Changes in the OGLE Light Curve for SXP 5.05

ATel #15837; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 31 Dec 2022; 16:33 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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The SMC X-ray source IGR J00569-7226 was discovered by Coe et. al. (ATel #5547). Kenna (ATel #5553) refined the X-ray position and identified a known Be star as the optical counterpart. From OGLE-II data, Schmidtke et al. (ATel #5557) found evidence for low-amplitude optical variations with P=17.2 d (that might be orbital) or its alias at P=0.94 d (that could be nonradial pulsations). A 17-d modulation in X-ray data (Coe et al., ATel #5631) reinforced the orbital interpretation. Further analysis revealed X-ray pulsations with P=5.05 s (Coe et al., ATel #5662). Hence, this Be/X-ray pulsar is known as SXP 5.05. Pulse timing analysis during the 2013 Type II outburst yielded an orbital solution with an eccentricity of 0.155 and a period of 17.13 d (Coe et al. 2015, MNRAS, 447, 2387). Concurrent OGLE data showed periodic brightenings, superimposed on the large outburst. These features are likely to be an orbital signature. However, there has been no period search of the entire OGLE data set.

We examined all OGLE I-band observations, including recent measurements (2022 August 9 to December 2) taken after resumption of operations that followed the 2.4-yr coronavirus shutdown. The long-term light curve up to OGLE-IV Season 4 is shown in Fig. 1 of Coe et al. (2015). Subsequent observations are available at the XROM web site (https://ogle.astrouw.edu.pl/ogle4/xrom/igr_j00569-7226.html). Smooth trends in data from individual observing seasons were removed by subtracting low-order polynomial fits. Observations from OGLE-IV Season 4, which coincide with the large outburst, were not used. Combining detrended data from OGLE-III Season 5 onward (1542 points. spanning 17 years), PDM analysis yields a period of 17.148 +/- 0.003 days. The estimated time of minimum light (T0) is JD 2459870.47 +/- 0.25 and comes from a fit to a well-defined dip in recent data. The accompanying figure shows folded light curves plotted on this ephemeris, with individual panels for each segment studied.

Early data (OGLE-II and OGLE-III Seasons 1-4) show only smooth, low-amplitude variations that are consistent with prior reports. Structure in the light curve starts to appear midway through OGLE-III and continues to become more prominent up to the present. The increase in structure is somewhat correlated with overall system brightness, in the sense that when bright there tends to be little or no structure. Comparing OGLE-II data (top panel of figure) with the most recent data (bottom panel), the r.m.s. of the detrended light curve increased from 0.014 to 0.020 mag, while the mean system brightness dimmed from I=15.74 to 15.92.

Multiple dips and brightenings are apparent in the most recent light curve. The deepest minimum occurs at phase zero and is followed by the brightest maximum. A shallower dip falls near phase 0.3, which is followed (possibly) by an even shallower dip near phase 0.7. The primary dip is broader than the X-ray eclipse (Coe et al. 2015), which indicates an extended region is being occulted, possibly an accretion stream or a disk surrounding the compact object. In the system geometry proposed by Brown et al. (2019, MNRAS, 486, 3078), the orbital plane of the Be and neutron star binary is seen nearly edge on, with the equatorial disk of the Be star oriented roughly perpendicular to the orbital plane. If the eccentricity of the orbit is small, then two, unequally spaced, occultations can occur during each orbit. This geometry is consistent with general features of the observed light curve.

OGLE Light Curves for SXP 5.05