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

Tiziana Di Salvo, Rosario Iaria, Riggio, Bozzo, Pintore, Sanna, Sánchez-Fernández, Burderi, Mereghetti

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5 Citations (Scopus)


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 ~3 × 1036erg s-1. This is the weakest outburst ever reported for this source to date. We analysed 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 (kTbb~ 0.6 keV, Rbb~ 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. © 2018 The Author(s) Published by Oxford University Press on behalf of The Royal Astronomical Society.
Original languageEnglish
Pages (from-to)4084-4090
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Publication statusPublished - 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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