Publications (62)
Intranight variability of UV emission from powerful blazars
Krishan Chand, Gopal-Krishna, Amitesh Omar +4
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 m…
Signature of outflows in strong MgII absorbers in quasar sightlines
Mahavir Sharma, Biman B. Nath, Hum Chand
We report a correlation between velocity offset (beta = v/c) of strong Mg II absorption systems and the bolometric luminosity (L_bol) of quasars in SDSS-DR7. We find that, beta sho…
Dependence of Residual Rotation Measure (RRM) on Intervening MgII Absorbers at Cosmic Distances
Ravi Joshi, Hum Chand
We investigate the dependence of residual rotation measure (RRM) on intervening absorption systems at cosmic distances by using a large sample of 539 SDSS quasars in conjunction wi…
Are there broad absorption-line blazars?
Sapna Mishra, Gopal-Krishna, Hum Chand +2
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 candi…
A Catalog of Narrow Line Seyfert 1 Galaxies from the Sloan Digital Sky Survey Data Release 12
Suvendu Rakshit, C. S. Stalin, Hum Chand +1
We present a new catalog of narrow-line Seyfert 1 (NLSy1) galaxies from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12). This was obtained by a systematic analysis through…
C IV absorbers tracing cool gas in dense galaxy group/cluster environments
Aditya Manuwal, Anand Narayanan, Sowgat Muzahid +3
We present analysis on three intervening H I-C IV absorption systems tracing gas within galaxy group/cluster environments, identified in the /COS far-UV spectra of the backgro…
Probing spectral properties of radio-quiet quasars searched for optical microvariability
Hum Chand, Paul J. Wiita, Alok C. Gupta
We obtained SDSS spectra for a set of 37 radio-quiet quasars (RQQSOs) that had been previously examined for rapid small scale optical variations, or microvariability. Their H-beta…
Unraveling the mysteries of Jets in peculiar NLSy1 galaxies through multi-wavelength variability
Vineet Ojha, Xue-Bing Wu, Luis C. Ho +4
Radio-quiet narrow-line Seyfert 1 galaxies (RQ-NLSy1s) are generally considered to be dominated by thermal emission from the accretion disk. However, recurring 37 GHz radio flares…
Probing magnetic fields with Square Kilometre array and its precursors
Subhashis Roy, Sharanya Sur, Kandaswamy Subramanian +3
Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has hampered its study. The first p…
Intranight optical variability of TeV blazars with parsec-scale jets dominated by slow-moving radio knots
Vibhore Negi, Gopal-Krishna, Hum Chand +1
BL Lac objects detected at TeV energies preferentially belong to the subclass called 'high-frequency-peaked' BL Lacs (HBLs). Parsec-scale radio jets in these TeV-HBLs often show do…
Role of intervening Mg II absorbers on the rotation measure and fractional polarisation of the background quasars
Sunil Malik, Hum Chand, T. R. Seshadri
We probed the magnetic fields in high-redshift galaxies using excess extragalactic contribution to residual rotation measure (RRM) for quasar sightlines with intervening Mg II abso…
Intra-Night Optical monitoring of three γ-ray detected Narrow-line Seyfert 1 galaxies
Vineet Ojha, Gopal-Krishna, Hum Chand
For 3 radio-loud -ray detected Narrow-Line Seyfert 1 (-ray NLSy1) galaxies, we report optical variability on intra-night and/or week-like time scales, based on five …
Appearance vs Disappearance of broad absorption line troughs in quasars
Sapna Mishra, M. Vivek, Hum Chand +1
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 0.34.8 rest-frame years by comparing the…
Constraining fundamental constants of physics with quasar absorption line systems
Patrick Petitjean, Raghunathan Srianand, Hum Chand +3
We summarize the attempts by our group and others to derive constraints on variations of fundamental constants over cosmic time using quasar absorption lines. Most upper limits res…
Evidence of under-developed torus and broad-line region of weak emission line quasars based on their spectral energy distribution
Ritish Kumar, Hum Chand, Ravi Joshi
To unravel the dominant cause of the weak emission line in a subset of optically selected radio-quiet 'weak emission line quasars' (WLQs), we have investigated the possibility of a…
Investigating kpc-scale radio emission properties of narrow-line Seyfert 1 galaxies
Veeresh Singh, Hum Chand
In recent years, several Radio-Loud Narrow-Line Seyfert 1 galaxies (RL-NLS1) possessing relativistic jets have come into attention with their detections in Very Large Baseline Arra…
