arXiv subjects

Sapna Mishra

Publications and source records attributed to Sapna Mishra.

18 recordsLinked to original sources

arXiv2026

HRMOS: A High-Resolution Multi-Object Spectrograph for the VLT

This White Paper presents the scientific rationale and instrument concept for HRMOS (High-Resolution Multi-Object Spectrograph), a next-generation instrument proposed for the ESO Very Large Telescope within the VLT 2030 roadmap. Current and planned facilities offer either multi-object spectroscopy or ultra-high spectral resolution, but not both. HRMOS fills this gap by combining very high spectral resolution, multi-object capability, and radial-velocity stability, enabling transformative studies in Galactic and extragalactic astrophysics. The baseline design provides a resolving power of R = 80000, radial-velocity precision of 10 m s-1 (goal: 5 m s-1), simultaneous observations of 50-60 targets, and broad optical coverage down to 385 nm. These capabilities enable precise measurements of elemental abundances, isotopic ratios, line profiles, and radial velocities for large stellar samples, including crowded fields, star clusters, the Galactic bulge, and nearby dwarf galaxies. HRMOS will address key questions on the age of the oldest stellar populations through nucleocosmochronology, the formation and survival of planetary systems, the assembly history of the Milky Way and satellites, the origin of the heaviest elements, stellar evolution, and the chemical and dynamical properties of the interstellar and circumgalactic medium. It will bridge large spectroscopic surveys and the next generation of extremely large telescopes, with strong synergies with 4MOST, Gaia, TESS, PLATO, the proposed Haydn mission, and future ELT instruments. Building on VLT/FLAMES heritage, HRMOS represents a strategic investment for European astronomy in the 2030s.

astro-ph.IM↗

arXiv2026

Astrometric Calibration of the 4-m International Liquid Mirror Telescope Observations

The 4-m International Liquid Mirror Telescope (ILMT) is a dedicated time domain survey telescope that continuously scans the zenithal sky over the Indian Himalayas in the g', r' and i' optical bands. Its unique capability to repeatedly image the same strip of sky every night makes it a highly useful instrument for the photometric and astrometric studies of Solar System, Galactic and extragalactic objects. We present a robust astrometric calibration pipeline developed for the ILMT data obtained in the time delay integration (TDI) mode. The pipeline uses a linear transformation model from pixel to world coordinates, with a second order correction for the asymmetric optical distortions introduced by the telescope's optical corrector, and ties the astrometric solution to the Gaia DR3 reference frame. The pipeline is integrated to the routine ILMT data processing workflow. Using data from the first four observing cycles (2022-2025), we present the first assessment of the astrometric performance of the pipeline based on positional residuals of sources cross-matched with Gaia DR3. The pipeline achieves a typical astrometric precision of ~100 milliarcseconds (mas), reaching ~70-80 mas for moderately bright sources (G~16.5-18.5). These results, based on 347 nights of data, demonstrate the stability and reliability of ILMT astrometry over multi-year timescales. The astrometrically calibrated data from these four observing cycles have been made publicly available to the astronomical community. This work establishes a validated framework for precision astrometry with zenith-pointing TDI surveys and provides a foundation for future time-domain studies with ILMT, including variability characterization, transient localization, and long-term positional monitoring.

astro-ph.IM↗

arXiv2026

High-S/N Quasar Observations with HST/COS: Deep Fields for Spectroscopy

Hubble is still in prime observing condition for making transformative discoveries in UV astronomy. In this white paper we describe the science case for a deep (S/N>30) UV spectroscopic survey with HST/COS targeting approximately 20 QSOs at 0.5<z<1.5 at good resolution (20 km/s). This survey would capitalize on our current UV capability, produce a legacy dataset enabling community science in many areas of galactic and extragalactic research, and pioneer a path for future UV science with the Habitable Worlds Observatory. Such high-S/N spectra are largely missing from the MAST archives, and would be analogous to the deep Hubble imaging fields (HDF, UDF, Frontier Fields) that have been enormously successful and far-reaching in their science impact. This legacy dataset would enable frontier science programs in several areas, including (1) studies of the CGM and IGM at unparalleled sensitivity, covering a wide range of UV metal lines and reaching very low H I column densities of log N=12.6 and low metallicities near [Z/H]=-2, enabling precision studies of the chemical abundances, ionization, temperature, and baryon and metal budgets of the CGM and IGM; (2) diffuse gas in the Milky Way and Local Group, including high-velocity clouds and gas streams from satellite mergers; (3) AGN outflows, which would be probed in the rest-frame extreme ultraviolet (EUV), covering continuum-generation mechanisms and diagnostics of gas in accretion-disk outflows.

