A faint outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021 in NGC 6440
Date Issued
2018
Author(s)
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Riggio, A.
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Di Salvo, T.
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Bozzo, E.
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Sánchez-Fernández, C.
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Burderi, L.
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Iaria, R.
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 ∼3 × 1036 erg 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.
Volume
479
Issue
3
Start page
4084
Issn Identifier
0035-8711
Ads BibCode
2018MNRAS.479.4084P
Rights
open.access
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