Anjali Gupta

The North Polar Spur (NPS) is a prominent diffuse X-ray feature whose origin has remained uncertain for decades. Using a uniform analysis of archival Suzaku and XMM-Newton data with new Chandra observations, we constrain its thermal and chemical properties. The NPS emission is fully absorbed by the neutral interstellar medium, demonstrating that the plasma lies beyond the Galactic disk and is not a local supernova remnant or nearby superbubble. The spectra require a two-temperature model with a warm–hot component ( kT ≈ 0.2 keV) and a hotter component ( kT = 0.4–0.7 keV), with emission measures of (41.8 ± 4.9) × 10 ⁻³ and (12.9 ± 2.2) × 10 ⁻³ cm ⁻⁶ pc, respectively. A key result is the detection of supersolar abundance ratios in the warm–hot phase, with N/O =3.6 ± 0.3 and Ne/O =1.9 ± 0.1 solar. A Suzaku observation of the outer South Polar Spur (SPS) shows similar absorption, temperatures, and enhanced abundances (N/O =2.9 ± 0.4, Ne/O =1.6 ± 0.2), though with lower emission measures. The similar supersolar abundance ratios suggest a common enrichment history. These properties are consistent with those measured along other sightlines through the X-ray-bright shells of the Galactic bubbles. Together, these results support that the NPS and SPS trace opposite limbs of the Galactic bubbles. The chemical properties suggest a strong contribution from stellar feedback in shaping the Galactic bubbles.

We present a comprehensive analysis of 475 ks (438 ks unpublished and 37 ks archival) XMM-Newton/EPIC-pn observations of a nearby, highly inclined, star-forming, luminous infrared galaxy NGC 3221 through spatial, temporal, and spectral information. We confirm the presence of a low-luminosity (presumably Compton-thick) active galactic nucleus (AGN). The 0.4–12 keV luminosity and the hardness ratio of the six ultraluminous X-ray sources previously identified in Chandra data exhibit diverse variability on day scales. The collective emission from unresolved sources exhibits a different day-scale variability. We have also discovered two new predominantly soft (<1 keV) sources. One of these has an enigmatic spectral shape featuring a soft component, which we interpret as a superbubble in NGC 3221, and a variable hard component from a compact object, unresolved from the superbubble. We do not confidently detect any X-ray emission from SN 1961L. The hot gas in the interstellar medium (ISM, out to ±6 kpc from the disk plane) and that in the extraplanar region (6–12 kpc) both require two thermal phases at ∼0.15 keV and ∼0.55 keV. The ∼0.55 keV component is fainter in the ISM than the ∼0.15 keV component, but the emission from the latter falls off more steeply with disk height than the emission from the former. This makes the extraplanar region hotter and less dense than the ISM. The proximity of NGC 3221 and the occurrence of the underluminous AGN offer a unique observing opportunity to study the hot diffuse medium along with nuclear and diskwide point sources.

Shadow observations provide a powerful tool to separate foreground components of the soft diffuse X-ray background (SDXB) from the background components. Such observations have now established that the “local” foreground is made of the solar wind charge exchange and the Local Bubble, and the background emission is from the extended circumgalactic medium (CGM) of the Milky Way and from the unresolved extragalactic sources. New data and careful analyses of the SDXB led to two new discoveries in recent years: (1) excess emission near 0.5 keV that is identified as the N vii emission line, and (2) excess emission near 0.8−1.0 keV that is identified with an additional, supervirial temperature hot thermal component of the CGM. The goal of this paper is to use Suzaku shadow observations along six sightlines to determine whether either of these components is from the “local” sources. We eliminate the ambiguity regarding the origin of N vii emission, ruling out the local origin. We confirm that the Milky Way CGM contains nitrogen-rich plasma, with a supersolar average (N/O) of 2.6 ± 0.5, and suggest that nitrogen-enhanced plasma is widespread throughout the CGM. We find supersolar Ne abundance in two sightlines, also from the CGM. Similarly, we rule out the local origin of the hot thermal component and confirm that it is present beyond the shadowing clouds. Furthermore, we provide a revised model of the SDXB, which is crucial for extragalactic astronomy.

Shadow observations provide a powerful tool to separate foreground components of the soft diffuse X-ray background (SDXB) from the background components. Such observations have now established that the ``local'' foreground is made of the solar wind charge exchange and the local bubble, and the background emission is from the extended circumgalactic medium (CGM) of the Milky Way and from the unresolved extragalactic sources. New data and careful analyses of the SDXB led to two new discoveries in recent years: (1) excess emission near 0.5 keV that is identified as the NVII emission line, and (2) excess emission near 0.8-1.0 keV that is identified with an additional, super-virial temperature hot thermal component of the CGM. The goal of this paper is to use Suzaku shadow observations along six sightlines to determine whether either of these components is from the ``local'' sources. We eliminate the ambiguity regarding the origin of NVII emission, ruling out the local origin. We confirm that the Milky Way CGM contains nitrogen-rich plasma, with a super-solar average (N/O) of 2.6+-0.5, and suggest that nitrogen-enhanced plasma is widespread throughout the CGM. We find super-solar Ne abundance in two sighlines, also from the CGM. Similarly, we rule out the local origin of the hot thermal component and confirm that it is present beyond the shadowing clouds. Furthermore, we provide a revised model of the soft diffuse X-ray background, which is crucial for extragalactic astronomy.

