http://arxiv.org/abs/1906.08364
AR Scorpii (AR Sco) is a binary-star system containing the only known white dwarf pulsar. Previously reported photometric datasets only provide coverage back to 2005, but we extend the observational baseline for AR Sco back to the beginning of the twentieth century by analyzing observations from the Digital Access to a Sky Century at Harvard project (DASCH). We find that the orbital waveform of AR Sco remained constant across that baseline with no significant deviations from its average brightness. This result strongly suggests that the absence of accretion in modern observations is a long-term feature of AR Sco. Additionally, the DASCH light curve provides an opportunity to test an earlier prediction that an obliquity of the white dwarf would result in a precessional period observable in long-term studies of the orbital light curve. The DASCH observations do not indicate the presence of such a period, and we show that previous, inconclusive tests of this hypothesis were insensitive to the existence of a precessional period. Furthermore, the long DASCH baseline enables us to constrain the rate of change of the orbital frequency to $\dot{\nu} \lesssim 3.8\times10^{-20}$ Hz/s, constraining the efficacy of magnetic braking as a mechanism of angular-momentum loss in this system. Finally, we discuss how the combination of the orbital waveform’s stability, high amplitude, and short period should make it possible to identify additional white-dwarf pulsars in all-sky survey data.
E. Peterson, C. Littlefield and P. Garnavich
Fri, 21 Jun 19
52/56
Comments: Accepted for publication in AJ on June 17, 2019, 10 pages, 7 figures