arXiv subjects

A. R. Rao

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

At least 73 records · Page 4Linked to original sources

arXiv2013

Observing Galactic Black Hole Sources in Hard X-rays

Observations of Galactic black hole sources are traditionally done in the classical X-ray range (2 -- 10 keV) due to sensitivity constraints. Most of the accretion power, however, is radiated above 10 keV and the study of these sources in hard X-rays has the potential to unravel the radiation mechanisms operating at the inner region of the accretion disk, which is believed to be the seat of a myriad of fascinating features like jet emission, high frequency QPO emission etc. I will briefly summarise the long term hard X-ray observational features like spectral state identification, state transitions and hints of polarised emission, and describe the new insights that would be provided by the forthcoming Astrosat satellite, particularly emphasising the contributions expected from the CZT-Imager payload.

astro-ph.HE↗

arXiv2013

Pulse-wise Amati correlation in Fermi GRBs

We make a detailed pulse-wise study of gamma-ray bursts (GRBs) with known redshift detected by \emph{Fermi}/Gamma Ray Burst Monitor (GBM). The sample contains 19 GRBs with 43 pulses. We find that the average peak energy is correlated to the radiated energy (the Amati relation) for individual pulses with a correlation coefficient of 0.86, which is slightly better than the correlation for the full GRBs. As the present correlation holds within GRBs, it is a strong evidence supporting the reliability of such a correlation. We investigate several aspects of this correlation. (i) We divide our sample into redshift bins and study the evolution of the correlation. Though there is a marginal indication of evolution of the correlation, we can conclude that the present data is consistent with no evolution. (ii) We compare the correlation in the first or single pulses of these GRBs to that of the rest of the pulses, and confirm that the correlation is unaffected by the fact that first/single pulses are generally harder than the rest. Finally, we conclude that the pulse-wise Amati correlation is more robust and it has the potential of refining the correlation so that GRB study could be used as a cosmological tool.

astro-ph.HE↗

arXiv2013

A lingering non-thermal component in the GRB prompt emission: predicting GeV emission from the MeV spectrum

The high energy GeV emission of gamma-ray bursts (GRBs), detected by \emph{Fermi}/LAT, has a significantly different morphology compared to the lower energy MeV emission, detected by \emph{Fermi}/GBM. Though the late time GeV emission is believed to be synchrotron radiation produced via an external shock, this emission as early as the prompt phase is puzzling. Meaningful connection between these two emissions can be drawn only by an accurate description of the prompt MeV spectrum. We perform a time-resolved spectroscopy of the GBM data of long GRBs having significant GeV emission, using a model consisting of 2 blackbodies and a power-law. We examine in detail the evolution of the spectral components and found that GRBs having high GeV emission (GRB 090902B and GRB 090926A) have a delayed onset of the power-law component, in the GBM spectrum, which lingers at the later part of the prompt emission. This behaviour mimics the flux evolution in LAT. In contrast, bright GBM GRBs with an order of magnitude lower GeV emission (GRB 100724B and GRB 091003) show a coupled variability of the total and the power-law flux. Further, by analyzing the data for a set of 17 GRBs, we find a strong correlation between the power-law fluence in the MeV and the LAT fluence (Pearson correlation: r=0.88 and Spearman correlation: $ρ=0.81$). We demonstrate that this correlation is not influenced by the correlation between the total and the power-law fluences at a confidence level of 2.3$σ$. We speculate the possible radiation mechanisms responsible for the correlation.

astro-ph.HE↗

arXiv2013

Time resolved spectral analysis of the prompt emission of long gamma ray bursts with GeV Emission

