No eclipses in the double WD binary SDSS J125733.63+542850.5

ATel #2778; A. Shporer (LCOGT, UCSB), D. Kaplan (KITP), L. Bildsten (KITP), S. Howell (NOAO), J. Steinfadt (UCSB)
on 7 Aug 2010; 06:43 UT
Credential Certification: Avi Shporer (shporer@wise.tau.ac.il)

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We have carried out a search with the Faulkes Telescope North (FTN) for eclipses in the binary white dwarf (WD) system SDSS J125733.63+542850.5, resulting in a non-detection.

SDSS J125733.63+542850.5 was identified by Marsh et al. (2010) and Kulkarni & van Kerkwijk (2010) as a binary system consisting of a low-mass He WD (the primary) and a high-mass CO WD (the secondary), with an orbital period of 4.56 hours. We used the accurate ephemeris given by Marsh et al. (2010) to schedule FTN observations at the expected primary and secondary eclipse times. So as to improve the cadence, we used no filter (clear), resulting in an exposure time of 10 seconds and a median cycle time of 30.5 seconds during our observations, where each of the primary and secondary eclipses were observed twice.

The expected eclipse duration (3.1 minutes) and depths were estimated using the system parameters given by Kulkarni & van Kerkwijk (2010). The eclipse durations are much longer than the uncertainty on the eclipse time of 19 seconds. Expected eclipse depths are 31.3 ± 7.4 % for the secondary and 2.2 ± 0.5 % for the primary, where uncertainties account for the uncertain radii ratio and effective temperatures of the two WDs. In our calculations we included the measured CCD quantum efficiency and the effect of lensing during the primary eclipse (see Marsh 2001).

After normalizing our light curves to unit flux while ignoring points within the predicted eclipse time, we measured the average intensity of these points to be 100.2 ± 1.5 % and 99.7 ± 0.6 %, for the secondary and primary respectively. Therefore, we can reject at the 3 sigma level any variability larger than 5 % (2 %) during the secondary (primary) eclipse time. Given the expected eclipse depth this leads to a rejection of the eclipsing nature of the system at 20 (3) sigma for the secondary (primary) eclipse, although we acknowledge that the wide range in predicted eclipse depth for he secondary leads to a range in confidence for the rejection.

The resulting light curves are visually shown here. Top panel shows light curves during the expected time of secondary eclipse, with vertical dashed lines marking the predicted start and end times of the partial eclipse, and dotted lines the start and end of the full eclipse. The expected eclipse depth is out of the scale of the plot. Bottom panel shows the primary eclipse, where a schematic eclipse shape is overplotted as a solid line. The dot-dashed line shows the schematic shape of a full eclipse that is borderline grazing.

The rejection of eclipses allows us to put an upper boundary on the system's inclination of 88.7 ± 0.3 degrees, at a confidence level higher than 3 sigma. This inclination corresponds to a borderline grazing eclipse whose impact parameter equals the difference in the WDs radii, according to Kulkarni & van Kerkwijk (2010). Expected eclipse depth decreases fast with decreasing inclination angle below the value above. An inclination smaller by 1 sigma results in shallower eclipses by a factor of about 2, which can not be ruled out at high significance by our data and the current understanding of the system.

We note that the timing of our observations and the estimated eclipse durations were done assuming a circular orbit.

FTN light curves