Pintore, FabioFabioPintoreSanna, A.A.SannaDi Salvo, T.T.Di SalvoDEL SANTO, MELANIAMELANIADEL SANTORiggio, A.A.RiggioD'AI', ANTONINOANTONINOD'AI'Burderi, L.L.BurderiScarano, F.F.ScaranoIaria, R.R.Iaria2020-05-262020-05-2620160035-8711http://hdl.handle.net/20.500.12386/25188We analysed a 115-ks XMM-Newton observation and the stacking of 8 d 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 (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 XMM-Newton data. The estimated 1.0-50 keV unabsorbed luminosity of the source is ∼5 × 10<SUP>37</SUP> erg s<SUP>-1</SUP>, about 25 per cent of the Eddington limit assuming a 1.4 M<SUB>☉</SUB> 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.STAMPAenArticle10.1093/mnras/stw1762-s2.0-84976877708000373583900053https://academic.oup.com/mnras/article/457/3/2988/25889642016MNRAS.457.2988PFIS/05 - ASTRONOMIA E ASTROFISICA