The XMM-Newton view of GRS 1915+105

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: 0.9div1.1) × 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.