arXiv subjects

Anjali Gupta

Publications and source records attributed to Anjali Gupta.

At least 37 records · Page 2Linked to original sources

arXiv2022

FairFoody: Bringing in Fairness in Food Delivery

Along with the rapid growth and rise to prominence of food delivery platforms, concerns have also risen about the terms of employment of the gig workers underpinning this growth. Our analysis on data derived from a real-world food delivery platform across three large cities from India show that there is significant inequality in the money delivery agents earn. In this paper, we formulate the problem of fair income distribution among agents while also ensuring timely food delivery. We establish that the problem is not only NP-hard but also inapproximable in polynomial time. We overcome this computational bottleneck through a novel matching algorithm called FairFoody. Extensive experiments over real-world food delivery datasets show FairFoody imparts up to 10 times improvement in equitable income distribution when compared to baseline strategies, while also ensuring minimal impact on customer experience.

cs.SI↗

arXiv2022

UV counterpart of an X-ray ultra-fast outflow in IRAS 17020+4544

We report on the discovery of a UV absorption counterpart of a low-ionization X-ray ultra-fast outflow (UFO) in the Narrow-Line Seyfert-1 galaxy IRAS 17020+4544. This UV signature of the UFO is seen as a narrow and blueshifted Lyman-alpha absorption feature in the far-UV spectrum, taken with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). The Lyman-alpha feature is found to be outflowing with a velocity of -23430 km/s (0.078 c). We carry out high-resolution UV spectroscopy and photoionization modeling to study the UFO that is seen in the HTS/COS spectrum. The results of our modeling show that the UV UFO corresponds to a low-ionization, low-velocity component of the X-ray UFO found previously with XMM-Newton's Reflection Grating Spectrometer (RGS). The other higher-velocity and higher-ionization components of the X-ray UFOs are not significantly detected in the HST/COS spectrum, consistent with predictions of our photoionization calculations. The multiple ionization and velocity components of the UFOs in IRAS 17020+4544 suggest a scenario where a powerful primary UFO entrains and shocks the ambient medium, resulting in formation of weaker secondary UFO components, such as the one found in the UV band.

astro-ph.HE↗

arXiv2021

The hot circumgalactic medium of the Milky Way: evidence for super-virial, virial, and sub-virial temperature, non-solar chemical composition, and non-thermal line broadening

For the first time, we present the simultaneous detection and characterization of three distinct phases at $>10^5$ K in $z=0$ absorption, using deep $\it{Chandra}$ observations toward Mrk 421. The extraordinarily high signal-to-noise ratio ($\geqslant60$) of the spectra has allowed us to detect a $\it{hot}$ phase of the Milky Way circumgalactic medium (CGM) at 3.2$^{+1.5}_{-0.5}\times$ 10$^7$ K, coexisting with a $\textit{warm-hot}$ phase at 1.5$\pm$0.1$\times$10$^6$ K and a $\textit{warm}$ phase at 3.0$\pm$0.4$\times$10$^5$ K. The $\textit{warm-hot}$ phase is at the virial temperature of the Galaxy, and the $\textit{warm}$ phase may have cooled from the $\textit{warm-hot}$ phase, but the super-virial $\textit{hot}$ phase remains a mystery. We find that [C/O] in the $\textit{warm}$ and $\textit{warm-hot}$ phases, [Mg/O] in the $\textit{warm-hot}$ phase and [Ne/O] in the $\textit{hot}$ phase are super-solar, and the $\textit{hot}$ and the $\textit{warm-hot}$ phases are $α-$enhanced. Non-thermal line broadening is evident in the $\textit{warm-hot}$ and the $\textit{hot}$ phases and it dominates the total line broadening. Our results indicate that the $>10^5$ K CGM is a complex ecosystem. It provides insights on the thermal and chemical history of the Milky Way CGM, and theories of galaxy evolution.

astro-ph.GA↗

arXiv2021

Super-virial temperature or Neon overabundance?: Suzaku observations of the Milky Way circumgalactic Medium

We analyzed Suzaku and Chandra observations of the soft diffuse X-ray background toward four sightlines with the goal of characterizing the X-ray emission from the Milky Way circumgalactic medium (CGM). We identified two thermal components of the CGM, one at a uniform temperature of $\rm kT = 0.176\pm0.008 ~keV$ and the other at temperatures ranging between $\rm kT = 0.65-0.90~ keV$. The uniform lower temperature component is consistent with the Galaxy's virial temperature ($ \sim10^{6}~ K$). The temperatures of the hotter components are similar to that recently discovered ($\rm \sim 10^{7}~ K$; Das et al.) in the sightline to blazar 1ES1553+113, passing close to the Fermi bubble. Alternatively, the spectra can be described by just one lower-temperature component with super-solar Neon abundance, once again similar to that found in the 1ES1553+113 sightline. The additional hot component or the overabundance of Ne is required at a significance of $>4σ$, but we cannot distinguish between the two possibilities. These results show that the super-virial temperature gas or an enhanced Ne abundance in the warm-hot gas in the CGM is widespread, and these are not necessarily related to the Fermi bubble.

