arXiv subjects

C. S. Stalin

Publications and source records attributed to C. S. Stalin.

At least 127 records · Page 7Linked to original sources

arXiv2016

Broadband Observations of High Redshift Blazars

We present a multi-wavelength study of four high redshift blazars, S5 0014+81 ($z=3.37$), CGRaBS J0225+1846 ($z=2.69$), BZQ J1430+4205 ($z=4.72$), and 3FGL J1656.2$-$3303 ($z=2.40$), using the quasi-simultaneous data from {\it Swift}, {\it NuSTAR}, and {\it Fermi}-Large Area Telescope (LAT) and also the archival {\it XMM-Newton} observations. Other than 3FGL J1656.2$-$3303, none of the sources were known as $γ$-ray emitters and our analysis of $\sim$7.5 years of LAT data reveals the first time detection of the statistically significant $γ$-ray emission from CGRaBS J0225+1846. We generate the broadband spectral energy distributions (SED) of all the objects, centering at the epoch of {\it NuSTAR} observations and reproduce them using a one zone leptonic emission model. The optical$-$UV emission in all the objects can be explained by the radiation from the accretion disk, whereas, X-ray to $γ$-ray window of the SEDs are found to be dominated by the inverse Compton scattering off the broad line region photons. All of them host billion solar mass black holes. Comparing the accretion disk luminosity and the jet power of these sources with a large sample of blazars, we find them to occupy high disk luminosity-jet power regime. We also investigate the X-ray spectral properties of the sources in detail with a major focus on studying the causes of soft X-ray deficit, a feature generally seen in high redshift radio-loud quasars. We summarize that this feature could be explained based on the intrinsic curvature in the jet emission rather than due to external effects predicted in the earlier studies, such as host galaxy and/or warm absorption.

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arXiv2016

Hard X-ray flux variations in AGN from NuSTAR

We present here our results on the hour like time scale X-ray flux variations in a sample of active galactic nuclei using data from the Nuclear Spectroscopic Telescope Array (NuSTAR). We find that in the 3-79 keV band, BL Lacs are more variable than flat spectrum radio quasars and Seyfert galaxies. Among Seyferts, Seyfert 2s are more variable than Seyfert 1s. Also, radio-loud quasars are more variable in the hard (10-79 keV) band than the soft (3-10 keV) band while, Seyfert galaxies tend to show more variations in the soft band relative to the hard band.

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arXiv2016

Lepto$-$Hadronic Origin of $γ$-ray Outbursts of 3C 279

The blazar 3C 279 exhibited a giant $γ$-ray outburst in 2013 December and 2014 April. Apart from the very fast $γ$-ray flux variability, the spectral nature of the flares were also found to vary significantly between these two flaring events. A prominent curvature in the $γ$-ray spectrum was noticed in 2014 April flare, on the other hand, the 2013 December displayed an extreme hardening of the spectrum. These observations, thus, put strong constraints on our understanding of the underlying particle acceleration mechanisms.

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arXiv2016

Broadband Observations of the Gamma-ray Emitting Narrow Line Seyfert 1 Galaxy SBS 0846+513

We present the results of our broadband study of the $γ$-ray emitting narrow line Seyfert 1 (NLSy1) galaxy SBS 0846+513 ($z=0.585$). This includes multi-band flux variations, $γ$-ray spectral analysis, broad band spectral energy distribution (SED) modeling, and intranight optical variability (INOV) observations carried over 6 nights between 2012 November and 2013 March using the 2 m Himalayan Chandra Telescope and the 1.3 m telescope at Devasthal. Multiple episodes of flaring activity are seen in the $γ$-ray light curve of the source which are also reflected in the observations at lower frequencies. A statistically significant curvature is noticed in the seven years averaged $γ$-ray spectrum, thus indicating its similarity with powerful flat spectrum radio quasars (FSRQs). Modeling the SEDs with a one zone leptonic emission model hints the optical-UV spectrum to be dominated by synchrotron radiation, whereas, inverse Compton scattering of broad line region photons reproduces the $γ$-ray part of the SEDs. The source was found to be variable on all the six nights of optical observations with a variation of $\sim$0.3 magnitude within a single night, coinciding with a high $γ$-ray activity state. The observed large amplitude INOV clearly indicates the presence of a closely aligned beamed relativistic jet in SBS 0846+513. Our broadband study supports the recent claims in literature that $γ$-ray emitting NLSy1 galaxies are similar to blazars and constitute the low black hole mass counterparts to FSRQs.

