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 millisecond pulsar, HETE J1900.1-2455, based on observations made simultaneously by XMM-Newton and RXTE. The 0.33-50 keV energy spectrum is described by the sum of a hard Comptonized component produced in an optically thin τ ≃ 1 corona, a soft thermal kTin ≃ 0.2 keV component interpreted as accretion disc emission, and of disc reflection of the hard component. Two emission features are detected at energies of 0.98(1) and 6.58(7) keV, respectively. The latter is identified as Kα transition of Fe XXIII-XXV. A simultaneous detection in the European Photon Imaging Camera (EPIC-pn), EPIC-MOS2 and Reflection Grating Spectrometer (RGS) spectra favours an astrophysical origin also for the latter, which has an energy compatible with Fe-Lα and helium-like Ne-Kα transitions. The broadness of the two features, σ/E ≃ 0.1, suggests a common origin, resulting from reflection in an accretion disc with inclination of (30+ 4- 3)°, and extending down to Rin = 25+ 16- 11 gravitational radii from the compact object. However, the strength of the feature at lower energy measured by EPIC-pn cannot be entirely reconciled with the amplitude of the Fe-Kα line, hampering the possibility of describing it in terms of a broad-band reflection model, and preventing a firm identification. Pulsations at the known 377.3 Hz spin frequency could not be detected with an upper limit of 0.4 per cent at 3σ confidence level on the pulsed fractional amplitude. We interpret the value of the inner disc radius estimated from spectral modelling and the lack of significant detection of coherent X-ray pulsations as an indication of a disc accretion flow truncated by some mechanism connected to the overall evolution of the accretion disc, rather than by the neutron star magnetic field. This is compatible with the extremely close similarity of spectral and temporal properties of this source with respect to other, non-pulsing atoll sources in the hard state.