Recent observations have revealed a supervirial temperature gas phase at log(T/K) ∼ 7 in the Milky Way, challenging existing galaxy formation models. This hot gas phase was discovered toward extragalactic absorption sightlines and blank-sky emission fields, both at high Galactic latitudes. The location of this hot component is unknown; is it in the extended circumgalactic medium (CGM) or in the interstellar medium (ISM) instead? We analyzed X-ray spectra from Chandra’s High-Energy Transmission Grating observations of 27 Galactic X-ray binaries (XRBs) to investigate whether the hot gas component is present in the ISM. We searched for absorption lines of S xvi Kα, Si xiv Kα, and Ne x Kα, which are the telltale signatures of the hot gas and which have been detected toward extragalactic sightlines. Of the 27 targets, these lines were detected in the spectra of only 7, with two sources displaying broad line features likely intrinsic to the XRB systems. Additionally, most of the detected lines are time variable, reinforcing their likely association with the XRBs. Our results suggest that the supervirial temperature gas is not a widespread component of the ISM but may instead be located in extraplanar regions or the extended CGM, which aligns with some recent simulation results.

The number of baryons hosted in the disks of galaxies is lower than expected based on the mass of their dark matter halos and the fraction of baryon-to-total matter in the Universe, giving rise to the so-called galaxy missing-baryon problem. The presence of cool circumgalactic matter gravitationally bound to its galaxy’s halo up to distances of at least 10 times the size of the galaxy’s disk mitigates the problem but is far from being sufficient for its solution. It has instead been suggested that the galaxy’s missing baryons may hide in a much hotter gaseous phase of the circumgalactic medium, possibly near the halo virial temperature and coexisting with the cool phase. Here we exploit the best available X-ray spectra of known cool circumgalactic absorbers of L* galaxies to report the first direct high statistical significance (best estimates ranging from 4.2σ to 5.6σ, depending on fitting methodology) detection of associated O vii absorption in the stacked XMM-Newton and Chandra spectra of three quasars. We show that these absorbers trace the hot medium in the X-ray halo of these systems at logT(in K) ≃ 5.8–6.3 (comprising the halo virial temperature T _vir ≃ 10⁶ K). We estimate masses of the X-ray halo within one virial radius within the interval M hot − CGM ≃ ( 1 – 1.7 ) × 10 11 ( Z / 0.3 Z ⊙ ) − 1 M _⊙. For these systems, this corresponds to galaxy missing-baryon fractions in the range ξ b = M hot − CGM / M missing ≃ ( 0.7 – 1.2 ) ( Z / 0.3 Z ⊙ ) − 1 , thus potentially closing the galaxy baryon census in typical L* galaxies. Our measurements contribute significantly to the solution of the long-standing galaxy missing-baryon problem and to the understanding of the continuous cycle of baryons in-and-out of galaxies throughout the life of the Universe.

We present XMM-Newton observations around the sight line of Mrk 421. The emission spectrum of the Milky Way (MW) circumgalactic medium (CGM) shows that a two-phase model is a better fit to the data compared to a single-phase model; in addition to the warm-hot virial phase at log (T/K) = 6.30 ± 0.02, a hot super-virial phase at log (T/K) = 6.88 − 0.06 + 0.07 is required. Furthermore, we present observations of five fields within 5 degrees of the primary field. Their spectra also require a two-phase model at warm-hot and hot temperatures. The hot phase, which was first discovered in Das et al. in 2019, appears to be widespread. By chemical tagging, we show that the emission from the supevirial phase comes from the L-shell transitions of Fe xviii–Fe xxii, and that the range of temperatures probed in emission is distinct from that in absorption. We detect scatter in temperature and emission measure in both the phases, and deduce that there is small-scale density inhomogeneity in the MW CGM. The emitting gas likely has higher density, possibly from regions close to the disk of the MW, while the absorption in the virial phase may arise from low-density gas extended out to the virial radius of the MW. The presence of the super-virial phase far from the regions around the Galactic center implicates physical processes unrelated to the activity at the Galactic center. Hot outflows resulting from star formation activity throughout the Galactic disk are likely to be responsible for producing this phase.

We studied the spectral signature of different components of the Diffuse X-ray Background (DXB), including Local Hot Bubble (LHB), Solar Wind Charge Exchange (SWCX), Galactic Halo, and typically unresolved point sources (galaxies and AGN), in the direction of the Chandra Deep Field South (CDFS) using the 4 Ms XMM-Newton survey and Chandra 4 Ms Source Catalog. In this paper, we present our results showing how the different components contribute to the DXB below 1 keV. In particular, we have found that ~6% of the emission at 3/4 keV (all-sky average value ~ 3$\times10^{-3}$ cm$^{-6}$pc), which is typically associated with Galactic Halo (GH) and Circum-galactic medium (CGM) is, in fact, due to emission from typically unresolved galaxies. We will discuss the effect that this has on our understanding of GH and CGM, and to our understanding of the missing CGM baryons.

The X-ray bright bubbles at the Galactic Center provide an opportunity to understand the effects of feedback on galaxy evolution. The shells of the eROSITA bubbles show enhanced X-ray emission over the sky background. Previously, these shells were assumed to have a single temperature component and to trace the shock-heated lower-temperature halo gas. Using Suzaku observations, we show that the X-ray emission of the shells is more complex and best described by a two-temperature thermal model: one component close to the Galaxy's virial temperature and the other at super-virial temperatures. Furthermore, we demonstrate that temperatures of the virial and super-virial components are similar in the shells and in the ambient medium, although the emission measures are significantly higher in the shells. This leads us to conclude that the eROSITA bubble shells are X-ray bright because they trace denser gas, not because they are hotter. Given that the pre- and post-shock temperatures are similar and the compression ratio of the shock is high, we rule out that the bubble shells trace adiabatic shocks, in contrast to what was assumed in previous studies. We also observe non-solar Ne/O and Mg/O ratios in the shells, favouring stellar feedback models for the formation of the bubbles and settling a long-standing debate on their origin.