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arXiv2016

The First GeV Outburst of the Radio-loud Narrow Line Seyfert 1 galaxy PKS 1502+036

The $γ$-ray loud narrow line Seyfert 1 ($γ$-NLSy1) galaxy PKS 1502+036 ($z=0.409$) exhibited its first $γ$-ray outburst on 2015 December 20. In the energy range of 0.1-300 GeV, the highest flux measured by {\it Fermi}-Large Area Telescope is (3.90 $\pm$ 1.52) $\times$ 10$^{-6}$ ph cm$^{-2}$ s$^{-1}$, which is the highest $γ$-ray flux ever detected from this object. The associated spectral shape is soft ($Γ_{0.1-300~{\rm GeV}}=2.57\pm0.17$) and this corresponds to an isotropic $γ$-ray luminosity of (1.2 $\pm$ 0.6) $\times$ 10$^{48}$ erg s$^{-1}$. We generate the broadband spectral energy distribution (SED) during the GeV flare and reproduce it using a one zone leptonic emission model. The optical-UV spectrum can be explained by a combination of synchrotron and the accretion disk emission, whereas, the X-ray to $γ$-ray SED can be satisfactorily reproduced by inverse-Compton scattering of thermal photons originated from the torus. The derived SED parameters hint for the increase in the bulk Lorentz factor as a major cause of the flare and the location of the emission region is estimated as outside the broad line region but still inside torus. A comparison of the GeV flaring SED of PKS 1502+036 with that of two other $γ$-NLSy1 galaxies, namely, 1H 0323+342 ($z=0.061$) and PMN J0948+0022 ($z=0.585$), and also with FSRQ 3C 279 ($z=0.536$) has led to the conclusion that the GeV flaring SEDs of $γ$-NLSy1 galaxies resemble with FSRQs and a major fraction of their bolometric luminosity is emitted at $γ$-ray energies.

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arXiv2015

A hard gamma-ray flare from 3C 279 in 2013 December

The blazar 3C 279 exhibited twin $γ$-ray flares of similar intensity in 2013 December and 2014 April. In this work, we present a detailed multi-wavelength analysis of the 2013 December flaring event. Multi-frequency observations reveal the uncorrelated variability patterns with X-ray and optical-UV fluxes peaking after the $γ$-ray maximum. The broadband spectral energy distribution (SED) at the peak of the $γ$-ray activity shows a rising $γ$-ray spectrum but a declining optical-UV flux. This observation along with the detection of uncorrelated variability behavior rules out the one-zone leptonic emission scenario. We, therefore, adopt two independent methodologies to explain the SED: a time dependent lepto-hadronic modeling and a two-zone leptonic radiative modeling approach. In the lepto-hadronic modeling, a distribution of electrons and protons subjected to a randomly orientated magnetic field produces synchrotron radiation. Electron synchrotron is used to explain the IR to UV emission while proton synchrotron emission is used to explain the high energy $γ$-ray emission. A combination of both electron synchrotron self Compton emission and proton synchrotron emission is used to explain the X-ray spectral break seen during the later stage of the flare. In the two-zone modeling, we assume a large emission region emitting primarily in IR to X-rays and $γ$-rays to come primarily from a fast moving compact emission region. We conclude by noting that within a span of 4 months, 3C 279 has shown the dominance of a variety of radiative processes over each other and this reflects the complexity involved in understanding the physical properties of blazar jets in general.

