Discovery of an 18-d Photometric Period in the Optical Counterpart of Swift J0208.4-7428
ATel #5776; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 18 Jan 2014; 23:39 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary
Swift J0208.4-7428 was identified in Integral/IBIS monitoring of the Magellanic Clouds and, by positional coincidence, correlated with a transient X-ray source in archival Swift/XRT data (McBride et al., 2010, MNRAS, 403, 709). The likely optical counterpart is a bright, blue star in the Magellanic Bridge (Demers & Irwin, 1991, A&AS, 91, 171). Weak H-alpha emission was detected by Masetti et al. (2010, A&A, 519, A96). The X-ray and optical properties suggest the source is one of the few known high-mass X-ray binaries in the Magellanic Bridge. To date, no X-ray pulsations have been reported.
The optical counterpart of Swift J0208.4-7428 falls in one of the OGLE-IV fields (MBR109.12, star 1333), and 4 seasons of photometry are currently available on the XROM web site (Udalski, 2008, Acta Astron., 58, 187). The long-term light curve shows gradual changes in mean brightness (I~15.0-14.9) on a time scale of about 700 days. Other, more rapid variations (up to ~0.07 mag) also are present.
The OGLE observations of Swift J0208.4-7428 were detrended by subtracting low-order polynomial fits to seasonal segments. Periodogram analysis of the flattened data set reveals power near 18.18 d and its aliases at 1.06 and 0.51 d. These signals are strong in seasons 1 and 2, weak in season 3, and insignificant in season 4 (only 21 data points are currently available). When folded on the shorter periods, the resulting light curves are roughly sinusoidal in shape with considerable scatter. The 18.18-d curve, however, shows variations that are consistent with a small outburst, suggesting this might be an orbital signature. Using phase-dispersion minimization, a refined period of P=18.153 d was obtained. The I-band light curve for Swift J0208.4-7428, folded on this period, is available at the URL listed below. A small dip in brightness is present near the beginning of the outburst. While most prominent in seasons 1 and 2, the dip also is apparent in data from seasons 3 and 4. Dipping behavior similar to this has been observed in the orbital outbursts of several Be/X-ray pulsar systems (see Schmidtke et al., 2013, MNRAS, 431, 252).
OGLE-IV Light Curve for Swift J0208.4-7428
