Chandra and RXTE spectroscopy of the accreting msec pulsar IGR J00291+5934

¹ Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland e-mail: Ada.Paizis@obs.unige.ch
² INAF-IASF, Sezione di Milano, Via Bassini 15, 20133 Milano, Italy
³ Center for Space Research, MIT, Cambridge, MA, USA
⁴ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁵ Observatoire de Genève, 51 chemin des Mailletes, 1290 Sauverny, Switzerland
⁶ NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
⁷ CEA Saclay, DSM/DAPNIA/SAp (CNRS UMR 7158 AIM), 91191 Gif-Sur-Yvette, France
⁸ INAF-IASF, Sezione di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 12 May 2005
Accepted: 14 July 2005

Abstract

We report on an observation of the recently discovered accreting millisecond X-ray pulsar IGR J00291+5934 performed with the RXTE-Proportional Counter Array (PCA) and Chandra-High Energy Transmission Grating Spectrometer (HETGS). The RXTE data are from a two-week follow-up of the source, while the Chandra observation took place around the end of the follow-up, about 12 days after the discovery of the source, when the source flux had decreased already by a factor of ten. The analysis of the Chandra data allowed us to extract the most precise X-ray position of IGR J00291+5934, RA =  , and Dec =  34′ 19.2″ (0.6″ error), compatible with the optical and radio ones. We find that the spectra of IGR J00291+5934 can be described by a combination of a thermal component and a power-law. Along the outburst detected by PCA, the power-law photon index showed no particular trend, while the thermal component (~1 keV, interpreted as a hot spot on the neutron star surface) became weaker until non-detection. In the simultaneous observation of the weak Chandra /RXTE spectrum, there was no longer any indication of the ~1 keV thermal component, while we detected a colder thermal component (~0.4 keV) that we interpret as the emission from the cold disc. A hint of a 6.4 keV iron line was detected, together with an excess around 6.8 keV and absorption feature around 7.1 keV. The last two features have never been detected in the spectra of accretion-driven millisecond pulsars before and, if confirmed, would suggest the presence of an expanding hot corona with high outflow velocities.