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

Abstract

We analysed a 115-ks <it>XMM–Newton</it> observation and the stacking of 8 d of <it>INTEGRAL</it> 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 <it>XMM–Newton</it> 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 (Γ ∼ 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 (σ = 0.1–0.4 keV) emission features likely associated with reflection processes have been observed in the <it>XMM–Newton</it> data. The estimated 1.0–50 keV unabsorbed luminosity of the source is ∼5 × 1037 erg s−1, about 25 per cent of the Eddington limit assuming a 1.4 M<inf>⊙</inf> 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 an NS radius of ∼7–8 km, consistent with previous results.