arXiv subjects

A. R. Rao

Publications and source records attributed to A. R. Rao.

At least 37 records · Page 2Linked to original sources

arXiv2021

Sub-MeV spectroscopy with AstroSat-CZT Imager for Gamma Ray Bursts

Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100 - 300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (around 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100 - 600 keV for the GRBs detected in the first year of operation. CZTI in last five years has detected around 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

astro-ph.HE↗

arXiv2019

Estimation of the jet inclination angle for the TDE Swift J1644+57

An estimate of the jet inclination angle relative to the accreting black hole's spin can be useful to probe the jet triggering mechanism and the disc--jet coupling. A Tidal Disruption Event (TDE) of a star by a supermassive spinning black hole provides an excellent astrophysical laboratory to study the jet direction through the possibility of jet precession. In this work, we report a new method to constrain the jet inclination angle $β$ and apply it to the well-sampled jetted TDE Swift J1644+57. This method involves X-ray data analysis and comparisons of jet models with broad properties of the observed X-ray dips, to estimate the upper limit of the extent of the contribution of a plausible jet precession to these X-ray dips. From this limit, we find that $β$ is very likely to be less than $\sim 15^\circ$ for Swift J1644+57. Such a well-constrained jet inclination angle could be useful to probe the jet physics. The main advantage of our method is that it does not need to assume an origin of the observed X-ray dips, and the conclusion does not depend on any particular type of jet precession (e.g., the one due to the Lense-Thirring effect) or any specific value of precession frequency or any particular jet model. These make this method reliable and applicable to other jetted TDEs, as well as to other jetted accreting systems.

astro-ph.HE↗

arXiv2019

Development of Instruments for Space Exploration Using Meteorological-balloons

Indian Centre for Space Physics is engaged in studying terrestrial and extra-terrestrial high energy phenomena from meteorological balloon borne platforms. A complete payload system with such balloons is at the most about five kilograms of weight. One has to adopt innovative and optimal design for various components of the experiment, so that the data can be procured at decent heights of ~ 35-42 km and at the same time, some scientific goals are achieved. In this paper, we mainly describe the instruments in detail and present their test and calibration results. We discuss, how we implemented and tested three major instruments, namely, a Geiger-Müller counter, a single crystal scintillator detector and a phoswich type scintillator detector for our missions. We also present some flight data of a few missions to demonstrate the capability of such experiments.

astro-ph.IM↗

arXiv2019

Prompt emission polarimetry of Gamma Ray Bursts with ASTROSAT CZT-Imager

X-ray and Gamma-ray polarization measurements of the prompt emission of Gamma-ray bursts (GRBs) are believed to be extremely important for testing various models of GRBs. So far, the available measurements of hard X-ray polarization of GRB prompt emission have not significantly constrained the GRB models, particularly because of the difficulty of measuring polarization in these bands. The CZT Imager (CZTI) onboard {\em AstroSat} is primarily an X-ray spectroscopic instrument that also works as a wide angle GRB monitor due to the transparency of its support structure above 100 keV. It also has experimentally verified polarization measurement capability in the 100 $-$ 300 keV energy range and thus provides a unique opportunity to attempt spectro-polarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during its first year of operation. Among these, 5 GRBs show polarization signatures with $\gtrapprox$3$σ$, and 1 GRB shows $\>$2$σ$ detection significance. We place upper limits for the remaining 5 GRBs. We provide details of the various tests performed to validate our polarization measurements. While it is difficult yet to discriminate between various emission models with the current sample alone, the large number of polarization measurements CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission.

astro-ph.HE↗

arXiv2019

Time varying polarized gamma-rays from GRB 160821A: evidence for ordered magnetic fields

GRB 160821A is the third most energetic gamma ray burst observed by the {\it Fermi} gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27} \%$; $5.3 σ$ detection), and (ii) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at $3.5 σ$ and $3.1 σ$ detections respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of $1/Γ$, where $Γ$ is the Lorentz factor of the outflow.

astro-ph.HE↗

arXiv2019

Possible formation of lowly luminous highly magnetized white dwarfs by accretion leading to SGRs/AXPs

