arXiv subjects

J. S. Yadav

Publications and source records attributed to J. S. Yadav.

36 records · Page 2Linked to original sources

arXiv2017

Calibration of the Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat

We present the calibration and background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on-board AstroSat. LAXPC instrument has three nominally identical detectors to achieve large collecting area. These detectors are independent of each other and in the event analysis mode, they record the arrival time and energy of each photon that is detected. The detectors have a time-resolution of 10 $μ$s and a dead-time of about 42 $μ$s. This makes LAXPC ideal for timing studies. The energy resolution and peak channel to energy mapping were obtained from calibration on ground using radioactive sources coupled with GEANT4 simulations of the detectors. The response matrix was further refined from observations of the Crab X-ray source after launch. At around 20 keV the energy resolution of detector is about 10--15\%, while the combined effective area of the 3 detectors is about 6000 cm$^2$.

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arXiv2017

Large Area X-ray Proportional Counter (LAXPC) Instrument on AstroSat and Some Preliminary Results from its performance in the orbit

Large Area X-ray Propositional Counter (LAXPC) instrument on AstroSat is aimed at providing high time resolution X-ray observations in 3 to 80 keV energy band with moderate energy resolution. To achieve large collecting area, a cluster of three co-aligned identical LAXPC detectors, is used to realize an effective area in access of about 6000 cm2 at 15 keV. The large detection volume of the LAXPC detectors, filled with xenon gas at about 2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. In this article, we present salient features of the LAXPC detectors, their testing and characterization in the laboratory prior to launch and calibration in the orbit. Some preliminary results on timing and spectral characteristics of a few X-ray binaries and other type of sources, are briefly discussed to demonstrate that the LAXPC instrument is performing as planned in the orbit.

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arXiv2017

Large Area X-ray Proportional Counter (LAXPC) Instrument on AstroSat

Large Area X-ray Proportional Counter (LAXPC) is one of the major AstroSat payloads. LAXPC instrument will provide high time resolution X-ray observations in 3 to 80 keV energy band with moderate energy resolution. A cluster of three co-aligned identical LAXPC detectors is used in AstroSat to provide large collection area of more than 6000 cm2 . The large detection volume (15 cm depth) filled with xenon gas at about 2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. With its broad energy range and fine time resolution (10 microsecond), LAXPC instrument is well suited for timing and spectral studies of a wide variety of known and transient X-ray sources in the sky. We have done extensive calibration of all LAXPC detectors using radioactive sources as well as GEANT4 simulation of LAXPC detectors. We describe in brief some of the results obtained during the payload verification phase along with LXAPC capabilities.

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arXiv2014

X-ray spectral state evolution in IGR J17091-3624 and comparison of its heartbeat oscillation properties with those of GRS 1915+105

In this work, we study the X-ray timing and spectral evolution of the transient low mass X-ray binary IGR J17091-3624 during first 66 days of its 2011 outburst. We present results obtained from observations with two instruments, Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) and SWIFT/X-ray telescope (XRT), between 09 February, 2011 and 15 April, 2011. Using quasi-periodic oscillation classifications, power density spectrum characteristics, time-lag behavior and energy spectral properties, we determine source states and their transitions at different times of the outburst. During the first part of the evolution, the source followed trends usually observed from transient black hole X-ray binaries (BHXBs). Interestingly, a gradual transition is observed in IGR J17091-3624 from the low-variability SIMS, commonly seen in BHXBs, to a high-variability state with regular, repetitive and structured pulsations, seen only from GRS 1915+105 (also known as 'rho' class variability/'heartbeat' oscillations). We study the time evolution of characteristic time-scale, quality factor and rms amplitude of heartbeat oscillations in IGR J17091-3624. We also present a detailed comparison of the timing and spectral properties of heartbeat oscillations and their evolution in IGR J17091-3624 and GRS 1915+105.

