arXiv subjects

A. R. Rao

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

133 records · Page 8Linked to original sources

arXiv1997

Observations of Cygnus X-1 during the two spectral states with the Indian X-ray Astronomy Experiment (IXAE)

We present the time variability characteristics of Cygnus X-1 in its two spectral states. The observations were carried out using the Pointed Proportional Counters (PPC) on-board the Indian X-ray Astronomy Experiment (IXAE). The details of the instrument characteristics, the observation strategy, and the background modeling methods are described. In the soft state of Cyg X-1, we confirm the general trend of the Power Density Spectrum (PDS) obtained using the Proportional Counter Array (PCA) on-board the RXTE satellite. The hard state of the source just prior to the spectral transition was not observed by the PCA and we present the PDS obtained in this state. We find that the low frequency end of the PDS is flatter than that observed during the spectral transition. Additionally, we find that there is one more component in the low frequency end of the PDS, which is independent of the spectral state of the source. The time variability is also examined by taking the statistics of the occurrence of shots and it is found that the shot duration and shot energy follow an exponential distribution, with time constants significantly different in the two spectral states. In the soft state of the source a shot with a very large strength has been identified and it has exponential rise and decay phases with time constants of 0.4 s. We examine these results in the light of the current models for accretion onto black holes.

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arXiv1997

Broad band X-ray spectrum of Cygnus X-1

We present the hard X-ray (20-100 keV) observations of Cygnus X-1 obtained using a large area balloon-borne Xenon filled Multi-anode Proportional Counter (XMPC) telescope. The observations were carried out during the gamma_2 state of the source and we obtain a power law photon index of 1.62 +- 0.07. To constrain the spectral shape of the source, we have analyzed the archival EXOSAT ME argon and GSPC data in the low energies (2-20 keV band) as well as the archival OSSE data in the high energies (50-500 keV). The data in different energy bands are not obtained in simultaneous observations, but they pertain to the gamma_2 state of the source. We have attempted a combined fit to the wide band data using appropriate mutual detector calibrations. A combined fit to the EXOSAT and XMPC data (2-100 keV) shows that the observed spectrum requires a low energy absorption corresponding to the Galactic interstellar absorption, a low energy excess modeled as blackbody, a narrow emission line due to iron K_alpha and a continuum. The continuum can be either modeled as a power law with a reflection bump or a Comptonisation model with an additional bump which can be modeled as the partial covering with a heavy absorber. To resolve between these two models, we have attempted combined fit to the 2-500 keV data obtained from EXOSAT, XMPC and OSSE. We find that two component Comptonisation model adequately represents the data. We explore the possible emission region that is responsible for the observed spectrum.

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arXiv1997

X-ray variability of GRS 1915+105 during the low-hard state observed with the Indian X-ray astronomy experiment (IXAE)

The galactic superluminal transient X-ray source GRS 1915+105 was observed with the pointed proportional counters (PPCs) onboard the Indian satellite IRS-P3 during 1996 July 23-27. We report here details of the behavior of this source during the relatively quiet and low luminosity state. Large intensity variations by a factor of 2 to 3, generally seen in black-hole candidates, are observed at a time scale of 100 ms to few seconds. No significant variation is detected over larger time scale of minute or more. The intensity variations are described as sum of shots in the light curve, and the number distribution of the shots are found to be exponential function of the fluence and duration of the shots. The cross correlation spectrum between 6-18 keV and 2-6 keV X-rays is found to have asymmetry signifying a delay of the hard X-rays by about 0.2 to 0.4 sec. This supports the idea of hard X-rays being generated by Compton up-scattering from high energy clouds near the source of soft X-rays. Very strong and narrow quasi periodic oscillations in the frequency range 0.62 to 0.82 Hz are observed. We discuss about a model which explains a gradual change in the QPO frequencies with corresponding changes in the mass accretion rate of the disk.

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arXiv1997

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 onboard the Indian satellite IRS-P3. The observations were carried out from 1997 June 12 to June 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 time scale of about 7 s to 10 s and a sharp linear decay in 2 to 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 due to oscillations in a shock front far away from the compact object.

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arXiv1997

Torque-luminosity correlation and possible evidence for core-crust relaxation in the X-ray pulsar GX 1+4

We present the detection of a positive correlation between spin-down rate $\dot{P}$ and pulsed X-ray luminosity in the BATSE archival data of the bright hard X-ray pulsar GX 1+4. We have also seen a delay of 5.6 $\pm$ 1.2 days between the luminosity change and the corresponding change in the spin-down rate. The observed correlation between $\dot{P}$ and L_X is used to reproduce the period history of GX 1+4 based on the observed luminosity alone, and it is found that the spin period can be predicted correct to 0.026% when the luminosity is adequately sampled. The idea that at a higher luminosity more matter is accreted and the accretion disk extends closer to the neutron star thereby transferring more angular momentum to the system, seems not to be the case with GX 1+4. The observed lag between the spin-down rate and the luminosity is reported here for the first time in any such binary X-ray pulsar, and is found to be consistent with the time scale for the core-crust relaxation in a neutron star.

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arXiv1997

Rapid X-ray variability of the superluminal source GRS1915+105

The superluminal X-ray transient source GRS 1915+105 was observed during July 20-29, 1996 with the Indian X-ray Astronomy Experiment (IXAE) on the Indian satellite IRS-P3 launched on March 21, 1996 from Shriharikota Range in India. During our observations covering the energy band 2-18 keV, we have seen strong erratic intensity variations on time scale of 0.1s - 10s. Quasi Periodic Oscillations (QPOs) in a frequency range of 0.62 to 0.82 Hz were detected with a rms fraction of about 9%. The rapid X-ray intensity variations in GRS1915+105 are similar to those observed in some other black hole binaries and thus provide further support for the hypothesis that this source is likely to be a black hole. We discuss the possible emission region and mechanism of the observed quasi-periodic oscillations. Comparing the observed QPOs with the ones observed in other neutron star and black-hole systems, we argue that GRS1915+105 is possibly a black-hole.

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arXiv1997

A double peaked pulse profile observed in GX 1+4

The hard X-ray pulsar GX 1+4 was observed several times in the last few years with a pair of balloon-borne Xenon filled Multi-cell Proportional Counters (XMPC). In a balloon flight made on 22 March 1995, the source was detected in a bright state, the average observed source count rate being $8.0\pm0.2/s$ per detector. X-ray pulsations with a period of $121.9\pm0.1$ s were detected in the source with a broad double peak pulse feature. When observed in December 1993 with the same instrument, the pulse profile of GX 1+4 showed a single peak. This change in the pulse profile to a double pulse structure in about 15 months indicates either activation of the opposite pole of the neutron star if the magnetic field is asymmetric or possibly a change in the beam pattern, from a pencil beam to a fan beam. Assuming a fan beam configuration, the pulse profile is used to find the inclinations of the magnetic axis and the viewing axis with the spin axis. The derived angles support the GINGA observations of a dip in the pulse profile which was resolved to have a local maximum in one of the observations and was explained with resonance scattering of cyclotron line energy photons by the accretion column (Makishima et al., \markcite{maki1988}, Dotani et al., \markcite{dotani1989}.). Compared to our previous observation of the same source with the same telescope (Rao et al., \markcite{rao1994}) a period change rate of $0.72 \pm 0.40 s/yr$ is obtained which is the lowest rate of change of period for this source since its discovery. Average pulse fraction in the hard X-ray range is low (30%), consistent with its anti correlation with luminosity as reported by us earlier (Rao et al., \markcite{rao1994}) and the observed spectrum is very hard (power law photon index $1.67\pm0.12$).

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