J. S. Yadav

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. To investigate the properties of the source during a superluminal flare, we present Giant Metrewave Radio Telescope observations during a radio flare that turned out to be a preplateau flare, as shown by the contemporaneous Ryle Telescope observations. A major superluminal ejection was observed at the end of this plateau state (as described by V. Dhawan et al.), associated with highly variable X-ray emission showing X-ray soft dips. This episode thus has all three types of radio emission: a preplateau flare, a plateau state, and superluminal jets. We analyze all the available Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array data during this episode and show that (1) the preflare plateau state consists of a three-component X-ray spectrum that includes a multicolor disk-blackbody, a Comptonized component, and a power law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source, identify several candidate superluminal flare events, 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 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 the types of radio emission observed from this source in terms of its X-ray-emission characteristics.

Different Types of X-Ray Bursts from GRS 1915+105 and Their Origin

We report X-ray observations of the Galactic X-ray transient source GRS 1915+105 with the pointed proportional counters of the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3, which show remarkable richness in temporal variability. The observations were carried out on 1997 June 12-29 and August 7-10, in the energy range of 2-18 keV and revealed the presence of very intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and broadly can be put in two classes: irregular and quasi-regular bursts in one class, and regular bursts in the other. The regular bursts are found to have two distinct timescales and to 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 present a comprehensive picture of the various types of bursts observed in GRS 1915+105 in the light of the recent theories of advective accretion disks. We suggest that the peculiar bursts that we have seen are characteristic 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 timescale, giving rise to the irregular and quasi-regular bursts. The fast timescale for the transition of the state is explained by invoking the appearance and disappearance of the advective disk in its viscous timescale. The periodicity of the regular bursts is explained by matching the viscous timescale with the cooling timescale of the postshock region. 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. It is found that the 13-60 keV flux relative to the 2-13 keV flux shows clear evidence for state change between the quiescent phase and the burst phase. The value of this ratio during burst is consistent with the values observed during the high-soft state seen on 1997 August 19, while its value during quiescent phase is consistent with the values observed during the low-hard state seen on 1997 May 8.

Quasi-regular X-Ray Bursts from GRS 1915+105 Observed with the IXAE: Possible Evidence for Matter Disappearing into the Event Horizon of the Black Hole

Three different types of very intense, quasi-regular X-ray bursts have been observed from the Galactic superluminal X-ray transient source GRS 1915+105 with the pointed proportional counters of the Indian X-Ray Astronomy Experiment on board the Indian satellite IRS-P3. The observations were carried out from 1997 June 12 to 29 in the energy range of 2-18 keV and revealed the presence of persistent quasi-regular bursts with different structures. Only one of the three types of bursts is regular in occurrence, revealing a stable profile over extended durations. The regular bursts have an exponential rise with a timescale of about 7-10 s and a sharp linear decay in 2-3 s. The X-ray spectrum becomes progressively harder as the burst evolves, and it is the hardest near the end of the burst decay. The profile and energetics of the bursts in this black hole candidate source are distinct from both the type I and type II X-ray bursts observed in neutron star sources. We propose that the sharp decay in the observed burst pattern is a signature of the disappearance of matter through the black hole horizon. The regular pattern of the bursts can be produced by material influx into the inner disk that is due to oscillations in a shock front far away from the compact object.

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 3 months, and during this transition it was exhibiting a series of fast variations, which can be classified as bursts. During one type of burst, called the irregular burst, 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 quasi-periodic oscillation at ~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 and coworkers that during the irregular bursts the source shows spectral state variations at very fast timescales. 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 coexisting with an advection-dominated hot thick disk.

PULSE CHARACTERISTICS OF THE X-RAY PULSAR 4U 1907)09

The X-ray pulsar 4U 1907+09 was observed in the 2-18 keV energy band with the Indian X-Ray Astronomy Experiment (IXAE) during 1996 August and again in 1998 June. From the timing measurements, the spin-down rate of the neutron star is measured to be +0.23 ± 0.01 s yr-1. A straight-line fit to the historical pulse period data indicates that the pulsar has been in a monotonic phase of spin-down since its discovery. The day-to-day pulse profile indicates some variations during its 8.4 day binary period. The average profile obtained from these observations shows a double-pulse shape with a pronounced asymmetric primary separated by a dip from a relatively weaker but broad secondary. These profiles show a closer resemblance to the pulse shape obtained with the recent observations with the Rossi X-Ray Timing Explorer (RXTE) during 1996 February, as compared to the earlier observations carried out six to 13 years earlier. A secondary flare of 88 mcrab intensity was detected during the IXAE observations in 1996 August. A detailed analysis of the flare data shows the presence of transient 14.4 s oscillations, which may be quasi-periodic during the flaring activity but have a period different from the earlier reported oscillations of 18.2 s as detected by RXTE during the flare of 1996 February. These results, therefore, strengthen the evidence for the presence of a transient accretion disk around the neutron star during the flare, which may be responsible for the continuous slowing down of the pulsar. The studies of such transient quasi-periodic oscillations during flaring activities of 4U 1907+09 thus provide opportunities for understanding the transient behavior of the accretion disk and its physical characteristics.

DISK-JET CONNECTION IN THE MICROQUASAR GRS 1915)105 AND INFRARED 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 RXT E/PCA data with a ii spike ˇˇ in X-ray light curves. We —nd that the radio emission increases as the hardness ratio increases during the low hard state. We suggest that the ii spike,ˇˇ 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 (D10). This produces a major ejection episode of the synchrotron-emitting plasma termed as ii baby jets,ˇˇ which are associated with infrared (IR) and radio —ares of about half an hour period widely reported in the literature. Subsequent short but frequent soft dips produce overlapping faint —ares which result in an enhanced level of quasi-steady emission. We discuss the diUerences between ii baby jets ˇˇ and relativistic radio jets and especially investigate their signatures in X-rays. . . . . . .

AstroSat/LAXPC reveals the high energy variability of GRS 1915+105 in the chi class

We present the first quick look analysis of data from nine {\it AstroSat}s LAXPC observations of GRS 1915+105 during March 2016 when the source had the characteristics of being in Radio-quiet

Connection between the Accretion Disk and Superluminal Radio Jets and the Role of the Radio Plateau State in GRS 1915+105

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AstroSat/LAXPC observation of Cygnus X-1 in the hard state

class. We find that a simple empirical model of a disk blackbody emission, with Comptonization and a broad Gaussian Iron line can fit the time averaged 3--80 keV spectrum with a systematic uncertainty of 1.5\% and a background flux uncertainty of 4\%. A simple deadtime-corrected Poisson noise level spectrum matches well with the observed high frequency power spectra till 50 kHz and as expected the data show no significant high frequency (

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

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