  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.
