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

TL;DR

This paper reports the detection and analysis of the weakest outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021, revealing its spectral properties and state during a short, faint outburst in 2017.

Contribution

It provides detailed spectral analysis of a rare, low-luminosity outburst of SAX J1748.9-2021, enhancing understanding of its accretion state and emission mechanisms.

Findings

Outburst lasted only ~13 days, weaker than previous events.

Spectral analysis shows a hard state with Comptonization and blackbody components.

Source was in a typical hard spectral state for accreting millisecond pulsars.

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.