• C. S. STALIN

Articles written in Journal of Astrophysics and Astronomy

• Optical variability properties of high luminosity AGN classes

We present the results of a comparative study of the intranight optical variability (INOV) characteristics of radio-loud and radioquiet quasars, which involves a systematic intra-night optical monitoring of seven sets of high luminosity AGNs covering the redshift rangez ≃ 0.2 toz ≃ 2.2. The sample, matched in the optical luminosity—redshift(MB—z) plane, consists of seven radio-quiet quasars (RQQs), eight radio lobedominated quasars (LDQs), five radio core-dominated quasars (CDQs) and six BL Lac objects (BLs). Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to detect INOV with amplitudes as low as about 1%. Present observations cover a total of 113 nights (720 hours) with only a single quasar monitored as continuously as possible on a given night. Considering the cases of only unambiguous detections of INOV we have estimated duty cycles (DCs) of 17%, 12%, 20% and 61% for RQQs, LDQs, CDQs, and BLs, respectively. The much lower amplitude and DC of ESfOV shown by RQQs compared to BLs may be understood in terms of their having optical synchrotron jets which are modestly misdirected from us. From our fairly extensive dataset, no general trend of a correlation between the INOV amplitude and the apparent optical brightness of the quasar is noticed. This suggests that the physical mechanisms of INOV and long term optical variability (LTOV) do not have a one-to-one relationship and different factors are involved. Also, the absence of a clear negative correlation between the INOV and LTOV characteristics of blazars of our sample points toward an inconspicuous contribution of accretion disk fluctuations to the observed INOV. The INOV duty cycle of the AGNs observed in this program suggests that INOV is associated predominantly with the highly polarized optical emission components. We also report new VLA imaging of two RQQs (1029 + 329 & 1252 + 020) in our sample which has yielded a 5 GHz detection in one of them (1252 + 020;S5GHZ ≃ 1 mJy).

• Determination of the Mean HI Absorption of the Intergalactic Medium

In recent years, the Lyman-𝛼 forest in quasar spectra has been used, together with 𝑁-body simulations, to determine the underlying matter distribution in the intergalactic medium (IGM). One of the key parameters to be known in order to compare observations and numerical simulations is the mean HI absorption in the IGM. To derive the latter, one has first to fit the quasar continuum.We have observed 20 high redshift and highly luminous QSOs (𝑚V ≤ 17.5 and 2.40 ≤ 𝑧em ≤ 3.91) at intermediate spectral resolution, with either EMMI (ESO Multi-Mode Instrument) on the ESO-NTT telescope or CARELEC at the OHP (Observatoire de Haute-Provence), and applied different methods of determining the QSO continuum to this QSO sample. We have measured the amount of absorption, known as the flux decrement, 𝐷𝐴, in the Lyman-𝛼 forest for these different methods and compared the results. In addition, we have compared 𝐷𝐴 values measured along the same lines of sight observed at high and intermediate spectral resolutions.We discuss the systematics resulting from the use of automatic continuum fitting methods.

• Generation of a Near Infra-Red Guide Star Catalog for Thirty-Meter Telescope Observations

The requirements for the production of a near Infra-Red Guide Star Catalog (IRGSC) for Thirty Meter Telescope (TMT) observations are identified and presented. A methodology to compute the expected J band magnitude of stellar sources from their optical (𝑔, 𝑟 , 𝑖 ) magnitudes is developed. The computed and observed J magnitudes of sources in three test fields are compared and the methodology developed is found to be satisfactory for the magnitude range, JVega = 16–22 mag. From this analysis, we found that for the production of final TMT IRGSC (with a limiting magnitude of JVega = 22 mag), we need 𝑔, 𝑟, 𝑖 bands optical data which go up to 𝑖AB ∼ 23 mag. Fine tuning of the methodology developed, such as using Spectral Energy Distribution (SED) template fitting for optimal classification of stars in the fainter end, incorporating spectral libraries in the model, to reduce the scatter, and modification of the existing colour–temperature relation to increase the source density are planned for the subsequent phase of this work.

