arXiv subjects

T. P. Prabhu

Publications and source records attributed to T. P. Prabhu.

At least 19 recordsLinked to original sources

arXiv2019

VHE Gamma-ray Observation of Crab Nebula with HAGAR Telescope Array

HAGAR is a system of seven Non-imaging Atmospheric Cherenkov Telescopes located at Hanle in the Ladakh region of the Indian Himalayas at an altitude of 4270 meters {\it amsl}. Since 2008, we have observed the Crab Nebula to assess the performance of the HAGAR telescopes. We describe the analysis technique for the estimation of $γ$-ray signal amidst cosmic ray background. The consolidated results spanning nine years of the Crab nebula observations show long term performance of the HAGAR telescopes. Based on about 219 hours of data, we report the detection of $γ$-rays from the Crab Nebula at a significance level of about 20$σ$, corresponding to a time averaged flux of (1.64$\pm$0.09) $\times10^{-10}$ photons cm$^{-2}$ sec$^{-1}$ above 230 GeV. Also, we perform a detailed study of possible systematic effects in our analysis method on data taken with the HAGAR telescopes.

astro-ph.HE↗

arXiv2017

Highly reddened type Ia supernova SN 2004ab: another case of anomalous extinction

We present optical photometric and spectroscopic results of supernova SN 2004ab, a highly reddened normal type Ia supernova. The total reddening is estimated as $E(B-V)$ = 1.70 $\pm$ 0.05 mag. The intrinsic decline rate parameter, $Δm_{15}(B)_\text{true}$ is 1.27 $\pm$ 0.05, and $B$-band absolute magnitude at maximum $M_{B}^{\text{max}}$ is $-$19.31 $\pm$ 0.25 mag. The host galaxy NGC 5054 is found to exhibit anomalous extinction with very low value of $R_V$ = 1.41 $\pm$ 0.06 in the direction of SN 2004ab. Peak bolometric luminosity is derived as $\log L_\text{bol}^\text{max}$ = 43.10 $\pm$ 0.07 erg\,s$^{-1}$. The photospheric velocity measured from absorption minimum of Si\,{\sc ii} $λ$6355 line shows a velocity gradient of $\dot{v}$ = 90 km\,s$^{-1}$\,d$^{-1}$, indicating that SN 2004ab is a member of the high velocity gradient (HVG) subgroup. The ratio of strength of Si\,{\sc ii} $λ$5972 and $λ$6355 absorption lines, $\cal R$(Si\,{\sc ii}) is estimated as 0.37, while their pseudo equivalent widths suggest that SN 2004ab belongs to broad line (BL) type subgroup.

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arXiv2016

Long-term study of Mkn 421 with the HAGAR Array of Telescopes

Context:The HAGAR Telescope Array at Hanle, Ladakh has been regularly monitoring the nearby blazar Mkn 421 for the past 7yrs. Aims: Blazars show flux variability in all timescales across the electromagnetic spectrum. While there is abundant literature characterizing the short term flares from different blazars, comparatively little work has been done to study the long term variability. We aim to study the long term temporal and spectral variability in the radiation from Mkn 421 during 2009-2015. Methods: We quantify the variability and lognormality from the radio to the VHE bands, and compute the correlations between the various wavebands using the z-transformed discrete correlation function. We construct the Spectral Energy Distribution (SED) contemporaneous with HAGAR observation seasons and fit it with a one zone synchrotron self Compton model to study the spectral variability. Results: The flux is found to be highly variable across all time scales. The variability is energy dependant, and is maximum in the X-ray and Very High Energy (VHE) bands. A strong correlation is found between the Fermi-LAT (gamma) and radio bands, and between Fermi-LAT and optical, but none between Fermi-LAT and X-ray. Lognormality in the flux distribution is clearly detected. This is the third blazar, following BL~Lac and PKS~2155$+$304 to show this behavior. The SED can be well fit by a one zone SSC model, and variations in the flux states can be attributed mainly due to changes in the particle distribution. A strong correlation is seen between the break energy $γ_b$ of the particle spectrum and the total bolometric luminosity.

