The highly ionized disk wind of GRO J1655-40

Abstract

Aims:The galactic superluminal microquasar GRO J1655-40 started a new outburst in February 2005, after seven years in quiescence, rising to a high/soft state in March 2005. In this paper we study the X-ray spectra during this rise.
Methods: We observed GRO J1655-40 with XMM-Newton on 27 February 2005 in the low/hard state, and on three consecutive days in March 2005 during the rise of the source to its high/soft state. The EPIC-pn camera was used in the fast-read burst mode to avoid photon pile-up.
Results: First, we contributed to the improvement of the calibration of the EPIC-pn, since the high flux received from the source required some refinements in the correction of the charge transfer efficiency of the camera. Second, we find that the X-ray spectrum of GRO J1655-40 is dominated in the high/soft state by the thermal emission from the accretion disk, with an inner radius of 13-14[D/3.2 kpc] km and a maximum temperature of 1.3 keV. We detected two absorption lines in the EPIC-pn spectra at 6.7-6.8 and 7.8-8.0 keV, which can be identified as the Kα and Kβ lines of either blended Fe XXV and Fe XXVI or blueshifted Fe XXV. We find no orbital dependence on the X-ray properties, which provides an upper limit for the inclination of the system of 73°. The RGS spectrometers reveal interstellar absorption features at 17.2 Å, 17.5 Å (Fe L edges), and 23.54 Å (OI Kα). Finally, while checking the interstellar origin of the OI line, we found a general correlation of the OI Kα line equivalent width with the hydrogen column density using several sources available in the literature.