• On the Photometric Error Calibration for the Differential Light Curves of Point-like Active Galactic Nuclei

It is important to quantify the underestimation of rms photometric errors returned by the commonly used APPHOT algorithm in the IRAF software, in the context of differential photometry of point-like AGN, because of the crucial role it plays in evaluating their variability properties. Published values of the underestimation factor, 𝜂, using several different telescopes, lie in the range 1.3–1.75. The present study aims to revisit this question by employing an exceptionally large data set of 262 differential light curves (DLCs) derived from 262 pairs of non-varying stars monitored under our ARIES AGN monitoring program for characterizing the intra-night optical variability (INOV) of prominent AGN classes. The bulk of these data were taken with the 1-m Sampurnanad Telescope (ST). We find 𝜂 = 1.54 ± 0.05 which is close to our recently reported value of 𝜂 = 1.5. Moreover, this consistency holds at least up to a brightness mismatch of 1.5 mag between the paired stars. From this we infer that a magnitude difference of at least up to 1.5 mag between a point-like AGN and comparison star(s) monitored simultaneously is within the same CCD chip acceptable, as it should not lead to spurious claims of INOV.

• Tracking Galaxy Evolution Through Low-Frequency Radio Continuum Observations using SKA and Citizen-Science Research using Multi-Wavelength Data

• In-orbit Performance of UVIT and First Results

The performance of the ultraviolet telescope (UVIT) on-board AstroSat is reported. The performance in orbit is also compared with estimates made from the calibrations done on the ground. The sensitivity is found to be within ∼15% of the estimates, and the spatial resolution in the NUV is found to exceed significantly the design value of 1.8′′ and it is marginally better in the FUV. Images obtained from UVIT are presented to illustrate the details revealed by the high spatial resolution. The potential of multi-band observations in the ultraviolet with high spatial resolution is illustrated by some results.

• Measurement of coronal properties of Seyfert galaxies from NuSTAR’s hard X-ray spectrum

Precise measurement of the coronal properties of Active Galactic Nuclei (AGN) requires the availability of high signal-to-noise ratio data covering awide range of X-ray energies.The Nuclear Spectroscopic Telescope Array ($NuSTAR$) which is highly sensitive to earlier missions in its operational energy range of 3–79 keV, allows us to arrive at precise estimates of the coronal parameters such as cut-off energy ($E_{\rm cut}$), coronaltemperature ($kT_e$) and geometry of the corona at least for sources that have $E_{\rm cut}$ within the energy range of$NuSTAR$. In this paper,we present our preliminary results on the spectral analysis of two Seyfert galaxies namely 3C 120 and NGC 4151 using $NuSTAR$ observations in the 3–79 keV band. We investigated the continuum and coronal parameters, the photon index $\Gamma$, $E_{\rm cut}$ and $kT_e$. By fitting the X-ray spectrum of 3C 120 and NGC 4151 with a simple phenomenological model, we found that both the sources showed a clear cut-off in their spectrum.

• Long-term ultraviolet variability of Seyfert galaxies

Flux variability is one of the defining characteristics of Seyfert galaxies, a class of active galactic nuclei (AGN). Although these variations are observed over a wide range of wavelengths, results on their flux variability characteristics in the ultraviolet (UV) band are very limited.We present here the long-term UV fluxvariability characteristics of a sample of fourteen Seyfert galaxies using data from the International Ultraviolet Explorer acquired between 1978 and 1995. We found that all the sources showed flux variations with no statistically significant differences in the amplitude of UV flux variation between shorter and longer wavelengths. Also, the flux variations between different near-UV (NUV, 1850−3300 å) and far-UV (FUV, 1150−2000 å) pass bands in the rest frames of the objects are correlated with no time lag. The data show indications of (i) a mild negative correlation of UV variability with bolometric luminosity and (ii) weak positive correlation between UV variability and black hole mass. At FUV, about 50% of the sources show a strong correlation between spectral indices and flux variations with a hardening when brightening behaviour, while for the remaining sources the correlation is moderate. In NUV, the sources do show a harder spectrum when brighter, but the correlation is either weak or moderate.