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arXiv2015

Violent Hard X-ray Variability of Mrk 421 Observed by NuSTAR in 2013 April

The well studied blazar Markarian 421 (Mrk 421, $z$=0.031) was the subject of an intensive multi-wavelength campaign when it flared in 2013 April. The recorded X-ray and very high energy (VHE, E$>$100 GeV) $γ$-ray fluxes are the highest ever measured from this object. At the peak of the activity, it was monitored by the hard X-ray focusing telescope {\it Nuclear Spectroscopic Telescope Array} ({\it NuSTAR}) and {\it Swift} X-Ray Telescope (XRT). In this work, we present a detailed variability analysis of {\it NuSTAR} and {\it Swift}-XRT observations of Mrk 421 during this flaring episode. We obtained the shortest flux doubling time of 14.01$\pm$5.03 minutes, which is the shortest hard X-ray (3$-$79 keV) variability ever recorded from Mrk 421 and is on the order of the light crossing time of the black hole's event horizon. A pattern of extremely fast variability events superposed on slowly varying flares is found in most of the {\it NuSTAR} observations. We suggest that these peculiar variability patterns may be explained by magnetic energy dissipation and reconnection in a fast moving compact emission region within the jet. Based on the fast variability, we derive a lower limit on the magnetic field strength of $B \ge 0.73 δ_1^{-2/3} \, ν_{19}^{1/3}$~G, where $δ_1$ is the Doppler factor in units of 10, and $ν_{19}$ is the characteristic X-ray synchrotron frequency in units of $10^{19}$~Hz.

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arXiv2015

Long-term optical flux and colour variability in quasars

We have used optical V and R band observations from the Massive Compact Halo Object (MACHO) project on a sample of 59 quasars behind the Magellanic clouds to study their long term optical flux and colour variations. These quasars lying in the redshift range of 0.2 < z < 2.8 and having apparent V band magnitudes between 16.6 and 20.1 mag have observations ranging from 49 to 1353 epochs spanning over 7.5 years with frequency of sampling between 2 to 10 days. All the quasars show variability during the observing period. The normalized excess variance (Fvar) in V and R bands are in the range 0.2% < Fvar < 1.6% and 0.1% < Fvar < 1.5%. In a large fraction of the sources, Fvar is larger in the V-band compared to the R-band. From the z-transformed discrete cross correlation function analysis, we find that there is no lag between the V and R-band variations. Adopting the Markov Chain Monte Carlo (MCMC) approach, and properly taking into account the correlation between the errors in colours and magnitudes, it is found that majority of the sources show a bluer when brighter trend, while a minor fraction of quasars show the opposite behaviour. This is similar to the results obtained from other two independent algorithms namely the weighted linear least squares fit (FITEXY) and the bivariate correlated errors and intrinsic scatter regression (BCES). However, the ordinary least squares (OLS) fit normally used in the colour variability studies of quasars, indicates that all the quasars studied here show a bluer when brighter trend. It is therefore very clear that OLS algorithm cannot be used for the study of colour variability in quasars.

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arXiv2015

$H_0$ from ten well-measured time delay lenses

In this work, we present a homogeneous curve-shifting analysis using the difference-smoothing technique of the publicly available light curves of 24 gravitationally lensed quasars, for which time delays have been reported in the literature. The uncertainty of each measured time delay was estimated using realistic simulated light curves. The recipe for generating such simulated light curves with known time delays in a plausible range around the measured time delay is introduced here. We identified 14 gravitationally lensed quasars that have light curves of sufficiently good quality to enable the measurement of at least one time delay between the images, adjacent to each other in terms of arrival-time order, to a precision of better than 20% (including systematic errors). We modeled the mass distribution of ten of those systems that have known lens redshifts, accurate astrometric data, and sufficiently simple mass distribution, using the publicly available PixeLens code to infer a value of $H_0$ of 68.1 $\pm$ 5.9 km s$^{-1}$ Mpc$^{-1}$ (1$σ$ uncertainty, 8.7% precision) for a spatially flat universe having $Ω_m$ = 0.3 and $Ω_Λ$ = 0.7. We note here that the lens modeling approach followed in this work is a relatively simple one and does not account for subtle systematics such as those resulting from line-of-sight effects and hence our $H_0$ estimate should be considered as indicative.

