group. These observations reveal the presence of inner and outer X-ray cavities and extended radio lobes, thereby showing multiple epochs of AGN activity. We demonstrate that the inner cavities were produced about 75 Myears ago by a moderately powerful AGN outburst, while the outer cavity and the radio lobes originate from a very powerful 15 Myears old AGN outburst. We thus conclude that the younger AGN outburst overran the previously produced and now buoyantly growing inner X-ray cavities. This result has important implications for the AGN heating of galaxies and galaxy clusters.

Dwarf Galaxies with Active Massive Black Holes

Supermassive black holes (BHs) live at the heart of essentially all massive galaxies with bulges, power AGN, and are thought to be important agents in the evolution of their hosts. However, the birth and growth of the first supermassive BH "seeds" is far from understood. While direct observations of these distant BHs in the infant Universe are unobtainable with current capabilities, massive BHs in present-day dwarf galaxies can place valuable constraints on the masses, formation path, and hosts of supermassive BH seeds. Using optical spectroscopy from the SDSS, we have systematically assembled the largest sample of dwarf galaxies hosting active massive BHs to date. These dwarf galaxies have stellar masses comparable to the Magellanic Clouds and contain some of the least-massive supermassive BHs known.

Gravitational instabilities in Kepler Potentials

Closed orbits are efficient drivers of gravitational instabilities through secular and resonant dynamical interactions. There exist only two gravitational potentials in which bound orbits are closed, that of a simple harmonic oscillator and that of a Kepler potential. The first corresponds to a homogenous density distribution, relevant for stars orbiting in the central regions of galaxies. Instabilities in this potential have been well described, from the formation of bars to buckling and bending modes. The second corresponds to a point mass, relevant for stars orbiting massive black holes, planets orbiting stars, and moons and rings around planets. Few instabilities have been explored beyond the linear regime in a Kepler potential. In this talk I will present preliminary work on a new instability in a Kepler potential, generated by secular interactions between low angular momentum orbits in a disk around a point mass. I will show how this instability has direct analogies with those present in galactic disks, and will motivate future work to discover many more instabilities.

Tearing up circumbinary discs

Supermassive black holes (SMBH) reside at the centre of almost all galaxies. When these galaxies collide an SMBH binary is formed. Dynamical friction with background stars causes the binary to shrink to approximately a parsec. At this point the evolution stalls as dynamical friction becomes inefficient and gravitational wave losses are still too slow. It is often supposed that infalling gas removes angular momentum from the binary, facilitating it's transition into the gravitational wave regime. By exploring the evolution of misaligned circumbinary gas discs I will show that the gas can be delivered on to the binary up to 10,000 times faster than previously thought. This occurs through the process of disc tearing. This may allow both a rapid merging of the binary, and new observational signatures of accreting SMBH binaries.

Formation and Coalescence of Cosmological Supermassive Black Hole Binaries in Supermassive Star Collapse

The overwhelming observational evidence for supermassive black holes in the early Universe at high redshifts z>7 produces a serious problem: how could these black holes have formed? The two most promising pathways (hierarchical galaxy mergers and accretion onto collapsed Population III stars) have been ruled out by a number of recent studies. A possible alternative pathway for supermassive black hole formation at high redshifts is the collapse of a supermassive star that may have formed in the direct collapse of a primordial gas cloud. We self-consistently simulate the collapse of rapidly rotating supermassive stars from the onset of collapse using three-dimensional general-relativistic hydrodynamics with fully dynamical spacetime evolution. We show that seed perturbations in the progenitor can lead to the formation of a system of two high-spin supermassive black holes, which inspiral and merge under the emission of powerful gravitational radiation. The gravitational-wave signals could be observed at redshifts z>10 with the DECIGO or Big Bang Observer gravitational-wave observatories, assuming supermassive stars in the mass range 10^4-10^6 M_\odot. The potential detection of such signals may inform cosmology about the formation process of supermassive black holes at high redshifts.