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arXiv2026

Low Metallicity Gas on the Outskirts of the Local Group: the Circumgalactic Medium of Sextans B

We present a UV absorption-line analysis of the circumgalactic medium (CGM) of Sextans B, a dwarf irregular galaxy at 1.3 Mpc distance on the outer frontier of the Local Group. Using HST/COS spectroscopy of two AGN sightlines passing through the Sextans B CGM at small impact parameters of 4 kpc and 8 kpc (0.04 and 0.08 r_vir), we detect the CGM in Si II, Si III, Si IV, and C II absorption. All four ions show a column-density profile that declines with radius. The profiles fall below the average CGM profiles of other nearby dwarfs (by 0.3-0.6 dex, depending on ion), likely due to the low halo mass and low metallicity of Sextans B. Using Cloudy photoionization models and interferometric measurements of the H I column density, we find low gas-phase silicon and carbon abundances in the Sextans B CGM, [Si/H]=-1.7+/-0.2 and [C/H]=-2.1+/-0.2, among the lowest gas-phase abundances anywhere in the Local Group. We calculate a cool CGM gas mass within 8 kpc of $\sim4\times10^7 M_\odot$, comparable to the H I mass and the stellar mass of Sextans B.

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arXiv2025

Discovery of Weak O VI Absorption in Underdense Regions of the Low-Redshift Intergalactic Medium

We search for weak O VI absorption in the low-redshift intergalactic medium (IGM) using 82 high signal-to-noise quasar spectra from the Cosmic Origins Spectrograph on the Hubble Space Telescope. From this dataset we compile a clean sample of 396 intervening Lyman-alpha (Lya) lines with H I column densities log N (HI))< 14.5 and no individual O VI detections. Stacking at the location of the O VI doublet reveals absorption at $>5\sigma$ significance, with equivalent width of 1.7 $\pm$ 0.3 mA, corresponding to log N (O VI) = 12.14 $\pm$ 0.08. The stacked O VI signal associated with strong Lya (13.5 <= log N (HI) < 14.5) absorbers is significantly stronger than that associated with weaker Lya (12.5 <= log N (HI) < 13.5) absorbers. For the subset of 81 broad Lya absorbers (BLAs; b(HI) > 45 km/s), we obtain a marginal $\sim4\sigma$ O VI detection. Other than Si III, detected at 5$\sigma$, no metal lines are found. Cross-correlation with galaxies shows that 93% of Lya absorbers lack an associated bright galaxy ($>3L^\ast$) within 1~Mpc. A more complete low-redshift ($z<0.25$) search shows 66% Lya lack galaxies down to $L^\ast$. This suggests the O VI arises mainly from the diffuse IGM, not the circumgalactic medium. The stacked O VI signal suggests characteristic metallicities of $\approx 0.01Z_\odot$ under photoionisation and $\approx 0.001Z_\odot$ under collisional ionisation, though these estimates are model-dependent and assume O VI and H I trace the same phase. This study provides the first observational evidence for metal absorption in low-column-density Lya systems that individually show no detectable metals, placing important constraints on metal enrichment of the underdense IGM.

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arXiv2025

The LMC Corona Favors a First Passage

We use constrained idealized simulations of the LMC/Milky Way interaction to determine if the size of the LMC's gaseous halo (Corona) can be used to distinguish between first and second passage models $-$ an orbital trajectory for the LMC in which it has just recently approached the Milky Way for the first time (first passage), or one in which it has had a previous pericenter (second passage). Using live circumgalactic gas particles combined with analytic dark matter potentials evolved to follow previously published orbital trajectories, we find that the first passage model is able to reproduce the observed velocity profile and column density profile of the present day LMC Corona. On the other hand, in a second passage scenario the longer interaction time leads to the velocities and column densities around the LMC at the present day being significantly lower than observations. Based on this observed velocity profile, recent works have found that the LMC's Corona has been truncated to 17$-$20 kpc, and we find truncation radii of $16.6\pm 0.5$ kpc and $5.7^{+1.8}_{-2.2}$ kpc for the first and second passage models, respectively. Thus, based on the gas properties of the LMC's CGM at the present day, a second passage trajectory is strongly disfavored.