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arXiv2015

Awakening of The High Redshift Blazar CGRaBS J0809+5341

CGRaBS J0809+5341, a high redshift blazar at z = 2.144, underwent a giant optical outburst on 2014 April 19 when it brightened by $\sim$5 mag and reached an unfiltered apparent magnitude of 15.7 mag. This implies an absolute magnitude of -30.5 mag, making it one of the brightest quasars in the Universe. This optical flaring triggered us to carry out observations during the decaying part of the flare covering a wide energy range using the {\it Nuclear Spectroscopic Telescope Array}, {\it Swift}, and ground based optical facilities. For the first time, the source is detected in $γ$-rays by the Large Area Telescope onboard the {\it Fermi Gamma-Ray Space Telescope}. A high optical polarization of $\sim$10\% is also observed. Using the Sloan Digital Sky Survey spectrum, accretion disk luminosity and black hole mass are estimated as $1.5 \times 10^{45}$ erg s$^{-1}$ and $10^{8.4}~M_{\odot}$ respectively. Using a single zone leptonic emission model, we reproduce the spectral energy distribution of the source during the flaring activity. This analysis suggests that the emission region is probably located outside the broad line region, and the jet becomes radiatively efficient. We also show that the overall properties of CGRaBS J0809+5341 seems not to be in agreement with the general properties observed in high redshift blazars up to now.

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arXiv2015

Multi-wavelength observations of 3C 279 during the extremely bright gamma-ray flare in 2014 March-April

The well studied blazar 3C 279 underwent a giant $γ$-ray outburst in 2014 March-April. The measured $γ$-ray flux (1.21 $\pm$ 0.10 $\times$ 10$^{-5}$ ph cm$^{-2}$ s$^{-1}$ in 0.1-300 GeV energy range) is the highest detected from 3C 279 by Fermi Large Area Telescope. Hour scale $γ$-ray flux variability are observed, with a flux doubling time as short as 1.19 $\pm$ 0.36 hours detected during one flare. The $γ$-ray spectrum is found to be curved at peak of the flare suggesting low probability of detecting very high energy (VHE; E $>$ 100 GeV) emission, which is further confirmed by the Very Energetic Radiation Imaging Telescope Array System observations. The $γ$-ray flux increased by more than an order in comparison to low activity state and the flare consists of multiple sub-structures having fast rise and slow decay profile. The flux enhancement is seen in all the wavebands though at a lesser extent compared to $γ$-rays. During the flare, a considerable amount of the kinetic jet power gets converted to $γ$-rays and the jet becomes radiatively efficient. A one zone leptonic emission model is used to reproduce the flare and we find increase in the bulk Lorentz factor as a major cause of the outburst. From the observed fast variability, lack of VHE detection, and the curved $γ$-ray spectrum, we conclude that the location of the emission region cannot be far out from the broad line region (BLR) and contributions from both BLR and torus photons are required to explain the observed $γ$-ray spectrum.