We sketch a possible evolutionary scenario by which a highly magnetized super-Chandrasekhar white dwarf could be formed by accretion on to a commonly observed magnetized white dwarf. This is an exploratory study, when the physics in cataclysmic variables (CVs) is very rich and complex. Based on this, we also explore the possibility that the white dwarf pulsar AR Sco acquired its high spin and magnetic field due to repeated episodes of accretion and spin-down. We show that strong magnetic field dramatically decreases luminosity of highly magnetized white dwarf (B-WD), letting them below the current detection limit. The repetition of this cycle can eventually lead to a B-WD, recently postulated to be the reason for over-luminous type Ia supernovae. A spinning B-WD could also be an ideal source for continuous gravitational radiation and soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs). SGRs/AXPs are generally believed to be highly magnetized, but observationally not confirmed yet, neutron stars. Invoking B-WDs does not require the magnetic field to be as high as for neutron star based model, however reproducing other observed properties intact.

astro-ph.SR↗

arXiv2019

AstroSat-CZTI detection of variable prompt emission polarization in GRB 171010A

We present spectro-polarimetric analysis of \thisgrb\ using data from \asat, \fermi, and \swift, to provide insights into the physical mechanisms of the prompt radiation and the jet geometry. Prompt emission from \thisgrb\ was very bright (fluence $>10^{-4}$~ergs~cm$^{-2}$) and had a complex structure composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low energy peak $\sim 15$~keV --- which we interpret as a power-law with two breaks, with a synchrotron origin. Alternately, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. Time-resolved analysis confirms the presence of the low energy component, while the peak energy is found to be confined in the range of 100--200~keV. Afterglow emission detected by \fermi-LAT is typical of an external shock model, and we constrain the initial Lorentz factor using the peak time of the emission. \swift-XRT measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to $>$ 6$\degr$. Detection of a large number of Compton scattered events by \asat-CZTI provides an opportunity to study hard X-ray polarization of the prompt emission. We find that the burst has high, time-variable polarization, with the emission {\bf have higher polarization} at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising due to {\bf due to physical or geometric effects:} synchrotron emission from multiple shocks with ordered or random magnetic fields, Poynting flux dominated jet undergoing abrupt magnetic dissipation, sub-photospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.

astro-ph.HE↗

arXiv2018

Temporal variability and estimation of jet parameters for Ton 599

The TeV blazar Ton 599 has exhibited a peculiar flare in 2017 November. The temporal variation of the source is studied using simultaneous $γ$-ray data from $\textit{Fermi}$ Large Area Telescope and radio data from Owens Valley Radio Observatory's 40 m telescope, over the period of nine years. Four major flaring periods are observed in the $γ$-ray energy band of 0.1-300 GeV. These periods are studied on a shorter timescale and modeled with a time-dependent function containing exponential rising and decaying components. The physical parameters of the jet are estimated numerically and compared with those reported in the literature. During the fourth flare a bunch of high energy photons ($>$10 GeV) were detected. The two highest energy photons having an energy of 76.9 GeV and 61.9 GeV are detected on MJD 58059.0 and 58073.3, respectively. This observation possibly constrains the $γ$-ray emission region to lie near outer edge or outside the broad line region of size $\sim$0.08 pc. The variation of equivalent width of a Mg-II line is studied using the spectroscopic data from Steward observatory. It was observed that the equivalent width of the line varies inversely with the underlying power-law continuum.

astro-ph.HE↗

arXiv2018

Long term X-ray variability characteristics of the narrow-line Seyfert 1 galaxy RE~J1034+396

We present the results of our study of the long term X-ray variability characteristics of the Narrow Line Seyfert 1 galaxy RE J1034+396. We use data obtained from the AstroSat satellite along with the light curves obtained from XMM-Newton and Swift-XRT. We use the 0.3 - 7.0 keV and 3 - 20 keV data, respectively, from the SXT and the LAXPC of AstroSat. The X-ray spectra in the 0.3 - 20 keV region are well fit with a model consisting of a power-law and a soft excess described by a thermal-Compton emission with a large optical depth, consistent with the earlier reported results. We have examined the X-ray light curves in the soft and hard X-ray bands of SXT and LAXPC, respectively, and find that the variability is slightly larger in the hard band. To investigate the variability characteristics of this source at different time scales, we have used X-ray light curves obtained from XMM-Newton data (200 s to 100 ks range) and Swift-XRT data (1 day to 100 day range) and find that there are evidences to suggest that the variability sharply increases at longer time scales. We argue that the mass of the black hole in RE J1034+396 is likely to be $\sim$3 $\times$ 10$^6$ M$_\odot$, based on the similarity of the observed QPO to the high frequency QPO seen in the Galactic black hole binary, GRS 1915+105.

