Astronomers May Have Detected Second White Dwarf Pulsar (Cosmology)

Using Public Data Release of Zwicky Transient Facility observations, Japanese astronomers found that ZTF J185139.81+171430.3 (hereafter ZTF18abnbzvx) shows a very short very short (12.37 min) period and large-amplitude (0.8 mag) coherent variations. The unique white dwarf pulsar ‘AR sco’ is the only known object that shows similar very short period, large-amplitude and coherent variations. Their study recently appeared in Arxiv.

ZTF18abnbzvx is a variable object which was first detected by the Zwicky Transient Facility (ZTF) project. The object was classified as a dwarf nova by the Automatic Learning for the Rapid Classification of Events (ALeRCE) Alert Broker. The object was listed as a candidate variable star (ZTF J185139.81+171430.3, hereafter ZTF J1851) having a range of 17.561–18.836 and a period of 0.0086 d ~ 12.37 min. Automatic period detections often give spurious periods (either close to 1 d, 0.5 d or very short ones) and such periods are not usually considered seriously.

Figure 1: Short-period variation in ZTF J185139.81+171430.3 = ZTF18abnbzvx. © Kato and Kojiguchi

Now, by using Public Data Release 6 of the Zwicky Transient Facility observations, Kato and Kojiguchi found that this object showed large-amplitude (0.6–1.0 mag) and very short period variations during a time-resolved run on 2019 June 12 (BJD 2458646; figure 1). They first confirmed that the period of the star is in agreement with previously estimated one. Later, they also confirmed that the variations recorded for BJD 2458675–2458746, the segment with multiple nightly observations, could be expressed by the same period. Finally, by using the entire data BJD 2458646–2458746, they obtained a period of 0.0085 days ~ 12.37 min. The resultant phased light curve (figure 2) shows that the variations are coherent.

Figure 2: ZTF r phase-folded light curve of ZTF J185139.81+171430.3 = ZTF18abnbzvx for the segment BJD 2458675–2458746. The epoch was chosen as BJD 2458737.3078 © Kato and Kojiguchi

Additionally, it has been shown that the variation arising from the orbital variation (such as a reflection variable) is very unlikely considering the large amplitude (0.8 mag) and the very short (12.37 min) period. The only known class of binaries with comparably short periods are AM Canum Venaticorum stars (AM CVn stars). No known AM CVn stars with similar periods show such large-amplitude orbital modulations. The small parallax of 0.325(119) mas also does not favor an underluminous binary.

“Stellar pulsations are also unlikely. The large amplitude requires radial pulsations, but there is no known class of radially pulsating variables with this short period.”

— they said

Finally, they proposed that this short period may reflect the spin period of the white dwarf. The famous white dwarf pulsar AR Sco, which is a 3.56-hr binary consisting of a white dwarf and an M5 star. The spin period of the white dwarf in AR Sco is 1.97 min and amplitudes of the pulses reach 0.5–1.0 mag in the optical.

Figure 3: Long-term light curve of ZTF J185139.81+171430.3 = ZTF18abnbzvx from ZTF observations © Kato and Kojiguchi

In contrast to AR Sco, orbital modulations are not apparent in ZTF J1851 (figure 3). There were possible outbursts with amplitudes of ∼2 mag such as on BJD 2458685 and on BJD 2459042. The reality of these possible outbursts requires confirmation since there were only a few observations during these events A quasi-simultaneous rise in g and r was recorded on BJD 2459042. The object returned to the normal brightness on the subsequent day. These “outbursts”, if present, should have been short-lived as in outbursts in intermediate polars.

“Although further observations are needed to see whether the short-period coherent variations in ZTF J1851 indeed arises from a spinning white dwarf, this object would be a good candidate for a white dwarf pulsar and should deserve attention.”

— they concluded

Reference: Taichi Kuto and Naoto Kojiguchi, “ZTF J185139.81+171430.3 = ZTF18abnbzvx: the second white dwarf pulsar?”, pp. 1-3, 2021.

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