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arXiv2025

On the incidence of weak and strong Mg II absorbers towards the Flat and Steep Spectrum Radio quasars

The incidence rate of Mg II absorbers per unit redshift path (dN/dz) towards quasar sightlines has been used to probe the interaction between quasar jets and surrounding gas clouds. Studies using core- and lobe-dominated samples found a higher dN/dz for strong Mg II absorbers (rest equivalent width, W_r(2796) >= 1.0 A) in velocity offsets ranging from 5000 km/s < beta * c < 60000 km/s (with beta = v/c), toward core-dominated sources. In this study, we applied a stringent spectral index criterion: alpha_radio < -0.7 for steep-spectrum radio quasars (SSRQs) and alpha_radio > -0.3 for flat-spectrum radio quasars (FSRQs). Using this, we assembled the largest sample to date: 441 FSRQs and 464 SSRQs with suitable optical spectra to study both strong absorbers and weak (0.3 A < W_r(2796) < 1.0 A) Mg II absorbers. We conducted a detailed comparison of absorbers' incidence rate and offset velocity distributions. Our main findings are: (i) For both weak and strong absorbers, we found no significant excess in dN/dz towards FSRQ compared to SSRQ sightlines. (ii) The dN/dbeta distribution of Mg II absorbers along FSRQs and SSRQs are statistically similar. (iii) The cumulative distribution of weak Mg II absorbers is slightly lower for beta < 0.3, but shows an excess at higher beta. This suggests that, while intrinsic Mg II absorber abundance is comparable along both sightlines, FSRQs' more aligned relativistic jets cluster weak absorbers at high beta, consistent with the scenario of jet-driven acceleration of smaller gas clumps.

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arXiv2025

The Distance to the Magellanic Stream: Constraints from Optical Absorption along Stellar Sightlines

The Magellanic Stream (MS) is a large tail of neutral and ionized gas originating from tidal and hydrodynamical interactions between the Magellanic Clouds as they orbit the Milky Way (MW). It carries a significant gas reservoir that could impact the future evolution of the MW. Despite its importance, no direct observational constraints on the Stream's distance have been previously published. In this study, we analyze Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (VLT/UVES) spectra of five blue horizontal branch (BHB) stars in the MW halo located at distances ranging from 13 to 56 kpc near two regions of the Stream (near Stream longitudes of -79 and -98 degrees), with the aim of detecting Ca II and Na I absorption. No Ca II or Na I absorption is detected at Stream velocities in any of the individual spectra, or in higher signal-to-noise stacks of the spectra. The resulting limits on the Ca II absorption are significantly lower than the Ca II columns measured in the Stream along extragalactic sightlines. These non-detections establish a firm lower distance limit of 42 kpc for one region of the Stream. For the other region, we set a firm lower limit of 20 kpc and a tentative lower limit of 55 kpc from the most distant star, but deeper spectra are needed to confirm this. Our results provide the first observational constraints on the gaseous Stream's distance.

astro-ph.GA↗

arXiv2024

The Truncated Circumgalactic Medium of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the nearest massive galaxy to the Milky Way. Its circumgalactic medium is complex and multi-phase, containing both stripped HI structures like the Magellanic Stream and Bridge, and a diffuse warm corona seen in high-ion absorption. We analyze 28 AGN sightlines passing within 35 kpc of the LMC with archival HST/COS spectra to characterize the cool (T\approx10^4$ K) gas in the LMC CGM, via new measurements of UV absorption in six low ions (OI, FeII, SiII, AlII, SII, and NiII) and one intermediate ion (SiIII). We show that a declining column-density profile is present in all seven ions, with the low-ion profiles having a steeper slope than the high-ion profiles in CIV and SiIV reported by Krishnarao et al. 2022. Crucially, absorption at the LMC systemic velocity is only detected (in all ions) out to 17 kpc. Beyond this distance, the gas has a lower velocity and is associated with the Magellanic Stream. These results demonstrate that the LMC's CGM is composed of two distinct components: a compact inner halo extending to 17 kpc, and a more extended stripped region associated with the Stream. The compactness and truncation of the LMC's inner CGM agree with recent simulations of ram-pressure stripping of the LMC by the Milky Way's extended corona.

