2005A&A...436..647F

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 0.5-200keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature ∼40keV, considerably higher than the value (∼10keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200keV with a luminosity in the energy band (0.1-200)keV of 1.3x10³⁷erg/s (assuming a distance of 8kpc). 22-days later the luminosity dropped to 3.6x10³⁶erg/s. 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<0.022M_☉, 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.