arXiv subjects

A. R. Rao

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

At least 55 records · Page 3Linked to original sources

arXiv2016

Significantly super-Chandrasekhar limiting mass white dwarfs and their consequences

Since 2012, we have initiated a new idea showing that the mass of highly magnetized or modified Einstein's gravity induced white dwarfs could be significantly super-Chandrasekhar with a different mass-limit. This discovery has several important consequences, including explanation of peculiar, over-luminous type Ia supernovae, soft gamma-ray repeaters and anomalous X-ray pulsars without invoking extraordinarily strong, yet unobserved, magnetic fields. It further argues for a possible second standard candle. Based on simpler calculations, these white dwarfs are also shown to be much less luminous than their standard counter-parts (of low magnetic fields). This discovery altogether initiates a new field of research.

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arXiv2016

In-orbit performance of AstroSat CZTI

Cadmium-Zinc-Telluride Imager (CZTI) is one of the five payloads on-board recently launched Indian astronomy satellite AstroSat. CZTI is primarily designed for simultaneous hard X-ray imaging and spectroscopy of celestial X-ray sources. It employs the technique of coded mask imaging for measuring spectra in the energy range of 20 - 150 keV. It was the first scientific payload of AstroSat to be switched on after one week of the launch and was made operational during the subsequent week. Here we present preliminary results from the performance verification phase observations and discuss the in-orbit performance of CZTI.

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arXiv2016

AstroSat CZT Imager observations of GRB 151006A: timing, spectroscopy, and polarisation study

AstroSat is a multi-wavelength satellite launched on 2015 September 28. The CZT Imager of AstroSat on its very first day of operation detected a long duration gamma-ray burst (GRB) namely GRB 151006A. Using the off-axis imaging and spectral response of the instrument, we demonstrate that CZT Imager can localise this GRB correct to about a few degrees and it can provide, in conjunction with Swift, spectral parameters similar to that obtained from Fermi/GBM. Hence CZT Imager would be a useful addition to the currently operating GRB instruments (Swift and Fermi). Specifically, we argue that the CZT Imager will be most useful for the short hard GRBs by providing localisation for those detected by Fermi and spectral information for those detected only by Swift. We also provide preliminary results on a new exciting capability of this instrument: CZT Imager is able to identify Compton scattered events thereby providing polarisation information for bright GRBs. GRB 151006A, in spite of being relatively faint, shows hints of a polarisation signal at 100-300 keV (though at a low significance level). We point out that CZT Imager should provide significant time resolved polarisation measurements for GRBs that have fluence 3 times higher than that of GRB 151006A. We estimate that the number of such bright GRBs detectable by CZT Imager is 5 - 6 per year. CZT Imager can also act as a good hard X-ray monitoring device for possible electromagnetic counterparts of Gravitational Wave events.

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arXiv2016

AstroSat - a multi-wavelength astronomy satellite

AstroSat is a multi-wavelength astronomy satellite, launched on 2015 September 28. It carries a suite of scientific instruments for multi-wavelength observations of astronomical sources. It is a major Indian effort in space astronomy and the context of AstroSat is examined in a historical perspective. The Performance Verification phase of AstroSat has been completed and all instruments are working flawlessly and as planned. Some brief highlights of the scientific results are also given here.

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arXiv2016

Charged Particle Monitor on the AstroSat mission

Charged Particle Monitor (CPM) on-board the AstroSat satellite is an instrument designed to detect the flux of charged particles at the satellite location. A Cesium Iodide Thallium (CsI(Tl)) crystal is used with a Kapton window to detect protons with energies greater than 1 MeV. The ground calibration of CPM was done using gamma-rays from radioactive sources and protons from particle accelerators. Based on the ground calibration results, energy deposition above 1 MeV are accepted and particle counts are recorded. It is found that CPM counts are steady and the signal for the onset and exit of South Atlantic Anomaly (SAA) region are generated in a very reliable and stable manner.

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arXiv2016

Anisotropy of partially self-absorbed jets and the jet of Cyg X-1

We study the angular dependence of the flux from partially synchrotron self-absorbed conical jets (proposed by Blandford \& K{ö}nigl). We consider the jet viewed from either a side or close to on axis, and in the latter case, either from the jet top or bottom. We derive analytical formulae for the flux in each of these cases, and find the exact solution for an arbitrary angle numerically. We find that the maximum of the emission occurs when the jet is viewed from top on-axis, which is contrast to a previous result, which found the maximum at some intermediate angle and null emission on-axis. We then calculate the ratio of the jet-to-counterjet emission for this model, which depends on the viewing angle and the index of power-law electrons. We apply our results to the black-hole binary Cyg X-1. Given the jet-to-counterjet flux ratio of $\gtrsim$50 found observationally and the current estimates of the inclination, we find the jet velocity to be $\gtrsim 0.8c$. We also point out that when the projection effect is taken into account, the radio observations imply the jet half-opening angle of $\lesssim 1^{\rm o}$, a half of the value given before. When combined with the existing estimates of $Γ_{\rm j}$, the jet half-opening angle is low, $\ll 1/Γ_{\rm j}$, and much lower than values observed in blazars, unless $Γ_{\rm j}$ is much higher than currently estimated.

