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Astron. Astrophys. 320, L37-L40 (1997)

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4. Discussion

The only reported low energy observations (below 20 keV) are the one by Nagase et al. using the ASCA satellite (Nagase et al. 1994 ) and by Greiner using the ROSAT satellite (Greiner et al. 1993 ). Recently, observations using PCA and ASM onboard the RXTE satellite have shown intensity variation on a variety of time scales (Greiner et al. 1996 ). The maximum X-ray luminosity for an assumed distance of 12.5 kpc (Mirabel & Rodriguez 1994 ), is more than 1039 erg s-1 (Sazonov et al. 1994 ). This greatly exceeds the limiting Eddington luminosity for emission from a neutron star surface of reasonable mass (Lang 1980 ). No coherent pulsations in the frequency range 0.001 to 0.1 Hz were detected (Finoguenov et al. 1994 ). Radio outbursts seen with the Green Bank Interferometer (GBI) are found to be correlated with the X-ray flaring seen by the BATSE onboard the CGRO (Foster et al. 1996 ) during a previous outburst of the source. The long time light curve of the source as seen by the All Sky Monitor (ASM) on the RXTE after its recent outburst in January 1996, shows very strong variability during May 15 to 30 June 1996 and August 15 to very recent time and almost constant intensity between July 1 and August 15 1996 (http://space.mit.edu/XTE/ASM_lc.html). Our observations were made during the period of July 20-29 when ASM count rate shows no significant intensity variations. Though the source was found to be bright in radio during the current outburst too (Fender et al. 1996 ), there is no reported radio observation during the rarely occuring constant intensity state reported here.

The subsecond time variability seen in GRS1915+105 indicates that the emission is from a compact region of size much smaller than a light second. But the total X-ray intensity greatly exceeds the Eddington luminosity from a neutron star with permitted mass limit. Therefore the most likely place for the radiation to come from is the accretion disk. In neutron star binaries, even a small magnetic field of 108 gauss is sufficient to keep the inner disk away from the neutron star surface (Frank et al. 1992 ). Hence the subsecond variability that is seen in GRS1915+105 indicates that the compact object in this system is likely to be a black hole. The observed quasi-periodic oscillations when compared to QPOs seen in other black hole X-ray binaries also support the black hole picture.

The stability of the QPO frequency over 4 days indicates that the intensity oscillations are generated in an annular region in the disk. If the QPOs were to arise due to blob of material orbiting in the disk, the QPO frequency should have increased systematically with time as the blob of material spirals towards the inner part of the disk. If the inner disk is superheated it can emit very high energy radiation upto gamma rays. Such an emission process is needed for the plasmoids to be thrown away by the compact object in the form of jets by radiation pressure along the axis of the system (Liang & Li 1995 ). While calculating the emission pattern from a disk around a black-hole two factors are to be considered, the gravitational redshift experienced by the radiation from the innermost disk and for super Eddington luminosity systems like GRS1915+105 and the effect of radiation pressure in the inner disk structure. If the radiation pressure is efficient in reducing the effective gravity the radial structure of the disk can be very different from that of an ordinary thin disk. These two factors, can lead to the fact that the efficient radiation zone in a disk around a black-hole can be somewhat further away from the event horizon. The observed QPOs of 0.7 Hz and even smaller frequencies as seen by PCA (http://heasarc.gsfc.nasa.gov/docs/xte/SOF/ toonews.html) can then be from the most efficient zone of radiation in the disk whose radius changes because of various disk instabilities.

In low mass X-ray binaries usually two types of QPOs are seen, the nominal branch QPOs with a narrow frequency peak around 6 Hz with rms variation of 1-3% and the horizontal branch and flaring branch QPO peak in the frequency range 15 - 50 Hz and somewhat larger rms variation. Quasi-periodic oscillations are also seen in some of the pulsars but at a lower frequency of 0.02 to 0.2 Hz and these can be explained using the beat frequency model (van der Klis 1995 ). If the compact object in GRS1915+105 is a neutron star, the QPOs seen in this source are not like the ones in any of the other neutron star sources. In many Black Hole Candidates (BHC) QPOs associated with low frequency noise are seen at different frequencies in the range of 0.04 to 6 Hz. The type of QPOs seen in GRS1915+105 are similar to those seen in GX 339-4 (Grebenev et al. 1991 ).

Very strong subsecond intensity variations similar to GRS1915+105 are also seen in other black-hole candidates like Cyg X-1 and GX 339-4 (van der Klis 1995 ). Some neutron star sources like Cir X-1 (Toor 1977 ), 4U 1608-52 (van der Klis 1995 ), V0332+53 (Tanaka et al. 1983 ) also have shown subsecond variability but of smaller magnitude. So the short time variability seen in the present observation alone does not prove that GRS1915+105 is a black-hole source. However the Quasi-periodic oscillations at a frequency of 0.7 Hz and its first harmonic brings out the remarkable similarity of the power density spectrum (PSD) of this source with that of Cyg X-1 and GS 1124-68 in their very high state (van der Klis 1995 ). The flatness of the PSD below the QPO peak and the steep fall above the QPO frequency is also similar to that of other black-hole candidates in their very high state. So the identical nature of the PSD of GRS 1915+105 to that of Cyg X-1 and GS 1124-68 and subsecond intensity variations by a factor of 2 or more makes a strong case for this source to be a black hole. The hard X-ray tail of the spectrum of GRS1915+105 also supports its black-hole nature. Simultaneous observations in low and high energy X-rays in future will help in finding the true nature of this source.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998