We make a detailed time resolved spectroscopy of bright long gamma ray bursts (GRBs) which show significant GeV emissions (GRB 080916C, GRB 090902B, and GRB 090926A). In addition to the standard Band model, we also use a model consisting of a blackbody and a power-law to fit the spectra. We find that for the latter model there are indications for an additional soft component in the spectra. While previous studies have shown that such models are required for GRB 090902B, here we find that a composite spectral model consisting of two black bodies and a power law adequately fit the data of all the three bright GRBs. We investigate the evolution of the spectral parameters and find several generic interesting features for all three GRBs, like a) temperatures of the black bodies are strongly correlated to each other, b) flux in the black body components are strongly correlated to each other, c) the temperatures of the black body trace the profile of the individual pulses of the GRBs, and d) the characteristics of the power law component like the spectral index and the delayed onset bear a close similarity to the emission characteristics in the GeV regions. We discuss the implications of these results to the possibility of identifying the radiation mechanisms during the prompt emission of GRBs.

astro-ph.HE↗

arXiv2013

The uncorrelated long term gamma-ray and X-ray variability of blazars and its implications on disk-jet coupling

We examine the long term (~10 years) gamma-ray variability of blazars observed by EGRET and Fermi and find that for six sources the average flux varied by more than an order of magnitude. For two of these sources (PKS 0208-512 and PKS 0528+134), there were extensive observations (at various observing periods) by EGRET. Hence these dramatic variations are not due to a single short time-scale flare, but reflect long term changes in the average flux. Over the last twenty years, these two sources were also the target of several X-ray observatories (e.g. ROSAT, ASCA, RXTE, BeppoSAX, Chandra, Suzaku, XMM-Newton and Swift). While the ratios of the average gamma-ray fluxes between EGRET and Fermi observations are 22.9 +/- 1.9 and 12.6 +/- 1.5, their estimated 2-10 keV X-ray flux do not show such dramatic variations. The X-ray emission from such flat spectrum radio quasars (FSRQs) are believed to be due to synchrotron self Compton, while gamma-rays originate from inverse Comptonization of external soft photons from an accretion disk and/or broad line region. We argue that in this scenario, the only explanation for the uncorrelated variability is that there was an order of magnitude decrease in the external soft photons, while the jet parameters remained more or less constant. This result indicates that perhaps the accretion and jet processes are not tightly coupled in these sources.

astro-ph.CO↗

arXiv2013

A possible evolutionary scenario of highly magnetized super-Chandrasekhar white dwarfs: progenitors of peculiar type Ia supernovae

Several recently discovered peculiar type Ia supernovae seem to demand an altogether new formation theory that might help explain the puzzling dissimilarities between them and the standard type Ia supernovae. The most striking aspect of the observational analysis is the necessity of invoking super-Chandrasekhar white dwarfs having masses ~ 2.1-2.8M_sun, M_sun being the mass of Sun, as their most probable progenitors. Strongly magnetized white dwarfs having super-Chandrasekhar masses were already established to be potential candidates for the progenitors of peculiar type Ia supernovae. Owing to the Landau quantization of the underlying electron degenerate gas, theoretical results yielded the observationally inferred mass range. Here we sketch a possible evolutionary scenario by which super-Chandrasekhar white dwarfs could be formed by accretion on to a commonly observed magnetized white dwarf, invoking the phenomenon of flux freezing. This opens the multiple possible evolutions ending in supernova explosions of super-Chandrasekhar white dwarfs having masses within the range stated above. We point out that our proposal has observational support, like, the recent discovery of a large number of magnetized white dwarfs by SDSS.

astro-ph.HE↗

arXiv2013

A new method of pulse-wise spectral analysis of Gamma-ray Bursts

Time-resolved spectral analysis, though a very promising method to understand the emission mechanism of gamma-ray bursts (GRBs), is difficult to implement in practice because of poor statistics. We present a new method for pulse-wise time-resolved spectral study of the individual pulses of GRBs, using the fact that many spectral parameters are either constants or smooth functions of time. We use this method for the two pulses of GRB 081221, the brightest GRB with separable pulses. We choose, from the literature, a set of possible models which includes the Band model, blackbody with a power-law (BBPL), a collection of black bodies with a smoothly varying temperature profile, along with a power-law (mBBPL), and two blackbodies with a power-law (2BBPL). First, we perform time-resolved study to confirm the spectral parameter variations, and then construct the new model to perform a joint spectral fit. We find that any photospheric emission in terms of black bodies is required mainly in the rising parts of the pulses and the falling part can be adequately explained in terms of the Band model, with the low energy photon index within the regime of synchrotron model. Interestingly, we find that 2BBPL is comparable or sometimes even better, though marginally, than the Band model, in all episodes. Consistent results are also obtained for the brightest GRB of Fermi era --- GRB 090618. We point out that the method is generic enough to test any spectral model with well defined parameter variations.

