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

¹ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France e-mail: mfalanga@cea.fr
² Unité mixte de recherche Astroparticule et Cosmologie, 11 place Berthelot, 75005 Paris, France
³ Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
⁴ Laboratoire de l'Univers et de ses Théories, Observatoire de Paris, 92195 Meudon Cedex, France
⁵ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁶ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁷ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy

Received: 27 September 2006
Accepted: 2 January 2007

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 for the slab geometry. The source is detected by INTEGRAL up to 200 keV at a luminosity of  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.