The highly ionized disk wind of GRO J1655-40

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: gsala@mpe.mpg.de
² Astronomical Institute, University Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ Institut für Astronomie und Astrophysik, Abteilung Astronomie, Sand 1, 72076 Tübingen, Germany

Received: 9 March 2006
Accepted: 31 August 2006

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 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 , 17.5 (Fe L edges), and 23.54 (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.