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arXiv2016

Optical and NIR observations of the nearby type Ia supernova SN 2014J

Optical and NIR observations of the type Ia supernova SN 2014J in M82 are presented. The observed light curves are found to be similar to normal SNe Ia, with a decline rate parameter $Δm_{15}(B) = 1.08 \pm 0.03$. The supernova reached $B$-band maximum on JD 2456690.14, at an apparent magnitude $m_B(max) = 11.94$. The optical spectra show a red continuum with deep interstellar Na~{\sc i} absorption, but otherwise resemble those of normal SNe Ia. The Si~{\sc ii} $λ6355$ feature indicates a velocity of $\sim 12\,000$ km s$^{-1}$ at $B$-band maximum, which places SN 2014J at the border of the Normal Velocity and High Velocity group of SNe Ia. The velocity evolution of SN 2014J places it in the Low Velocity Gradient subclass, whereas the equivalent widths of Si~{\sc ii} features near $B$-band maximum place it at the border of the Core Normal and Broad Line subclasses of SNe Ia. An analytic model fit to the bolometric light curve indicates that a total of $\sim 1.3$ M$_{\odot}$ was ejected in the explosion, and the ejected $^{56}$Ni mass $M_{Ni} \sim 0.6$ M$_{\odot}$. The low [Fe~{\sc iii}] $\lambda4701$ to [Fe~{\sc ii}] $\lambda5200$ ratio in the nebular spectra of SN 2014J hints towards clumpiness in the ejecta. Optical broadband, linear polarimetric observations of SN 2014J obtained on four epochs indicate an almost constant polarization ($P_{\rm R} \sim$2.7 per cent; $θ$ $\sim$37$^\circ$), which suggests that the polarization signal is of interstellar origin.

astro-ph.SR↗

arXiv2015

Insights into the evolution of symbiotic recurrent novae from radio synchrotron emission: V745 Scorpii and RS Ophiuchi

We present observations at 610 MHz and 235 MHz using the Giant Metrewave Radio Telescope (GMRT) of the recurrent nova V745 Scorpii which recorded its last outburst on 6 February 2014. This is the second symbiotic recurrent nova whose light curve at low frequencies has been followed in detail, the first being RS Ophiuchi in 2006. We fitted the 610 MHz light curve by a model of synchrotron emission from an expanding shell being modified by radiative transfer effects due to local absorbing gas consisting of a uniformly distributed and a clumpy component. Using our model parameters, we find that the emission at 235 MHz peaked around day 35 which is consistent with our GMRT observations. The two main results of our study are: (1) The radio emission at a given frequency is visible sooner after the outburst in successive outbursts of both V745 Scorpii and RS Ophiuchi. The earlier detection of radio emission is interpreted to be caused by decreasing foreground densities. (2) The clumpy material, if exists, is close to the white dwarf and can be interpreted as being due to the material from the hot accretion disk. The uniform density gas is widespread and attributed to the winds blown by the white dwarf. We present implications of these results on the evolution of both novae. Such studies alongwith theoretical understanding have the potential of resolving several outstanding issues such as why all recurrent novae are not detectable in synchrotron radio and whether recurrent novae are progenitor systems of type 1a supernova.

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arXiv2015

Modelling the synchrotron light curves in recurrent novae V745 Scorpii and RS Ophiuchi

In this paper, we present the synchrotron light curve at 610 MHz from the recurrent nova V745 Sco following its outburst on 6 February 2014. The system has been detected and periodically monitored with the Giant Metrewave Radio Telescope (GMRT) since 9 February 2014 as part of the Galactic Nova with GMRT (GNovaG) project. The light curves are well fit by a model of synchrotron emitting region obscured by foreground thermal gas which eventually becomes optically thin to the low GMRT frequencies. We present the model fit to the 2014 data on V745 Sco and discuss it alongwith the model fit to the 1.4 GHz data of the recurrent nova RS Ophiuchi following its outburst in 1985.