astro-ph.HE↗

arXiv2020

Empirical estimates of the Galactic halo contribution to the dispersion measures of extragalactic fast radio bursts using X-ray absorption

We provide an empirical list of the Galactic dispersion measure ($DM_{Gal}$) contribution to the extragalactic fast radio bursts along 72 sightlines. It is independent of any model of the Galaxy, i.e., we do not assume the density of the disk or the halo, spatial extent of the halo, baryonic mass content, or any such external constraints to measure $DM_{Gal}$. We use 21-cm, UV, EUV and X-ray data to account for different phases, and find that $DM_{Gal}$ is dominated by the hot phase probed by X-ray absorption. We improve upon the measurements of N(\oviin) and f$_{OVII}$ compared to previous studies, thus providing a better estimate of the hot phase contribution. The median $DM_{Gal}$=64$^{+20}_{-23}$ cm$^{-3}$ pc, with a 68\% (90\%) confidence interval of 33--172 (23--660) cm$^{-3}$ pc. The $DM_{Gal}$ does not appear to follow any trend with the galactic longitude or latitude, and there is a large scatter around the values predicted by simple disk+spherical halo models. Our measurements provide more complete and accurate estimates of $DM_{Gal}$ independent from the previous studies. We provide a table and a code to retrieve $DM_{Gal}$ for any FRB localized in the sky.

astro-ph.GA↗

arXiv2020

Probing the Warm-Hot Circumgalactic Medium with broad OVI and X-rays

Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around $10^6$K. The narrow OVI absorption lines probe a somewhat cooler component at $\log \rm T(K)= 5.5$, but broad OVI absorbers have the potential to probe the hotter CGM. Here we present 376 ks Chandra LETG observations of a carefully selected galaxy in which the presence of broad OVI together with the non-detection of Lya was indicative of warm-hot gas. The strongest line expected to be present at $\approx 10^6$K is OVII $λ21.602$. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than $2σ$ significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has $\log \rm T(K) =6.3\pm 0.2$ and $\log \rm N_{H} (cm^{-2})=20.7^{+0.3}_{-0.5}$. These parameters are consistent with the warm-hot plasma model based on UV observations; other OVI models of cooler gas phases are ruled out at better than $99$% confidence. Thus we have suggestive, but not conclusive evidence for the broad OVI absorber probing the warm-hot gas from the shallow observations of this pilot program. About 800ks of XMM-Newton observations will detect the expected absorption lines of OVII and OVIII unequivocally. Future missions like XRISM, Arcus and Athena will revolutionize the CGM science.

astro-ph.GA↗

arXiv2020

Multiple temperature components of the hot circumgalactic medium of the Milky Way

We present a deep XMM-Newton observation of the Galactic halo emission in the direction of the blazar 1ES 1553+113. In order to extract the Galactic halo component from the diffuse soft X-ray emission spectrum, accurately modeling the foreground components is crucial. Here we present complex modeling of the foregrounds with unprecedented details. A careful analysis of the spectrum yields two temperature components of the halo gas (T$^{em}_1$= 10$^{6.25-6.42}$K, T$^{em}_2$= 10$^{6.68-6.92}$K). We find that these temperatures obtained from the emission spectrum are not consistent with those from the absorption spectrum (T$^{ab}_1$= 10$^{6.07-6.13}$K, T$^{ab}_2$= 10$^{6.96-7.15}$K), unlike the previous studies that found only one temperature component of the Milky Way circumgalactic medium. This provides us with interesting insights into the nature of emitting and absorbing systems. We discuss several possibilities objectively, and conclude that most likely we are observing multiple (3 to 4) discrete temperatures between 10$^{5.5}$K and $\geqslant$10$^7$K in the Milky Way circumgalactic medium.

astro-ph.GA↗

arXiv2020

The warm-hot, extended, massive circumgalactic medium of NGC 3221: an XMM-Newton discovery