Deceleration of CIV and SiIV broad absorption lines in X-ray bright quasar SDSS-J092345+512710
Ravi Joshi, Raghunathan Srianand, Hum Chand +4
We report a synchronized kinematic shift of CIV and SiIV broad absorption lines (BAL) in a high-ionization, radio-loud, and X-ray bright quasar SDSS-J092345+512710 (at $z_{em} \sim…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-III
Parveen Kumar, Gopal-Krishna, Hum Chand
This is continuation of our programme to search for the elusive radio-quiet BL Lacs, by carrying out a systematic search for intranight optical variability (INOV) in a subset of `w…
Incidence of Mg II absorption systems towards flat-spectrum radio quasars
Hum Chand, Gopal-Krishna
The conventional wisdom that the rate of incidence of Mg II absorption systems, dN/dz (excluding `associated systems' having velocity beta*c relative to the AGN of less than ~5000…
Probing the environment of high-z quasars using the proximity effect in projected quasar pairs
Priyanka Jalan, Hum Chand, Raghunathan Srianand
We have used spectra of 181 projected quasar pairs at separations arcmins from the Sloan Digital Sky-Survey Data Release 12 in the redshift range of 2.5 to 3.5 to probe t…
Probing jet-induced optical variability across timescales in radio-loud NLSy1 galaxies
Vivek Kumar Jha, Anshul Kumar Sharma, Madhu Sudan +1
We investigate optical variability across multiple timescales in a sample of radio-loud narrow-line Seyfert~1 (RL-NLSy1) galaxies, including -ray detected, -ray undetected,…
A Search for Blazar Activity in Broad-Absorption-Line Quasars
Sapna Mishra, Gopal-Krishna, Hum Chand +3
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…
Limits on the time variation of the electromagnetic fine-structure constant in the low energy limit from absorption lines in the spectra of distant quasars
Raghunathan Srianand, Hum Chand, Patrick Petitjean +1
Most of the successful physical theories rely on the constancy of few fundamental quantities (such as the speed of light, , the fine-structure constant, α, the proton to electr…
On the variation of the fine-structure constant: Very high resolution spectrum of QSO HE 0515-4414
Hum Chand, Raghunathan Srianand, Patrick Petitjean +3
We present a detailed analysis of a very high resolution (R\approx 112,000) spectrum of the quasar HE 0515-4414 obtained using the High Accuracy Radial velocity Planet Searcher (HA…
Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination
Roger Cayrel, Matthias Steffen, Hum Chand +6
Context: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheri…
In response to the comments by Murphy et al. (arxiv:0708.3677)
Raghunathan Srianand, Hum Chand, Patrick Petitjean +1
In their comment, Murphy et al. (arxiv:0708.3677) criticize the fitting procedure we used in two previous papers [Srianand et al. 2004(Paper I) and Chand et al. 2004 (Paper II)] an…
Probing the time-variation of the fine-structure constant: Results based on Si IV doublets from a UVES sample
Hum Chand, Patrick Petitjean, Raghunathan Srianand +1
We report a new constraint on the variation of the fine-structure constant based on the analysis of 15 Si IV doublets selected from a ESO-UVES sample. We find Îα/α= (+0.15+/-0.4…
The transience and persistence of high optical polarization state in beamed radio quasars
Krishan Chand, Gopal-Krishna, Amitesh Omar +2
We examine the long-term stability (on decade-like time scales) of optical `high polarization' (HP) state with , which commonly occurs in flat-spectrum (i.e., beam…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-IV
Parveen Kumar, Hum Chand, Gopal-Krishna
We report an extension of our program to search for radio-quiet BL Lac candidates using intra-night optical variability (INOV) as a probe. The present INOV observations cover a wel…
Optical Flux and Colour Variability of Blazars in the ZTF Survey
Vibhore Negi, Ravi Joshi, Krishan Chand +4
We investigate the temporal and colour variability of 897 blazars, comprising 455 BL Lacertae objects (BL Lacs) and 442 Flat Spectrum Radio Quasars (FSRQs), selected from the Roma-…
Brightness and colour variability in NLSy1s
Madhu Sudan, Hum Chand, Paul J. Wiita +1
The Zwicky Transient Facility (ZTF), with its extensive optical monitoring capabilities, has provided an unprecedented opportunity to study the long-term variability of active gala…
Probable low-frequency quasi-periodic oscillations in blazars from the ZTF survey
Anuvab Banerjee, Vibhore Negi, Ravi Joshi +6
