Rapid X-ray variability of the superluminal source GRS 1915+105

Astronomy and Astrophysics (Impact Factor: 4.38). 04/1997; 320(2).
Source: arXiv


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 2-18keV, 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 0.82Hz were detected with a rms fraction of about 9%. The rapid X-ray intensity variations in GRS 1915+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 GRS 1915+105 is possibly a black-hole.

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    ABSTRACT: 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.
    Full-text · Article · Nov 1997 · The Astrophysical Journal
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    ABSTRACT: 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 s. 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.
    Full-text · Article · Feb 1998 · Astronomy and Astrophysics Supplement Series
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    ABSTRACT: We have observed the black hole candidate and Galactic superluminal source GRS 1915+105 with ISO LWS and SWS in March 1996 and March 1997 when the source was at fairly low X-ray and radio flux levels. If the source is asso-ciated with a dense molecular cloud, then corresponding far-IR continuum and line fluxes should be easily detectable and the data should constrain models for accretion from ISM (Maloney et al. 1997). The integrated fluxes in the SWS (2.3 - 45.2 μm) and LWS (43.2 - 195.6 μm) are 2.5 × 10-17 and 8.9 × 10-17 W/cm2, respectively. The continuum above 50 μm is well represented by thermal black body radiation of T = (25.9 ± 0.8) K, modified by a dust emissivity component ε ∼ λ-1.5. This can be indicative of a dust cloud with radius rDust ∼ 0.3 pc, and mass MDust ∼ 5.5 M⊙. However, we do not detect significant line emission in the background subtracted LWS spectrum of GRS 1915+105, as predicted by Maloney et al. (1997) for far-IR lines of the ambient ISM excited by compact objects, and the 3σ upper limits are below 7.5 to 9.6 × 10-20 W/cm2 for prominent lines as [OI], [OIII], [NII] and [CII]. The non-detection of fine-strucure lines implies that either GRS 1915+105 is not embedded in a dense molecular cloud or, if the source would be indeed close to such a cloud, the source X-ray luminosity averaged over the past few hundred years is significantly below 1037 erg/s so that line intensities are low. We conclude that the far-IR continuum emission from the source region, with T = 26 K not unusual for the ISM, is most likely not related to GRS 1915+105 and that most probably GRS 1915+105 is not surrounded by dense ISM, so that the lack of detected line emission is a consequence of the absence of material suitable for irradation.
    Full-text · Article · Sep 1998 · Astronomy and Astrophysics
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