arXiv subjects

Krishan Chand

Publications and source records attributed to Krishan Chand.

15 recordsLinked to original sources

arXiv2026

Search for intranight optical variability in a large sample of intermediate-mass black holes

Using the extensive archival database of the Zwicky Transient Facility (ZTF) survey, we have investigated intranight optical variability (INOV) in a large sample of low-mass active galactic nuclei (LMAGN) powered by intermediate-mass black holes (IMBHs). After applying a sequence of well-motivated selection filters, we constructed an unbiased, representative sample of 62 IMBHs for which $r$-band intranight photometric sequences are available in the ZTF database, yielding 167 sessions with a minimum duration of 2 h. By performing aperture photometry on the sequence of exposures in each session, we derived differential light curves (DLCs) of the target LMAGN relative to two carefully selected non-varying comparison stars monitored simultaneously with the LMAGN in the same exposure sequence. Application of the widely used test based on $F$-statistics to all these DLCs revealed no case of statistically significant INOV. We discuss this result for the present large sample of LMAGN in the context of an earlier report of INOV detections in another, albeit much smaller and differently selected, sample of LMAGN that had been monitored in targeted observations.

astro-ph.GA↗

arXiv2026

Geometry-controlled heat transport in differentially heated cavity through the lens of DNS and resolvent analysis

We perform direct numerical simulations (DNS) and resolvent analysis of natural convection in a differentially heated cavity (DHC) at Prandtl number $Pr = 0.7$, systematically varying the aspect ratio $\Gamma$ over $0.1 \leq \Gamma \leq 60$ and Rayleigh number $Ra = 10^5$--$10^8$. Across this nearly three-decade range of $\Gamma$, the Nusselt number $Nu$ exhibits four distinct regimes driven purely by geometric confinement via altering the structure of large-scale circulation (LSC): $Nu$ rises in regime I, peaks and plateaus in regime II, falls monotonically in regime III, and reaches a minimum and plateaus again in regime IV. Resolvent analysis shows a corresponding shift in the flow's response structure across these regimes: the maximum gain drops monotonically through regime I, bottoms out in regime II, and rises sharply through regime III and IV. The dominant response mode also changes character---from a diagonal mode peaking near the adiabatic walls, to a horizontally oriented structure peaking in the bulk flow (regimes I--II), to a vertical stack of such structures in regimes III--IV. The resolvent response tracks the sensitivity of the LSC to perturbations, which is minimized in regime II and maximized in regimes III and IV. Notably, the LSC configuration with the \textit{lowest} sensitivity consistently corresponds to the \textit{highest} heat flux, across all $Ra$ examined. We link this sensitivity to the LSC's anisotropy, quantified by the ratio of horizontal-to-vertical velocity Reynolds numbers ($Re_u/Re_v$). Remarkably, this ratio stays nearly constant ($\approx 0.45$) at the optimal (heat-transport-maximizing) aspect ratio ($\Gamma_{\mathrm{opt}}$) across all $Ra$, and the corresponding $\Gamma_{\mathrm{opt}}$ follows the power-law scaling $\Gamma_{\mathrm{opt}} \sim Ra^{-0.17 \pm 0.01}$.

physics.flu-dyn↗

arXiv2025

The alternating 'changing-look' blazar OQ 334 (B2 1420+32): New observational clues to the blazar state transitions

The high-luminosity blazar OQ 334 is a leading exponent of the intriguing rare phenomenon of alternating between a flat-spectrum radio quasar (FSRQ) and a BL Lac (BLL) states. Its two optical continuum outbursts observed during the $\sim$ 1.5-year long time span, starting Jan 2018, had been shown to coincide with transition from the FSRQ to BLL state, manifested by a sharp drop in the equivalent width of MgII broad emission-line. Recently, a continuous monitoring of its blazar state, over a much longer duration (past $\sim$ 5 years) has become possible by deploying the observed $\gamma-ray$ spectral slope ($\Gamma_{\gamma}$) as a diagnostic. This opens prospects of making a much less biased and statistically more robust check on the association of optical flaring with FSRQ $\rightarrow$ BLL transition. We find that all 4 such transitions ($\Gamma_{\gamma}$ becoming < 2.0), observed during the past $\sim$ 5 years, were accompanied by an optical flare. While this appears consistent with the transition to BL Lac state happening purely due to an enhanced optical continuum (flaring) swamping out the broad emission-lines, this simple scheme may need additional ingredients, considering the hint found for a day-like offset between the flaring and the state transition.

astro-ph.HE↗

arXiv2024

Intranight optical monitoring of the rare quasar J0950+5128, the brightest known candidate for transition from radio-quiet to radio-loud state