We investigate the possible presence of quasi-periodic oscillation (QPO) signals in 2103 blazars from the Zwicky Transient Facility (ZTF) time-domain survey. We detect a low-freque…
Intra-night optical variability of ray detected narrow-line Seyfert 1 galaxies
Vineet Ojha, Hum Chand, Gopal-Krishna
We report the first attempt to systematically characterise intra-night optical variability (INOV) of the rare and enigmatic subset of Narrow-Line Seyfert1 galaxies (NLSy1s), which…
Circumgalactic medium of quasar host galaxies at 0.4<z<0.8 probed by strong Mg II absorption
Paryag Sharma, Raghunathan Srianand, Hum Chand +1
Using a sample of 166 projected quasar pairs we investigate the influence of active galactic nuclei on the circumgalactic medium (CGM) of the quasar host galaxies probed using stro…
Incidence of Strong MgII Absorbers Towards Different Types of Quasars
Ravi Joshi, Hum Chand, Gopal-Krishna
We report the first comparative study of strong MgII absorbers seen towards radio-loud quasars of core-dominated (CDQs) and lobe-dominated (LDQs) types and normal QSOs. The MgII as…
Exploring the AGN Accretion Disks using Continuum Reverberation Mapping
Vivek Kumar Jha, Ravi Joshi, Jayesh Saraswat +3
In the innermost regions of Active Galactic Nuclei (AGN), matter is understood to be flowing onto the Supermassive black hole (SMBH), which forms an accretion disk. This disk is re…
Intranight optical variability of low-mass Active Galactic Nuclei: A Pointer to blazar-like activity
Gopal-Krishna, Krishan Chand, Hum Chand +4
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} \…
A comparison of X-ray photon indices among the narrow and broad-line Seyfert 1 galaxies
Vineet Ojha, Hum Chand, Gulab Chand Dewangan +1
We present a detailed comparative systematic study using a sample of 221 Narrow-line Seyfert 1 (NLSy1) galaxies in comparison to a redshift matched sample of 154 Broad-line Seyfert…
C IV absorption line variability in X-ray bright BALQSOs
Ravi Joshi, Hum Chand, Raghunathan Srianand +1
We report kinematic shift and strength variability of C IV broad absorption line (BAL) trough in two high-ionization X-ray bright QSOs SDSS J085551+375752 (at zem ~ 1.936) and SDSS…
Do Low-Mass, Low-Luminosity AGNs Deviate from the Quasar Main Sequence?
Himanshu Sharma, Vivek Kumar Jha, Hum Chand +1
We present a comprehensive spectroscopic and variability-based characterisation of a sample of low-luminosity active galactic nuclei (AGNs) hosting low mass black holes, identified…
A comparative study of the physical properties for a representative sample of Narrow and Broad-line Seyfert galaxies
Vivek Kumar Jha, Hum Chand, Vineet Ojha +2
We present a comparative study of the physical properties of a homogeneous sample of 144 Narrow line Seyfert 1 (NLSy1) and 117 Broad-line Seyfert 1 (BLSy1) galaxies. These two samp…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-II
Hum Chand, Parveen Kumar, Gopal-Krishna
This is continuation of our search for the elusive radio-quiet blazars, by carrying out a systematic programme to detect intranight optical variability (INOV) in a subset of `Weak-…
Intranight optical variability of radio-loud broad absorption line quasars
Ravi Joshi, Hum Chand
We present the results of an optical photometric monitoring program of 10 extremely radio loud broad absorption line quasars (RL-BALQSOs) with radio-loudness parameter, R, greater…
Optical microvariability properties of BALQSOs
Ravi Joshi, Hum Chand, Alok C. Gupta +1
We present optical light curves of 19 radio quiet (RQ) broad absorption line (BAL) QSOs and study their rapid variability characteristics. Systematic CCD observations, aided by a c…
Uncertainty-Aware Deep Learning for the Ly Forest: CNN-Based Absorber Detection and Characterization
Paryag Sharma, Vikram Khaire, Ting-Yun Cheng +2
The Ly forest is a powerful probe of the intergalactic medium and small-scale matter distribution, but deriving absorber properties traditionally requires computationally expen…
Kpc-scale radio-jets in narrow-line Seyfert 1 galaxies
Veeresh Singh, Hum Chand, C. H. Ishwara-Chandra +1
Narrow-Line Seyfert 1 galaxies (NLS1s) are generally believed to be radio-quiet Active Galactic Nuclei (AGN) with relatively less-massive (1010~M) Super-Ma…
Evidence of jet induced optical microvariability in radio-loud Narrow Line Seyfert 1 Galaxies
Vineet Ojha, Vivek Kumar Jha, Hum Chand +1
To quantify the role of radio jets for Intra-Night Optical Variability (INOV) in Radio-Loud Narrow-Line Seyfert 1 (RLNLSy1) galaxies, we report the first systematic comparative INO…
