Where is the Supervirial hot gas? I: A pilot study with sightlines to Galactic X-ray binaries

Hot, log(𝑇 /K)∼7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study we investigate absorption features of SXVI, SiXIV, and NeX in the spectra of three XRBs, namely 4U 1735-44, 4U 1820-30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. We determine the 2𝜎 upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions. We note that the 2𝜎 upper limits for SXVI K𝛼 and SiXIV K𝛼 are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at log(𝑇 /K)> 7 is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that SXVI, SiXIV and NeX absorption

detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended CGM.

Monthly Notices of the Royal Astronomical Society, Volume 533, Issue 1, September 2024, Pages 287–291

DOI:  https://doi.org/10.1093/mnras/stae1845

Authors: Armando Lara-DI, Yair Krongold, Smita Mathur, Manami Roy, RebeccaL. McClain, Sanskriti Das, Anjali Gupta

Our study focuses on characterizing the highly ionized gas within the Milky Way’s (MW) Circumgalactic Medium (CGM) that gives rise to ionic transitions in the X-ray band 2–25 Å. Utilizing stacked Chandra/ACIS-S MEG and LETG spectra toward Quasar (QSO) sightlines, we employ the self-consistent hybrid ionization code PHASE to model our data. The stacked spectra are optimally described by three distinct gas phase components: warm [log (T/K) ∼ 5.5], warm-hot [log (T/K) ∼6], and hot [log (T/K) ∼ 7.5] components. These findings confirm the presence of the hot component in the MW’s CGM indicating its coexistence with warm and warm-hot gas phases. We find this hot component to be homogeneous in temperature but inhomogeneous in column density. The gas in the hot component requires overabundances relative to solar to be consistent with the Dispersion Measure (DM) from the Galactic halo reported in the literature. For the hot phase we estimated a DM = 55.1_{−23.7}^{+29.9} pc cm−3. We conclude that this phase is either enriched in Oxygen, Silicon, and Sulfur, or has metallicity over 6 times solar value, or a combination of both. We do not detect Fe L-shell absorption lines, implying O/Fe ≥ 4. The non-solar abundance ratios found in the supervirial gas component in the Galactic halo suggest that this phase arises from Galactic feedback.

Monthly Notices of the Royal Astronomical Society, Volume 531, Issue 3, July 2024, Pages 3034–3041

DOI: 10.1093/mnras/stae1051

Authors: Armando Lara-DI, Yair Krongold, Smita Mathur, Sanskriti Das, Anjali Gupta, O Segura Montero

The study of the elusive hot component (T  ≳ 10^7 K) of the Milky Way circumgalactic medium (CGM) is a novel topic for understanding Galactic formation and evolution. In this work, we use the stacking technique through 46 lines of sight with Chandra ACIS-S HETG totaling over 10 Ms of exposure time and nine lines of sight with ACIS-S LETG observations totaling over 1 Ms of exposure time, to study in absorption the presence of highly ionized metals arising from the supervirial temperature phase of the CGM. Focusing in the spectral range 4–8 Å, we were able to confirm the presence of this hot phase with high significance. We detected transitions of Si XIV Kα (with a total significance of 6.0σ) and, for the first time, S XVI Kα (total significance of 4.8σ) in the rest frame of our own galaxy. For S XVI Kα we found a column density of 1.50±0.44 x 10^16 cm^-2. For Si XIV Kα we measured a column density of 0.87 ± 0.16 × 10^16 cm^-2 . The lines of sight used in this work are spread across the sky, probing widely separated regions of the CGM. Therefore, our results indicate that this newly  discovered hot medium extends throughout the halo and is not related only to the Galactic bubbles. The hot gas location, distribution, and covering factor, however, remain unknown. This component might contribute significantly to the missing baryons and metals in the Milky Way.

The Astrophysical Journal

2023-03 | Journal article

DOI: 10.3847/1538-4357/acbf40

Authors: Armando Lara-DI; Smita Mathur; Yair Krongold; Sanskriti Das; Anjali Gupta

Using a recent homogeneous sample of 40 high quality velocity dispersion profiles for Galactic globular clusters, we study the low gravitational acceleration regime relevant to the outskirts of these systems. We find that a simple empirical profile having a central Gaussian component and a constant large radius asymptote, σ∞, accurately describes the variety of observed velocity dispersion profiles. We use published population synthesis models, carefully tailored to each individual cluster, to estimate mass to light ratios from which total stellar masses, M, are inferred. We obtain a clear scaling, reminiscent of the galactic Tully-Fisher relation of σ∞(kms−1)=0.084+0.075−0.040(M/M⊙)0.3±0.051, which is interesting to compare to the deep MOND limit of σ∞(kms−1)=0.2(M/M⊙)0.25. Under a Newtonian interpretation, our results constitute a further restriction on models where initial conditions are crafted to yield the outer flattening observed today. Within a modified gravity scheme, as the globular clusters studied are not isolated objects in the deep MOND regime, the results obtained point towards a modified gravity where the external field effect of MOND does not appear, or is much suppressed.

Monthly Notices of the Royal Astronomical Society, Volume 491, Issue 1, January 2020, Pages 272–280, https://doi.org/10.1093/mnras/stz3038

Authors: X.Hernandez, Armando Lara-DI