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arXiv2016

The Cadmium Zinc Telluride Imager on AstroSat

The Cadmium Zinc Telluride Imager (CZTI) is a high energy, wide-field imaging instrument on AstroSat. CZT's namesake Cadmium Zinc Telluride detectors cover an energy range from 20 keV to > 200 keV, with 11% energy resolution at 60 keV. The coded aperture mask attains an angular resolution of 17' over a 4.6 deg x 4.6 deg (FWHM) field of view. CZTI functions as an open detector above 100 keV, continuously sensitive to GRBs and other transients in about 30% of the sky. The pixellated detectors are sensitive to polarisation above ~100 keV, with exciting possibilities for polarisation studies of transients and bright persistent sources. In this paper, we provide details of the complete CZTI instrument, detectors, coded aperture mask, mechanical and electronic configuration, as well as data and products.

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arXiv2016

Pulse-wise GRB correlation: implication as a cosmological tool

Gamma-ray bursts (GRBs) are cosmological explosions which carry valuable information from the distant past of the expanding universe. One of the greatest discoveries in modern cosmology is the finding of the accelerated expansion of the universe using Type Ia supernovae (SN Ia) as standard candles. However, due to the interstellar extinction SN Ia can be seen only up to a redshift $z\sim 1.5$. GRBs are considered as the potential alternative to push this limit to as high as $z\sim 10$, a redshift regime corresponding to an epoch when the universe just started to form the first structures. There exist several correlations between the energy and an observable of a GRB which can be used to derive luminosity distance. In recent works, we have studied spectral evolution within the individual pulses and obtained such correlations within the pulses. Here we summarize our results of the pulse-wise GRB correlation study. It is worth mentioning that all GRB correlations are still empirical, and we cannot use them in cosmology unless we understand the basic physics of GRBs. To this end, we need to investigate the prompt emission spectrum which is so far generally described by the empirical Band function. We shall discuss our current understanding of the radiation process particularly the finding of two blackbodies and a powerlaw (the 2BBPL model) as the generic spectral model and its implication. This is a work in progress and we expect to obtain the most fundamental GRB correlation based on our improved spectral model.

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arXiv2016

Soft gamma-ray repeaters and anomalous X-ray pulsars as highly magnetized white dwarfs

We explore the possibility that soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are powered by highly magnetized white dwarfs (B-WDs). We take a sample of SGRs and AXPs and provide the possible parameter space in mass, radius, and surface magnetic field based on their observed properties (period and its derivative) and the assumption that these sources obey the mass-radius relation derived for the B-WDs. The radius and magnetic field of B-WDs are adequate to explain energies in SGRs/AXPs as the rotationally powered energy. In addition, B-WDs also adequately explain the perplexing radio transient GCRT J1745-3009 as a white dwarf pulsar. Note that the radius and magnetic fields of B-WDs are neither extreme (unlike of highly magnetized neutron stars) nor ordinary (unlike of magnetized white dwarfs, yet following the Chandrasekhar's mass-radius relation (C-WDs)). In order to explain SGRs/AXPs, while the highly magnetized neutron stars require an extra, observationally not well established yet, source of energy, the C-WDs predict large ultra-violet luminosity which is observationally constrained from a strict upper limit. Finally, we provide a set of basic differences between the magnetar and B-WD hypotheses for SGRs/AXPs.

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arXiv2016

The Multi-wavelength Characteristics of the TeV Binary LS I +61$^\circ$ 303

We study the characteristics of the TeV binary LS I +61$^\circ$ 303 in radio, soft X-ray, hard X-ray, and gamma-ray (GeV and TeV) energies. The long term variability characteristics are examined as a function of the phase of the binary period of 26.496 days as well as the phase of the super orbital period of 1626 days, dividing the observations into a matrix of 10$\times$10 phases of these two periods. It is found that the long term variability can be described by a sine function of the super orbital period, with the phase and amplitude systematically varying with the binary period phase. We also find a definite wavelength dependent change in this variability description. To understand the radiation mechanism, we define three states in the orbital/ super orbital phase matrix and examine the wide band spectral energy distribution. The derived source parameters indicate that the emission geometry is dominated by a jet structure showing a systematic variation with the orbital/ super orbital period. We suggest that LS I +61$^\circ$ 303 is likely to be a micro-quasar with a steady jet.

