Multiple Photometric Periods in SXP1323 (RX J0103.6-7201)

ATel #716; Paul C. Schmidtke and Anne P. Cowley (Arizona State University)
on 30 Jan 2006; 18:49 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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RX J0103.6-7201 (SXP1323) is a Be/neutron-star binary in the Small Magellanic Cloud with the longest known X-ray pulsation period (1323 s; Haberl & Pietsch 2005, A&A, 438, 211). The optical counterpart is #1393 from the emission-line catalogue of Meyssonnier & Azzopardi (1993, A&AS, 102, 451). Periodogram analysis of OGLE-II photometric data for this 14th magnitude star reveals three strong periodicities at 26.16 d, 0.88 d, and 0.41 d. All show approximately sinusoidal light curves with full amplitudes of 0.045, 0.035, and 0.03 mag, respectively. The analysis is complicated since all of these periods, as well as their aliases, are present simultaneously. We have divided the data into four seasons to search for changes in the periods or amplitudes, but they appear to be stable. The two shorter periods are most likely due to non-radial pulsations of the Be star, of the type seen in several other Be/neutron-star X-ray sources. The 26-d period is difficult to understand, since it is shorter than would be expected from the pulsation/orbital period relation for Be/X-ray binaries (e.g. Corbet 1984, A&A, 141, 91). Furthermore, the sinusoidal shape of the light curve argues against it being an orbital period, since generally such light curves show brief optical (and/or X-ray) outbursts as the neutron star passes periastron, rather than a smooth sinusoidal variation. An alternate interpretation is that the strong 26-day period is the alias of a shorter non-radial pulsation period at either 0.96 or 1.04 days, but the power is weaker at these periods. Further data are needed to resolve this ambiguity.