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arXiv2024

MUSEQuBES: Mapping the distribution of neutral hydrogen around low-redshift galaxies

We present a detailed study of cool, neutral gas traced by Lya around 4595 z<0.5 galaxies using stacks of background quasar spectra. The galaxies are selected from our MUSEQuBES low-z survey along with data from the literature. These galaxies, with a median stellar mass of log (M*/Msun)= 10.0, are probed by 184 background quasars giving rise to 5054 quasar-galaxy pairs. The median impact parameter is b = 1.5 pMpc (median b/Rvir=10.4) with 204 (419) quasar-galaxy pairs probing b/Rvir < 1 (2). We find excess absorption out to at least ~ 15 Rvir transverse distance and ~ 600 km/s along the line of sight. We show that the median stacked profile for the full sample, dominated by the pairs with b > Rvir, can be explained by a galaxy-absorber two-point correlation function with r0 = 7.6 pMpc and gamma = -1.57. There are strong indications that the inner regions (< Rvir) of the rest equivalent width profile are better explained by a log-linear (or a Gaussian) relation whereas the outer regions are well described by a power-law, consistent with galaxy-absorber large-scale clustering. Using a sub-sample of 339 galaxies (442 quasar-galaxy pairs, median b/Rvir = 1.6) with star formation rate measurements, we find that the Lya absorption is significantly stronger for star-forming galaxies compared to passive galaxies, but only within the virial radius. The Lya absorption at b ~ Rvir for a redshift-controlled sample peaks at M* ~ 10^9 Msun~ (Mhalo ~ 10^11 Msun).

astro-ph.GA↗

arXiv2023

Characterizing cool, neutral gas and ionized metals in the outskirts of low-z galaxy clusters

We present the first detection of cool, neutral gas in the outskirts of low-z galaxy clusters using a statistically significant sample of 3191 z$\approx$0.2 background quasar - foreground cluster pairs by cross-matching the Hubble Spectroscopic Legacy Archive quasar catalog with optically- and SZ-selected cluster catalogs. The median cluster mass of our sample is $\approx 10^{14.2}$ M_sun, with a median impact parameter ($ρ_{cl}$) of $\approx5$ Mpc. We detect significant Lya, marginal CIV, but no OVI absorption in the signal-to-noise ratio weighted mean stacked spectra with rest-frame equivalent widths of 0.096$\pm$0.011 A, 0.032$\pm$0.015 A, and <0.009 A (3$σ$) for our sample. The Lya REW shows a declining trend with increasing $ρ_{cl}$ ($ρ_{cl}$ / $R_{500}$) which is well explained by a power-law with a slope of -0.79 (-0.70). The covering fractions (CFs) measured for Lya (21\%), CIV (10\%) and OVI (10\%) in cluster outskirts are significantly lower than in the circumgalatic medium (CGM). We also find that the CGM of galaxies that are closer to cluster centers or that are in massive clusters is considerably deficient in neutral gas. The low CF of the Lya along with the non-detection of Lya signal when the strong absorbers (N(HI) > $10^{13} cm^{-2}$) are excluded, indicate the patchy distribution of cool gas in the outskirts. We argue that the cool gas in cluster outskirts in combination arises from the circumgalactic gas stripped from cluster galaxies and to large-scale filaments feeding the clusters with cool gas.

astro-ph.GA↗

arXiv2022

Intranight optical variability of low-mass Active Galactic Nuclei: A Pointer to blazar-like activity