Galactic Winds Driven by Active Galactic Nuclei

Observations have recently revealed clear evidence of galaxy-scale outflows of neutral, ionized, and molecular gas driven by active galactic nuclei. I will discuss the physics of these outflows, addressing in particular whether they conserve energy or momentum, and the implications for their impact on galaxy scales. In particular, I will argue that trapping of hot shocked gas explains the momentum fluxes >>L_AGN/c inferred from observations. Observational predictions of our model, from the radio to the gamma-rays, will be summarized.

The Origin of the Fermi Bubbles: AGN or starburst?

In 2010, Su, Slatyer, and I used data from the Fermi Gamma-ray Space Telescope to discover a 2-lobed structure extending 8 kpc above and below the Galactic plane (the Fermi Bubbles). The structure has a hard spectrum from 1-100 GeV, and relatively sharp edges, providing possible clues about its origin. Presently, the leading hypotheses involve (1) a burst of star formation at the Galactic center, or (2) AGN activity, though neither idea can explain all aspects of the data. I will review the latest observational data on the bubbles -- and a possible cocoon and jet feature -- and speculate about a path forward.

A Joint Analysis of PS1-10jh and PS1-11af: Comparing and Contrasting Two Near-Eddington Tidal Disruption Events

The full disruption of a star by a supermassive black hole can result in mass accretion rates that exceed the Eddington limit by a couple orders of magnitude. In such events, the black hole is likely incapable of accepting the full dose of mass provided by the disrupted star, and thus may eject a significant fraction of this mass through a wind. While the tidal disruption event (TDE) PS1-10jh appears to have no significant absorption features, a second TDE (PS1-11af) shows clear evidence of metal-line absorption features with blue-shifted velocities in excess of 10,000 km/s near peak luminosity. We show that the multiband light curves of both events are well-modeled as an elliptical debris disk with a reprocessing zone, and argue that the absorption features of PS1-11af are likely associated with a wind that developed when the accretion rate exceeded Eddington for a brief period. Such an interpretation suggests that PS1-11af is an important "transition" TDE, whose behavior may help us understand the conditions under which winds may be generated by accreting supermassive black holes.

The secret life of a pion in the Sun

Dark matter particles captured by the Sun through scattering may annihilate and produce neutrinos, which escape the Sun and are detectable. Current indirect dark matter searches with neutrinos aim to detect the few high-energy (> 1 GeV) neutrinos produced in the prompt decays of some final states. I will present a new channel for detecting dark matter annihilation in the Sun using low-energy (25-50 MeV) neutrinos. I will demonstrate that interactions of hadronic annihilation products in the solar medium lead to a large number of pions for nearly all final states. Positive pions and muons decay at rest, producing low-energy neutrinos with known spectra. This channel allows low-energy neutrino detectors such as the currently-operating Super-Kanionkande experiment to provide a new, complementary probe of the spin-dependent WIMP-proton cross section. I will discuss the sensitivity of the Super-Kamionkande detector to this signal, as well as the sensitivity of planned detectors.

The 3 Ms Chandra Campaign on Sgr A*: A Census of X-ray Flaring Activity from the Galactic Center

Over the last decade, X-ray observations of Sgr A* have revealed a black hole in a deep sleep, punctuated roughly once per day by brief flares. The extreme X-ray faintness of this supermassive black hole has been a long-standing puzzle in black hole accretion. To study the accretion processes in the Galactic Center, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012. With its excellent observing cadence, sensitivity, and spectral resolution, this Chandra X-ray Visionary Project (XVP) provides an unprecedented opportunity to study the behavior of our closest supermassive black hole. I will present a progress report from our ongoing study of X-ray flares, including the brightest flare ever seen from Sgr A*. Focusing on the statistics of the flares and the quiescent emission, I will discuss the physical implications of X-ray variability in the Galactic Center.

