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arXiv · 1806.10944

A faint outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021 in NGC 6440

F. Pintore·A. Sanna·A. Riggio·T. Di Salvo·S. Mereghetti·E. Bozzo·C. Sánchez-Fernández·L. Burderi·R. Iaria
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Abstract

SAX J1748.9-2021 is an accreting X-ray millisecond pulsar observed in outburst five times since its discovery in 1998. In early October 2017, the source started its sixth outburst, which lasted only ~13 days, significantly shorter than the typical 30 days duration of the previous outbursts. It reached a 0.3-70 keV unabsorbed peak luminosity of $\sim3\times10^{36}$ erg/s. This is the weakest outburst ever reported for this source to date. We analyzed almost simultaneous XMM-Newton, NuSTAR and INTEGRAL observations taken during the decaying phase of its 2017 outburst. We found that the spectral properties of SAX J1748.9-2021 are consistent with an absorbed Comptonization plus a blackbody component. The former, characterized by an electron temperature of ~20 keV, a photon index of ~1.6-1.7 keV and seed photon temperature of 0.44 keV, can be associated to a hot corona or the accretion column, while the latter is more likely originating from the neutron star surface (kT$_{bb}\sim0.6$ keV, R$_{bb}\sim2.5$ km). These findings suggest that SAX J1748.9-2021 was observed in a $hard$ spectral state, as it is typically the case for accreting millisecond pulsars in outburst.

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F. Pintore, A. Sanna, A. Riggio, T. Di Salvo, S. Mereghetti, E. Bozzo, C. Sánchez-Fernández, L. Burderi, R. Iaria. 2018-06-28. A faint outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021 in NGC 6440. https://doi.org/10.1093/mnras%2Fsty1735

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