This study aims to characterise, for the first time, intranight optical variability (INOV) of low-mass active galactic nuclei (LMAGN) which host a black hole (BH) of mass $M_{BH} \sim 10^6 M_{\odot}$, i.e., even less massive than the Galactic centre black hole Sgr A* and 2-3 orders of magnitude below the supermassive black holes (SMBH, $M_{BH}$ $\sim$ $10^8 - 10^9 M_{\odot}$) which are believed to power quasars. Thus, LMAGN are a crucial subclass of AGN filling the wide gap between SMBH and stellar-mass BHs of Galactic X-ray binaries. We have carried out a 36-session campaign of intranight optical monitoring of a well-defined, representative sample of 12 LMAGNs already detected in X-ray and radio bands. This set of LMAGN is found to exhibit INOV at a level statistically comparable to that observed for blazars (M$_{BH} \gtrsim$ 10$^{8-9}$ M$_{\odot}$) and for the $γ$-ray detected Narrow-line Seyfert1 galaxies (M$_{BH}\sim 10^7$ M$_{\odot}$) which, too, are believed to have relativistic jets. This indicates that the blazar-level activity can even be sustained by central engines with black holes near the upper limit for Intermediate Mass Black Holes ($M_{BH}$ $\sim$ $10^3 - 10^6 M_{\odot}$).

astro-ph.HE↗

arXiv2022

Discovery of a cool, metal-rich gas reservoir in the outskirts of z ~ 0.5 clusters

We built the first-ever statistically significant sample of ~ 80,000 background quasar - foreground cluster pairs to study the cool, metal-rich gas in the outskirts (> R500) of z ~ 0.5 clusters with a median mass of ~ 10^14.2 M_sun. The sample was obtained by cross-matching the SDSS cluster catalog of Wen & Han (2015) and SDSS quasar catalog of Lyke et al. (2020). The median impact parameter (rho_cl) of the clusters from the quasar sightlines is 2.4 Mpc (median rho_cl/R500 = 3.6). A strong MgII, along with marginal FeII, absorption is detected in the mean and median stacked spectra of the quasars with total MgII rest-frame equivalent width (W^(2796+2803)) of 0.034 pm 0.005 A (7simga) and 0.010 pm 0.003 A (3sigma), respectively. The W^(2796+2803) shows a declining trend with increasing rho_cl and rho_cl/R500, but does not show any significant trend with mass (M500) or redshift (zcl) within the small M500 and zcl ranges probed here. The MgII absorption signal and the trends persist even if we exclude the quasar-cluster pairs where the background quasars may be probing the circum-galactic medium (CGM) of bright galaxies with impact parameters < 300 kpc. The MgII (and FeII) absorption reported here is the first detection of its kind. It indicates the presence of a cool, metal-rich gas reservoir surrounding galaxy clusters out to several R500. We suggest that the metal-rich gas in the cluster outskirts arise from stripped materials and that gas stripping may be important out to large clustocentric distances (> 3R500 ).

astro-ph.GA↗

arXiv2021

Intranight variability of UV emission from powerful blazars

We report the first study to characterise intranight variability of the blazar class from the perspective of (rest-frame) UV emission. For this, we carried out intranight optical monitoring of 14 flat-spectrum radio quasars (FSRQs) located at high redshifts (1.5 < $z$ < 3.7), in 42 sessions of median duration $\sim$ 5.4 hr. These sources were grouped into two samples distinguished by published fractional optical polarisation: (i) nine low-polarisation sources with $p_{opt} < 3\%$ and (ii) five high-polarisation sources. Unexpectedly, a high duty cycle (DC $\sim$ 30$\%$) is found for intranight variability (with amplitude $ψ> 3\%$) of the low-polarisation sources. This DC is a few times higher than that reported for low-polarisation FSRQs located at moderate redshifts ($z$ $\sim$ 0.7) and hence typically monitored in the rest-frame blue-optical. Further, we found no evidence for an increased intranight variability of UV emission with polarisation, in contrast to the strong correlation found for intranight variability of optical emission. We briefly discuss this in the context of an existing scenario which posits that the nonthermal UV emission of blazars arises from a relativistic particle population different from that radiating up to near-infrared/optical frequencies.

