The XMM-Newton view of GRS 1915+105

¹ CNRS / Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France e-mail: andrea.martocchia@cesr.fr
² Dipartimento di Fisica, Università degli Studi “Roma Tre”, Via della Vasca Navale 84, 00146 Roma, Italy
³ INAF / Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁴ INAF / IASF, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ Astronomical Institute, Academy of Sciences, Boční II, 140 31 Prague, Czech Republic
⁶ INAF / IASF, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁷ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁸ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

Received: 17 May 2005
Accepted: 20 October 2005

Abstract

Two XMM-Newton observations of the black-hole binary GRS 1915+105 were triggered in 2004 (April 17 and 21), during a long “plateau” state of the source. We analyzed the data collected with EPIC-pn in timing and burst modes, respectively. Reflection Grating Spectrometers were used only on April 21st. The source 2–10 keV flux is ~0.6 (unabsorbed: )  10^-8 in cgs units. While the light curves show only small amplitude variations (a few percent) at timescales longer than a few seconds, a QPO is seen at about 0.6 Hz – as expected in χ variability modes of GRS 1915+105, when the phenomenological correlation with the source flux is taken into account – possibly with a harmonic signal at 1.2 Hz. The pn spectrum is well fitted without invoking thermal disk emission, on the basis of four main components: a primary one (either a simple power law or thermal Comptonization models), absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disk; emission and absorption lines; and a soft X-ray excess at ~1 keV. However, the last is not confirmed by the RGS spectra, whose difference from the EPIC-pn ones lacks a fully satisfactory explanation. If real, the soft X-ray excess may be due to reflection from an optically thin, photoionized disk wind; in this case it may lead to a way to disentangle intrinsic from interstellar absorption.