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arXiv2013

Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR J17091-3624 as modulations of an accretion disc

Using the flux resolved spectroscopy for the first time, we analyse the RXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR J17091-3624, when both sources show large amplitude, quasi-regular oscillations in 2.0-60.0 keV X-ray light curves. For different observations, we extract spectra during the peak (spectrally soft) and dip (spectrally hard) intervals of the oscillation, and find that their spectra are phenomenologically complex, requiring at least two distinct spectral components. Besides a thermal Comptonization component, we find that the disc emission is better modelled by an index-free multicolour disc blackbody component (p-free disc model) rather than that from a standard accretion disc. While the peak and dip spectra are complex, remarkably, their difference spectra constructed by treating dip spectra as the background spectra of the peak spectra, can be modelled as a single p-free disc component. Moreover, the variability at different time-scales and energy bands of the peak flux level is always greater than or equal to the variability of the dip flux level, which strengthens the possibility that the peak flux level may be due to an independent spectral component added to the dip one. We also verify it using joint spectral analysis of peak and dip spectra with a variable emission component. Our result substantiates the standard hypothesis that the oscillations are due to the limit cycle behaviour of an unstable radiation pressure dominated inner disc. However, in this interpretation, the flux variation of the unstable disc can be several order of magnitudes as expected from some theoretical simulations and need not be fine tuned to match the factor ~10 variation seen between the peak and dip levels.

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arXiv2013

Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR J17091-3624 and their implications

We report a comprehensive study on spectral and timing properties of hard X-ray dips uniquely observed in some so-called variability classes of the micro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by a sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12 simultaneous with the increase in hardness ratio by a factor of 2-4. Using 31 observations of GRS 1915+105 with RXTE/PCA, we show that different behavior are observed in different types of variability classes, and we find that a dichotomy is observed between classes with abrupt transitions vs those with smoother evolution. For example, both energy-lag spectra and frequency-lag spectra of hard X-ray dips in classes with abrupt transitions and shorter dip intervals show hard-lag (hard photons lag soft photons), while both lag spectra during hard dips in classes with smoother evolution and longer dip intervals show soft-lag. Both lag time-scales are of the order of 100-600 msec. We also show that timing and spectral properties of hard X-ray dips observed in light curves of IGR J17091-3624 during its 2011 outburst are consistent with the properties of the abrupt transitions in GRS 1915+105 rather than smooth evolution. A global correlation between the X-ray intensity cycle time and hard dip time is observed for both abrupt and smooth transition which may be due to two distinct physical processes whose time-scales are eventually correlated. We discuss implications of our results in the light of some generic models.

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arXiv2012

Evidence of two unique variability classes from IGR J17091-3624

IGR J17091-3624 is the second black hole X-ray binary after GRS 1915+105, which showed large and distinct variabilities. The study of these variability classes can be useful to understand the accretion-ejection mechanisms of accreting black holes, and hence to probe the strong gravity regime. We report the discovery of two new variability classes (C1 and C2) from IGR J17091-3624 from the 2011 outburst Rossi X-ray Timing Explorer data. These unique classes will be useful to have complete details about the source, and to learn new aspects about variabilities. For examples, the C1 class shows that the intensity and period of oscillations, energy spectrum and power spectrum can clearly evolve in tens of seconds. Moreover, in such a small time scale, soft-lag becomes hard-lag. The C2 class shows that the variability and the nonvariability can occur at similar energy spectrum, and a soft state is not required for variability to happen.

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arXiv2006

Connection between accretion disk and superluminal radio jets and the role of radio plateau state in GRS 1915+105

We investigate the association between the accretion disk during radio plateau state and the following superluminal relativistic radio jets with peak intensity varies from 200 mJy to 1000 mJy observed over a period of five years and present the evidences of direct accretion disc-jet connection in microquasar GRS 1915+105. We have analysed RXTE PCA/HEXTE X-ray data and have found that the accretion rate, $\dot{m}_{accr}$, as inferred from the X-ray flux, is very high during the radio plateaux. We suggest that the accretion disk during the radio plateaux always associated with radiation-driven wind which is manifested in the form of enhanced absorption column density for X-ray and the depleted IR emission. We find that the wind density increases with the accretion disk luminosity during the radio plateaux. The wind density is similar to the density of the warm absorber proposed in extragalactic AGNs and Quasars. We suggest a simple model for the origin of superluminal relativistic jets. Finally, We discuss the implications of this work for galactic microquasars and the extragalactic AGNs and Quasars.

