Multicolor Photometry of SXP 1062 in Outburst
ATel #13624; P. C. Schmidtke, A. P. Cowley (Arizona State University)
on 9 Apr 2020; 03:08 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary, Neutron Star, Pulsar
X-ray pulsations of the Be/X-ray pulsar SXP 1062 were discovered by Henault-Bruent et al. (2012, MNRAS, 420, L13). Its optical counterpart is the emission-line star 2dFS 3831 (Evans et al. 2004, MNRAS, 353, 601). OGLE-IV observations show recurrent optical outbursts with a probable orbital period near 656 days (Schmidtke et al. 2012, ATel #4596). I-band observations of the 2019 October outburst (Schmidtke & Cowley 2020, ATel #13426, hereafter SC20) reveal a complex structure that is not prominent in earlier outbursts. We report here on the analysis of images taken with BVRc filters of the Johnson-Cousins system on the same nights as the SC20 Ic data.
The new images (all 60-s exposures) were obtained with telescope T17 of the www.itelescope.net network and calibrated using the photometry of Massey (2002, ApJS, 141, 81). In http://www.public.asu.edu/~atpcs/SXP/SXP1062_2019_BVRI.pdf , we plot a phased light curve for each filter, using the updated ephemeris of SC20. The double-peak structure of the optical outburst is present in all filters. The outburst amplitude, relative to the mean of 5 pre-outburst observations, is largest in Ic (0.419+/-0.012) and smallest in B (0.241+/-0.018), which implies the temperature of the outbursting region is less than that of the combined light of the Be star and its disk. It is important to note that daily observations do not fully resolve the outburst, hence the quoted amplitudes are considered minimum values. Future observations need to be taken with improved time resolution to better resolve the structure and color changes.
To further study the nature of the outburst we converted the observed BVRcIc magnitudes to flux units and calculated the net flux from the outburst itself. At maximum light, the net flux is equivalent to an object with the following magnitudes: B=15.810+/-0.081, V=15.426+/-0.086, Rc=15.172+/-0.076, and Ic=14.996+/-0.041. The implied colors ((B-V)=+0.384, (V-Rc)=+0.254, and (Rc-Ic)=+0.176) are similar to those of an early F dwarf star and roughly consistent with a 6250 K blackbody. Unfortunately, a reliable color curve of the entire outburst can not be constructed due to the increase in photometric errors in declining portions of the outburst.
Double-peaked outbursts have been observed in several other sources, e.g. SXP 7.78 (SMC X-3), SXP 46.6, SXP 293, and SXP 327 (see Schmidtke et al. 2013, MNRAS, 431, 252). Often the first peak is stronger, but in some cases, like SMC X-3, the second peak is stronger. The variety of profiles can be explained if a single broad peak is temporarily obscured, perhaps by an accretion stream or disk associated with the pulsar. Alternatively, the impacted portion of the Be star's disk might be occulted by the star itself. In both scenarios, details of the resulting dip would depend on system geometry (for example, inclination of the Be disk, orientation of the pulsar's orbital plane, etc.). In most cases no multicolor data are available. Among the exceptions is SXP 327, for which MACHO V and R data show a systemic color that is redder during the double peaks but bluer in the intervening dip. High-quality time-resolved multicolor measurements throughout an outburst would help advance our understanding of these interesting systems