AGN feedback in clusters of galaxies

One of the most fascinating discoveries in modern astrophysics has been the realization that supermassive black holes can have a profound impact on their host galaxies. This impact appears mostly in the form of AGN feedback and during this talk, I will review the current status of this field while focusing on the most massive black holes in the Universe, those that lie at the centres of clusters of galaxies. I will address how AGN feedback evolves in such systems over the last 8 Gyrs, and present new results on one of the most extreme clusters of galaxies known so far, RX J1532.9+3021.

A Paradigm Shift for Type Ia Supernova Progenitors

For decades, most researchers believed that Type Ia supernovae (SNe Ia) occurred when a C/O white dwarf grew to the Chandrasekhar mass via accretion from a H-rich donor star. However, in the past several years, evidence against this scenario has mounted to the point that the majority of the SN Ia field thinks it can no longer be responsible for the bulk of SNe Ia. I will talk about the state of the field and our research into the most probable replacement scenario, which involves two detonations triggered during the merger of two WDs. Study of this scenario is still in its infancy, but unique predictions can be made to test its success in the coming years.

Understanding gamma-ray flares of blazars

Blazars are the primary sources of extragalactic gamma-ray radiation detected by Fermi/LAT. They are constantly variable, occasionally producing spectacular flares. These flares are excellent laboratories of dissipation processes operating in AGN jets. I will present a systematic study of the brightest gamma-ray flares in the Fermi/LAT data, comparing their temporal and spectral characteristics. I will discuss the following problems: what are the most basic components of blazar flares, what is the nature of gamma-ray spectral breaks in blazars, and what can we learn about the underlying particle acceleration mechanism.

Signatures of Single and Dual Narrow-Line Active Galactic Nuclei

It is well-established that merging galaxies have a higher incidence of nuclear activity than their isolated counterparts. Single or dual active galactic nuclei (AGN) observed in ongoing galaxy mergers provide direct evidence of supermassive black hole (SMBH) -- galaxy co-evolution and place indirect constraints on the rate of SMBH mergers. Recent progress in spectroscopic searches for dual AGN has created, for the first time, a statistical sample of candidates. We describe efforts to characterize the signatures of AGN in mergers, focusing especially on their narrow-line profiles. Our previous work indicates that double-peaked narrow-lines, which are used as a selection criterion for dual AGN candidates, can arise from a variety of gas kinematic effects, as well as from dual SMBH motion during mergers. By combining hydrodynamic simulations with dust radiative transfer calculations, including our model for the narrow-line region, we are able to characterize the spectral signatures of single and dual AGN during galaxy mergers. In particular, we describe the substantial effects of dust scattering and obscuration in gas-rich mergers, as well as the contribution of stellar photo-ionization in the nuclear region to the narrow-line profiles. We discuss the implications of this modeling for future follow-up studies of candidate dual AGN.

The Role of Stellar Feedback in the Dynamics of HII Regions

Stellar feedback is often cited as the biggest uncertainty in galaxy formation models today. This uncertainty stems from a dearth of observational constraints as well as the great dynamic range between the small scales (<1 pc) where the feedback occurs and the large scales of galaxies (>1 kpc) that are shaped by this feedback. To bridge this divide, in this talk I highlight how to assess observationally the role of stellar feedback at the intermediate scales of HII regions. In particular, I employ multiwavelength data to examine several stellar feedback mechanisms in a sample of 32 HII regions in the Large and Small Magellanic Clouds (LMC and SMC, respectively). Using optical, infrared, radio, and X-ray images, I measure the pressures exerted on the shells from the direct stellar radiation, the dust-processed radiation, the warm ionized gas, and the hot X-ray emitting gas. I find that the warm ionized gas dominates over the other terms in all of the sources, although two have comparable dust-processed radiation pressures to their warm gas pressures. The hot gas pressures are comparatively weak, while the direct radiation pressures are 1-2 orders of magnitude below the other terms. I discuss the implications of these results, particularly highlighting evidence for hot gasleakage from the HII shells and regarding the momentum deposition from the dust-processed radiation to the warm gas. Furthermore, I emphasize that similar observational work should be done on very young HII regions to test whether direct radiation pressure and hot gas can drive the dynamics at early times.

