Updates from the Low Energy Transmission Grating

Brad Wargelin for the LETG Team

Back on Schedule

With the HRC’S Return to Science in April 2023 (see Issue 33) now well behind us, the associated backlog in LETG observations has been fully cleared. The last observations from Cycles 22, 23, and 24 were all completed by Oct 2023, and as of June 2024 there is only one more non-Calibration target from Cycle 25 remaining (for 59 ks), scheduled for fall 2024. Another milestone is the recently completed (mid June) increase in the HRC high voltage to restore detector gain and QE to their pre-pandemic levels. As with previous HV increases in 2012 (HRC-S only) and 2021, it will take a few months for results of the new QE calibration to be included in the CALDB, but observation scheduling should not be significantly affected.

By the Numbers

The current lull in LETG observations, along with Chandra's 25th anniversary, inspires a look back at how observers have been using the LETG. To date, the total non-Calibration LETG exposure time is 25038 ks (1.13 years assuming 70% Chandra observing efficiency), accumulated over 593 ObsIDs. The distribution among source categories is listed in Table 1.

Sources Total Exposure (ks) TOO & DDT Exposure (ks)
Active Galaxies & Quasars 34 6658 984
Stars & White Dwarfs 52 6037 19
Supernovae, Supernova Remnants, & Isolated Neutron Stars 17 4251 958
White Dwarf Binaries & Cataclysmic Variables 41 2560 1371
Extragalactic: Diffuse & Surveys 8 2110 746
Black Hole & Neutron Star Binaries 20 1323 370
Galactic: Diffuse & Surveys 8 610 94
Solar System & Exoplanets 6 228 113

TOOs account for 3398 ks of LETG exposure, while DDT observations contribute an additional 1256 ks. The source with the most LETG DDT exposure is RX J1856.5–3754, a nearby isolated neutron star, which was observed for 441 ks in Oct 2001. Including GTO and calibration observations, RX J1856.5–3754 has a total LETG exposure of 1206 ks. Other sources with substantial LETG Calibration time include Capella (808 ks, all from Calibration) and Mkn 421 (4833 ks total, of which 4463 ks was for Calibration). The former source is used for calibration of the line response function and dispersion relation monitoring, as well as effective area cross-calibration between HETG and LETG for HRC-S and ACIS-S, while the latter source is observed primarily to monitor contaminant buildup on ACIS; Mkn 421 therefore has the greatest exposure time of any target observed by the LETG. As well as being calibration targets, Capella and Mkn 421 are also notable as exemplars of the two broad classes of LETG science: studying line emission and absorption.

Coronae and Capella

Capella is a stellar binary consisting of G8 and G1 giants, with most of the X-ray emission coming from the G8; at a distance of 13.12 pc, Capella is the brightest coronal source in the sky—other than the Sun, that is. The first LETG/HRC-S observations of Capella were focusing observations taken in early Sep 1999, with longer observations following on Nov 9 (85 ks using LETG/HRC-S) and Nov 10 (54 ks using LETG/ACIS-S).

Its Chandra spectra were first presented in Brinkman et al. (2000) and cited in Issue 8 but, apart from a few narrow glimpses, the Capella spectrum has amazingly never appeared in this august publication. LETG observers have probably seen those data many times, however, in Figure 9.26 of the Proposers' Observatory Guide. And for those who prefer a Ken Burns-style presentation, Vinay Kashyap and Thomas Connor have created a movie of the Capella spectrum from HETG and LETG data at https://youtu.be/a6rCzrGOQys. With resolving power of up to R~2000 and wavelength coverage out to ~175 Angstroms, Capella spectra have been invaluable for improving the atomic data used in spectral modeling (including energy levels, electron collision excitation cross sections, and dielectronic recombination resonance strengths), allowing much more accurate use of temperature and density line ratio diagnostics in high-resolution spectra and higher-quality fits to lower-resolution CCD spectra.

Backlit Baryons and Mkn 421

Mkn 421, a BL Lac object (roughly synonymous with “blazar,” an AGN with its relativistic jet aimed at us) illustrates the other prime use of the LETG: studying interstellar or intergalactic gaseous material such as the Warm Hot Intergalactic Medium (WHIM) or Galactic dust grains through their absorption of a bright continuum source. Due to its abundant data, Mkn 421 has been a valuable target for both sets of searches, directly (Nicastro et al. 2005) and indirectly (Staunton & Paerels 2023).

