arXiv subjects

Anjali Gupta

Publications and source records attributed to Anjali Gupta.

At least 19 recordsLinked to original sources

arXiv2026

Thermal Structure and Chemical Enrichment of the North and South Polar Spurs: Supersolar N/O and Ne/O in the X-ray Plasma

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 \textit{Suzaku} and \textit{XMM--Newton} data with new \textit{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 ($kT \approx 0.2$ keV) and a hotter component ($kT = 0.4$--$0.5$ keV), with emission measures of $(41.8 \pm 4.9) \times 10^{-3}$ and $(12.9 \pm 2.2) \times 10^{-3} \mathrm{cm^{-6}~pc}$, respectively. A key result is the detection of super-solar abundance ratios in the warm--hot phase, with N/O $= 3.6 \pm 0.3$ and Ne/O $= 1.9 \pm 0.1$ solar. A Suzaku observation of the outer South Polar Spur (SPS) shows similar absorption, temperatures, and enhanced abundances (N/O $= 2.9 \pm 0.4$, Ne/O $= 1.6 \pm 0.2$), though with lower emission measures. The similar super-solar 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.

astro-ph.HE↗

arXiv2025

Deep X-ray observation of NGC 3221: everything everywhere all at once

We present a comprehensive analysis of 475 ks (438 ks unpublished & 37 ks archival) XMM-Newton/EPIC-pn observation of a nearby, highly inclined, star-forming, luminous infrared galaxy NGC 3221 through spatial, temporal, and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) AGN. The 0.4$-$12 keV luminosity and the hardness ratio of the six ultra-luminous X-ray sources (ULX) previously identified in Chandra data exhibit diverse variability on day-scale. The collective emission from unresolved sources exhibits a different day-scale variability. We have also discovered two new predominantly soft ($<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, which is unresolved from the superbubble. We do not confidently detect any X-ray emission from SN 1961L. The hot gas in the ISM (out to $\pm$6 kpc from the disk plane) and the extraplanar region (6$-$12 kpc) both require two thermal phases at $\sim 0.15$ keV and $\sim 0.55$ keV. The $\sim 0.55$ keV component is fainter in the ISM than the $\sim 0.15$ keV component, but the emission from the latter falls off more steeply with disk height than 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 in conjunction with nuclear and disk-wide point sources.

astro-ph.GA↗

arXiv2025

Detection of 'super-virial' gas in the Circumgalactic medium of the Milky Way towards PKS 2155-304

We present the first simultaneous detection of four distinct highly ionized $z=0$ absorbing phases using Chandra and XMM-Newton grating spectra toward the blazar PKS 2155$\hbox{-}$304. We detect the MgXII K$\alpha$ absorption line for the first time in the circumgalactic medium (CGM) of the Milky Way. Along with MgXII K$\alpha$, we detect SiXIV K$\alpha$ absorption, which are the tell-tale signatures of the hot 'super-virial' gas in the CGM. Both from the model-independent calculations and hybrid-ionization modeling, we infer four phases at distinct temperatures, hot 'super-virial' ($5.4^{+1.9}_{-0.8} \times 10^7$ K), warm-hot 'virial' ($1.8^{+0.3}_{-0.2} \times 10^6$ K), warm 'sub-virial' ($2.2\pm 0.5 \times 10^5$ K), and cool phase ($<1.7 \times 10^5$ K). The warm-hot and hot phases are $\alpha$-enhanced, and [C/O] and [Ne/O] are super-solar in the warm-hot phase, while [Mg/O] and [Si/O] are super-solar in the hot phase. The low-ionization lines are blue-shifted (v$_{\rm los} \approx -100$ km s$^{-1}$), whereas the high-ionization lines are red-shifted. It suggests a scenario of infalling sub-virial, quasi-static virial, and outflowing super-virial phases along this sightline. Earlier studies on individual sightlines were confined to the Northern Hemisphere. Our sightline is located in the Southern hemisphere, demonstrating that hot super-virial gas is also present at Southern Galactic latitudes as well. This confirms a more widespread distribution of the super-virial gas across both hemispheres.

