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