astro-ph.HE↗

arXiv2012

Pulse spectral evolution of GRBs: implication as standard candle

Using an \emph{empirical} description of a prompt GRB pulse, we analyze the individual pulses of all Fermi/GBM GRBs with known redshifts, till July 2009. This description is simultaneous in time and energy and allows one to determine the peak energy of Band spectrum at zero fluence ($E_{peak,0}$). We demonstrate, for the first time, that the $E_{peak,0}$ bears a very strong correlation with the isotropic energy of the individual pulses, and hence, each pulse can be used as a luminosity indicator. As a physical description is needed in order to use GRB pulses for cosmological purposes, we explore other physical spectral models. As pulses are the building blocks of a GRB, we choose another sample of Fermi/GBM GRBs having bright, long and single/ separable pulse(s) and fit the time-resolved spectra of the individual pulses with the Band model and a model consisting of a blackbody and a power-law. Both these models give acceptable fits. We find that the peak energy/ temperature always decreases exponentially with fluence in the later part of a pulse. We investigate multiple spectral components in the initial rising part and provide a comprehensive empirical description of the spectral and timing behaviour of prompt GRB pulses. This work strongly extends the possibility of using GRB pulses as standard candles and the spectral parameters as proxy for redshift.

astro-ph.HE↗

arXiv2012

Anti-correlated time lags in the Z source GX 5-1: Possible evidence for a truncated accretion disk

We investigate the nature of the inner accretion disk in the neutron star source GX 5-1 by making a detailed study of time lags between X-rays of different energies. Using the cross-correlation analysis, we found anti-correlated hard and soft time lags of the order of a few tens to a few hundred seconds and the corresponding intensity states were mostly the horizontal branch (HB) and upper normal branch (NB). The model independent and dependent spectral analysis showed that during these time lags the structure of accretion disk significantly varied. Both eastern and western approaches were used to unfold the X-ray continuum and systematic changes were observed in soft and hard spectral components. These changes along with a systematic shift in the frequency of quasi-periodic oscillations (QPOs) made it substantially evident that the geometry of the accretion disk is truncated. Simultaneous energy spectral and power density spectral study shows that the production of the horizontal branch oscillations (HBOs) are closely related to the Comptonizing region rather than the disk component in the accretion disk. We found that as the HBO frequency decreases from the hard apex to upper HB, the disk temperature increases along with an increase in the coronal temperature which is in sharp contrast with the changes found in black hole binaries where the decrease in QPO frequency is accompanied by a decrease in the disk temperature and a simultaneous increase in the coronal temperature. We discuss the results in the context of re-condensation of coronal material in the inner region of the disk.

astro-ph.HE↗

arXiv2012

Fast transition of type-B QPO in the black hole transient XTE J1817-330

The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is associated with spectral variations. Such studies in other black hole binaries are important to understand the QPO phenomenon. For the black hole transient XTE J1817-330, we study fast QPO transitions and accompanying spectral variations to investigate what causes the spectral variation during the QPO transition. Roy et al. (2011) found QPOs in ten RXTE observations of XTE J1817-330. We found that, among the ten observations, only one observation shows erratic dips in its X-ray light curve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of the light curve. We found that type-B $\sim$6 Hz QPO changes into type-A QPO in a few tens of seconds along with a flux decrease. This transient evolution is accompanied with a significant spectral variation. We report a transient QPO feature and accompanying spectral variation in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral variations.

astro-ph.HE↗

arXiv2012

Correlation between the isotropic energy and the peak energy at zero fluence for the individual pulses of GRBs: towards an universal physical correlation for the prompt emission