astro-ph.GA↗

arXiv2025

Where is the Supervirial Gas? III. Insights from X-ray Shadow Observations and a revised Model for the Soft Diffuse X-ray Background

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.

astro-ph.GA↗

arXiv2024

Where is the Super-virial Gas? II: Insight from the Survey of Galactic Sightlines

Recent observations have revealed a super-virial temperature gas phase at log(T/K) $\sim7$ 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 (HETG) 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 SXVI K$\alpha$, SiXIV K$\alpha$, and NeX K$\alpha$, which are the tell-tale 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 super-virial temperature gas is not a widespread component of the ISM but may instead be located in extraplanar regions or the extended CGM, in line with some recent simulation results.

astro-ph.GA↗

arXiv2024

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

Hot, \logT\ $\sim$ 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 2$\sigma$ upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions.} We note that the 2$\sigma$ upper limits for \SXVI\ \Ka\ and \SiXIV\ \Ka\ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at \logT\ $>7$ 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.

astro-ph.GA↗

arXiv2024

A Sub-solar Fe/O, logT~7.5 Gas Component Permeating the Milky Way's CGM

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 \AA. Utilizing stacked \Chandra/\ACISS\ \MEG\ and \LETG\ spectra toward 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: a \warm\ (\logT\ $\sim$ 5.5), \warmhot\ (\logT\ $\sim 6$), and \hot\ (\logT\ $\sim$ 7.5) components. These findings confirm the presence of the \hot\ component in the MW's CGM indicating its coexistence with a \warm\ and a \warmhot\ gas phases. We find this \hot\ component to be homogeneous in temperature but inhomogeneous in column density. The gas in the \hot\ component requires over-abundances 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 = $55.1^{+29.9}_{-23.7}$ 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 $\geq$ 4. The non-solar abundance ratios found in the super-virial gas component in the Galactic halo suggest that this phase arises from Galactic feedback.

astro-ph.GA↗

arXiv2024

GRAPHGINI: Fostering Individual and Group Fairness in Graph Neural Networks

Graph Neural Networks (GNNs) have demonstrated impressive performance across various tasks, leading to their increased adoption in high-stakes decision-making systems. However, concerns have arisen about GNNs potentially generating unfair decisions for underprivileged groups or individuals when lacking fairness constraints. This work addresses this issue by introducing GraphGini, a novel approach that incorporates the Gini coefficient to enhance both individual and group fairness within the GNN framework. We rigorously establish that the Gini coefficient offers greater robustness and promotes equal opportunity among GNN outcomes, advantages not afforded by the prevailing Lipschitz constant methodology. Additionally, we employ the Nash social welfare program to ensure our solution yields a Pareto optimal distribution of group fairness. Extensive experimentation on real-world datasets demonstrates GraphGini's efficacy in significantly improving individual fairness compared to state-of-the-art methods while maintaining utility and group fairness.

cs.LG↗

arXiv2024

Towards Fair Allocation in Social Commerce Platforms

Social commerce platforms are emerging businesses where producers sell products through re-sellers who advertise the products to other customers in their social network. Due to the increasing popularity of this business model, thousands of small producers and re-sellers are starting to depend on these platforms for their livelihood; thus, it is important to provide fair earning opportunities to them. The enormous product space in such platforms prohibits manual search, and motivates the need for recommendation algorithms to effectively allocate product exposure and, consequently, earning opportunities. In this work, we focus on the fairness of such allocations in social commerce platforms and formulate the problem of assigning products to re-sellers as a fair division problem with indivisible items under two-sided cardinality constraints, wherein each product must be given to at least a certain number of re-sellers and each re-seller must get a certain number of products. Our work systematically explores various well-studied benchmarks of fairness -- including Nash social welfare, envy-freeness up to one item (EF1), and equitability up to one item (EQ1) -- from both theoretical and experimental perspectives. We find that the existential and computational guarantees of these concepts known from the unconstrained setting do not extend to our constrained model. To address this limitation, we develop a mixed-integer linear program and other scalable heuristics that provide near-optimal approximation of Nash social welfare in simulated and real social commerce datasets. Overall, our work takes the first step towards achieving provable fairness alongside reasonable revenue guarantees on social commerce platforms.