• Quasar catalogue for the astrometric calibration of the forthcoming ILMT survey

Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys as they have negligible proper motion and parallax. The forthcoming 4-m International Liquid Mirror Telescope (ILMT) will survey the sky that covers a width of about 27$'$. To carry out astrometric calibration of theILMT observations, we aimed to compile a list of quasars with accurate equatorial coordinates and falling in the ILMT stripe. Towards this, we cross-correlated all the quasars that are known till the present date with the sources in the Gaia-DR2 catalogue, as the Gaia-DR2 sources have position uncertainties as small as a few milli arcsec (mas). We present here the results of this cross-correlation which is a catalogue of 6738 quasars that is suitable for astrometric calibration of the ILMT fields. In this work, we present this quasar catalogue. This catalogue of quasars can also be used to study quasar variability over diverse time scales when the ILMT starts its observations. While preparing this catalogue, we also confirmed that quasars in the ILMT stripe have proper motion and parallax lesser than 20 mas yr$^{-1}$ and 10 mas, respectively.

• In-orbit performance of UVIT over the past 5 years

Over the last 5 years, UVIT has completed observations of more than 500 proposals with $\sim$800 unique pointings. In addition, regular planned monitoring observations have been made and from their analysis various key parameters related to in orbit performance of UVIT have been quantified. The sensitivities of the UV channels have remained steady indicating no effect of potential molecular contamination confirming the adequacy of all the protocols implemented for avoiding contamination. The quality of the PSF through the years confirms adequacy of thermal control measures. The early calibrations obtained during the Performance Verification (PV) phase have been further revised for more subtle effects. These include flat fields and detector distortions with greater precision. The operations of UVIT have also evolved through inorbit experience, e.g. tweaking of operational sequencing, protocol for recovery from bright object detection (BOD) shutdowns, parameters for BOD thresholds, etc. Finally, some effects of charged particle hits on electronics led to optimised strategy for regular resetting. The Near-UV channel was lost in one of suchoperations. All the above in-orbit experiences are presented here.

• Curvit: An open-source Python package to generate light curves from UVIT data

Curvit is an open-source Python package that facilitates the creation of light curves from the data collected by the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat, India’s first multi-wavelength astronomical satellite. The input to Curvit is the calibrated events list generated by the UVIT-Payload Operation Center (UVIT-POC) and made available to the principal investigators through the Indian Space Science Data Center. The features of Curvit include: (i) automatically detecting sources and generating light curves for all the detected sources and (ii) custom generation of light curve for any particular source of interest. We present here the capabilities of Curvit and demonstrate its usability on the UVIT observations ofthe intermediate polar FO Aqr as an example. Curvit is publicly available on GitHub at https://github.com/prajwel/curvit.

• Performance of the UVIT Level-2 pipeline

Performance of the Level-2 pipeline, which translates the UVIT data created by the ISRO’s ground segment processing systems (Level-1) into astronomer ready scientific data products, is described. This pipeline has evolved significantly from experiences during the in orbit mission. With time, the detectormodules of UVIT developed certain defects which led to occasional corruption of imaging and timing data. This article will describe the improvements and mitigation plans incorporated in the pipeline and report its efficacy and quantify the performance.

• The sharpest ultraviolet view of the star formation in an extreme environment of the nearest Jellyfish Galaxy IC 3418

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418,due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milky Way. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical imagesfrom Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66$+$112227.8, the farthest star($\sim$17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate ($4–7.4 \times 10^{–4} \ M_{\odot}$ yr$^{–1}$) in these fireballs, estimated from UVIT flux densities, to beincreasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain thepuzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy.

• # Journal of Astrophysics and Astronomy

Volume 42, 2021
All articles
Continuous Article Publishing mode

• # Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019