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arXiv2016

Soft gamma-ray repeaters and anomalous X-ray pulsars as highly magnetized white dwarfs

We show that the soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) can be explained as recently proposed highly magnetized white dwarfs (B-WDs). The radius and magnetic field of B-WDs are perfectly adequate to explain energies in SGRs/AXPs as the rotationally powered energy. While the highly magnetized neutron stars require an extra, observationally not well established yet, source of energy, the magnetized white dwarfs, yet following Chandrasekhar's theory (C-WDs), exhibit large ultra-violet luminosity which is observationally constrained from a strict upper limit.

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arXiv2015

Discovery of Smoothly Evolving Blackbodies in the Early Afterglow of GRB 090618 : Evidence for a Spine-Sheath Jet?

GRB 090618 is a bright GRB with multiple pulses. It shows evidence of thermal emission in the initial pulses as well as in the early afterglow phase. As high resolution spectral data of \emph{Swift}/XRT is available for the early afterglow, we investigate the shape and evolution of the thermal component in this phase using data from the \emph{Swift}/BAT, the \emph{Swift}/XRT, and the \emph{Fermi}/GBM detectors. An independent fit to the BAT and XRT data reveals two correlated blackbodies with monotonically decreasing temperatures. Hence we investigated the combined data with a model consisting of two blackbodies and a power-law (2BBPL), a model suggested for several bright GRBs. We elicit the following interesting features of the 2BBPL model: a) the same model is applicable from the peak of the last pulse in the prompt emission to the afterglow emission, b) the ratio of temperatures and the fluxes of the two black bodies remains constant throughout the observations, c) the black body temperatures and fluxes show a monotonic decrease with time, with the BB fluxes dropping about a factor of two faster than that of the power-law emission, d) attributing the blackbody emission to photospheric emissions, we find that the photospheric radii increase very slowly with time, and the lower temperature blackbody shows a larger emitting radius than that of the higher temperature black body. We find some evidence that the underlying shape of the non-thermal emission is a cut-off power-law rather than a power-law. We sketch a spine-sheath jet model to explain our observations.

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arXiv2015

Study of underlying particle spectrum during huge X-ray flare of Mkn 421 in April 2013

Context: In April 2013, the nearby (z=0.031) TeV blazar, Mkn 421, showed one of the largest flares in X-rays since the past decade. Aim: To study all multiwavelength data available during MJD 56392 to 56403, with special emphasis on X-ray data, and understand the underlying particle energy distribution. Methods: We study the correlations between the UV and gamma bands with the X-ray band using the z-transformed discrete correlation function. We model the underlying particle spectrum with a single population of electrons emitting synchrotron radiation, and do a statistical fitting of the simultaneous, time-resolved data from the Swift-XRT and the NuSTAR. Results: There was rapid flux variability in the X-ray band, with a minimum doubling timescale of $1.69 \pm 0.13$ hrs. There were no corresponding flares in UV and gamma bands. The variability in UV and gamma rays are relatively modest with $ \sim 8 \% $ and $\sim 16 \% $ respectively, and no significant correlation was found with the X-ray light curve. The observed X-ray spectrum shows clear curvature which can be fit by a log parabolic spectral form. This is best explained to originate from a log parabolic electron spectrum. However, a broken power law or a power law with an exponentially falling electron distribution cannot be ruled out either. Moreover, the excellent broadband spectrum from $0.3-79$ keV allows us to make predictions of the UV flux. We find that this prediction is compatible with the observed flux during the low state in X-rays. However, during the X-ray flares, the predicted flux is a factor of $2-50$ smaller than the observed one. This suggests that the X-ray flares are plausibly caused by a separate population which does not contribute significantly to the radiation at lower energies. Alternatively, the underlying particle spectrum can be much more complex than the ones explored in this work.

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arXiv2015

A Review of Annual Review of Astronomy and Astrophysics, Volume 52, 2014

A review of the Annual Review of Astronomy and Astrophysics Volume 52, 2014 (Ed. S.M. Faber, Ewine van Dishoeck, and John Kormendy) is given, with a perspective of understanding the current trends in Astronomy and Astrophysics. The impact of high volume data, high connectivity, and fast computations is clearly seen in the various research areas discussed in this volume. This has provided unprecedented development in the understanding of various astrophysical phenomena. At the same time, some negative trends like commodification of science, ignoring dissenting views are also evident.

