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Photometric Periods for SMC Be/X-ray Pulsars SXP645 and SXP726

ATEL # 1632; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University)
on 25 Jul 2008; 21:20 UT
Password Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

Subjects: Optical, X-ray, Binaries, Neutron Stars, Pulsars

SXP645: Recently, Haberl et al. (arXiv:0806.4132v1) discovered X-ray pulsations with a period of 645 s in the transient source XMMU J005535.2-722906. They identified the optical counterpart in the MACHO and OGLE-II data sets with a V=14.6 star that brightened by ~0.85 mag near MJD 50100 and faded to its original brightness at MJD 51800 (see their Fig. 7). We have searched the early, low-state, MACHO R data and found a strong sinusoidal signal (with a normalized power of 52 in the periodogram) at P=0.41606 d. A weaker signal is also present (power 13) at P=1.03490 d (and its aliases near 27.5 and 0.96 d). These very short periods are probably due to non-radial pulsations of the Be star, as found in similar systems. However, analysis of OGLE I data taken when the source was bright does not show any significant periodocities.

SXP726: RX J0105.9-7203 has been found by Eger & Haberl (A&A 485, 807, 2008) to show 726 s X-ray pulsations. The optical counterpart for this source had been identified with the emission line star [MA93]1557 by Sasaki et al. (A&A 403, 901, 2003). Using OGLE-II data Eger & Haberl found peaks in the periodogram at 2.35 and 3.2 days. We have reanalyzed the OGLE I data extending the analysis to higher frequencies. There is a very prominent signal (power 78) at P=0.30195 d and a second signal (power 51) at P=0.67271 d. Folded light curves for both periods are sinusoidal, also suggesting they are due to non-radial pulsations of the optical star. Peaks in the periodogram for these short periods are much more prominent than ones in the 2 to 4 day range, which are aliases of the short periods reported here.


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