arXiv subjects

C. D. Ravikumar

Publications and source records attributed to C. D. Ravikumar.

At least 19 recordsLinked to original sources

arXiv2025

The long-term optical flux variations of Compact Symmetric Objects

Compact Symmetric Objects (CSOs) are a distinct category of jetted active galactic nuclei (AGN) whose optical variability characteristics have not been well investigated. We present here the results of our investigation on the optical flux and colour variability properties of a bona fide sample of 38 CSOs. We used the g-, r- and i-bands data from the Zwicky Transient Facility survey that spans a duration of about 5 years. We also considered a comparison sub-sample of blazars that includes 5 flat spectrum radio quasars and 12 BL Lac objects with redshifts and g-band magnitudes similar to the limited sub-sample of 9 CSOs. These two sub-samples of AGN, chosen for this comparative study of their long-term optical variability, represent different orientations of their relativistic jets with respect to the observer. We found that both CSOs and blazars exhibit optical flux variations, although variability of CSOs is lower than that of blazars. The observed variability in both CSOs and blazars is attributed to the relativistic jets and the increased optical variations in blazars relative to CSOs are likely due to beaming effects. CSOs and blazars exhibit similar colour variations, with both of them showing a bluer when brighter trend. Such a colour variability pattern is expected due to processes associated with their relativistic jets.

astro-ph.GA↗

arXiv2024

Signature of Particle Diffusion on the X-ray Spectra of the blazar Mkn 421

The curvature in blazar spectrum has the potential to understand the particle dynamics in jets. We performed a detailed analysis of simultaneous Swift-XRT (0.3-10 keV) and NuSTAR (3-79 keV)} observations of Mkn 421. Our analysis of NuSTAR observations alone reveals that, during periods of low flux, the hard X-ray spectra are best represented by a steep power-law with photon index reaching $\sim$ 3. However, the spectrum exhibits significant curvature during its high flux states. To investigate this, we explore plausible diffusion processes facilitating shock acceleration in the emission region that can contribute to the observed spectral curvature. Particularly, such processes can cause gradual fall of the photon spectrum at high energies which can be represented by a sub-exponential function. The parameter that decides this spectral change can be used to characterise the energy dependence of the diffusive process. Our results suggest that the X-ray spectra of Mkn 421 are consistent with a scenario where particle acceleration is mediated through Bohm-type diffusion and the spectra beyond the synchrotron peak is modulated by the radiative loss process.

astro-ph.HE↗

arXiv2023

Understanding the Broadband Emission Process of 3C 279 through Longterm Spectral Analysis

The long term broadband spectral study of Flat Spectrum Radio Quasars during different flux states has the potential to infer the emission mechanisms and the cause of spectral variations. To scrutinize this, we performed a detailed broadband spectral analysis of 3C 279 using simultaneous Swift-XRT/UVOT and Fermi-LAT observations spanning from August 2008 to June 2022. We also supplement this with the simultaneous NuSTAR observations of the source. The optical/UV, X-ray, and gamma-ray spectra were individually fitted by a power-law to study the long-term variation in the flux and the spectral indices. A combined spectral fit of simultaneous optical/UV and X-ray spectra was also performed to obtain the transition energy at which the spectral energy distribution is minimum. The correlation analysis suggests that the long-term spectral variations of the source are mainly associated with the variations in the low energy index and the break energy of the broken power-law electron distribution which is responsible for the broadband emission. The flux distribution of the source represents a log-normal variability while the gamma-ray flux distribution showed a clear double log-normal behavior. The spectral index distributions were again normal except for gamma-ray which showed a double-Gaussian behavior. This indicates that the log-normal variability of the source may be associated with the normal variations in the spectral index. The broadband spectral fit of the source using synchrotron and inverse Compton processes indicates different emission processes are active at optical/UV, X-ray, and gamma-ray energies.

astro-ph.HE↗

arXiv2023

Study of central light distribution in nearby early-type galaxies hosting nuclear star clusters