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 from a radio-quiet to radio-loud state, on a decadal time scale. Such rare transitions may signify a recurrence, or conceivably the first switch-on of jet activity in optically luminous quasars. The newly formed jet could well be jittery and unsteady, both in power and direction. The optically brightest among such radio-state transition candidates, the quasar J0950+5128 ($z = 0.2142$), was monitored by us with dense sampling in the R-band, during 2020-21 in 6 sessions, each lasting $>$ 4 hours. This is the first attempt to characterise the INOV properties associated with this recently discovered, extremely rarely observed phenomenon of quasar radio-state transition. The non-detection of INOV in any of the 6 sessions, down to the 1-2% level, amounts to a lack of evidence for a blazar-like optical activity, $\sim$ 2 years after its transition to radio-loud state was found. The only INOV feature detected in J0950+5128 during our observational campaign was a $\sim$ 0.15-mag spike lasting < 6 minutes, seen at 13.97 UT on 18-March-2021. We also report the available optical light curves of this quasar from the Zwicky Transient Facility (ZTF) survey, which indicate that it had experienced a phase of INOV activity around the time its transition to the radio-loud state was detected, however that phase did not sustain until the launch of our INOV campaign $\sim$ 2 years later.

astro-ph.HE↗

arXiv2024

Spectrophotometric reverberation mapping of Intermediate-mass black hole NGC 4395

Understanding the origins of massive black hole seeds and their co-evolution with their host galaxy requires studying intermediate-mass black holes (IMBHs) and estimating their mass. However, measuring the mass of these IMBHs is challenging due to the high spatial resolution requirement. A spectrophotometric reverberation monitoring is performed for a low-luminosity Seyfert 1 galaxy NGC 4395 to measure the size of the broad line region (BLR) and black hole mass. The data were collected using the 1.3-m Devasthal fast optical telescope (DFOT) and 3.6-m Devasthal optical telescope (DOT) at ARIES, Nainital, over two consecutive days in March 2022. The analysis revealed strong emission lines in the spectra and light curves of merged 5100{\AA} spectroscopic continuum flux ($f_{\mathrm{5100}}$) with photometric continuum V-band and H$\alpha$, with fractional variabilities of 6.38\% and 6.31\% respectively. In comparison to several previous studies with lag estimation $<$ 90 minutes, our calculated H$\alpha$ lag supersedes by $125.0^{+6.2}_{-6.1}$ minutes using ICCF and {\small JAVELIN} methods. The velocity dispersion ($\sigma_{\mathrm{line}}$) of the broad line clouds is measured to be $544.7^{+22.4}_{-25.1}$ km s$^{-1}$, yielding a black hole mass of $\sim$ $2.2^{+0.2}_{-0.2}\times 10^{4}M_{\mathrm{\odot}}$ and an Eddington ratio of 0.06.

astro-ph.GA↗

arXiv2024

Intranight variability of ultraviolet emission from high-$z$ blazars

Rapid intranight variability of continuum and polarization in blazars is a very useful tool to probe the beaming of a relativistic jet and the associated population of the relativistic particles. Such intranight variability in the rest-frame optical continuum has been carried out extensively, but there is a scarcity of such information in the rest-frame ultra-violet (UV), where the cause of variability might be due to {a} secondary population of relativistic particles. To fill this gap, recently in Chand et al. (2022) we reported for the first time intranight variability study of a sample of high-$z$ blazars so that the monitored optical radiation is their rest-frame UV radiation. Here we discuss in detail the implication of this investigation with a proper comparison of high-$z$ blazar samples with fractional optical polarization ($p_{opt}$) smaller and higher than 3\%. In this context, we also report intranight variability study of an additional high-$z$ blazar at $z$=2.347, namely J161942.38+525613.41, monitored over three sessions each with a duration of $\sim$5 hr. Our investigation does not reveal any compelling evidence for a stronger intranight variability of UV emission for high polarization blazars, in contrast to the blazars monitored in the rest-frame blue-optical. We also discuss this trend in light of the proposal that the synchrotron radiation of blazar jets in the UV/X-ray regime may arise from a relativistic particle population different from that radiating up to near-infrared/optical frequencies.

astro-ph.HE↗

arXiv2023

A pressure-free long-time stable reduced-order model for two-dimensional Rayleigh-B\'enard convection

