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

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 $\sim 3\times 10^{36}$ 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${}_{bb}\sim 0.6$ keV, R${}_{bb}\sim 2.5$ 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.

arXiv commentAccepted for publication in MNRAS, 7 pages, 5 figures, 1 table

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• Indications of non-conservative mass-transfer in AMXPs
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• A broadband look of the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton
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• Study of the reflection spectra of SAX J1748.9-2021