Small Magellanic Cloud - X-ray Sources

X-ray Sources

The Small Magellanic Cloud contains a large and active population of X-ray binaries. Recent star formation has led to a large population of massive stars and high-mass X-ray binaries (HMXBs) which are the relics of the short-lived upper end of the initial mass function. The young stellar population and the majority of the known X-ray binaries are concentrated in the SMC’s Bar. HMXB pulsars are rotating neutron stars (NSs) in binary systems with Be-type (spectral type 09-B2, luminosity classes V–III) or supergiant stellar companions. Most HMXBs are of the Be type which account for 70% in the Milky Way an 98% in the SMC (Coe et al. 2005). The Be-star equatorial disk provides a reservoir of matter that can be accreted onto the NS during periastron passage (most known systems have large orbital eccentricity) or during large-scale disk ejection episodes. This scenario leads to strings of X-ray outbursts with typical luminosities LX=10^36-10^37 erg s−1, spaced at the orbital period, plus infrequent giant outbursts of greater duration and luminosity (see Negueruela 1998 for a review). Monitoring surveys of the SMC performed with NASA's Rossi X-ray Timing Explorer (RXTE) (Laycock et al. 2005, hereafter L05; Galache et al. 2008) see X-ray pulsars in outburst at >1036 erg s−1 and have counted 50 by the end of 2008. The ROSAT and ASCA missions detected many faint X-ray point sources (e.g., Haberl & Sasaki 2000), but the typical positional uncertainties frequently made positive identification difficult. Recent studies using XMM-Newton (Haberl et al. 2008; Haberl & Pietsch 2004) and Chandra (Antoniou et al. 2009; Edge et al. 2004, and Laycock et al. 2010) have now cataloged several hundred X-ray sources in the direction of the SMC, the of which perhaps half are considered likely HMXBs, and the remainder a mix of foreground stars, and background AGN.

No X-rays above background were observed from the Magellanic Clouds during the September 20, 1966, Nike Tomahawk flight. Balloon observation from Mildura, Australia, on October 24, 1967, of the SMC set an upper limit of X-ray detection. An X-ray astronomy instrument was carried aboard a Thor missile launched from Johnston Atoll on September 24, 1970, at 12:54 UTC for altitudes above 300 km, to search for the Small Magellanic Cloud. The SMC was detected with an X-ray luminosity (L_x) of 5 x 1038 ergs/s in the range 1.5-12 keV, and 2.5 x 1039 ergs/s in the range 5-50 keV for an apparently extended source.

The fourth Uhuru catalog lists an early X-ray source within the constellation Tucana: 4U 0115-73 (3U 0115-73, 2A 0116-737, SMC X-1). Uhuru observed the SMC on January 1, 12, 13, 16, and 17, 1971, and detected one source located at 01149-7342, which was then designated SMC X-1. Some X-ray counts were also received on January 14, 15, 18, and 19, 1971. The third Ariel 5 catalog (3A) also contains this early X-ray source within Tucana: 3A 0116-736 (2A 0116-737, SMC X-1). The SMC X-1, a HMXRB, is at J2000 right ascension (RA) 01h 15m 14s declination (Dec) 73° 42′ 22″.

Two additional sources detected and listed in 3A include SMC X-2 at 3A 0042-738 and SMC X-3 at 3A 0049-726.