Swift-XRT detection of the candidate white dwarf pulsar ZTF J185139.81+171430.3

ATel #14965; Noel Klingler (NASA Goddard Space Flight Center / University of Maryland Baltimore County), George Pavlov (Pennsylvania State University)
on 12 Oct 2021; 04:50 UT
Credential Certification: George Pavlov (pavlov@astro.psu.edu)

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ZTF J185139.81+171430.3 = ZTF18abnbzvx (hereafter, J1851) is a candidate white dwarf (WD) pulsar exhibiting optical (R- and G-bands) pulsations with a ~0.8 magnitude amplitude at a period P=742.173(3) s (Kato & Kojiguchi 2021; arXiv:2107.09913). Swift observed J1851 on 2021-08-15 (for 0.85 ks) and again on 2021-08-29 (for 2.07 ks; total exposure ~2.9 ks). The count rates in the UVOT B- and UVW1-bands were too low to resolve the sinusoidal variations seen in the (ZTF) R- and G-bands with the above exposure. However, significant X-ray emission from J1851 was detected with the XRT, which collected 51 +/- 7 net counts in the 2.9 ks exposure (PC mode). The X-ray spectrum can be described by an absorbed power-law model, with N_H ~< 1e21 cm^-2 and Gamma = 0.9 +/- 0.4, with a corresponding observed flux (0.3-10 keV) F_X = (1.5 +/- 0.5) * 10^-12 erg cm^-2 s^-1. At the object's distance d ~ 3.1 kpc (parallax = 0.325(119) mas; Gaia Collaboration 2021), the flux corresponds to an X-ray luminosity L_X = (1.7 +/- 0.6) * 10^33 erg/s. This is a factor of ~400 more luminous than the WD pulsar AR Sco (L_X ~ 4 * 10^30 erg/s).