The present work presents a stable POD-Galerkin based reduced-order model (ROM) for two-dimensional Rayleigh-B\'enard convection in a square geometry for three Rayleigh numbers: $10^4$ (steady state), $3\times 10^5$ (periodic), and $6 \times 10^6$ (chaotic). Stability is obtained through a particular (staggered-grid) full-order model (FOM) discretization that leads to a ROM that is pressure-free and has skew-symmetric (energy-conserving) convective terms. This yields long-time stable solutions without requiring stabilizing mechanisms, even outside the training data range. The ROM's stability is validated for the different test cases by investigating the Nusselt and Reynolds number time series and the mean and variance of the vertical temperature profile. In general, these quantities converge to the FOM when increasing the number of modes, and turn out to be a good measure of accuracy. However, for the chaotic case, convergence with increasing numbers of modes is relatively difficult and a high number of modes is required to resolve the low-energy structures that are important for the global dynamics.

physics.flu-dyn↗

arXiv2023

The transience and persistence of high optical polarization state in beamed radio quasars

We examine the long-term stability (on decade-like time scales) of optical `high polarization' (HP) state with $p_{opt}$ $> 3\%$, which commonly occurs in flat-spectrum (i.e., beamed) radio quasars (FSRQs) and is a prominent marker of blazar state. Using this clue, roughly a quarter of the FSRQ population has been reported to undergo HP $\leftrightarrow$ non-HP state transition on year-like time scales. This work examines the extent to which HP (i.e., blazar) state can endure in a FSRQ, despite these `frequent' state transitions. This is the first attempt to verify, using purely opto-polarimetric data for a much enlarged sample of blazars, the recent curious finding that blazar state in individual quasars persists for {\it at least} a few decades, despite its changing/swinging observed fairly commonly on year-like time scales. The present analysis is based on a well-defined sample of 83 radio quasars, extracted from the opto-polarimetric survey RoboPol (2013-17), for which old opto-polarimetric data taken prior to 1990 could be found in the literature. By a source-wise comparison of these two datasets of the same observable ($p_{opt}$), we find that $\sim$ 90% of the 63 quasars found in blazar state in our RoboPol sample, were also observed to be in that state about 3 decades before. On the other hand, within the RoboPol survey itself, we find that roughly a quarter of the blazars in our sample migrated to the other polarization state on year-like time scales, by crossing the customary $p_{opt}$ = 3% threshold. Evidently, these relatively frequent transitions (in either direction) do not curtail the propensity of a radio quasar to retain its blazar (i.e., HP) state for at least a few decades. The observed transitions/swings of polarization state are probably manifestation of transient processes, like ejections of synchrotron plasma blobs (VLBI radio knots) from the active nucleus.

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 $\gamma$-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

Persistence of blazar state in flat-spectrum radio quasars

Flat-spectrum radio quasars (FSRQs) whose brightness is dominated by a relativistically beamed core, are frequently found in 'blazar state' commonly inferred from a high optical polarization ( > 3%), and/or a large continuum variability. Here we use these two prime optical markers to investigate continuance of an FSRQ in blazar (or non-blazar) state over an exceptionally long time baseline spanning 4 decades. Our basic sample is a well-defined, unbiased set of 80 FSRQs whose blazar state stood confirmed during 1980s from optical polarimetry. Four decades later, blazar state of each FSRQ is ascertained here from variability of their optical light-curves of typical duration $\sim$ 3.5 years, a low noise (rms $\sim$ 2%) and good cadence ($\sim$ 3 days), obtained under the Zwicky Transient Facility project ongoing since 2018. For about 40% of these FSRQs, blazar state could be ascertained additionally from the opto-polarimetric survey RoboPol (2013-2017). From both these databases it is found that only $\sim$ 10% of the FSRQs have undergone a blazar $\leftrightarrow$ non-blazar state transition over the past 3 - 4 decades. This reinforces the case for a long-term stability of blazar state in individual FSRQs, despite their state fluctuating more commonly on year-like time scales.

astro-ph.GA↗

arXiv2021

Pluto's atmosphere in plateau phase since 2015 from a stellar occultation at Devasthal

A stellar occultation by Pluto was observed on 6 June 2020 with the 1.3-m and 3.6-m telescopes located at Devasthal, Nainital, India, using imaging systems in the I and H bands, respectively. From this event, we derive a surface pressure for Pluto's atmosphere of $p_{\rm surf}= 12.23^{+0.65}_{-0.38} $~$\mu$bar. This shows that Pluto's atmosphere is in a plateau phase since mid-2015, a result which is in excellent agreement with the Pluto volatile transport model of Meza et al. (2019}. This value does not support the pressure decrease reported by independent teams, based on occultations observed in 2018 and 2019, see Young et al. (2021} and Arimatu et al. (2020), respectively.

astro-ph.EP↗

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 $\psi > 3\%$) 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

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

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↗

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.

astro-ph.GA↗