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Astron. Astrophys. 351, 985-992 (1999)


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Inner accretion disk disappearance during a radio flare in GRS 1915+105

M. Feroci 1, G. Matt 2, G. Pooley 3, E. Costa 1, M. Tavani 4,5 and T. Belloni 6,7

1 Istituto di Astrofisica Spaziale, CNR, Via Fosso del Cavaliere, 00133 Roma, Italy
2 Dipartimento di Fisica, Universita' `Roma Tre', Via della Vasca Navale 84, 00146 Roma, Italy
3 Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
4 Istituto Fisica Cosmica e Tecnologie Relative, CNR, Via Bassini 15, 20133 Milano, Italy
5 Columbia Astrophysics Laboratory, New York, NY, USA
6 Astronomical Institute "A. Pannekoek" and Center for High-Energy Astrophysics, Kruislaan 403, 109855 Amsterdam, The Netherlands
7 Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy

Received 17 May 1999 / Accepted 28 September 1999

Abstract

Simultaneous X-ray (BeppoSAX, 0.1-300 keV) and radio (Ryle Telescope, 15 GHz) observations of the superluminal galactic source GRS 1915+105 in high flux state were carried out starting October 1996. The source was in a phase of large X-ray intensity and spectral variability. Among the simultaneous observations, an isolated radio flare lasting [FORMULA]2000 s was detected accompanied by X-ray outbursting activity. Here we present the simultaneous radio/X-ray light curves of the flare event and the spectral and timing analysis of the relevant BeppoSAX data. The broad band (0.1-200 keV) energy spectrum of the source can be described in terms of the `standard' spectral model for black hole candidates. We obtain evidence for a temporary disappearance and subsequent restoring of the inner portion of the accretion disk during the flare from the spectral analysis of the X-ray data. We find a variable power law photon index positively correlated to the variability of the exponential cut-off energy. In addition, frequency and intensity variations of a [FORMULA]4-5 Hz quasi-periodic oscillation (QPO) are detected during the same time interval. The centroid frequency of the QPO is found to be correlated to the spectral power law photon index, and to the X-ray flux from the disk and the high energy flux from the power law. These observational results are discussed in the framework of the disk-instability model proposed for this source.

Key words: accretion, accretion disks – stars: individual: GRS 1915+105 – gamma rays: bursts – X-rays: bursts

Send offprint requests to: feroci@ias.rm.cnr.it

© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999

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