New SMC X-ray Pulsar in 37 d binary and showing large amplitude super-orbital variations.

ATel #3154; A. F. Rajoelimanana (SAAO/UCT); P. A. Charles (SAAO/Southampton University); A. Udalski (Warsaw University Observatory, Poland)
on 9 Feb 2011; 16:36 UT
Credential Certification: Phil Charles (pac@saao.ac.za)

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We have detected a new X-ray pulsar in the direction of the SMC using archival Chandra data (Observation ID 2758) which we designate SXP707. This observation (time interval 2002-08-27 04:04:23 - 18:15:24; useful exposure time 49.45 ks) shows a very significant detection of an X-ray source at RA= 01:04:28.7; Dec.= -72:31:34 (J2000) with an observed flux of 3.2 x 10^-13 ergs/cm^2/s and luminosity of 1.4 x 10^35 ergs/s (0.2 - 10.0 keV, assuming d= 60 kpc). The power spectrum of its light curve shows a highly significant peak at a period of 707.464 s +/- 0.250 s (see ftp://ftp.saao.ac.za/outgoing/andry/sxp707/scargle707.eps).

The position of this new X-ray pulsar is coincident with MACHO object 206.16885.25, OGLE II object SMC_SC10 57262 and OGLE III object SMC110.5 9503 at RA= 01:04:29.0; Dec.= -72:31:36(J2000) with a magnitude of V=15.780 +/- 0.029, B=15.578 +/- 0.018, I=15.955 +/- 0.024. The MACHO and OGLE light curves both show a strong modulation with a period of 37.15 d +/- 0.02 d which we interpret as the orbital period of the system.

The combined MACHO and OGLE light curve shows a longer term (super-orbital) variation of about half a magnitude on timescales of 500-800 d, with two very large outbursts (with an amplitude exceeding ~1 mag) on a timescale of 2000 d. We suggest that this behaviour is related to the formation and depletion of the circumstellar disc around the presumed Be donor star, similar to that seen in other SMC BeX systems recently described in Rajoelimanana, Charles& Udalski (2010, arXiv:1012.4610). The combination of these two periods (37 d and 500-800 d) fits very well in their super-orbital vs orbital period diagram.

The source is not present in the catalogue of emission line stars by Meyssonnier& Azzopardi, 1993. However, its long-term light curve is similar to that of other SMC Be/X-ray binaries sources (see ftp://ftp.saao.ac.za/outgoing/andry/sxp707/lcsxp707.eps). We therefore encourage optical spectroscopic observations to provide a refinement of the spectral type of the donor star and confirm its BeX nature.

ftp://ftp.saao.ac.za/outgoing/andry/sxp707/lcsxp707.eps