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arXiv2014

Fermi Monitoring of Radio-loud Narrow Line Seyfert 1 Galaxies

We present detailed analysis of the gamma-ray flux variability and spectral properties of the five radio-loud narrow line Seyfert 1 (RL-NLSy1) galaxies, detected by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope, namely 1H 0323+342, SBS 0846+513, PMN J0948+0022, PKS 1502+036, and PKS 2004$-$447. The first three sources show significant flux variations including the rapid variability of a few hours by 1H 0323+342. The average gamma-ray spectrum of 1H 0323+342 and PMN J0948+0022 shows deviation from a simple power law (PL) behavior, whereas for other three sources, PL model gives better fit. The spectra of 1H 0323+342, SBS 0846+513, and PMN J0948+0022, respectively in low, flaring, and moderately active states, show significant curvature. Such curvature in the gamma-ray spectrum of 1H 0323+342 and PMN J0948+0022 can be due to emission region located inside the broad line region (BLR) where the primary mechanism of the gamma-ray emission is inverse-Compton (IC) scattering of BLR photons occurring in the Klein-Nishina regime. The gamma-ray emission of SBS 0846+513 is explained by IC scattering of dusty torus photons which puts the emission region to be outside the BLR and thus under the Thomson regime. Therefore, the observed curvature of SBS 0846+513 could be intrinsic to the particle energy distribution. The presence of curvature in the gamma-ray spectrum and flux variability amplitudes of some of the RL-NLSy1 galaxies suggest that these sources could be akin to low/moderate jet power flat spectrum radio quasars.

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arXiv2014

The Peculiar Radio-Loud Narrow Line Seyfert 1 Galaxy 1H 0323+342

We present a multi-wavelength study of the radio-loud narrow line Seyfert 1 galaxy (NLSy1), 1H 0323+342, detected by Fermi Gamma Ray Space Telescope. Multi-band light curves show many orphan X-ray and optical flares having no corresponding γ-ray counterparts. Such anomalous variability behavior can be due to different locations of the emission region from the central source. During a large flare, γ-ray flux doubling time scale as small as $\sim$ 3 hours is noticed. We built spectral energy distribution (SED) during different activity states and modeled them using an one-zone leptonic model. The shape of the optical/UV component of the SEDs is dominated by accretion disk emission in all the activity states. In the X-ray band, significant thermal emission from the hot corona is inferred during quiescent and first flaring states, however, during subsequent flares, non-thermal jet component dominates. The γ-ray emission in all the states can be well explained by inverse-Compton scattering of accretion disk photons reprocessed by the broad line region. The source showed violent intra-night optical variability, coinciding with one of the high γ-ray activity states. An analysis of the overall X-ray spectrum fitted with an absorbed power-law plus relativistic reflection component hints for the presence of Fe K-α line and returns a high black hole spin value of a=0.96 $\pm$ 0.14. We argue that 1H 0323+342 possesses dual characteristics, akin to flat spectrum radio quasars (FSRQs) as well as radio-quiet NLSy1s, though at a low jet power regime compared to powerful FSRQs.

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arXiv2014

New results on the exotic galaxy `Speca' and discovering many more Specas with RAD@home network

We present the first report on an innovative new project named "RAD@home", a citizen-science research collaboratory built on free web-services like Facebook, Google, Skype, NASA Skyview, NED, TGSS etc.. This is the first of its kind in India, a zero-funded, zero-infrastructure, human-resource network to educate and directly involve in research, hundreds of science-educated under-graduate population of India, irrespective of their official employment and home-location with in the country. Professional international collaborators are involved in follow up observation and publication of the objects discovered by the collaboratory. We present here ten newly found candidate episodic radio galaxies, already proposed to GMRT, and ten more interesting cases which includes, bent-lobe radio galaxies located in new Mpc-scale filaments, likely tracing cosmological cluster accretion from the cosmic web. Two new Speca-like rare spiral-host large radio galaxies have also been been reported here. Early analyses from our follow up observations with the Subaru and XMM-Newton telescopes have revealed that Speca is likely a new entry to the cluster and is a fast rotating, extremely massive, star forming disk galaxy. Speca-like massive galaxies with giant radio lobes, are possibly remnants of luminous quasars in the early Universe or of first supermassive black holes with in first masssve galaxies. As discoveries of Speca-like galaxies did not require new data from big telescopes, but free archival radio-optical data, these early results demonstrate the discovery potential of RAD@home and how it can help resource-rich professionals, as well as demonstrate a model of academic-growth for resource-poor people in the underdeveloped regions via Internet.