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arXiv2005

Near room temperature X-ray and Gamma ray spectroscopic detectors for future space experiments

New generation Cadmium Telluride (CZT & CdTe) solid state detectors can provide high quantum efficiency with reasonably good energy resolution and can operate at near room temperature; an unique advantage for space experiments. We present here results of our study of small diode detectors as well as large area pixel detectors. Our study is aimed at developing near room temperature hard X-ray spectroscopy detectors for ASTROSAT and other future Indian space science missions.We have studied a Si-PIN detector in the energy range 5 - 60 keV and CZT & CdTe Schottky diode detectors in the energy region 10 - 1330 keV. We have studied large area (64 cm$^2$) CZT pixel detectors with pixel size close to the small CZT detector. We have studied individual pixels as well as CZT detector as a whole (summed over all the 1024 pixels). The energy resolution behaviour of the large area CZT detector is similar to that of small diode detectors in the low energy region. The change in operating temperature from room temperature ~20 to ~70$ degree C drastically affects both the energy resolutionas well as the peak area due to a sharp rise in the thermal noise.These results suggest that the cooling of detector system will provide better energy resolution as well as detection efficiency.

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arXiv2003

On the origin of the various types of radio emission in GRS 1915+105

We investigate the association between the radio ``plateau'' states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. We identify several candidate superluminal flare events from available monitoring data on this source and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the ``plateau'' state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are observed due to the interaction of the matter blob ejected during the X-ray soft dips, with the steady jet already established during the ``plateau'' state. This picture can explain all types of radio emission observed from this source in terms of its X-ray emission characteristics.

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arXiv2002

GMRT radio observations of microquasar GRS 1915+105

We have observed microquasar GRS 1915+105 at 1.28 GHz for 8 days from June 18 to July 1, 2001 using Giant Metrewave Radio Telescope (GMRT). We have seen several isolated radio baby flares of varying intensity and duration. We have also observed broad composite flares with rise and decay times of few hours on June 28-29, 2001 few days prior to when source went to the ``plateau radio state'' on 3rd July, 2001. These broad radio flares consist of several overlapping baby radio flares. The source was in the low-hard X-ray state during this period. We compare these results with 15 GHz radio data from the Ryle telescope.

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arXiv2002

Evidence for Synchrotron Bubbles from GRS 1915+105

We present GMRT observations of the Galactic microquasar GRS 1915+105 at 1.28 GHz for 8 days from 2001 June 18 to July 1. We have seen several isolated radio flares of varying magnitudes (20 - 50 mJy) and durations (6 - 35 min) and we model them as due to adiabatically expanding synchrotron emitting clouds (synchrotron bubbles) ejected from the accretion disk. By applying this model, we provide a new method to estimate the electron power-law index p, hence the spectral index, from single frequency radio observations. This method does not require correction for the optical depth time delay effects which may be important in the case of optically thick radio emission. Using our estimated value of p and simultaneous multiwavelength data from literature, we have calculated the time of ejection of the synchrotron plasma and the time delays at different observed frequencies. Our estimates are in good agreement with the observed time delays.

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arXiv2001

X-ray flares/bursts from GRS 1915+105 and the two component accretion flow

We present results of our analysis of a set of RXTE/PCA observations of X-ray flares/bursts with burst cycle ranging from 30 to 1300 s from the Galactic X-ray transient source GRS 1915+105 during last four years. These flares/bursts can be classified into four types: 1) regular bursts with short burst phase of about 20 s (R1), 2) regular bursts with short quiescent phase of about 20 s (R2), 3) long regular bursts (R3) with burst cycle > 1000 s, and 4) irregular bursts (IR). For all the observed bursts, the duration of the quiescent phase is inversely proportional to the square of the QPO frequency (2-10 Hz).

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arXiv2001

X-Ray Bursts from the Galactic X-Ray Transient Source GRS 1915+105

We have analyzed publicly available RXTE/PCAs archival data of GRS 1915+105 during its burst/flaring state. The burst cycle ranges from 30 to 1300 s. These bursts are different from the type I and type II classical bursts seen in Low Mass X-ray Binaries (LMXBs) in terms of their temporal and spectral properties. We have classified these bursts on the basis of properties observed during the quiescent (low flux) phase. The 2 - 10 Hz QPOs are present during the quiescent phase and disappear during the burst phase of all types of these X-ray bursts. The duration of the quiescent phase can be explained assuming an outflow from the post-shock regions and the catastrophic Compton cooling.

