arXiv subjects

Vibhore Negi

Publications and source records attributed to Vibhore Negi.

43 records · Page 3Linked to original sources

arXiv2022

First light preparations of the 4m ILMT

The 4m International Liquid Mirror Telescope (ILMT) is a zenith-pointing optical observing facility at ARIES Devasthal observatory (Uttarakhand, India). The first light preparatory activities of the ILMT were accomplished in April 2022 followed by on-sky tests that were carried out at the beginning of May 2022. This telescope will perform a multi-band optical (SDSS $g'$, $r'$ and $i'$) imaging of a narrow strip (~$22'$) of sky utilizing the time-delayed integration technique. Single-scan ILMT images have an integration time of 102 sec and consecutive-night images can be co-added to further improve the signal-to-noise ratio. An image subtraction technique will also be applied to the nightly recorded observations in order to detect transients, objects exhibiting variations in flux or position. Presently, the facility is in the commissioning phase and regular operation will commence in October 2022, after the monsoon. This paper presents a discussion of the main preparation activities before first light, along with preliminary results obtained.

astro-ph.IM↗

arXiv2022

Astrometric and photometric standard candidates for the upcoming 4-m ILMT survey

The International Liquid Mirror Telescope (ILMT) is a 4-meter class survey telescope that has recently achieved first light and is expected to swing into full operations by 1st January 2023. It scans the sky in a fixed 22' wide strip centered at the declination of $+29^o21'41''$ and works in Time Delay Integration (TDI) mode. We present a full catalog of sources in the ILMT strip that can serve as astrometric calibrators. The characteristics of the sources for astrometric calibration are extracted from Gaia EDR3 as it provides a very precise measurement of astrometric properties such as RA ($α$), Dec ($δ$), parallax ($π$), and proper motions ($μ_{α^{*}}$ & $μ_δ$). We have crossmatched the Gaia EDR3 with SDSS DR17 and PanSTARRS-1 (PS1) and supplemented the catalog with apparent magnitudes of these sources in g, r, and i filters. We also present a catalog of spectroscopically confirmed white dwarfs with SDSS magnitudes that may serve as photometric calibrators. The catalogs generated are stored in a SQLite database for query-based access. We also report the offsets in equatorial positions compared to Gaia for an astrometrically calibrated TDI frame observed with the ILMT.

astro-ph.IM↗

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↗

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

Optical Flux and Colour Variability of Blazars in the ZTF Survey

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-BZCAT catalogue, using the multi-band light curves from the Zwicky Transient Facility (ZTF DR6) survey. Assessing the colour variability characteristics over ~2 year timescales, we found that 18.5 per cent (84 out of 455) BL Lacs showed a stronger bluer when brighter (BWB) trend, whereas 9.0 per cent (41 out of 455) showed a redder when brighter (RWB) trend. The majority (70 per cent) of the BL Lacs showing RWB are host galaxy dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442) objects showed a strong BWB trend and 17.6 per cent (78 out of 442) showed a strong RWB trend. Hence we find that BL Lacs more commonly follow a BWB trend than do FSRQs. This can be attributed to the more dominant jet emission in the case of BL Lacs and the contribution of thermal emission from the accretion disc for FSRQs. In analysing the colour behaviour on shorter time windows, we find many blazars evince shorter partial trends of BWB or RWB nature (or occasionally both). Some of such complex colour behaviours observed in the colour-magnitude diagrams of the blazars may result from transitions between the jet-dominated state to the disc-dominated state and vice versa.

astro-ph.GA↗

arXiv2021

Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars

We present XMM-Newton X-ray observations of nine confirmed lensed quasars at $1 \lesssim z \lesssim 3$ identified by the Gaia Gravitational Lens program. Eight systems are strongly detected, with 0.3--8.0 keV fluxes $F_{0.3-8.0} \gtrsim 5 \times 10^{-14}\ {\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}$. Modeling the X-ray spectra with an absorbed power law, we derive power law photon indices and 2--10 keV luminosities for the eight detected quasars. In addition to presenting sample properties for larger quasar population studies and for use in planning for future caustic crossing events, we also identify three quasars of interest: a quasar that shows evidence of flux variability from previous ROSAT observations, the most closely-separated individual lensed sources resolved by XMM-Newton, and one of the X-ray brightest quasars known at $z>3$. These sources represent the tip of discovery that will be enabled by SRG/eROSITA.

astro-ph.GA↗

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↗