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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

Investigating the AGN activity and black hole masses in Low Surface brightness galaxies

We present an analysis of the optical nuclear spectra from the active galactic nuclei (AGN) in a sample of giant low surface brightness (GLSB) galaxies. GLSB galaxies are extreme late type spirals that are large, isolated and poorly evolved compared to regular spiral galaxies. Earlier studies have indicated that their nuclei have relatively low mass black holes. Using data from the Sloan Digital Sky Survey (SDSS), we selected a sample of 30 GLSB galaxies that showed broad H$α$ emission lines in their AGN spectra. In some galaxies such as UGC 6284, the broad component of H$α$ is more related to outflows rather than the black hole. One galaxy (UGC 6614) showed two broad components in H$α$, one associated with the black hole and the other associated with an outflow event. We derived the nuclear black hole (BH) masses of 29 galaxies from their broad H$α$ parameters. We find that the nuclear BH masses lie in the range $10^{5}-10^{7} M_{\odot}$. The bulge stellar velocity dispersion $σ_{e}$ was determined from the underlying stellar spectra. We compared our results with the existing BH mass - velocity dispersion ($M_{BH}-σ_{e}$) correlations and found that the majority of our sample lie in the low BH mass regime and below the $M_{BH}-σ_{e}$ correlation. The effects of galaxy orientation in the measurement of $σ_e$ and the increase of $σ_e$ due to the effects of bar are probable reasons for the observed offset for some galaxies, but in many galaxies the offset is real. A possible explanation for the $M_{BH}-σ_{e}$ offset could be lack of mergers and accretion events in the history of these galaxies which leads to a lack of BH-bulge co-evolution. \keywords{galaxies: active, galaxies: bulges, galaxies: nuclei}

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arXiv2015

Multi-frequency, Multi-Epoch Study of Mrk 501: Hints for a two-component nature of the emission

Since the detection of very high energy (VHE) $γ$-rays from Mrk 501, its broad band emission of radiation was mostly and quite effectively modeled using one zone emission scenario. However, broadband spectral and flux variability studies enabled by the multiwavelength campaigns carried out during the recent years have revealed rather complex behavior of Mrk 501. The observed emission from Mrk 501 could be due to a complex superposition of multiple emission zones. Moreover new evidences of detection of very hard intrinsic $γ$-ray spectra obtained from {\it Fermi}--LAT observations have challenged the theories about origin of VHE $γ$-rays. Our studies based on {\it Fermi}--LAT data indicate the existence of two separate components in the spectrum, one for low energy $γ$-rays and the other for high energy $γ$-rays. Using multiwaveband data from several ground and space based instruments, in addition to HAGAR data, the spectral energy distribution of Mrk~501 is obtained for various flux states observed during 2011. In the present work, this observed broadband spectral energy distribution is reproduced with a leptonic, multi-zone Synchrotron Self-Compton model.

astro-ph.HE↗

arXiv2014

TIRSPEC : TIFR Near Infrared Spectrometer and Imager

We describe the TIFR Near Infrared Spectrometer and Imager (TIRSPEC) designed and built in collaboration with M/s. Mauna Kea Infrared LLC, Hawaii, USA, now in operation on the side port of the 2-m Himalayan Chandra Telescope (HCT), Hanle (Ladakh), India at an altitude of 4500 meters above mean sea level. The TIRSPEC provides for various modes of operation which include photometry with broad and narrow band filters, spectrometry in single order mode with long slits of 300" length and different widths, with order sorter filters in the Y, J, H and K bands and a grism as the dispersing element as well as a cross dispersed mode to give a coverage of 1.0 to 2.5 microns at a resolving power R of ~1200. The TIRSPEC uses a Teledyne 1024 x 1024 pixel Hawaii-1 PACE array detector with a cutoff wavelength of 2.5 microns and on HCT, provides a field of view of 307" x 307" with a plate scale of 0.3"/pixel. The TIRSPEC was successfully commissioned in June 2013 and the subsequent characterization and astronomical observations are presented here. The TIRSPEC has been made available to the worldwide astronomical community for science observations from May 2014.

astro-ph.IM↗

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.

astro-ph.CO↗

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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arXiv2012

An HI shell-like structure associated with nova V458 Vulpeculae?