We present analysis of 63 nearby ($<$ 44 Mpc) early-type galaxies hosting nuclear star clusters using the recently discovered parameter Central Intensity Ratio (CIR$_I$) determined from near-infra-red (3.6 $μ$m) observations with the Infra-red-array-camera of \emph{Spitzer} space telescope. The CIR$_I$, when combined with filters involving age and $B-K$ colour of host galaxies, helps identify two distinct classes of galaxies hosting nuclear star clusters. This is independently verified using Gaussian Mixture Model. CIR shows a positive trend with faint, low mass, and blue galaxies in the sample, while the opposite is true for bright, high mass, and red galaxies, albeit with large scatter. The variation of CIR$_I$ with central velocity dispersion, absolute B band magnitude, dynamical mass, and stellar mass of host galaxies suggests that the mass of nuclear star clusters increases with that of host galaxies, for faint, low mass, young and blue galaxies in the sample. In bright, high-mass, old and red galaxies, on the other hand, the evolution of nuclear star clusters appears complex, with no apparent trends.The analysis also reveals that redder galaxies ($B-K > 3.76$) are more likely to be dominated by the central black-hole than the nuclear star clusters, while for bluer galaxies ($B-K < 3.76$) in the sample the situation is quite opposite.

astro-ph.GA↗

arXiv2022

Do radiative losses determine the characteristic emission of the blazar Mkn 421?

The radiative loss interpretation for the broken power-law spectra of blazars is often questioned since the difference between the indices does not support this inference. Using the blazar Mkn 421 as a case study, we performed a detailed analysis of its characteristic photon energy where the spectral index changes significantly. We used the observations of the source by Swift-XRT from 2008 to 2019 to identify the characteristic photon energy and the corresponding spectral indices. The spectra in the energy range 0.3-10.0 keV can be well fitted by a log parabola as well as a smooth broken power-law. From the smooth broken power-law spectral fit we show that the spectral indices before and after the characteristic photon energy are strongly anti-correlated. Further, the spectral curvature measured at the characteristic photon energy indicates an anti-correlation with the low energy spectral index while the high energy spectral index shows a positive correlation. These findings are at variance with a simple radiative loss interpretation for the characteristic photon energy, and alternative scenarios are thus discussed. Though these scenarios are in principle capable of reproducing the correlation results, they deviate significantly from the observed properties.

astro-ph.HE↗

arXiv2021

Convex X-ray Spectra of PKS 2155-304 and Constraints on the Minimum Electron Energy

The convex (concave upward) high-energy X-ray spectra of the blazar PKS\,2155-304, observed by \emph{XMM-Newton}, is interpreted as the signature of sub-dominant inverse Compton emission. The spectra can be well fitted by a superposition of two power-law contributions which imitate the emission due to synchrotron and inverse Compton processes. The methodology adopted enables us to constrain the photon energy down to a level where inverse Compton emission begins to contribute. We show that this information supplemented with knowledge of the jet Doppler factor and magnetic field strength can be used to constrain the low-energy cutoff $γ_{\rm min}m_{\rm e} c^2$ of the radiating electron distribution and the kinetic power $P_{\rm j}$ of the jet. We deduce these quantities through a statistical fitting of the broadband spectral energy distribution of PKS\,2155-304 assuming synchrotron and synchrotron self Compton emission mechanisms. Our results favour a minimum Lorentz factor for the non-thermal electron distribution of $γ_{\rm min} \gtrsim 60$, with a preference for a value around $γ_{\rm min} \simeq 330$. The required kinetic jet power is of the order of $P_{\rm j} \sim 3\times 10^{45}$ erg s$^{-1}$ in case of a heavy, electron-proton dominated jet, and could be up to an order of magnitude less in case of a light, electron-positron dominated jet. When put in context, our best fit parameters support the X-ray emitting part of blazar jets to be dominated by an electron-proton rather than an electron-positron composition.

astro-ph.HE↗

arXiv2020

Study of central intensity ratio of early-type galaxies from low density environment

We present correlations involving central intensity ratio (CIR) of 52 early type galaxies, including 24 ellipticals and 28 lenticulars, selected from low density environment in the nearby (< 30 Mpc) universe. CIR is found to be negatively and significantly correlated with the mass of the central super massive black hole, central velocity dispersion, absolute B band magnitude, stellar bulge mass and central Mg2 index of the host galaxy. The study proposes the use of CIR as a simple, fast and efficient photometric tool for exploring the co-evolution scenario existing in galaxies.

astro-ph.GA↗

arXiv2019

Co-evolution of Nuclear Rings, Bars and the Central Intensity Ratio of their Host Galaxies

