Characterizing a new class of variability in GRS 1915+105 with simultaneous INTEGRAL/RXTE observations

¹ Observatory, PO Box 14, 00014 University of Helsinki, Finland e-mail: diana@astro.helsinki.fi
² Centre d'Études de Saclay, DAPNIA/Service d'Astrophysique (CNRS FRE 2591), Bât. 709, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
³ Science Data Center, Chemin d'Écogia 16, 1290 Versoix, Switzerland
⁴ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland
⁵ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁶ Astronomy Division, University of Oulu, PO Box 3000, 90014 Finland
⁷ MSSL, University College London, Holmbury St. Mary, Surrey, RH5 6NT, UK
⁸ Dept. of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
⁹ NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA
¹⁰ Joint Center for Astrophysics, Department of Physics, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
¹¹ School of Physics, University of Sydney, NSW 2006, Australia
¹² Astrophysics Group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK
¹³ Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹⁴ European Southern Observatory, Santiago 10, Chile
¹⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain


Received: 25 October 2004
Accepted: 12 February 2005

Abstract

We report on the analysis of 100 ks INTEGRAL observations of the Galactic microquasar GRS 1915+105. We focus on INTEGRAL Revolution number 48 when the source was found to exhibit a new type of variability as preliminarily reported in Hannikainen (2003, A&A, 411, L415). The variability pattern, which we name ξ, is characterized by a pulsing behaviour, consisting of a main pulse and a shorter, softer, and smaller amplitude precursor pulse, on a timescale of 5 min in the JEM-X 3–35 keV lightcurve. We also present simultaneous RXTE data. From a study of the individual RXTE/PCA pulse profiles we find that the rising phase is shorter and harder than the declining phase, which is opposite to what has been observed in other otherwise similar variability classes in this source. The position in the colour-colour diagram throughout the revolution corresponds to State A (Belloni et al. 2000, A&A, 355, 271) but not to any previously known variability class. We separated the INTEGRAL data into two subsets covering the maxima and minima of the pulses and fitted the resulting two broadband spectra with a hybrid thermal–non-thermal Comptonization model. The fits show the source to be in a soft state characterized by a strong disc component below ~6 keV and Comptonization by both thermal and non-thermal electrons at higher energies.