The accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1−2455, as observed by XMM–Newton and RXTE

Abstract

We present a study of the accretion flow to the intermittent accreting\nmillisecond pulsar, HETE J1900.1-2455, based on observations performed\nsimultaneously by XMM-Newton and RXTE. The 0.33-50 keV spectrum is described by\nthe sum of a hard Comptonized component originated in an optically thin\n{\\tau}~1 corona, a soft kTin~0.2 keV component interpreted as accretion disc\nemission, and of disc reflection of the hard component. Two emission features\nare detected at energies of 0.98(1) and 6.58(7) keV, respectively. The latter\nis identified as K{\\alpha} transition of Fe XXIII-XXV. A simultaneous detection\nin EPIC-pn, EPIC-MOS2, and RGS spectra favours an astrophysical origin also for\nthe former, which has an energy compatible with Fe-L{\\alpha} and helium-like\nNe-K{\\alpha} transitions. Broadness of the two features suggests a common\norigin, resulting from reflection in an accretion disc with inclination of\n(30+4{\\deg}), and extending down to Rin=25(+16,-11) gravitational radii from\nthe compact object. However, the strength of the feature at lower energy\nmeasured by EPIC-pn cannot be entirely reconciled with the amplitude of the Fe\nK{\\alpha} line, hampering the possibility of describing it in terms of a\nbroad-band reflection model, and preventing a firm identification. Pulsations\nat the 377.3 Hz spin frequency could not be detected, with an upper limit of\n0.4% at 3-{\\sigma} c.l. on the fractional amplitude. We interpret the inner\ndisc radius estimated from spectral modelling and the lack of significant\ndetection of coherent X-ray pulsations as an indication of a disc accretion\nflow truncated by some mechanism connected to the overall evolution of the\naccretion disc, rather than by the neutron star magnetic field. This is\ncompatible with the extremely close similarity of spectral and temporal\nproperties of this source with respect to other, non pulsing atoll sources in\nthe hard state.\n