C. S. STALIN
Articles written in Journal of Astrophysics and Astronomy
Volume 25 Issue 1-2 March 2004 pp 1-55
Optical variability properties of high luminosity AGN classes
C. S. Stalin Gopal-Krishna Ram Sagar Paul J. Wiita
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 range
Volume 31 Issue 2 June 2010 pp 59-79
Determination of the Mean HI Absorption of the Intergalactic Medium
A. Aghaee P. Petitjean R. Srianand C. S. Stalin R. Guimarães
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.
Volume 34 Issue 2 June 2013 pp 175-192
Generation of a Near Infra-Red Guide Star Catalog for Thirty-Meter Telescope Observations
Smitha Subramanian Annapurni Subramaniam Luc Simard Kim Gillies A. N. Ramaprakash G. C. Anupama C. S. Stalin Swara Ravindranath B. Eswar Reddy
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.
Volume 34 Issue 3 September 2013 pp 273-296
Arti Goyal Mukul Mhaskey Gopal-Krishna Paul J. Wiita C. S. Stalin Ram Sagar
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.
Volume 37 Issue 4 December 2016 Article ID 0041 Review
Ananda Hota C. Konar C. S. Stalin Sravani Vaddi Pradeepta K. Mohanty Pratik Dabhade Sai Arun Dharmik Bhoga Megha Rajoria Sagar Sethi
We present a brief review of progress in the understanding of general spiral and elliptical galaxies, through merger, star formation and AGN activities. With reference to case studies performed with the GMRT, we highlight the unique aspects of studying galaxies in the radio wavelengths where powerful quasars and bright radio galaxies are traditionally the dominating subjects. Though AGN or quasar activity is extremely energetic, it is extremely short-lived. This justify focussing on transitional galaxies to find relic-evidences of the immediate past AGN-feedback which decide the future course of evolution of a galaxy. Relic radio lobes can be best detected in low frequency observations with the GMRT, LOFAR and in future SKA. The age of these relic radio plasma can be as old as a few hundred Myr. There is a huge gap between this and what is found in optical bands. The very first relic-evidences of a past quasar activity (Hanny’s Voorwerp) was discovered in 2007 by a Galaxy Zoo citizen-scientist, a school teacher, in the optical bands. This relic is around a few tens of thousand years old. More discoveries needed to match these time-scales with star formation time-scales in AGN host galaxies to better understand black hole galaxy co-evolution process via feedback-driven quenching of star formation. It is now well-accepted that discovery and characterization of such faint fuzzy relic features can be more efficiently done by human eye than a machine. Radio interferometry images are more complicated than optical and need the citizen-scientists to be trained. RAD@home, the only Indian citizen-science research project in astronomy, analysing TIFR GMRT Sky Survey (TGSS) 150 MHz data and observing from the Giant Meterwave Radio Telescope (GMRT), was launched in April 2013. Unique, zero-infrastructure zero-funded design of RAD@home as a collaboratory of 69 trained e-astronomers is briefly described. Some of the new-found objects like episodic radio galaxies, radio-jet and companion galaxy interaction, radio galaxy bent by motion of the intra-filament medium in a Mpc-scale galaxy filament etc. are briefly presented as demonstration of its potential. Citizen-science has not only opened up a new way for astronomy research but also possibly the only promising way to extract maximum science out of the Big Data in the SKA-era. This possibly can convert the Big Data problem into a prospect. Citizen-science can contribute to the knowledge creation in never-seen-before speed and in approach. As it is based on internet, it can provide an equal opportunity of academic-growth to people even in the under-developed regions where we always need to put our optical and radio telescopes. This can liberate the research-activity of city-based research-institutes out of the four brick walls and alleviate various socio-economic and geo-political constraints on growth of citizens educated in undergraduate-level science but located in remote areas.
Volume 38 Issue 2 June 2017 Article ID 0028 Review Article
In-orbit Performance of UVIT and First Results
S. N. Tandon J. B. Hutchings S. K. Ghosh A. Subramaniam G. Koshy V. Girish P. U. Kamath S. Kathiravan A. Kumar J. P. Lancelot P. K. Mahesh R. Mohan J. Murthy S. Nagabhushana A. K. Pati J. Postma N. Kameswara Rao K. Sankarasubramanian P. Sreekumar S. Sriram C. S. Stalin F. Sutaria Y. H. Sreedhar I. V. Barve C. Mondal S. Sahu
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.
Volume 39 Issue 1 February 2018 Article ID 0015
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.
Volume 39 Issue 6 December 2018 Article ID 0065
Long-term ultraviolet variability of Seyfert galaxies
N. SUKANYA C. S. STALIN P. JOSEPH S. RAKSHIT D. PRAVEEN R. DAMLE
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.
Volume 41 All articles Published: 25 September 2020 Article ID 0022
Quasar catalogue for the astrometric calibration of the forthcoming ILMT survey
AMIT KUMAR MANDAL BIKRAM PRADHAN JEAN SURDEJ C. S. STALIN RAM SAGAR BLESSON MATHEW
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
Volume 41, 2020
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