Using Suzaku data, we had found a $3.4σ$ evidence for the X-ray emitting warm-hot circumgalactic medium (CGM) in the L$^\star$ galaxy NGC 3221. Here we present XMM-Newton data and outline an efficient, rigorous and well-defined method to extract the faint CGM signal. We confirm the CGM detection at $4σ$ significance within 30-200 kpc of the galaxy. We claim with $99.62\%$ confidence that the CGM is extended beyond $150$ kpc. The average temperature of the CGM is 2.0$^{+0.2}_{-0.3} \times 10^6$ K, but it is not isothermal. We find suggestive evidence for a declining temperature gradient out to 125 kpc and for super-virial temperature within 100 kpc. While a super-virial temperature component has been detected in the Milky Way CGM, this is the first time a temperature gradient has been observed in the warm-hot CGM of any spiral galaxy. The emission measure profile is well-fit with either a $β-$ model or a constant density profile. Deeper data are required to constrain the temperature and density profiles. We also confirm the Suzaku result that the warm-hot CGM is one of the most massive baryon components of NGC 3221 and can account for the missing galactic baryons.

astro-ph.GA↗

arXiv2019

Evidence for massive warm-hot circumgalactic medium around NGC 3221

We report a 3.4$σ$ detection of the warm-hot, massive, extended circumgalactic medium (CGM) around an L$^\star$ star-forming spiral galaxy NGC 3221, using deep Suzaku observations. The temperature of the gas is $10^{6.1}$ K, comparable to that of the Milky Way CGM. The spatial extent of the gas is at least $150$ kpc. For a $β$-model of density profile with solar abundance, the central emission measure is EM = $3\pm 1 \times 10^{-5}$ cm$^{-6}$ kpc and the central electron density is $n_{eo} = 4\pm 1 \times10^{-4}$ cm$^{-3}$, with a slope of $β= 0.56$. We investigate a range of $β$ values, and find that the details of the density profile do not change our results significantly. The mass of the warm-hot gas, assuming a metallicity of $\frac{1}{3}$ Z$_\odot$ is $16 \pm 3 \times 10^{10}$ M$_\odot$, being the most massive baryon component of NGC 3221. The baryon fraction is $f_b$ = 0.120 $\pm$ 0.036 (statistical) $^{+0.104}_{-0.048}$ (systematic), consistent with the cosmological mean value, closing the baryon budget of this galaxy. We also investigated the missing metals problem in conjunction with the missing baryons problem and conclude that metals are likely to be preferentially expelled from the galaxy. Ours is the first detection of an extended warm-hot CGM around an external L$^\star$ star-forming spiral galaxy, where the CGM likely accounts for the missing galactic baryons.

astro-ph.GA↗

arXiv2019

A High Signal-to-Noise HST Spectrum Toward J1009+0713: Precise Absorption Measurements in the CGM of Two Galaxies

High signal-to-noise spectra toward background quasars are crucial for uncovering weak absorption in the circumgalactic medium (CGM) of intervening galaxies, such as the diagnostic lines of N V that provide insight to the ionization process of warm gas but typically have low equivalent widths. We present a new spectrum from the Hubble Space Telescope with a signal-to-noise ratio of $\sim20-35$ toward the quasar SDSS J1009+0713 and analyze absorption systems in the CGM of two $L^\star$ galaxies close to the line of sight. We identify additional absorption in the CGM of these galaxies that was not reported by the previous lower signal-to-noise spectrum, as well as Milky Way absorbers and quasar outflows from J1009+0713. We measure $\log (N_\mathrm{NV}/N_\mathrm{OVI})\sim-1.1$ for two CGM absorbers, inconsistent with gas in collisional ionization equilibrium and consistent with a radiatively cooling bulk flow of $\sim50-150$ km s$^{-1}$, which could be produced by galactic winds. These column density ratios are also consistent with those found for other $L^\star$ galaxies and for some gas in the Milky Way's halo. We place upper limits of $\log (N_\mathrm{NV}/N_\mathrm{OVI})<-1.8$ to $-1.2$ for other O VI absorbers in the same halos, which suggests that O VI is produced by different processes in different parts of the CGM, even within the same galactic halo. Together with the kinematically different structure of high- and low-ionization lines, these results indicate there are many components to a single galaxy's gaseous halo. We find the redshift number density of Ly-$α$ forest absorbers and broad Ly-$α$ absorbers are consistent with expectations at this redshift.

astro-ph.GA↗

arXiv2018

A Null-model Exhibiting Synchronized Dynamics in Uncoupled Oscillators

The phenomenon of phase synchronization of oscillatory systems arising out of feedback coupling is ubiquitous across physics and biology. In noisy, complex systems, one generally observes transient epochs of synchronization followed by non-synchronous dynamics. How does one guarantee that the observed transient epochs of synchronization are arising from an underlying feedback mechanism and not from some peculiar statistical properties of the system? This question is particularly important for complex biological systems where the search for a non-existent feedback mechanism may turn out be an enormous waste of resources. In this article, we propose a null model for synchronization motivated by expectations on the dynamical behaviour of biological systems to provide a quantitative measure of the confidence with which one can infer the existence of a feedback mechanism based on observation of transient synchronized behaviour. We demonstrate the application of our null model to the phenomenon of gait synchronization in free-swimming nematodes, C. elegans.