We report the radio detection of a shell-like HI structure in proximity to, and probably associated with, the nova V458 Vul. High spectral resolution observation with the Giant Metrewave Radio Telescope has made it possible to study the detailed kinematics of this broken and expanding shell. Unlike the diffuse Galactic HI emission, this is a single velocity component emission with significant clumping at ~ 0.5' scales. The observed narrow line width of ~ 5 km/s suggests that the shell consists of mostly cold gas. Assuming a distance of 13 kpc to the system, as quoted in the literature, the estimated HI mass of the nebula is about 25 M_sun. However, there are some indications that the system is closer than 13 kpc. If there is a physical association of the HI structure and the nova system, the asymmetric morphology and the off-centred stellar system indicates past strong interaction of the mass loss in the asymptotic giant branch phase with the surrounding interstellar medium. So far, this is the second example, after GK Per, of a large HI structure associated with a classical nova.

astro-ph.GA↗

arXiv2012

Multiwavelength study of TeV Blazar Mrk421 during giant flare

Context: The nearby (z=0.031) TeV blazar Mrk421 was reported to be in a high state of flux activity since November, 2009. Aims: To investigate possible changes in the physical parameters of Mrk421 during its high state of activity using multiwavelength data. Methods: We have observed this source in bright state using High Altitude GAmma Ray (HAGAR) telescope array at energies above 250 GeV during February 13 - 19, 2010. Optical, X-ray and gamma-ray archival data are also used to obtain the SEDs and light curves. Results: Mrk421 was found to undergo one of its brightest flaring episodes on February 17, 2010 by various observations in X-rays and gamma-rays. HAGAR observations during February 13 - 19, 2010 at the energies above 250 GeV show an enhancement in the flux level, with a maximum flux of ~ 7 Crab units being detected on February 17, 2010. We present the spectral energy distributions during this flaring episode and investigate the correlation of the variability in X-ray and gamma-ray bands. Conclusions: Our multiwavelength study suggests that the flare detected during February 16 and 17, 2010 could arise due to a passing shock in the jet.

astro-ph.HE↗

arXiv2011

Multiwavelength study of TeV Blazar Mrk421 during giant flare and observations of TeV AGNs with HAGAR

The radiation mechanism of very high energy $γ$-ray emission from blazars and crucial parameters like magnetic field, and size of the emitting region are not well understood yet. To understand the above mentioned properties of blazars, we observed five nearby TeV $γ$-ray emitting blazars (Mrk421, Mrk501, 1ES2344+514, 1ES1218+304 and 3C454.3) and one radio galaxy (M87) using the High Altitude GAmma Ray (HAGAR) telescope. HAGAR is an array of seven telescopes located at Hanle, India to detect Cherenkov light caused by extensive air showers initiated by $γ$-rays. Mrk421 was observed to undergo one of its brightest flaring episodes on 2010 February 17, and detected by various experiments in X-rays and $γ$- rays. HAGAR observations of this source during 2010 February 13 - 19, in the energies above 250 GeV show an enhancement in the flux level, with a flux of 6-7 Crab units being detected on 2010 February 17. We present the spectral energy distribution of the source during this flaring episode. In addition to this, the analysis procedure to extract $γ$-ray signal from HAGAR data is discussed and preliminary results on all the AGNs are presented.