astro-ph.HE↗

arXiv2018

Violation of synchrotron line of death by the highly polarized $GRB~160802A$

$GRB~160802A$ is one of the brightest gamma-ray bursts (GRBs) observed with $Fermi$ Gamma-ray Burst Monitor (GBM) in the energy range of $10~-~1000$ keV, while at the same time it is surprisingly faint at energies $\gtrsim2$ MeV. An observation with $AstroSat$/CZT Imager (CZTI) also provides the polarisation which helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the $Fermi$/GBM data, and find two main bursting episodes that are clearly separated in time, one particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behaviour corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90 per cent cases, cross even the slow cooling limit ($α=-2/3$) of an optically thin synchrotron shock model (SSM). Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarisation in 100--300\,keV is too high, $π=85\pm29\%$, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to $α=+0.5$, in principle can produce high polarisation but only beyond the spectral peak, which in our case lies close to $200~-~300$ keV during the time when most of the polarisation signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.

astro-ph.HE↗

arXiv2018

A comprehensive study of high-energy gamma-ray and radio emission from Cyg X-3

We study high-energy $γ$-rays observed from Cyg X-3 by the Fermi Large Area Telescope and the 15-GHz emission observed by the Ryle Telescope and the Arcminute Microkelvin Imager. We measure the $γ$-ray spectrum averaged over strong flares much more accurately than before, and find it well modelled by Compton scattering of stellar radiation by relativistic electrons with the power law index of $\simeq$3.5 and a low-energy cutoff at the Lorentz factor of $\sim\!10^3$. We find a weaker spectrum in the soft spectral state, but only upper limits in the hard and intermediate states. We measure strong orbital modulation during the flaring state, well modelled by anisotropic Compton scattering of blackbody photons from the donor by jet relativistic electrons. We discover a weaker orbital modulation of the 15 GHz radio emission, which is well modelled by free-free absorption by the stellar wind. We then study cross-correlations between radio, $γ$-ray and X-ray emissions. We find the cross-correlation between the radio and $γ$-ray emissions peaks at a lag less than 1 d, while we detect a distinct radio lag of $\sim$50 d with respect to the soft X-rays in the soft spectral state.

astro-ph.HE↗

arXiv2017

A precise measurement of the orbital period parameters of Cygnus X-3

We present X-ray light curves of Cygnus X-3 as measured by the recently launched AstroSat satellite. The light curve folded over the binary period of 4.8 hours shows a remarkable stability over the past 45 years and we find that we can use this information to measure the zero point to better than 100 s. We revisit the historical binary phase measurements and examine the stability of the binary period over 45 years. We present a new binary ephemeris with the period and period derivative determined to an accuracy much better than previously reported. We do not find any evidence for a second derivative in the period variation. The precise binary period measurements, however, indicate a hint of short term episodic variations in periods. Interestingly, these short term period variations coincide with the period of enhanced jet activity exhibited by the source. We discuss the implications of these observations on the nature of the binary system.

astro-ph.HE↗

arXiv2017

General Physical Properties of CGRaBS Blazars

We present the results of a multi-frequency, time-averaged analysis of blazars included in the Candidate Gamma-ray Blazar Survey catalog. Our sample consists of 324 $γ$-ray detected ($γ$-ray loud) and 191 non $γ$-ray detected ($γ$-ray quiet) blazars, and we consider all the data up to 2016 April 1. We find that both the $γ$-ray loud and the $γ$-ray quiet blazar populations occupy similar regions in the WISE color-color diagram, and in the radio and X-ray bands $γ$-ray loud sources are brighter. A simple one-zone synchrotron inverse-Compton emission model is applied to derive the physical properties of both populations. We find that the central black hole mass and the accretion disk luminosity ($L_{\rm disk}$) computed from the modeling of the optical-UV emission with a Shakura-Sunyaev disk reasonably matches with that estimated from the optical spectroscopic emission-line information. A significantly larger Doppler boosting in the $γ$-ray loud blazars is noted, and their jets are more radiatively efficient. On the other hand, the $γ$-ray quiet objects are more MeV-peaked, thus could be potential targets for next-generation MeV missions. Our results confirm the earlier findings about the accretion-jet connection in blazars; however, many of the $γ$-ray quiet blazars tend to deviate from the recent claim that the jet power exceeds $L_{\rm disk}$ in blazars. A broadband study, considering a larger set of $γ$-ray quiet objects and also including BL Lacs, will be needed to confirm/reject this hypothesis and also to verify the evolution of the powerful high-redshift blazars into their low-power nearby counterparts.