cs.CY↗

arXiv2024

Test-Time Adaptation for Depth Completion

It is common to observe performance degradation when transferring models trained on some (source) datasets to target testing data due to a domain gap between them. Existing methods for bridging this gap, such as domain adaptation (DA), may require the source data on which the model was trained (often not available), while others, i.e., source-free DA, require many passes through the testing data. We propose an online test-time adaptation method for depth completion, the task of inferring a dense depth map from a single image and associated sparse depth map, that closes the performance gap in a single pass. We first present a study on how the domain shift in each data modality affects model performance. Based on our observations that the sparse depth modality exhibits a much smaller covariate shift than the image, we design an embedding module trained in the source domain that preserves a mapping from features encoding only sparse depth to those encoding image and sparse depth. During test time, sparse depth features are projected using this map as a proxy for source domain features and are used as guidance to train a set of auxiliary parameters (i.e., adaptation layer) to align image and sparse depth features from the target test domain to that of the source domain. We evaluate our method on indoor and outdoor scenarios and show that it improves over baselines by an average of 21.1%.

cs.CV↗

arXiv2023

Super-virial Hot Phase in Milky Way Circumgalactic Medium: Further Evidences

Recent discoveries of a super-virial hot phase of the Milky Way circumgalactic medium (CGM) has launched new questions regarding the multi-phase structure of the CGM around the Galaxy. We use 1.05 Ms of archival Chandra/HETG observations to characterize highly ionized metal absorption at z=0 along the line of sight of the quasar NGC 3783. We detect two distinct temperature phases with T$_1 = 5.83^{+0.15}_{-0.07}$ K, warm-hot virial temperature, and T$_2=6.61^{+0.12}_{-0.06}$ K, hot super-virial temperature. The super-virial hot phase coexisting with the warm-hot virial phase has been detected in absorption along only two other sightlines and in one stacking analysis. There is scatter in temperature of the hot as well as warm-hot gas. Similar to previous observations, we detect super-solar abundance ratios of metals in the hot phase, with a Ne/O ratio 2$σ$ above solar mixtures. These new detections continue the mystery of the mechanism behind the super-virial hot phase, but provide evidence that this is a true property of the CGM rather than an isolated observation. The super-virial CGM could hold the key to understanding the physical and chemical history of the Milky Way.

astro-ph.GA↗

arXiv2023

X-Ray Detection of the Galaxy's Missing Baryons in the Circum-Galactic Medium of L$^*$ Galaxies

The amount 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 circum-galactic matter gravitationally bound to its galaxy's halo up to distances of at least ten 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 missing baryons may hide in a much hotter gaseous phase of the circum-galactic medium, possibly near the halo virial temperature and co-existing with the cool phase. Here we exploit the best available X-ray spectra of known cool circum-galactic absorbers of L$^*$ galaxies to report the first direct high-statistical-significance (best estimates ranging from $4.2-5.6σ$, depending on fitting methodology)} detection of associated OVII absorption in the stacked XMM and Chandra spectra of three quasars. We show that these absorbers trace hot medium in the X-ray halo of these systems, at logT(in k)$\simeq 5.8-6.3$ K (comprising the halo virial temperature T$_{vir} \simeq 10^6$ K). We estimate masses of the X-ray halo within 1 virial radius within the interval M$_{hot-CGM}\simeq (1-1.7)\times 10^{11} (Z/0.3 Z_{\odot})^{-1}$ M$_{\odot}$. For these systems, this corresponds to galaxy missing baryon fractions in the range $ξ_b = M_{hot-CGM}/M_{missing}\simeq (0.7-1.2) (Z/0.3 Z_{\odot})^{-1}$, 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.

astro-ph.GA↗

arXiv2023

Probing the hot circumgalactic medium of external galaxies in X-ray absorption II: a luminous spiral galaxy at $z\approx 0.225$