Parameterizing and constraining scalar corrections to general relativity

General relativity passes all tests of gravity so far, but none of these are in the truly strong-field, dynamical regime of the theory. On general grounds we expect the theory to be incomplete and require corrections. I will discuss some forms that corrections may take and where to look for them. The workhorse will be the phenomenology of the compact binary system's inspiral, with dynamical Chern-Simons theory as a prototypical example of a correction. If nature is kind, we can constrain or measure these corrections with future pulsar timing or gravitational wave detections.

Following TeV Photons in Blazar Jets from Birth to Death

By means of first-principles particle-in-cell (PIC) kinetic simulations, we investigate the origin and the fate of TeV photons in blazar jets. In Poynting-dominated jets, magnetic reconnection is often invoked as a mechanism to dissipate the jet magnetic energy into plasma thermal energy, thus powering the observed emission. With 2D and 3D PIC simulations, we show that magnetic reconnection in blazar jets can efficiently accelerate the particles up to extreme energies, generating a flat power-law tail with slope between -2 and -1.5. The upper energy cutoff of the particle spectrum grows linearly in time, at a rate that becomes faster for higher magnetizations or colder plasma temperatures, everything else being fixed. TeV photons generated by the highest energy electrons resulting from reconnection will interact in the intergalactic medium (IGM) with the extragalactic background light, producing a dilute beam of ultra-relativistic electron-positron pairs. We study the relaxation of such beams in the IGM, by means of 2D and 3D PIC simulations. We find that about 10% of the beam energy is ultimately transferred to the IGM plasma, irrespective of the beam number density or Lorentz factor. It follows that roughly 90% of the beam energy is still available to power the GeV emission produced by inverse Compton up-scattering of the CMB by the beam pairs. The energy of TeV photons should then re-appear in the GeV band.

Neutrinos from Proto-Neutron Stars

After a successful core collapse supernova explosion, a neutron star is often left as a compact remnant. During the first minute of the neutron star's life, it cools and contracts by emitting a prodigious number of neutrinos. Additionally, a significant amount of matter is ejected The properties of the neutrino signal depend on the properties of dense matter inside the neutron star, which are beginning to be constrained by terrestrial laboratory experiments. I will discuss recent advances in modelling this environment and potential impact on the synthesis of heavy elements.

Laboratory Astrophysics: Producing Scaled Hydrodynamic Systems Using High-power Lasers

Historically, the field of astrophysics has been dominated by observational and theoretical investigation, but technological advancements in the past two decades have opened this field to more intimate exploration. Of particular interest, are the so-called high-energy-density environments where pressures in these systems exceed one-million times standard atmospheric levels. This type of environment is prevalent throughout the universe: within stellar interiors and exteriors, in planetary nebulae, in astrophysical jets and shock waves, and in mass accretion disks near dying stars or black holes, to name a few. These phenomena have been studied using observations and theoretical modeling, but today’s high-power laser facilities offer a unique opportunity to recreate and study scaled systems under controlled conditions. Using magnetohydrodynamic equations to describe the dynamics allows for similarity criteria to be defined such that some portion of an astrophysical system can be accurately scaled down to an achievable plasma state in the laboratory. This theoretical framework is presented and the capabilities of present-day experimental facilities addressed. Examples of scaled astrophysical systems in the laboratory are given.

