Mon. Not. R. Astron. Soc., 457, 2988-2998 (2016/April-2)
Broad-band spectral analysis of the accreting millisecond X-ray pulsar SAX J1748.9-2021.
PINTORE F., SANNA A., DI SALVO T., DEL SANTO M., RIGGIO A., D'AI A., BURDERI L., SCARANO F. and IARIA R.
Abstract (from CDS):
We 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 x 1037 erg/s, about 25 per cent of the Eddington limit assuming a 1.4 M☉ 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.
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
accretion, accretion discs - X-rays: binaries - X-rays: galaxies - X-rays: individual: SAX J1748.9-2021
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