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arXiv2015

Thermal Emissions Spanning the Prompt and the Afterglow Phase of the Ultra-long GRB 130925A

GRB 130925A is an ultra-long GRB, and it shows clear evidences for a thermal emission in the soft X-ray data of \emph{Swift}/XRT ($\sim0.5$\,keV), lasting till the X-ray afterglow phase. Due to the long duration of the GRB, the burst could be studied in hard X-rays with high-resolution focusing detectors (\emph{NuSTAR}). The blackbody temperature, as measured by the \emph{Swift}/XRT, shows a decreasing trend till the late phase (Piro et al. 2014) whereas the high-energy data reveals a significant blackbody component during the late epochs at an order of magnitude higher temperature ($\sim5$\,keV), as compared to the contemporaneous low energy data (Bellm et al. 2014). We resolve this apparent contradiction by demonstrating that a model with two black bodies and a power-law (2BBPL) is consistent with the data right from the late prompt emission to the afterglow phase. Both the blackbodies show a similar cooling behaviour upto the late time. We invoke a structured jet, having a fast spine and a slower sheath layer, to identify the location of these blackbodies. Independent of the physical interpretation, we propose that the 2BBPL model is a generic feature of the prompt emission of all long GRBs, and the thermal emission found in the afterglow phase of different GRBs reflects the lingering thermal component of the prompt emission with diverse time-scales. We strengthen this proposal by pointing out a close similarity between the spectral evolutions of this GRB and GRB~090618, a source with significant wide band data during the early afterglow phase.

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arXiv2014

Time-resolved Spectral Study Of Fermi GRBs Having Single Pulses

We analyze gamma-ray bursts (GRBs) detected by \textit{Fermi}/Gamma-Ray Burst Monitor (GBM) and having single pulse. We fit the light curves with a model having exponential rise and decay parts. We perform a detailed time-resolved spectroscopy using four models: Band, blackbody with a power-law (BBPL), multicolour blackbody with a power-law (mBBPL) and two blackbodies with a power-law (2BBPL). We find that models other than the BBPL give better $χ_{red}^2$ for the "hard-to-soft" (HTS) pulses, while for the "intensity tracking" (IT) pulses, the BBPL model is statistically as good as the other models. Interestingly, the energy at the peak of the spectrum resulting from the BBPL model ($\sim3kT$), is always lower than that of the $νF_ν$ spectrum of the Band function. The values of the low energy photon index ($α$) of the Band function are often higher than the fundamental single particle synchrotron limit, especially for the HTS pulses. Specifically we find two extreme cases --- for GRB~110817A (HTS GRB) $α$ is always higher, while for GRB~100528A (IT GRB) $α$ is always within the synchrotron regime. The PL component of the BBPL model always starts with a delay compared to the BB component, and it lingers at the later part of the prompt emission phase. For three HTS GRBs, namely, GRB~081224, GRB~100707A and GRB~110721A this behaviour is particularly significant and interestingly there are reported LAT detections for them. Finally, we argue that various evidences hint that neither BBPL nor Band model is acceptable, while 2BBPL and mBBPL are the most acceptable models for the set of GRBs we have analyzed.

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arXiv2014

GRB as luminosity indicator

Gamma Ray Bursts (GRBs) are found at much higher redshifts (z>6) than Supernova Ia (z~1), and hence, they can be used to probe very primitive universe. However, radiation mechanism of GRB remains a puzzle, unlike Supernova Ia. Through comprehensive description, both empirical and physical, we shall discuss the most likely way to use the constituent pulses of a GRB to find the radiation mechanism as well as using the pulses as luminosity indicators.

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arXiv2013

Nature of GRBs observed by RT-2 on-board CORONAS-PHOTON satellite

The RT-2 Experiment, a low energy gamma-ray telescope, onboard CORONAS-PHOTON satellite is designed to study the temporal, spectral and spatial properties of the hard X-ray solar flares mainly in the energy range of 15 - 100 keV, which is extendable upto 1000 keV. During the operational period of ~ 9 months, it has been able to detect a few solar flares and at least four Gamma-Ray Bursts (GRBs). In this paper, we discuss the properties of the GRBs as observed by RT-2 in the energy band of 15 to ~ 1000 keV. We will present the results of spectral and timing properties of the GRBs (specially for the GRB 090618) using RT-2 detectors. Temporal analysis suggests that all four GRBs belong to the category of long duration bursts.

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