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arXiv2011

AGN Activity and Black Hole Masses in Low Surface Brightness Galaxies

We present medium resolution optical spectroscopy of a sample of nine Low Surface Brightness (LSB) galaxies. For those that show clear signatures of AGN emission, we have disentangled the AGN component from stellar light and any Fe I and Fe II contribution. We have decomposed the H_alpha line into narrow and broad components and determined the velocities of the broad components; typical values lie between 900--2500 km/s. Of the galaxies in our study, UGC 6614, UGC 1922, UGC 6968 and LSBC F568-6 (Malin~2) show clear signatures of AGN activity. We have calculated the approximate black hole masses for these galaxies from the H_alpha line emission using the virial approximation. The black hole masses are ~3x10^{5} M_sun for three galaxies and lie in the intermediate mass black holes domain rather than the supermassive range. UGC 6614 harbors a BH of mass 3.8x10^{6} M_sun; it also shows an interesting feature blueward of H_alpha and H_beta implying outflow of gas or a one-sided jet streaming towards us. We have also measured the bulge stellar velocity dispersions using the Ca II Triplet lines and plotted these galaxies on the M-sigma plot. We find that all the three galaxies UGC 6614, UGC 6968 and F568-6 lie below the M-sigma relation for nearby galaxies. Thus we find that although the bulges of LSB galaxies may be well evolved, their nuclear black hole masses are lower than those found in bright galaxies and lie offset from the M-sigma correlation.

astro-ph.CO↗

arXiv2010

Radio Continuum and HI study of Blue Compact Dwarf Galaxies

The multifrequency radio continuum and 21cm HI observations of five blue compact dwarf (BCD) galaxies, Mrk 104, Mrk 108, Mrk 1039, Mrk 1069 and I Zw 97 using the Giant Meterwave Radio Telescope (GMRT) are presented here. Radio continuum emission at 610 MHz and 325 MHz is detected from all the observed galaxies whereas only a few are detected at 240 MHz. In our sample, three galaxies are members of groups and two galaxies (Mrk 1069 and I Zw 97) are isolated galaxies. The radio emission from Mrk 104 and Mrk 108 is seen to encompass the entire optical galaxy whereas the radio emission from Mrk 1039, Mrk 1069, I Zw 97 is confined to massive HII regions. This, we suggest, indicates that the star formation in the latter group of galaxies has recently been triggered and that the environment in which the galaxy is evolving plays a role. Star formation rates (SFR) calculated from 610 MHz emission is in the range 0.01-0.1 M_sun/yr; this is similar to the SFR obtained for individual star forming regions in BCDs. The integrated radio spectra of four galaxies are modelled over the frequency range where data is available. We find that two of the galaxies Mrk 1069 and Mrk 1039, show a turnover at low frequencies which is well fitted by free-free absorption whereas the other two galaxies, Mrk 104 and Mrk 108, show a power law at the lowest GMRT frequencies. The flatter spectrum, localized star formation and radio continuum in isolated galaxies lend support to stochastic self-propagating star formation (SSPSF). The HI observations of four galaxies Mrk 104, Mrk 108, Mrk 1039 and Mrk 1069 show extended disks as large as ~1.1-6 times the optical size. All the observed BCDs (except Mrk 104) show rotating disk with a half power width of ~50-124 km/s. Solid body rotation is common in our sample. We note that the tidal dwarf (TD) origin is possible for two of the BCDs in our sample.

astro-ph.CO↗

arXiv2008

Optical photometry and spectroscopy of the type Ibn supernova SN 2006jc until the onset of dust formation

We present optical UBVRI photometric and spectroscopic data of the type Ibn supernova SN 2006jc, until the onset of the dust forming phase. The optical spectrum shows a blue continuum and is dominated by the presence of moderately narrow (velocity ~2500 km/s) He I emission lines superimposed over a relatively weak supernova spectrum. The helium lines are produced in a pre-existing He rich circumstellar shell. The observed helium line fluxes indicate the circumstellar shell is dense, with a density of ~10^9 - 10^{10} cm^{-3}. The helium mass in this shell is estimated to be <~0.07 Msun. The optical light curves show a clear signature of dust formation, indicated by a sharp decrease in the magnitudes around day 50, accompanied by a reddening of the colours. The evolution of the optical light curves during the early phase and that of the uvoir bolometric light curve at all phases is reasonably similar to normal Ib/c supernovae.

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