The circumgalactic medium (CGM) is the most massive baryonic component of a spiral galaxy, shock heated to about $10^6$K for an $\rm L^{\star}$ galaxy. The CGM of the Milky Way has been well-characterized through X-ray absorption line spectroscopy. However, the paucity of bright background sources makes it challenging to probe the CGM of external galaxies. Previously, using broad OVI absorption as a signpost, we successfully detected the CGM of one galaxy in X-rays. Here we report on the detection of the OVII $Kα$ absorption line at the redshift of a spiral galaxy at $z\approx0.225$ using 1.2 Ms of Chandra observations. This is a robust detection, clearly showing the presence of the hot gas. The mass in the hot phase is at least an order of magnitude larger than that in the cooler phases detected in the UV. The presence of hot gas $116h^{-1}$kpc from the center of this galaxy provides credence to the existence of the extended CGM of the Milky Way. There has been a report of the detection of OVII absorption from the warm-hot intergalactic medium in this sightline using stacking analysis on an older dataset. We argue that the absorption line is from the CGM of the $z\approx0.225$ galaxy instead.

astro-ph.GA↗

arXiv2023

Point source contribution to the Diffuse X-ray Background below 1 keV and its effect on our understanding of the circum-galactic medium

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.

astro-ph.HE↗

arXiv2023

Thermal and chemical properties of the eROSITA bubbles from Suzaku observations

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.

astro-ph.GA↗

arXiv2023

Detection of a Super-Virial Hot Component in the Milky Way Circumgalactic Medium Along Multiple Sight-Lines by Using the Stacking Technique

The study of the elusive hot component ($T \gtrsim 10^7$ K) of the Milky Way circumgalactic medium (CGM) is a novel topic to understand Galactic formation and evolution. In this work, we use the stacking technique through 46 lines of sight with Chandra ACIS-S HETG totaling over 10Ms of exposure time and 9 lines of sight with ACIS-S LETG observations totaling over 1Ms of exposure time, to study in absorption the presence of highly ionized metals arising from the super-virial 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 K$α$ (with total significance of 6.0$σ$) and, for the first time, SXVI K (total significance 4.8$σ$) in the rest frame of our own Galaxy. For S XVI K$α$ we found a column density of $1.50^{+0.44}_{-0.38} \times 10^{16} \mathrm{cm}^{-2}$. For Si XIV K$α$ we measured a column density of $0.87\pm{0.16} \times 10^{16} \mathrm{cm}^{-2}$. 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.

astro-ph.GA↗

arXiv2022

The hot circumgalactic medium of the Milky-Way: new insights from XMM-Newton observations

We present XMM-Newton observations around the sightline of Mrk 421. The emission spectrum of the Milky Way 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) = $6.33_{-0.02}^{+0.03}$, a hot super-virial phase at log ($T/$K) = $6.88_{-0.07}^{+0.08}$ 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, first discovered in Das et al. 2019, appears to be widespread. By chemical tagging we show that emission from the supevirial phase comes from the L-shell transitions of Fe XVIII-FeXXII, and that the range of temperatures probed in emission is distinct from that in absorption. We detect scatter in temperature and emission measure (EM) 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 responsible for producing this phase.

astro-ph.GA↗

arXiv2022

Gigs with Guarantees: Achieving Fair Wage for Food Delivery Workers

With the increasing popularity of food delivery platforms, it has become pertinent to look into the working conditions of the 'gig' workers in these platforms, especially providing them fair wages, reasonable working hours, and transparency on work availability. However, any solution to these problems must not degrade customer experience and be cost-effective to ensure that platforms are willing to adopt them. We propose WORK4FOOD, which provides income guarantees to delivery agents, while minimizing platform costs and ensuring customer satisfaction. WORK4FOOD ensures that the income guarantees are met in such a way that it does not lead to increased working hours or degrade environmental impact. To incorporate these objectives, WORK4FOOD balances supply and demand by controlling the number of agents in the system and providing dynamic payment guarantees to agents based on factors such as agent location, ratings, etc. We evaluate WORK4FOOD on a real-world dataset from a leading food delivery platform and establish its advantages over the state of the art in terms of the multi-dimensional objectives at hand.

cs.AI↗