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

Abstract

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 τ_T ∼ 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 ~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.