We find a strong correlation between the peak energy at zero fluence ($\rm E_{peak,0}$) and the isotropic energy ($\rm E_{γ,iso}$) of the 22 pulses of 9 Gamma Ray Bursts (GRB) detected by the Fermi satellite. The correlation holds for the individual pulses of each GRB, which shows the reality of the correlation. The derived correlation (Spearman correlation coefficient, $r$, is 0.96) is much stronger compared to the correlations using $\rm E_{peak}$ (in place of $\rm E_{peak,0}$) determined from the time-integrated spectrum ($r$ = 0.8), or the time-resolved spectrum not accounting for broad pulse structures ($r$ = 0.37), or the pulse-wise spectrum ($r$ = 0.89). Though the improvement in the $\rm E_{peak}$ - $\rm E_{γ,iso}$ relation (the Amati relation) for a pulse-wise analysis is known earlier, this is the first time a parameter derived from a joint spectral and timing fit to the data is shown to improve the correlation. We suggest that $\rm E_{peak,0}$, rather than $\rm E_{peak}$, is intrinsic to a GRB pulse and a natural choice as the parameter in the pulse-wise correlation studies.

astro-ph.HE↗

arXiv2011

Coupled HBO and NBO variations in the Z source GX 5-1: inner accretion disk as the location of QPOs

The simultaneous and coupled evolution of horizontal branch oscillation (HBO) and normal branch oscillation (NBO) in Z-type sources suggests that the production of HBO is connected to NBO and is caused by changes in the physical/radiative properties of the inner accretion disk, although there is a lack of substantial spectral evidence to support this. In this {\it Letter}, we present the results of an analysis of a RXTE observation of a Z source GX~5-1, where the 6 Hz NBO is simultaneously detected along with a HBO at 51 Hz. The variations in the intensity and the associated power density spectrum indicate that the HBO and NBO are strongly coupled, originating from the same location in the inner accretion disk. The absence of HBO and NBO in the lower energy bands, an increase in the rms amplitude with energy and a smooth transition among them suggest that they are produced in the hot inner regions of the accretion disk. Based on a spectral analysis, we found a signature of changing or physically modified inner disk front during the coupled HBO and NBO evolution. We explore the various models to explain the observed phenomenon and propose that the NBO is affiliated to the oscillations in the thick/puffed-up inner region of the accretion disk.

astro-ph.HE↗

arXiv2011

Measuring the pulse of GRB 090618: A Simultaneous Spectral and Timing Analysis of the Prompt Emission

We develop a new method for simultaneous timing and spectral studies of Gamma Ray Burst (GRB) prompt emission and apply it to make a pulse-wise description of the prompt emission of GRB 090618, the brightest GRB detected in the Fermi era. We exploit the large area (and sensitivity) of Swift/BAT and the wide band width of Fermi/GBM to derive the parameters for a complete spectral and timing description of the individual pulses of this GRB, based on the various empirical relations suggested in the literature. We demonstrate that this empirical model correctly describes the other observed properties of the burst like the variation of the lag with energy and the pulse width with energy. The measurements also show an indication of an increase in pulse width as a function of energy at low energies for some of the pulses, which is naturally explained as an off-shoot of some particular combination of the model parameters. We argue that these model parameters, particularly the peak energy at the beginning of the pulse, are the natural choices to be used for correlation with luminosity. The implications of these results for the use of GRBs as standard candles are briefly described.

astro-ph.HE↗

arXiv2010

Instruments of RT-2 Experiment onboard CORONAS-PHOTON and their test and evaluation III: Coded Aperture Mask and Fresnel Zone Plates in RT-2/CZT Payload

Imaging in hard X-rays of any astrophysical source with high angular resolution is a challenging job. Shadow-casting technique is one of the most viable options for imaging in hard X-rays. We have used two different types of shadow-casters, namely, Coded Aperture Mask (CAM) and Fresnel Zone Plate (FZP) pair and two types of pixellated solid-state detectors, namely, CZT and CMOS in RT-2/CZT payload, the hard X-ray imaging instrument onboard the CORONAS-PHOTON satellite. In this paper, we present the results of simulations with different combinations of coders (CAM & FZP) and detectors that are employed in the RT-2/CZT payload. We discuss the possibility of detecting transient Solar flares with good angular resolution for various combinations. Simulated results are compared with laboratory experiments to verify the consistency of the designed configuration.

