INTEGRAL spectroscopy of the accreting millisecond pulsar XTE J1807-294 in outburst

¹ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France e-mail: mfalanga@cea.fr
² Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
³ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
⁴ Observatoire Astronomique, 11 rue de l'Université, 67000 Strasbourg, France
⁵ Laboratory for Particle Astrophysics, Institute of High Energy Physics, CAS, Beijing 100039, PR China
⁶ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received: 20 December 2004
Accepted: 7 March 2005

Abstract

The transient X-ray accreting millisecond pulsar XTE J1807-294 was observed during its February/March 2003 outburst by INTEGRAL, partly simultaneously with the XMM-Newton and RXTE satellites. We present here the first study of the 0.5-200 keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature ~40 keV, considerably higher than the value (~10 keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200 keV with a luminosity in the energy band (0.1-200) keV of  erg s^-1 (assuming a distance of 8 kpc). 22 days later the luminosity dropped to  erg s^-1. A re-analysis of XMM-Newton data yields the orbital Doppler variations of the pulse period and refines the previous ephemeris. For this source, with shortest orbital period of any known binary radio or X-ray millisecond pulsar, we constrain the companion mass , assuming minimum mass transfer driven by gravitational radiation. Only evolved dwarfs with a C/O composition are consistent with the Roche lobe and gravitational radiation constraints, while He dwarfs require an unlikely low inclination.