physics.bio-ph↗

arXiv2017

The OVI mystery: mismatch between X-ray and UV column densities

The UV spectra of Galactic and extragalactic sightlines often show OVI absorption lines at a range of redshifts, and from a variety of sources from the Galactic circumgalactic medium to AGN outflows. Inner shell OVI absorption is also observed in X-ray spectra (at lambda=22.03 AA), but the column density inferred from the X-ray line was consistently larger than that from the UV line. Here we present a solution to this discrepancy for the z=0 systems. The OII K-beta line ^4S^0 --> (^3D)3p ^4P at 562.40 eV (==22.04 AA) is blended with the OVI K-alpha line in X-ray spectra. We estimate the strength of this OII line in two different ways and show that in most cases the OII line accounts for the entire blended line. The small amount of OVI equivalent width present in some cases has column density entirely consistent with the UV value. This solution to the OVI discrepancy, however, does not apply to the high column density systems like AGN outflows. We discuss other possible causes to explain their UV/X-ray mismatch. The OVI and OII lines will be resolved by gratings on-board the proposed mission Arcus and the concept mission Lynx and would allow detection of weak OVI lines not just at z=0 but also at higher redshift.

astro-ph.GA↗

arXiv2016

Probing the Anisotropy of the Milky Way Gaseous Halo-II: sightline toward Mrk509

Hot, million degree gas appears to pervade the Milky way halo, containing a large fraction of the Galactic missing baryons. This circumgalactic medium (CGM) is probed effectively in X-rays, both in absorption and in emission. The CGM also appears to be anisotropic, so we have started a program to determine CGM properties along several sightlines by combining absorption and emission measurements. Here we present the emission measure close to the \mrk509 sightline using new \suzaku and \xmm observations. We also present new analysis and modeling of \chandra HETG spectra to constrain the absorption parameters. The emission measure in this sightline is high, EM$=0.0165\pm0.0008\pm0.0006~$cm$^{-6}~$pc, five times larger than the average. The observed \ovii column density N(\ovii)$= 2.35\pm0.4 \times 10^{16}$cm$^{-2}$, however, is close to the average. We find that the temperature of the emitting and absorbing gas is the same: $\log T (\rm K) = 6.33\pm0.01$ and $\log T (\rm K)=6.33\pm0.16$ respectively. We fit the observed column density and emission measure with a $β-$model density profile. The the central density is constrained to be between $n_0=2.8$--$6.0\times 10^{-4}$ cm$^{-3}$ and the core radius of the density profile has a lower limit of 40 kpc. This shows that the hot gas is mostly in the CGM of the galaxy, not in the Galactic disk. Our derived density profile is close to the \citet{Maller2004} profile for adiabatic gas in hydrostatic equilibrium with an NFW dark matter potential well. Assuming this density profile, the minimum mass of the hot CGM is $3.2 \times 10^{10}~$M$_{\odot}$.

astro-ph.GA↗

arXiv2014

Detection of high velocity outflows in Seyfert 1 galaxy Mrk 590

We report on the detection of ultra-fast outflows in the Seyfert~1 galaxy Mrk 590. These outflows are identified through highly blue-shifted absorption lines of OVIII and NeIX in the medium energy grating spectrum and SiXIC and MgXII in the high energy grating spectrum on board Chandra X-ray observatory. Our best fit photoionization model requires two absorber components at outflow velocities of 0.176c and 0.0738c and a third tentative component at 0.0867c. The components at 0.0738c and 0.0867c have high ionization parameter and high column density, similar to other ultra-fast outflows detected at low resolution by Tombesi et al. These outflows carry sufficient mass and energy to provide effective feedback proposed by theoretical models. The component at 0.176c, on the other hand, has low ionization parameter and low column density, similar to those detected by Gupta et al. in Ark~564. These absorbers occupy a different locus on the velocity vs. ionization parameter plane and have opened up a new parameter space of AGN outflows. The presence of ultra-fast outflows in moderate luminosity AGNs poses a challenge to models of AGN outflows.

astro-ph.HE↗

arXiv2012

Detection of X-ray Emission from the Warm-Hot Intergalactic Medium through the Angular Autocorrelation Function with Chandra