astro-ph.IM↗

arXiv2010

Instruments of RT-2 Experiment onboard CORONAS-PHOTON and their test and evaluation V: Onboard software, Data Structure, Telemetry and Telecommand

The onboard software and data communication in the RT-2 Experiment onboard the Coronas-Photon satellite is organized in a hierarchical way to effectively handle and communicate asynchronous data generated by the X-ray detectors. A flexible data handling system is organized in the X-ray detector packages themselves and the processing electronic device, namely RT-2/E, has the necessary intelligence to communicate with the 3 scientific payloads by issuing commands and receiving data. It has direct interfacing with the Satellite systems and issues commands to the detectors and processes the detector data before sending to the satellite systems. The onboard software is configured with several novel features like a) device independent communication scheme, b) loss-less data compression and c) Digital Signal Processor. Functionality of the onboard software along with the data structure, command structure, complex processing scheme etc. are discussed in this paper.

astro-ph.IM↗

arXiv2010

Instruments of RT-2 Experiment onboard CORONAS-PHOTON and their test and evaluation II: RT-2/CZT payload

Cadmium Zinc Telluride (CZT) detectors are high sensitivity and high resolution devices for hard X-ray imaging and spectroscopic studies. The new series of CZT detector modules (OMS40G256) manufactured by Orbotech Medical Solutions (OMS), Israel, are used in the RT-2/CZT payload onboard the CORONAS-PHOTON satellite. The CZT detectors, sensitive in the energy range of 20 keV to 150 keV, are used to image solar flares in hard X-rays. Since these modules are essentially manufactured for commercial applications, we have carried out a series of comprehensive tests on these modules so that they can be confidently used in space-borne systems. These tests lead us to select the best three pieces of the 'Gold' modules for the RT-2/CZT payload. This paper presents the characterization of CZT modules and the criteria followed for selecting the ones for the RT-2/CZT payload. The RT-2/CZT payload carries, along with three CZT modules, a high spatial resolution CMOS detector for high resolution imaging of transient X-ray events. Therefore, we discuss the characterization of the CMOS detector as well.

astro-ph.IM↗

arXiv2010

Instruments of RT-2 Experiment onboard CORONAS-PHOTON and their test and evaluation IV: Background Simulations using GEANT-4 Toolkit

Hard X-ray detectors in space are prone to background signals due to the ubiquitous cosmic rays and cosmic diffuse background radiation that continuously bombards the satellites which carry the detectors. In general, the background intensity depends on the space environment as well as the material surrounding the detectors. Understanding the behavior of the background noise in the detector is very important to extract the precise source information from the detector data. In this paper, we carry out Monte Carlo simulations using the GEANT-4 toolkit to estimate the prompt background noise measured with the detectors of the RT-2 Experiment onboard the CORONAS-PHOTON satellite.

astro-ph.IM↗

arXiv2010

Detection of GRB 090618 with RT-2 Experiment Onboard the Coronas-Photon Satellite

We present the results of an analysis of the prompt gamma-ray emission from GRB 090618 using the RT-2 Experiment onboard the Coronas-Photon satellite. GRB 090618 shows multiple peaks and a detailed study of the temporal structure as a function of energy is carried out. As the GRB was incident at an angle of 77 degree to the detector axis, we have generated appropriate response functions of the detectors to derive the spectrum of this GRB. We have augmented these results using the publicly available data from the Swift BAT detector and show that a combined spectral analysis can measure the spectral parameters quite accurately. We also attempt a spectral and timing analysis of individual peaks and find evidence for a systematic change in the pulse emission characteristics for the successive pulses. In particular, we find that the peak energy of the spectrum, E_p, is found to monotonically decrease with time, for the successive pulses of this GRB.

astro-ph.HE↗