astro-ph.GA↗

arXiv2021

The upcoming 4m ILMT facility and data reduction pipeline testing

The 4m International Liquid Mirror Telescope (ILMT) installation activities have recently been completed at the Devasthal observatory (Uttarakhand, India). The ILMT will perform continuous observation of a narrow strip of the sky ($\sim$27$'$) passing over the zenith in the SDSS $g'$, $r'$ and $i'$ bands. In combination with a highly efficient 4k $\times$ 4k CCD camera and an optical corrector, the images will be secured at the prime focus of the telescope using the Time Delayed Integration technique. The ILMT will reach $\sim$22.5 mag ($g'$-band) in a single scan and this limiting magnitude can be further improved by co-adding the nightly images. The uniqueness of the one-day cadence and deeper imaging with the ILMT will make it possible to discover and study various galactic and extra-galactic sources, especially variable ones. Here, we present the latest updates of the ILMT facility and discuss the preparation for the first light, which is expected during early 2022. We also briefly explain different steps involved in the ILMT data reduction pipeline.

astro-ph.IM↗

arXiv2021

A Search for Blazar Activity in Broad-Absorption-Line Quasars

Our recently reported lack of Intra-Night Optical Variability (INOV) among Broad-Absorption-Line (BAL) quasars exhibiting some blazar-like radio properties, either questions polar ejection of BAL clouds, and/or hints at a physical state of the relativistic jet modified due to interaction with the thermal BAL wind on the innermost sub-parsec scale. As a robust check on this unexpected finding for the BAL_blazar candidates, we report here the INOV study of a new and much more rigorously defined comparison sample consisting of 9 normal (non-BAL) blazars, matched in both magnitude and redshift to the aforementioned sample of BAL_blazar candidates. The present campaign spanning 27 sessions yields an INOV duty cycle of ~23% for the comparison sample of normal blazars, employing the enhanced F-test. However, even this more sensitive test does not detect INOV for the sample of BAL_blazar candidates. This stark INOV contrast found here between the BAL_blazar candidates and normal blazars can probably be traced to a physical interaction of the relativistic jet with the thermal wind, within sub-parsec range from the nucleus. The consequent enfeebling of the jet would additionally explain the striking deficiency among BAL quasars of powerful FR II radio lobes on the much larger scale of 10-100 kpc.

astro-ph.GA↗

arXiv2021

Appearance vs Disappearance of broad absorption line troughs in quasars

We present a new set of 84 Broad absorption line (BAL) quasars ( 1.7 $<$ \zem $<$ 4.4) exhibiting an appearance of \civ BAL troughs over 0.3$-$4.8 rest-frame years by comparing the Sloan Digital Sky Survey Data Release (SDSSDR)-7, SDSSDR-12, and SDSSDR-14 quasar catalogs. We contrast the nature of BAL variability in this appearing BAL quasar sample with a disappearing BAL quasar sample studied in literature by comparing the quasar's intrinsic, BAL trough, and continuum parameters between the two samples. We find that appearing BAL quasars have relatively higher redshift and smaller probed timescales as compared to the disappearing BAL quasars. To mitigate the effect of any redshift bias, we created control samples of appearing and disappearing BAL quasars that have similar redshift distribution. We find that the appearing BAL quasars are relatively brighter and have shallower and wider BAL troughs compared to the disappearing BAL sample. The distribution of quasar continuum variability parameters between the two samples is clearly separated, with the appearance of the BAL troughs being accompanied by the dimming of the continuum and vice versa. Spectral index variations in the two samples also point to the anti-correlation between the BAL trough and continuum variations consistent with the "bluer when brighter" trend in quasars. We show that the intrinsic dust model is less likely to be a favorable scenario in explaining BAL appearance/disappearance. Our analysis suggests that the extreme variations of BAL troughs like BAL appearance/disappearance are mainly driven by changes in the ionization conditions of the absorbing gas.

astro-ph.GA↗

arXiv2019

Are there broad absorption-line blazars?

We report the first systematic search for blazars among broad-absorption-line (BAL) quasars. This is based on our intranight optical monitoring of a well-defined sample of 10 candidates selected on the criteria of a flat spectrum and an abnormally high linear polarization at radio wavelengths. A small population of BAL blazars can be expected in the 'polar model' of BAL quasars. However, no such case is found, since none of our 30 monitoring sessions devoted to the 10 candidates yielded a positive detection of intra-night optical variability (INOV), which is uncharacteristic of blazars. This lack of INOV detection contrasts with the high duty cycle of INOV observed for a comparison sample of 15 'normal' (i.e., non-BAL) blazars. Some possible implications of this are pointed out.

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