Simultaneous INTEGRAL and RXTE observations of the accreting millisecond pulsar HETE J1900.1-2455

Aims. HETE J1900.1 - 2455 is the seventh known X-ray transient accreting millisecond pulsar and has been in outburst for more than one year. We compared the data on HETE J1900.1 - 2455 with other similar objects and made an attempt at deriving constraints on the physical processes responsible for a spectral formation. Methods. The broad-band spectrum of the persistent emission in the 2 - 300 keV energy band and the timing properties were studied using simultaneous INTEGRAL and publicly available RXTE data obtained in October 2005. The properties of the X-ray bursts observed from HETE J1900.1 - 2455 were also investigated. Results. The spectrum is well described by a two- component model consisting of a blackbody- like soft X- ray emission at 0.8 keV temperature and a thermal Comptonized spectrum with electron temperature of 30 keV and Thomson optical depth τT similar to 2 for the slab geometry. The source is detected by INTEGRAL up to 200 keV at a luminosity of 5 * 1036 erg s-1 (assuming a distance of 5 kpc) in the 0.1 - 200 keV energy band. We have also detected one type I X- ray burst which shows photospheric radius expansion. The burst occurred at an inferred persistent emission level of similar to 3 - 4% of the Eddington luminosity. Using data for all X- ray bursts observed to date from HETE J1900.1 - 2455, the burst recurrence time is estimated to be about 2 days. No pulsations have been detected either in the RXTE or in the INTEGRAL data which puts interesting constraints on theories of magnetic field evolution in neutron star low- mass X- ray binaries. (authors)