astro-ph.HE↗

arXiv2017

High Energy Transients: the Millisecond Domain

Search for high energy transients in the millisecond domain has come to the focus in recent times due to the detection of Gravitational Wave events and the identification of Fast Radio Bursts as cosmological sources. I will highlight the sensitivity limitations in the currently operating hard X-ray telescopes and give some details of the search for millisecond events in the AstroSat CZT Imager data.

astro-ph.HE↗

arXiv2017

Cadmium Zinc Telluride Imager onboard AstroSat : a multi-faceted hard X-ray instrument

The AstroSat satellite is designed to make multi-waveband observations of astronomical sources and the Cadmium Zinc Telluride Imager (CZTI) instrument of AstroSat covers the hard X-ray band. CZTI has a large area position sensitive hard X-ray detector equipped with a Coded Aperture Mask, thus enabling simultaneous background measurement. Ability to record simultaneous detection of ionizing interactions in multiple detector elements is a special feature of the instrument and this is exploited to provide polarization information in the 100 - 380 keV region. CZTI provides sensitive spectroscopic measurements in the 20 - 100 keV region, and acts as an all sky hard X-ray monitor and polarimeter above 100 keV. During the first year of operation, CZTI has recorded several gamma-ray bursts, measured the phase resolved hard X-ray polarization of the Crab pulsar, and the hard X-ray spectra of many bright Galactic X-ray binaries. The excellent timing capability of the instrument has been demonstrated with simultaneous observation of the Crab pulsar with radio telescopes like GMRT and Ooty radio telescope.

astro-ph.IM↗

arXiv2017

AR Sco as a possible seed of highly magnetised white dwarf

We explore the possibility that the recently discovered white dwarf pulsar AR Sco acquired its high spin and magnetic field due to repeated episodes of accretion and spin-down. An accreting white dwarf can lead to a larger mass and consequently a smaller radius thus causing an enhanced rotation period and magnetic field. This spinning magnetic white dwarf temporarily can inhibit accretion, spin down, and, eventually, the accretion can start again due to the shrinking of the binary period by gravitational radiation. A repeat of the above cycle can eventually lead to a high magnetic field white dwarf, recently postulated to be the reason for over-luminous type Ia supernovae. We also point out that these high magnetic field spinning white dwarfs are attractive sites for gravitational radiation.

astro-ph.HE↗

arXiv2017

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

We present a detailed analysis of GRB 151006A, the first GRB detected by Astrosat CZT Imager (CZTI). We study the long term spectral evolution by exploiting the capabilities of \emph{Fermi} and \emph{Swift} satellites at different phases, which is complemented by the polarization measurement with the CZTI. While the light curve of the GRB in different energy bands show a simple pulse profile, the spectrum shows an unusual evolution. The first phase exhibits a hard-to-soft (HTS) evolution until $\sim16-20$\,s, followed by a sudden increase in the spectral peak reaching a few MeV. Such a dramatic change in the spectral evolution in case of a single pulse burst is reported for the first time. This is captured by all models we used namely, Band function, Blackbody+Band and two blackbodies+power law. Interestingly, the \emph{Fermi} Large Area Telescope (LAT) also detects its first photon ($>100$\,MeV) during this time. This new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself which, however, is not seen in the otherwise smooth pulse profile. By constructing Bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. The \emph{Swift} data at late epochs ($>T_{90}$ of the GRB) also shows a significant spectral evolution consistent with the early second phase. The CZTI data (100--350\,keV), though having low significance ($1σ$), show high values of polarization in the two epochs ($77\%$ to $94\%$), in agreement with our interpretation.

astro-ph.HE↗

arXiv2017

A tale of two transients: GW170104 and GRB170105A

We present multi-wavelength follow-up campaigns by the AstroSat-CZTI and GROWTH collaborations to search for an electromagnetic counterpart to the gravitational wave event GW170104. At the time of the GW170104 trigger, the AstroSat CZTI field-of-view covered 50.3\% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of $\approx 4.5 \times 10^{-7}~{\rm erg~cm}^{-2}{\rm~s}^{-1}$ for a 1\,s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB~170105A, with only a fortuitous spatial coincidence with GW170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models.

astro-ph.HE↗