Constraining the ratio of median pixel optical depth profile around z ~ 4 quasars using the longitudinal proximity effect
Priyanka Jalan, Hum Chand, Raghunathan Srianand
We present a detailed study of the longitudinal proximity effect using a sample of 85 quasars spanning an emission redshift range of and Lyman continuum…
Intranight Optical Variability of Blazars and Radio-quiet Quasars using the ZTF Survey
Vibhore Negi, Gopal-Krishna, Ravi Joshi +4
We explore the potential of the ongoing Zwicky-Transient-Facility (ZTF) survey for studying Intra-Night Optical Variability (INOV) of active galactic nuclei (AGN), in particular fo…
Investigating the Circumgalactic Medium through Mg II absorption coincidence
Paryag Sharma, Raghunathan Srianand, Hum Chand +1
We present a statistical measurement of the transverse coherence of Mg II absorption using a large sample of 9204 absorber-centric quasar sightline pairs from…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars
Gopal-Krishna, Ravi Joshi, Hum Chand
Based on a recently started programme, we report the first search for intranight optical variability among radio-quiet weak-line-quasars (RQWLQs). Eight members of this class were…
Probing spectral properties of radio-quiet quasars searched for optical microvariability-II
Ravi Joshi, Hum Chand, Paul J. Wiita +2
In the context of AGN unification scheme rapid variability properties play an important role in understanding any intrinsic differences between sources in different classes. In thi…
Properties of Narrow line Seyfert 1 galaxies
Suvendu Rakshit, C. S. Stalin, Hum Chand +1
Narrow line Seyfert 1 (NLSy1) galaxies constitute a class of active galactic nuclei characterized by the full width at half maximum (FWHM) of the H broad emission line < 2000 k…
Exploring the Origins of Optical Variability in AGNs: Correlations with Black Hole Properties, X-ray, and Radio Emission
Vivek Kumar Jha, Debbijoy Bhattacharya, Hum Chand
We study the optical variability characteristics of Active Galactic Nuclei (AGN) from the Swift Burst Alert Telescope (BAT) AGN catalogue by utilising approximately five years of o…
Star-Formation in Neutral Hydrogen Gas Reservoirs at Cosmic Noon
Dharmender, Ravi Joshi, Michele Fumagalli +3
We aim to constrain the average star formation associated with neutral hydrogen gas reservoirs at cosmic noon. Using a unprecedented sample of 1716 high column density Damped Ly-$Î…
VR CCD photometry of variable stars in globular cluster NGC 4147
Sneh Lata, A. K. Pandey, J. C. Pandey +20
We present results of a search for variable stars in a region of the globular cluster NGC 4147 based on photometric observations with 4k x 4k CCD imager mounted at the axial port o…
The Origin of UV-optical Variability in AGN and Test of Disc Models: XMM-Newton and ground based observations of NGC4395
Ian McHardy, Sam Connolly, Brad Peterson +12
The origin of short timescale (weeks/months) variability of AGN, whether due to intrinsic disc variations or reprocessing of X-ray emission by a surrounding accretion disc, has bee…
Weak-emission-line quasars: A new clue from their optical variability
Ritish Kumar, Gopal-Krishna, Hum Chand +1
Weak-emission-line QSOs (WLQs) are an enigmatic subclass of the QSO population, as their optical/UV spectra are marked by abnormally weak (or absent) emission lines. To obtain much…
Status of Astronomy Education in India: A Baseline Survey
Moupiya Maji, Surhud More, Aniket Sule +28
We present the results of a nation-wide baseline survey, conducted by us, for the status of Astronomy education among secondary school students in India. The survey was administere…
Intranight optical monitoring of the rare quasar J0950+5128, the brightest known candidate for transition from radio-quiet to radio-loud state
Krishan Chand, Gopal-Krishna, Hum Chand
We report a novel pilot project to characterise intra-night optical variability (INOV) of an extremely rare type of quasar, which has recently been caught in the act of transiting…
On the incidence of weak and strong Mg II absorbers towards the Flat and Steep Spectrum Radio quasars
Ritish Kumar, Sapna Mishra, Hum Chand
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…
Accretion Disk Sizes from Continuum Reverberation Mapping of AGN Selected from the ZTF Survey
Vivek Kumar Jha, Ravi Joshi, Hum Chand +4
We present the accretion disk size estimates for a sample of 19 active galactic nuclei (AGN) using the optical , , and band light curves obtained from the Zwicky Transien…