We have used the angular Autocorrelation Function (AcF) in a pointing with Chandra's ACIS-S instrument. However, we determined that the contribution of Chandra's background below 1 keV is too high to achieve reasonable sensitivity.

astro-ph.CO↗

arXiv2011

Search for X-Ray Emission Associated with the Shapley Supercluster with Suzaku

Suzaku performed observations of 3 regions in and around the Shapley supercluster: a region located between A3558 and A3556, at ~0.9 times the virial radii of both clusters, and two other regions at 1°and 4°away from the first pointing. The 4°-offset observation was used to evaluate the Galactic foreground emission. We did not detect significant redshifted Oxygen emission lines (O VII and O VIII) in the spectra of all three pointings, after subtracting the contribution of foreground and background emission. An upper limit for the redshifted O VIII Ka line intensity of the warm-hot intergalactic medium (WHIM) is 1.5 \times 10^-7 photons s^-1 cm^-2 arcmin^-2, which corresponds to an overdensity of ~380 (Z/0.1 Z_solar)^{-1/2} (L/3 Mpc)^{-1/2}, assuming T=3\times10^6 K. We found excess continuum emission in the 1°-offset and on-filament regions, represented by thermal models with kT ~1 keV and ~2 keV, respectively. The redshifts of both 0 and that of the supercluster (0.048) are consistent with the observed spectra. The ~1 keV emission can be also fitted with Ne-rich Galactic (zero redshift) thin thermal emission. Radial intensity profile of 2 keV component suggests contribution from A3558 and A3556, but with significant steepening of the intensity slope in the outer region of A3558. Finally, we summarized the previous Suzaku search for the WHIM and discussed the feasibility of constraining the WHIM. An overdensity of < 400 can be detectable using O VII and O VIII emission lines in a range of 1.4\times10^6 K < T < 5\times10^6 K or a continuum emission in a relatively high temperature range T > 5\times10^6 K with the Suzaku XIS. The non detection with Suzaku suggests that typical line-of-sight average overdensity is < 400.

astro-ph.CO↗

arXiv2011

Contemporaneous XMM-Newton investigation of a giant X-ray flare and quiescent state from a cool M-class dwarf in the local cavity

We report the serendipitous detection of a giant X-ray flare from the source 2XMM J043527.2-144301 during an XMM-Newton observation of the high latitude molecular cloud MBM20. The source has not been previously studied at any wavelength. The X-ray flux increases by a factor of more than 52 from quiescent state to peak of flare. A 2MASS counterpart has been identified (2MASS J04352724-1443017), and near-infrared colors reveal a spectral type of M8-M8.5 and a distance of (67\pm 13) pc, placing the source in front of MBM20. Spectral analysis and source luminosity are also consistent with this conclusion. The measured distance makes this object the most distant source (by about a factor of 4) at this spectral type detected in X-rays. The X-ray flare was characterized by peak X-ray luminosity of ~8.2E28 erg s-1 and integrated X-ray energy of ~2.3E32 erg. The flare emission has been characterized with a 2-temperature model with temperatures of ~10 and 46 MK (0.82 and 3.97 keV), and is dominated by the higher temperature component.

astro-ph.SR↗

arXiv2010

Study of the Intracluster and Intergalactic Medium in the Sculptor Supercluster with Suzaku

We studied the high temperature plasma in the direction of the Sculptor supercluster at z=0.108 with Suzaku. Suzaku carried out four observations in the supercluster: namely, A2811, A2811 offset, A2804, A2801 regions in 2005 Nov.--Dec., including the regions beyond the virial radii of these clusters. The study needed precise background estimation because the measured intensity of the redshifted lines, especially those from oxygen, were strongly affected by the the Galactic emission. The spectra taken in the regions outside of the virial radii of the member clusters were used as the background which included both the Galactic and Cosmic X-ray Background (CXB) components. We also used the background data which were taken near the Sculptor supercluster. Temperature and metal abundance profiles were determined to the virial radii of the member clusters, and then we searched for the oxygen line emission in the region outside of the virial radii of the clusters. As a result, the temperature of the clusters decreased toward the virial radii, and the spectral fits for the filament region did not require extra component other than the Galactic and CXB components. We constrained the intensities of O VII and O VIII lines to be less than 8.1 and 5.1 photons cm^-2 s^-1 arcmin^-2, respectively, as 2-sigma upper limits. The intensity of O VII indicates n_H < 1.6e-5 cm^-3 (Z/0.1 Z_solar)^-1/2 (L/25 Mpc)^-1/2, which corresponds to an over density, delta < 60 (Z/0.1 Z_solar)^-1/2 (L/25 Mpc)^-1/2.

astro-ph.CO↗