arXiv subjects

Ashutosh Tomar

Publications and source records attributed to Ashutosh Tomar.

3 recordsLinked to original sources

arXiv2026

Intranight optical monitoring of eight quasars with superluminal jets and low optical polarization: Probing the mechanism of rapid flux variability

Intranight optical variability (INOV) of blazars whose emission arises predominantly from a relativistic jet, has been widely linked to a combination of two key observational diagnostics: the fractional optical polarization ($P_{opt}$) and the Doppler boost arising from relativistic beaming of the jet's emission. To discern which of these two factors plays the dominant role, we investigate INOV of a well-defined representative sample of 8 flat-spectrum radio quasars (FSRQs, which are `low-synchrotron-peaked' blazars) whose jets exhibit a low degree of polarization ($P_{opt}$ < 3$\%$) but a large superluminal motion with c$β_{app}$ = 5c $-$ 31c (median = 10c). We report sensitive optical monitoring of these 8 `low-polarization superluminal quasars' (LPSQs) using a 1.3-metre telescope, in 20 sessions of minimum 3-hour duration. These data were supplemented with similarly high-quality monitoring data/results available in the literature for this sample, thus adding another 19 sessions. Duty cycle (DC) of strong INOV (amplitude $ψ> 5\%$) is found to be zero, applying the normally used $F_η$ test, and $\sim 7\%$ using the more powerful `enhanced $F$-test'. The small DC, despite the strong Doppler boost expected from the observed high superluminality, shows that strong INOV is much rarer among LPSQs, compared to high-polarization quasars, for which DC $(ψ> 5\%$) is found to be around 30%, even using the $F_η$ test. This provides the first compelling evidence that optical polarization is the primary diagnostic of INOV, while relativistic beaming plays a supportive role. Possible implications for the proposed mechanisms for INOV are pointed out.

astro-ph.HE↗

arXiv2025

Spectroscopic Reverberation Mapping for SARM: The Case of Mrk 1048 and Mrk 618

Robust extragalactic distance measurements are crucial for resolving the persistent discrepancy in the value of the Hubble constant (H$_0$)). Active Galactic Nuclei (AGNs), through their compact and variable broad-line regions (BLRs), enable the determination of geometric distances when reverberation mapping (RM) is combined with spectroastrometry(SA). We report results from a spectroscopic RM campaign (October 2022 to March 2023) targeting two narrow-line Seyfert 1 galaxies, Mrk 1048 and Mrk 618, using 3.6-m DOT and 2-m HCT. High-cadence spectro-photometric monitoring was carried out using onboard instruments such as ADFOSC, HFOSC, and TANSPEC, resulting in well-sampled continuum and emission line light curves. The observed fractional variability ($F_{\mathrm{var}}$) ranged from 4% to 14% across the $g$-band, H$β$, and H$α$ light curves. The time lags were measured using the interpolated cross-correlation function (ICCF), PyI$^{2}$CCF, and \textsc{JAVELIN} methods. In the rest frame, the ICCF analysis yields H$β$ lags of $10.5^{+2.6}_{-4.2}$ days for Mrk 1048 and $10.2^{+3.4}_{-2.9}$ days for Mrk 618, while the corresponding H$α$ lags are $18.7^{+5.3}_{-5.4}$ and $14.4^{+4.6}_{-10.5}$ days, respectively. The emission-line widths, measured from the rms spectra using $σ_{\mathrm{line}}$, give virial black hole mass estimates of $6.3^{+2.0}_{-2.1} \times 10^7\,M_\odot$ for Mrk 1048 and $1.2^{+0.4}_{-0.6} \times 10^7\,M_\odot$ for Mrk 618. These results will serve as a basis for absolute geometric distance calibration when combined with VLTI/GRAVITY spectro-astrometric measurements, thereby contributing to the development of AGNs as standardizable cosmological probes.

astro-ph.GA↗

arXiv2025

Probing dust torus radius--luminosity relation: An WISE view

We present measurements of the dusty torus sizes of 51 active galactic nuclei (AGNs) with a redshift of $z<$ 0.8. Our analysis utilizes about 16 years of optical photometric data of 146 AGNs from various time-domain surveys, including ASAS-SN, CRTS, and ZTF, along with 14 years of infrared data in the $W$1 ($\sim$ 3.4 $μ$m) and $W$2 ($\sim$ 4.6 $μ$m) bands obtained from the Wide-Field Infrared Survey Explorer (WISE). The estimated dust torus size ranges from 1000 to 3000 days, using both the cross-correlation analysis and lightcurve modeling through `MICA'. The measured lag has been corrected by $(1+z)^{-0.37}$, to account for cosmological time dilation and the torus temperature-gradient scaling. We conduct a linear regression analysis for both the $W$1 and $W$2 bands to examine the radius--luminosity ($R$--$L_{BOL}$) relationship under two conditions: one where the slope is fixed at 0.5 and one where it is allowed to vary. For the fixed slope of 0.5, we find the ratio of R$_{\mathrm{BLR}}$: R$_{W1}$: R$_{W2}$ to be 1: 9: 12, indicating that the torus lies outside the BLR and that its size increases with wavelength. Furthermore, we determine the relationship between torus size and L$_{BOL}$, yielding best-fit slopes of $0.413\pm0.047$ for the $W$1 band and $0.397\pm0.058$ for the $W$2 band. Both slopes are shallower than predicted by the dust radiation equilibrium model. Furthermore, our findings indicate that the torus size systematically decreases as the Eddington ratio increases, a trend that can be explained by the self-shadowing effects of slim disks.

astro-ph.GA↗