Using a sample of 13 early-type spiral galaxies hosting nuclear rings, we report remarkable correlations between the properties of the nuclear rings and the central intensity ratio (CIR) of their host galaxies. The CIR, a function of intensity of light within the central 1.5 and 3 arcsec region, is found to be a vital parameter in galaxy evolution, as it shares strong correlations with many structural and dynamical properties of early-type galaxies, including mass of the central supermassive black hole (SMBH). We use archival HST images for aperture photometry at the centre of the galaxy image to compute the CIR. We observe that the relative sizes of nuclear rings and ring cluster surface densities strongly correlate with the CIR. These correlations suggest reduced star formation in the centres of galaxies hosting small and dense nuclear rings. This scenario appears to be a consequence of strong bars as advocated by the significant connection observed between the CIR and bar strengths. In addition, we observe that the CIR is closely related with the integrated properties of the stellar population in the nuclear rings associating the rings hosting older and less massive star clusters with low values of CIR. Thus, the CIR can serve as a crucial parameter in unfolding the coupled evolution of bars and rings as it is intimately connected with both their properties.

astro-ph.GA↗

arXiv2018

Establishing the Spectral Turnover of Blazar PKS 2155-304 as an Outcome of Radiative Losses

The broad band optical/UV and X-ray spectra of blazars have been often modeled as synchrotron component arising from a broken power-law distribution of electrons. A broken power-law distribution is expected, since the high energy electrons undergo radiative losses effectively. The change in the energy index should then be approximately 1 and corresponds to a spectral index difference of 0.5. However, one of the long outstanding problems has been that the observed index change is significantly different. On the other hand, recent high quality observations of blazars suggest that their local spectra may not be a power-law, instead have a slight curvature and often represented by a log parabola model. Using XMM-Newton observations spanning over 12 years for the BL Lac PKS 2155-304, we show that the optical/UV and X-ray spectra can be well represented by a broken log parabola model. Further, we show that such a spectrum can indicate the energy dependence of the electron escape timescale from the main acceleration zone. This novel approach, besides addressing the observed difference in the photon spectral indices, also tries to explain the spectral turn over in far UV/soft X-rays as a result of the radiative losses.

astro-ph.HE↗

arXiv2018

Study of central light concentration in nearby galaxies

We propose a novel technique to estimate the masses of super massive black holes (SMBHs) residing at the centres of massive galaxies in the nearby Universe using simple photometry. Aperture photometry using SEXTRACTOR is employed to determine the central intensity ratio (CIR) at the optical centre of the galaxy image for a sample of 49 nearby galaxies with SMBH mass estimations. We find that the CIR of ellipticals and classical bulges is strongly correlated with SMBH masses whereas pseudo bulges and ongoing mergers show significant scatter. Also, the CIR of low luminosity AGNs in the sample shows significant connection with the 5 GHz nuclear radio emission suggesting a stronger link between the former and the SMBH evolution in these galaxies. In addition, it is seen that various structural and dynamical properties of the SMBH host galaxies are correlated with the CIR making the latter an important parameter in galaxy evolution studies. Finally, we propose the CIR to be an efficient and simple tool not only to distinguish classical bulges from pseudo bulges but also to estimate the mass of the central SMBH.

astro-ph.GA↗

arXiv2017

Physics Potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies and path lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial to address some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations. We describe the simulation framework, the neutrino interactions in the detector, and the expected response of the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Its charge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.

physics.ins-det↗

arXiv2016

SN 2015bp: adding to the growing population of transitional type Ia supernovae

Photometric and spectroscopic observations of type Ia supernova 2015bp are presented, spanning $\sim -6$ to $\sim +141$ days since $B$-band maximum. Also presented are unpublished HCT spectra of type Ia iPTF13ebh between $-11$ to +34 days since $B$-band maximum. SN 2015bp shows rapidly declining light curves with $Δm_{15}(B)=1.72 \pm 0.04$. The $I$-band light curve shows a clear secondary maximum and peaks before the $B$-band maximum, placing SN 2015bp in the transitional category of SNe Ia. The spectral evolution of SN 2015bp resembles other transitional SNe Ia rather than 1991bg-like events. The C~{\sc ii} $λ6580$ feature is detected in both SN 2015bp and iPTF13ebh, though it is present till the epoch of $B$-band maximum in the case of SN 2015bp. The velocity gradients of Si~{\sc ii} $λ6355$ place SN 2015bp and iPTF13ebh in the FAINT subclass, whereas pseudo-equivalent widths of Si~{\sc ii} features place them in the Cool (CL) subclass of SNe Ia. The bolometric light curve of SN 2015bp indicates that $\sim 0.2$ M$_{\odot}$ of $^{56}$Ni was synthesized in the explosion, with a total ejected mass of $\sim 0.9$ M$_{\odot}$, suggesting a sub-Chandrasekhar mass white dwarf progenitor.

astro-ph.SR↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2014

Long term optical variability of bright X-ray point sources in elliptical galaxies