Pegasus: A New Hybrid-Kinetic Particle-in-Cell Code for Astrophysical Plasma Dynamics

Many astrophysical systems, such as hot accretion flows, the intracluster medium, the solar wind, and some phases of the interstellar medium, are weakly collisional and magnetized. As a result, their mean, global properties are vastly separated in both space and time from the detailed kinetic microphysics that governs the transport and dissipation of momentum, energy, and magnetic fields. As these systems are more often than not highly nonlinear, one is faced with the daunting task of developing a rigorous numerical approach that can simultaneously grapple with these nonlinearities while respecting the scale hierarchy. To this end we offer a new hybrid (kinetic ions, fluid electrons) particle-in-cell code, named Pegasus. Pegasus supplements a second-order-accurate integration scheme with a variety of techniques to allow the study of astrophysical systems, including the shearing-sheet formalism to study accretion disks and a delta-f scheme to facilitate a reduced-noise study of systems in which only small changes in the distribution function are expected. Pegasus is efficiently parallelized to run on thousands of processors, and is currently being used to study a variety of plasma problems in contemporary astrophysics. These include collisionless magnetorotational turbulence, perpendicular heating in solar-wind turbulence, saturation of microscale kinetic instabilities, and particle acceleration in non-relativistic shocks. Our hope is that Pegasus will become an essential numerical tool of the astrophysical plasma community and, in doing so, will serve to strengthen a growing appreciation for the impact of kinetic-scale plasma physics on the large-scale behavior of the Universe.

Multi-wavelength observations of the Fermi bubbles and the implication to its nature

Data from the Fermi-LAT revealed two large gamma-ray bubbles, extending 50 degrees above and below the Galactic center, with a width of about 40 degrees in longitude. Such structure has been revealed with WMAP data and recently confirmed by Planck in microwave. I will show multi-wavelength studies of the Fermi bubbles including features of polarization emission and a rotation measure study of the Fermi bubbles. We observe the edge of the bubbles using XMM-Newton which confirm a sharp edge in X-ray. I will discuss the observational implications to our understanding of the Fermi bubbles.

Radio Observations of Distant Merging Galaxy Clusters

In a few dozen merging galaxy clusters diffuse extended radio emission has been found, implying the presence of relativistic particles and magnetic fields in the intracluster medium. These diffuse radio sources are referred to as radio halos and relics. At low-z, the basic observational properties of halos and relics have been established. However, no systematic search for halos and relics beyond z > 0.6 has been carried out so far. Until recently, progress was not possible due to the lack of sufficiently large high-z massive cluster samples. Since these samples are now available we are carrying out a large observing campaign with the Giant Metrewave Radio Telescope of distant massive clusters. In this talk I will present some of the first results of these observations.

Reflection from beyond our local Universe

The co-evolution of a black hole with its host galaxy through cosmic time is encoded in the spin of the black hole. Here, we report on a black hole, located over 6 billion light years away (z=0.658), which is found to be rapidly rotating. A large fraction of its radiation comes from a compact region of less than 6 gravitational radii and we show that there are times when the system appears to be reflection dominated. We also discuss some of the implications for the cosmic X-ray background and black hole-host galaxy co-evolution which arise from this results.

Gravitational-wave detection with precise timing of millisecond pulsars.

Astrophysical gravitational waves (GWs) in the nHz regime could become observable within a decade by projects utilising high-precision long term timing of millisecond pulsars. Among the expected dominant sources of GWs in those frequencies are inspiraling supermassive black-hole binaries at the centres of galaxies. One of the current outstanding problems is whether this source population should be modelled as a stochastic background, or as a superposition of individually resolvable sources. By analysing pulsar timing observations with such more realistic models, and by using improved noise mitigating techniques, pulsar timing array projects are currently obtaining astrophysically interesting results. Current preliminary results of a combined international search are encouraging.

On the Outer Pulsar Magnetosphere

We have found time-dependent, non-axisymmetric exact solutions to force-free electrodynamics that should describe the outer magnetosphere of pulsars, including those that are accelerated or torqued. This description includes the exact dynamics of the current sheet, where gamma-ray emission may originate. For an accelerated aligned pulsar, the luminosity is enhanced relative to what would be expected for a magnetic dipole in vacuum.

What sets jet power in black hole accretion systems?

