CXOU J005047.9−731817: a 292-s X-ray binary pulsar in the Small Magellanic Cloud

• Published in: Monthly notices of the Royal Astronomical Society Vol. 433; no. 4; pp. 3464 - 3471
• Main Authors: Esposito, P., Israel, G. L., Sidoli, L., Rodríguez Castillo, G. A., Masetti, N., D'Avanzo, P., Campana, S.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21.08.2013
• Subjects: Double stars; Luminosity; Pulsars; Starbursts; X-ray astronomy; Magellanic Clouds; X-rays: individual: CXOU J005047.9−731817; stars: emission-line, Be; X-rays: binaries
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt1010

We report on the discovery of a transient X-ray pulsar, located in the Small Magellanic Cloud, with a pulse period of 292 s. A series of Chandra pointings fortuitously recorded in 2010 April-May the occurrence of a two-weeks-long outburst, during which the source luminosity increased by a factor of about 100, reaching a peak of ∼1036 erg s−1 (for a distance of 61 kpc). Complex-shape and energy-dependent pulsations were detected close to the outburst peak and during the very first part of its decay phase. During the outburst, the phase-averaged spectrum of the pulsar was well described by an absorbed power law with photon index Γ ∼ 0.6, but large variations as a function of phase were present. The source was also detected by Chandra several times (during 2002, 2003, 2006 and 2010) at a quiescent level of ∼1034 erg s−1. In 2012, we performed an infrared photometric follow-up of the R ∼ 15 mag optical counterpart with the European Southern Observatory/Very Large Telescope and a spectroscopic observation by means of the CTIO telescope. The optical spectra suggest a late-Oe or early Be V-III luminosity-class star, though a more evolved companion cannot be ruled out by our data (we can exclude a luminosity class I and a spectral type later than B2). Finally, we show that the outburst's main parameters (duration and peak luminosity) can be accounted for by interpreting the source transient activity as a type I outburst in a Be X-ray binary.