We present long term optical variability studies of bright X-ray sources in four nearby elliptical galaxies with {\it Chandra} Advanced CCD Imaging Spectrometer array (ACIS-S) and {\it Hubble Space Telescope (HST)} Advanced Camera for Surveys observations. Out of the 46 bright (X-ray counts $> 60$) sources that are in the common field of view of the {\it Chandra} and {\it HST} observations, 34 of them have potential optical counterparts, while the rest of them are optically dark. After taking into account of systematic errors, estimated using the field optical sources as reference, we find that four of the X-ray sources (three in NGC1399 and one in NGC1427) have variable optical counterparts at a high significance level. The X-ray luminosities of these source are $\sim 10^{38}$ $\rm ergs~s^{-1}$ and are also variable on similar time-scales. The optical variability implies that the optical emission is associated with the X-ray source itself rather than being the integrated light from a host globular cluster. For one source the change in optical magnitude is $> 0.3$, which is one of the highest reported for this class of X-ray sources and this suggests that the optical variability is induced by the X-ray activity. However, the optically variable sources in NGC1399 have been reported to have blue colours ($g - z > 1$). All four sources have been detected in the infra-red (IR) by {\it Spitzer} as point sources, and their ratio of $5.8$ to $3.6 μm$ flux are $> 0.63$ indicating that their IR spectra are like those of Active Galactic Nuclei (AGN). While spectroscopic confirmation is required, it is likely that all four sources are background AGNs. We find none of the X-ray sources having optical/IR colours different from AGNs, to be optically variable.

astro-ph.HE↗

arXiv2013

Mid-Infrared and X-ray luminosity correlations of X-ray point sources in NGC 1399

It is known that the IR and X-ray luminosities of Active Galactic Nuclei (AGN) are correlated with L_{IR} ~ L_{X}. Moreover, the IR flux ratio between the 5.8 and 3.6 μm bands is a good distinguishing characteristic of AGN or AGN-like behaviour. On the other hand, Galactic X-ray binaries (GXB) are under-luminous in the IR with L_{IR} << L_{X}. Since Ultra-luminous X-ray sources in nearby galaxies may be an intermediate class between AGN and GXB, it is interesting to study if their IR properties indicate which kind of objects they resemble. We use Spitzer IRAC images to identify mid-IR counterparts of bright X-ray sources, detected by Chandra in the elliptical galaxy NGC 1399. We find that for sources with AGN-like IR flux ratios, the IR luminosity strongly correlates with that in X-rays, L_{IR} ~ L_{X}, while for the others, there is no correlation between the two. Some of the former objects may be background AGN. If they are not strongly contaminated by background AGN, this result extends the IR-X-ray luminosity correlation down to L_{X} ~ 10^{39} ergs/s. We calculate their g-z colours and find that the bright X-ray sources with IR counterparts are typically blue in optical color. This is in contrast to typical X-ray sources, without IR counterparts which have predominantly red optical counterparts. We highlight the need for IR or optical spectra of these sources to distinguish background AGN and unveil the effect of the X-ray emission on the different environments of these systems.

astro-ph.HE↗

arXiv2013

X-ray spectral cut-off and the lack of hard X-ray emission from two ultraluminous X-ray sources M81 X-6 and Holmberg IX X-1

We present broadband X-ray spectral study of two ultraluminous X-ray sources (ULXs), M81 X-6 and Holmberg IX X-1 based on Suzaku & XMM-Newton observations. We perform joint broadband spectral analysis of the brightest sources in the field, i.e. the two ULXs and the active galactic nucleus (AGN) in M81, and demonstrate that the X-ray spectra of the ULXs cut off at energies more than about 3keV with negligible contribution at high energies in the Suzaku HXD/PIN band. The 90% upper limit on the 10-30keV band luminosity of an underlying broadband power-law component is 3.5e38 ergs/s for M81 X-6 and 1.2e39ergs/s for Holmberg IX X-1. These limits are more than an order of magnitude lower than the bolometric (0.1-30 keV) luminosity of 6.8e39ergs/s for M81 X-6 and 1.9e40 ergs/s for Holmberg IX X-1. Our results confirm earlier indications of spectral cut-offs inferred from the XMM-Newton observations of bright ULXs and show that there isn't an additional high energy power-law component contributing significantly to the X-ray emission. The spectral form of the two ULXs are very different from those of Galactic black hole X-ray binaries (BHBs) or AGNs. This implies that the ULXs are neither simply scaled-up versions of stellar mass BHBs or scaled-down versions of AGNs.

astro-ph.HE↗