In the broader sample of LETG observations, Galactic absorption features have been confidently measured (e.g., Paerels et al. 2001; Wang et al. 2005; Williams et al. 2007), but WHIM detections remain tantalizingly tentative with mostly marginal S/N (Fang et al. 2002; Nicastro et al. 2005; Zappacosta et al. 2010; Kovács et al. 2019). Likewise, there are Chandra grating detections of dust absorption (Valencic & Smith 2013; Rogatini et al. 2020; Staunton & Paerels 2023), but uncertainties in laboratory spectral data—particularly energy—prevent secure conclusions regarding dust grain compositions. More compelling WHIM detections will probably have to await a future X-ray observatory with significantly larger effective area (a disappointingly distant prospect), while progress on studies of dust composition will also require better energy calibration of lab measurements.

Notable Novae

In addition to over 4 Ms of calibration observations, Mkn 421 has a substantial 393 ks of TOO/DDT exposure in observations intending to catch the source when it is particularly bright. Yet even with that boost, the “Active Galaxies and Quasars” category is not the most popular category for LETG TOO/DDT observations, by either total time or fraction of time. The winning category is “White Dwarf Binaries and Cataclysmic Variables,” which, with over 50% of its total 2560 ks of LETG exposure devoted to TOOs and DDTs (mostly of novae), could also be well described as “Things That Go Boom.” Spectra of novae tend to be a messy mix of temporally evolving components and are therefore extremely challenging to model in detail. Soft X-rays become visible a week to several weeks after the optical eruption; known as the supersoft source phase (SSS), the ejecta has thinned enough at this point that a blackbody-like spectrum from the bloated photosphere of the white dwarf is seen. Steady nuclear burning continues on the white dwarf surface during the SSS phase, and the spectrum shows mostly blue-shifted absorption lines from the radiatively-driven outflow. Sometimes emission lines are also observed, likely resulting from an edge-on view of the outflow in the presence of the accretion disk or, at later times, from shocked and photoionized ejecta. A recent paper by Milla & Paerels (2023) analyzes LETG spectra from Nova Delphini 2013 (see Figure 1) and compares them with other novae including V3890 (Ness et al. 2022), KT Eri 2009 (Pei et al. 2021), and LMC 2009a (Orio et al. 2021).

One of the main motivations in studying novae is to better understand how and which white dwarfs become supernovae, a process that depends on the balance between mass accretion gains and explosive losses. Observed features such as ejecta velocity, dispersal patterns, shell mass, metal enrichment, and nova recurrence rate can vary widely, with poorly understood dependencies on white dwarf mass, temperature, and binary mass transfer rate. Although many questions remain, LETG spectra are providing valuable glimpses of order amid the chaos. Hopes are high that the explosion of T Coronae Borealis (T CrB), eagerly expected later this year after a wait of 78 years, will answer a good number of those questions.

 Plot of flux versus wavelength showing a spectrum and a model fit. Flux is presented in counts per second per Angstrom logarithmically spaced from 0.01 to 10 and wavelength is plotted linearly from 22 to 35 Angstroms. Both model and spectrum have similar shapes, beginning from a steep rise from 0.01 to 1 just redward of 22 Angstroms; this plateaus until 24 Angstroms, when there is a trough followed by a rise to around 3 to 5 flux units. The flux stays mostly constant redward of 26 Angstroms, other than numerous absorption lines, some absorbing 90% of the flux or more. The model mostly follows the spectrum through absorption lines and overall continuum strength, but the model does not appear to track all of the absorption lines visible in the spectrum.


Figure 1: Model fit to spectrum of Nova Delphini 2013 (ObsID 15742), assuming a 6.3×105 K photosphere surrounded by two shells of absorbing material with temperatures of approximately 1.6×106 K. These shells have blueshifted velocities of 1300 and 4000 km/s, with the second shell having a much higher N/C abundance ratio  than the first. This figure is adapted from Milla & Paerels (2023).

Facing Forward

Because of the steady loss of ACIS QE at low energies, the rate of non-calibration LETG/ACIS observations has declined (only two since May 2015), but the LETG/HRC-S wavelength coverage remains unchanged since the beginning of the mission. LETG also remains the only high-resolution X-ray observatory instrument able to observe energies below the C-K edge (0.28 keV, 44 Å), and good resolution can be achieved out to ~200 Angstroms using offset pointing. The LETG has provided unique capabilities in X-ray astronomy for a quarter century, and we look forward to more discoveries in the coming years.

References

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