With the recent advances in computational methodology and increase in computational resources, we can now construct ever more realistic computational models of black hole accretion and outflows (jets). The jets can be launched by magnetic fields that thread the black hole and the inner regions of the accretion disk. I will present the results of time-dependent 3D general relativistic magnetohydrodynamic simulations that for the first time enable us to probe the maximum power attainable by the jets at a given mass accretion rate. This maximum power can be achieved when the accretion flow floods the black hole with large-scale magnetic flux. The rest of the flux remains outside of the black hole, impedes the accretion and forms a magnetically-arrested disk (MAD). In this talk, I will overview the properties of MADs and their jets and discuss the observational consequences.

The Evolution of Massive Stars towards their Death: Rotation, Binarity and Mergers

Although they are rare and short-lived, massive stars play a major role in Universe. With their large luminosities, strong stellar winds and spectacular explosions they act as cosmic engines, heating and enriching their surroundings, where the next generation of stars and their planets are forming. For this reason, a wide variety of astrophysical problems depends directly or indirectly on stellar models. Typically these are the classic stellar evolution models, in which stars are assumed to be non rotating and single. Massive stars rotate and do not live their lives alone. They form in close pairs or higher order multiple systems. Recent new constraints indicate that the majority (~70%) of massive stars will experience a severe interaction with a companion before ending drastically changing the properties of both stars (brightness, color, ionizing flux, chemical yields, X-rays etc.) as well as their final fate as core-collapse and pair-instability supernovae and gamma-ray bursts. These developments call for a critical reconsideration of the validity of the application of the classic stellar stellar evolutionary models.

The Seeds of a Magnetic Universe

Magnetic fields appear to be present in all galaxies and galaxy clusters, and perhaps even in the smooth low density intergalactic medium. One explanation for these observations is that a seed magnetic field was generated by some unknown mechanism early in the life of the Universe, and was later amplified by various dynamos in nonlinear objects like galaxies and clusters. I will show that a primordial magnetic fields are expected to be generated in the early Universe on purely linear scales through vorticity induced by scale-dependent temperature fluctuations. Residual free electrons left over after recombination tap into this vorticity to generate magnetic field via the Biermann battery process. At redshifts of order a few tens, we estimate a root mean square field strength of order 1e-25–1e-24G on comoving scales ~10kpc. This field, which is generated purely from linear perturbations, is expected to be amplified significantly after reionization, and to be further boosted by dynamo processes during nonlinear structure formation.

The Indirect Detection of Dark Matter at the Galactic Center

Recent data taken at TeV energies (by Atmospheric Cherenkov Telescopes) and at GeV energies (by the Fermi-LAT) have opened a new window into studies of the Galactic center. The high angular resolution of these observations make them especially well-suited to understanding the many energetic processes occurring in this dense region. Interestingly, Fermi-LAT observations have uncovered an apparent excess of ~1 GeV photons peaked around the galactic center compared to the smooth power-law observed by TeV telescopes. In this talk, I will discuss both dark matter and astrophysical models for this emission, giving particular attention to recent developments in the modeling of this signal, which are able to differentiate dark matter and astrophysical interpretations of this excess.

Understanding Accretion Disks through Three Dimensional Radiation MHD Simulations

Thermal properties of the black hole accretion disks in the radiation pressure dominated regime and a new approach to form the corona above the accretion disks will be discussed. Angular momentum transfer in the disk is provided by the turbulence generated by the magneto-rotational instability (MRI), which is calculated self-consistently with a recently developed 3D radiation magneto-hydrodynamics (MHD) code based on Athena. The thermal instability of the radiation pressure dominated disks is studied with vertically stratified shearing box simulations. I always find that the radiation pressure dominated disks either collapse or expand until we have to stop the simulations. During the thermal runaway, the heating and cooling rates from the simulations are consistent with the general criterion of thermal instability. However, details of the thermal runaway are different from the predictions of the standard alpha disk model. We also identify the key reasons why previous simulations do not find the instability. A new mechanism to generate high temperature corona above the accretion disk is also found from these simulations. The thermal instability has many important implications for understanding the observations of both X-ray binaries and Active Galactic Nuclei (AGNs).