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

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 similar to 40 keV, considerably higher than the value (similar to 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 1.3 x 10(37) erg s(-1) ( assuming a distance of 8 kpc). 22 days later the luminosity dropped to 3.6 x 10(36) 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 M-c < 0.022 M-circle dot, 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.