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arXiv2013

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XIV. Time delay of the doubly lensed quasar SDSS J1001+5027

This paper presents optical R-band light curves and the time delay of the doubly imaged gravitationally lensed quasar SDSS J1001+5027 at a redshift of 1.838. We have observed this target for more than six years, between March 2005 and July 2011, using the 1.2-m Mercator Telescope, the 1.5-m telescope of the Maidanak Observatory, and the 2-m Himalayan Chandra Telescope. Our resulting light curves are composed of 443 independent epochs, and show strong intrinsic quasar variability, with an amplitude of the order of 0.2 magnitudes. From this data, we measure the time delay using five different methods, all relying on distinct approaches. One of these techniques is a new development presented in this paper. All our time-delay measurements are perfectly compatible. By combining them, we conclude that image A is leading B by 119.3 +/- 3.3 days (1 sigma, 2.8% uncertainty), including systematic errors. It has been shown recently that such accurate time-delay measurements offer a highly complementary probe of dark energy and spatial curvature, as they independently constrain the Hubble constant. The next mandatory step towards using SDSS J1001+5027 in this context will be the measurement of the velocity dispersion of the lensing galaxy, in combination with deep Hubble Space Telescope imaging.

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arXiv2013

Improved characterisation of intra-night optical variability of prominent AGN classes

The incidence of intra-night optical variability (INOV) is known to to differ significantly among different classes of powerful active galactic nuclei (AGN). A number of statistical methods have been employed in the literature for testing the presence of INOV in the light curves, sometimes leading to discordant results. In this paper we compare the INOV characteristics of six prominent classes of AGN, as evaluated using three commonly used statistical tests, namely the $χ^2-$test, the modified $C-$test and the $F-$test, which has recently begun to gain popularity. The AGN classes considered are: radio-quiet quasars (RQQs), radio-intermediate quasars (RIQs), lobe-dominated quasars (LDQs), low optical polarization core-dominated quasars (LPCDQs), high optical polarization core-dominated quasars (HPCDQs), and TeV blazars. Our analysis is based on a large body of AGN monitoring data, involving 262 sessions of intra-night monitoring of a total 77 AGN, using 1-2 metre class optical telescopes located in India. In order to compare the usefulness of the statistical tests, we have also subjected them to a `sanity check' by comparing the number of false positives yielded by each test with the corresponding statistical prediction. The present analysis is intended to serve as a benchmark for future INOV studies of AGN of different classes.

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arXiv2013

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 $\emph APPHOT$ algorithm in the $\emph IRAF$ 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$\pm$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.

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arXiv2013

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XII. Time delays of the doubly lensed quasars SDSS J1206+4332 and HS 2209+1914

Aims. Within the framework of the COSMOGRAIL collaboration we present 7- and 8.5-year-long light curves and time-delay estimates for two gravitationally lensed quasars: SDSS J1206+4332 and HS 2209+1914. Methods. We monitored these doubly lensed quasars in the R-band using four telescopes: the Mercator, Maidanak, Himalayan Chandra, and Euler Telescopes, together spanning a period of 7 to 8.5 observing seasons from mid-2004 to mid-2011. The photometry of the quasar images was obtained through simultaneous deconvolution of these data. The time delays were determined from these resulting light curves using four very different techniques: a dispersion method, a spline fit, a regression difference technique, and a numerical model fit. This minimizes the bias that might be introduced by the use of a single method. Results. The time delay for SDSS J1206+4332 is Delta_t AB = 111.3 +/- 3 days with A leading B, confirming a previously published result within the error bars. For HS 2209+1914 we present a new time delay of Delta_t BA = 20.0 +/- 5 days with B leading A. Conclusions. The combination of data from up to four telescopes have led to well-sampled and nearly 9-season-long light curves, which were necessary to obtain these results, especially for the compact doubly lensed quasar HS 2209+1914.

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