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arXiv2001

Different Types of X-ray Variability Observed in Micro-Quasar GRS 1915+105 with the IXAE

The galactic micro-quasar source GRS 1915+105 has been observed with the Indian X-ray Astronomy Experiment (IXAE) several times over a period extending from 1996 to 2000. The source has been observed in different luminosity states and exhibits a rich variety of random and regular X-ray variability over a wide range of time scales. It also shows a unique variability characteristic namely the occurrence of quasi-regular X-ray bursts with period ranging from 20 s to about 100 s which have slow rise and fast decay. The X-ray spectrum of these bursts is hardest near the end of decay. In the low-hard state and bright-soft state with rapid variability, the source exhibits quasi-periodic oscillations (QPOs) with variable frequency which is luminosity dependent. Regular X-ray intensity dips have been detected coincident with the occurrence of a giant radio outburst. The source made a transition from a low-state to a high-state in 1999 in less than a day and from a soft-bright state to a hard-low state in 2000 in less than 100 minutes during the IXAE observations. Details of these and other variability characteristics of the source are presented in this paper.

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arXiv2000

Disk-Jet Connection in the Microquasar GRS 1915+105 and IR and Radio Emission

We present evidence of a direct accretion disk-jet connection in the Galactic microquasar GRS 1915+105 based on our analysis of RXTE/PCA data with a ``spike'' in X-ray light curves. We find that the radio emission increases as the hardness ratio increases during the low hard state. We suggest that the ``spike'' which separates the dips with hard and soft spectra marks the beginning of the burst phase when the luminosity of the soft X-rays (5-15 keV) increases by a large factor (~ 10). This produces a major ejection episode of the synchrotron - emitting plasma termed as ``baby jets'' which are associated with infrared (IR) and radio flares of about half an hour period widely reported in the literature. Subsequent short but frequent soft dips produce overlapping faint flares which result in an enhanced level of quasi-steady emission. We discuss the dissipation of accretion energy in the inner part of the disk in terms of a comptonising cloud and the escaping mass which produces synchrotron radiation. We discuss the differences between ``baby jets'' and relativistic radio jets and especially investigate their signatures in X-rays. Our results support the relativistic jet model with quasi-continuous mass ejection and strong variations in the jet speeds required for the formation of strong internal shocks to continuously accelerate the particles.

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arXiv2000

Rapid State Transitions in the Galactic Black Hole Candidate Source GRS 1915+105

We examine the X-ray spectral and temporal behavior of the Galactic black hole candidate source GRS 1915+105 during its spectral state transition observed in 1997 March - August. The source was making a slow transition from a low-hard state to a high-soft state in about three months and during this transition it was exhibiting a series of fast variations which can be classified as bursts. During one type of bursts, called the irregular bursts, it was found that the source makes rapid transitions between two intensity states. We have analyzed the RXTE PCA data obtained on 1997 June 18 (when the source was making rapid state transitions) and compared the results with two sets of data each pertaining to the low hard-state and high-soft states. We find that the power density spectrum (PDS) during the burst shows remarkable similarity to that seen during the high-soft state of the source. The PDS during the quiescence, on the other hand, is quite flat and is very similar to the PDS commonly seen during the canonical low-hard state of black hole X-ray binaries. A low frequency QPO at $\sim$3 Hz is observed during the quiescence, a property seen during the low-hard state of the source. The X-ray spectrum of the source during quiescence is similar to the spectrum seen during the low-hard state of the source and the spectra during the burst and high-soft state are very similar to each other. These observations corroborate the suggestion made by Yadav et al. (1999) that during the irregular bursts the source shows spectral state variations at very fast time scales. We explain such fast state changes using the two component accretion flow model in which the accretion disk consists of a geometrically thin (and optically thick) standard disk co-existing with an advection dominated hot thick disk.

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arXiv1999

Different types of X-ray bursts from GRS 1915+105 and their origin

We report the X-ray observations of the Galactic X-ray transient source GRS 1915+105 with the PPCs of the Indian X-ray Astronomy Experiment(IXAE) onboard the Indian satellite IRS-P3 during 1997 June - August, which have revealed the presence of four types of intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and they can broadly be put in two classes: irregular and quasi-regular bursts in one class, and regular bursts in another class. The regular bursts are found to have two distinct time scales and they persist over extended durations. There is a strong correlation between the preceding quiescent time and the burst duration for the quasi-regular and irregular bursts. No such correlation is found for the regular bursts. The ratio of average flux during the burst time to the average flux during the quiescent phase is high and variable for the quasi- regular and irregular bursts while it is low and constant for the regular bursts. We suggest that the peculiar bursts that we have seen are charact- eristic of the change of state of the source. The source can switch back and forth between the low-hard state and the high-soft state near critical accretion rates in a very short time scale. A test of the model is presented using the publicly available 13-60 keV RXTE/PCA data for irregular and regular bursts concurrent with our observations.

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