OGLE-IV photometry of 1A0538-66: long-term brightness changes and behavior of optical outbursts

ATel #6135; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 14 May 2014; 20:21 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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1A0538-66 is a Be/X-ray pulsar in an eccentric 16.6-d binary orbit. Recurrent optical and X-ray outbursts occur when the 69-ms pulsar penetrates the Be star's disk. Due to its high orbital inclination, the system is brighter when the disk dissipates and fainter when it is active, thereby blocking some light from the Be star. Using archival photographs, Skinner (1981, Sp. Sci. Rev., 20, 441; Sk81) found the orbital period to be 16.6515 d.

Alcock et al. (2001, MNRAS, 321, 678) analyzed ~5 years of MACHO photometry and discovered an apparent 421-d activity cycle (see their Fig. 1). The light curve shows the system was bright (V~14.4) for extended intervals of time before dimming to V~14.9. McGowan & Charles (2003, MNRAS, 339, 748; MC03) linked the MACHO data with plate material from Sk81, suggesting stability of 421-d periodicity. However, the photographic data alone show other long-term periodicities, with the strongest signal near P~340 d. Linking outbursts in MACHO data and Sk81's photometry, MC03 found an updated orbital period of 16.6460 d.

We have examined recent (2010-2014) observations of 1A0538-66 from the OGLE-IV project to study the long-term brightness changes and behavior of the optical outbursts. Approximately 5 seasons of I-band photometry are available from the XROM web site (Udalski, 2008, Acta. Astron., 58, 187). The data (see http://ogle.astrouw.edu.pl/ogle4/xrom/1a0538-66.html ) show a smaller range of underlying system brightness (I~15.3 to 15.5). The irregular variations are superimposed with outbursts of variable amplitude. The 421-d cycle, present in the 1990s, is not seen in OGLE-IV observations, and the prolonged bright states of the MACHO data are noticeably absent.

A detrended light curve was constructed by clipping the brightest points and subtracting low-order polynomials from short data segments (100-250 d). These fitted values define the underlying system brightness, I_sys. Linking 8 bright OGLE-IV outbursts with 4 outbursts observed in 1981 by Densham et al. (1983, MNRAS, 205, 1117), we find P=16.6429+/-0.0007 d and T0=JD 2445674.3+/-0.2. The OGLE-IV data folded on this period are shown at the URL below (Fig. 1).

Our orbital period is slightly shorter than that of MC03 (based on independent data sets), but it results in a better fit to the new data. When folded on the MC03 ephemeris, the bright OGLE-IV outbursts fall before phase 0 and the phase dispersion of these peaks is greater. While the outbursts in Fig. 1 appear to be double peaked, this is an artifact of the orbital period and daily sampling. The "second" peak falls ~0.06 cycle, or ~1 d, after the "first" one. All bright points in the secondary peak are immediately preceded by either a brighter data point or missing observation.

To investigate the relation between outburst amplitude and system brightness, we separated the OGLE-IV data into 4 bins, based on I_sys values. A folded light curve for each bin is shown in Fig. 1. When the system is bright (I_sys < 15.41), the outbursts are of very low amplitude (< 0.1 mag). The outbursts are considerably larger (up to 0.7 mag) when the system is faint (I_sys > 15.45). At intermediate system brightness, the outbursts have moderate amplitude (up to ~0.4 mag). This relationship is consistent with a high orbital-inclination model. We note that the largest outbursts observed by MACHO also occurred when the system was near minimum brightness, as can be seen in Fig. 3 of Alcock et al..

In summary, OGLE-IV photometry of 1A0538-66 reveals the MACHO 421-d activity cycle is no longer present. Instead, long-term variations in system brightness occur on irregular time scales of a few hundred days. The outburst amplitude depends on system brightness, with the largest outbursts occurring when the system is faintest.