Broad-band spectral analysis of the accreting millisecond X-ray pulsar SAX J1748.9−2021

Abstract

We analyzed a 115 ks XMM-Newton observation and the stacking of 8 days of\nINTEGRAL observations, taken during the raise of the 2015 outburst of the\naccreting millisecond X-ray pulsar SAX J1748.9-2021. The source showed numerous\ntype-I burst episodes during the XMM-Newton observation, and for this reason we\nstudied separately the persistent and burst epochs. We described the persistent\nemission with a combination of two soft thermal components, a cold thermal\nComptonization component (~2 keV) and an additional hard X-ray emission\ndescribed by a power-law (photon index ~2.3). The continuum components can be\nassociated with an accretion disc, the neutron star (NS) surface and a thermal\nComptonization emission coming out of an optically thick plasma region, while\nthe origin of the high energy tail is still under debate. In addition, a number\nof broad (~0.1-0.4 keV) emission features likely associated to reflection\nprocesses have been observed in the XMM-Newton data. The estimated 1.0-50 keV\nunabsorbed luminosity of the source is ~5x10^37 erg/s, about 25% of the\nEddington limit assuming a 1.4 solar mass NS. We suggest that the spectral\nproperties of SAX J1748.9-2021 are consistent with a soft state, differently\nfrom many other accreting X-ray millisecond pulsars which are usually found in\nthe hard state. Moreover, none of the observed type-I burst reached the\nEddington luminosity. Assuming that the burst ignition and emission are\nproduced above the whole NS surface, we estimate a neutron star radius of ~7-8\nkm, consistent with previous results.\n