HMXB candidate RX J0101.8-7223 is a Be/X-ray binary pulsar in the SMC
ATel #3311; L. J. Townsend (Southampton), S. P. Drave (Southampton), R. H.D. Corbet (UMBC/NASA GSFC), M. J. Coe (Southampton), A. J. Bird (Southampton)
on 28 Apr 2011; 16:08 UT
Credential Certification: L. J. Townsend (ljt203@soton.ac.uk)
The RXTE/PCA detected 175.4 +/- 0.1s pulsations coming from the direction of the SMC on 2011 March 19 (MJD 55639). This period was confirmed by a second detection at 175.1 +/- 0.1s on 2011 March 26 (MJD 55646). It does not coincide with the spin period of any known pulsar in the RXTE field of view. An INTEGRAL follow up observation was performed on 2011 April 2 (MJD 55653) to localise any X-ray sources in the RXTE field of view. A source was detected in the IBIS revolution map at the 6.7 sigma significance level, with an error circle of radius 3.6 arcmin. This permitted an XMM-Newton ToO observation to identify the nature of the source. A 16ks XMM observation was carried out on 2011 April 8 (MJD 55659) with the EPIC cameras in full frame, imaging mode. Source detection and light curve extraction were carried out using standard SAS tools. A single bright X-ray source was found within the IBIS error circle at the position RA = 01:01:52.53, dec = -72:23:34.98 (J2000.0) with a 1-sigma error circle of radius 1.1 arcsec. A Lomb-Scargle periodogram of the 0.2--10 keV light curve revealed a period of 175.1 +/- 0.1s, confirming the RXTE, INTEGRAL and XMM detections are of the same source.
The X-ray spectrum was fit with an absorbed power law with Galactic absorption fixed to 6 x 10^20 cm^-2 resulting in a photon index of 0.97 +/- 0.02 and intrinsic SMC absorption of (4.6 +/- 0.4) x 10^21 cm^-2. The 0.2--10 keV flux from the fit is 8.2 x 10^-12 ergs/cm^2/s, corresponding to 3.5 x 10^36 ergs/s at the distance of the SMC.
The pulsar is coincident with the HMXB candidate RX J0101.8-7223 (e.g. Haberl & Sasaki, 2000, A&A, 359, 573; Yokogawa et al., 2003, PASJ, 55, 161) and is believed to be the same object. The counterpart is likely the emission line star [MA93] 1288. Initial analysis of the MACHO and OGLE II optical light curves of this star show no obvious periodicities that can be attributed to the binary period.
In summary, we have shown that the HMXB candidate RX J0101.8-7223 is a Be/X-ray binary pulsar with a 175.1s period. We would like to thank the INTEGRAL and XMM teams for scheduling the ToO observations.
