1No.77

Jul. 2021

ISSN 0917-2211

Variable Star Bulletin
ZTF J185139.81+171430.3 = ZTF18abnbzvx: the second white dwarf pulsar?
Taichi Kato1 and Naoto Kojiguchi1
1

Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

Received 2021 Jul. 21

Abstract
We found that ZTF J185139.81+171430.3 = ZTF18abnbzvx shows large-amplitude (0.8 mag) coherent
variations with a very short [0.00858995(3) d = 12.37 min] period using Public Data Release of Zwicky
Transient Facility observations. The only known object that shows similar very short period, large-amplitudeand coherent variations is the unique white dwarf pulsar AR Sco. The variations in ZTF J185139.81+171430.3
may arise from a mechanism as in AR Sco and this object should deserve attention.

ZTF18abnbzvx is a variable object 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) AlertBroker (Förster et al., 2021). 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 in table 4 of Ofek et al. (2020).
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.
We used Public Data Release 6 of the Zwicky Transient Facility (Masci et al., 2019) observations1 and foundthat 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). We found that the period is in agreement with the candidate
period in Ofek et al. (2020). We also conﬁrmed that the variations recorded for BJD 2458675–2458746, thesegment with multiple nightly observations, could be expressed by the same period. By using the entire data
BJD 2458646–2458746, we obtained a period of 0.00858995(3) d. The resultant phased light curve (ﬁgure 2)
shows that that the variations are coherent.
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 withcomparably short periods are AM CVn stars (see e.g. Solheim 2010). No known AM CVn stars with similar periods
show such large-amplitude orbital modulations. The small parallax of 0.325(119) mas in Gaia Collaboration et al.
(2021) also does not favor an underluminous binary.
Stellar pulsations are also unlikely. The large amplitude requires radial pulsations, but there is no knownclass of radially pulsating variables with this short period.
We propose that this short period may reﬂect the spin period of the white dwarf. The famous white dwarf
pulsar AR Sco (Marsh et al., 2016), 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 theoptical (Marsh et al. 2016; Stiller et al. 2018).
1 The ZTF data can be obtained from IRSA <https://irsa.ipac.caltech.edu/Missions/ztf.html> using the interface <https://irsa.ipac.caltech.edu/docs/program_interface/ztf_api.html> or using a wrapper of the above IRSA API<https://github.com/MickaelRigault/ztfquery>.

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17.5

18.0

18.5

19.0
0.84

0.86

0.88

0.90

Figure 1: Short-period variation in ZTF J185139.81+171430.3 = ZTF18abnbzvx.
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 subsequentday. These “outbursts”, if present, should have been short-lived as in outbursts in intermediate polars. Althoughfurther 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.

Acknowledgments
This work was supported by JSPS KAKENHI Grant Number 21K03616.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as
part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant
No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar
Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches
Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortiumof Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations
are conducted by COO, IPAC, and UW.
The ztfquery code was funded by the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement n◦ 759194 – USNAC, PI: Rigault).

References
Förster, F., et al. 2021, AJ, 161, 242
Gaia Collaboration, et al. 2021, A&A, 649, A1
Marsh, T. R., et al. 2016, Nature, 537, 374

317.0

17.5

18.0

18.5

19.0

P=0.00858995 d
19.5

−0.4

−0.20.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

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.

16

ZTF r
ZTF g

17

18

1920

58200

58400

58600

58800

59000

59200

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

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Masci, F.-J., et al. 2019, PASP, 131, 018003
Ofek, E. O., Soumagnac, M., Nir, G., Gal-Yam, A., Nugent, P., Masci, F., & Kulkarni, S. R. 2020, MNRAS, 499,5782
Solheim, J.-E. 2010, PASP, 122, 1133
Stiller, R. A., Littleﬁeld, C., Garnavich, P., Wood, C., Hambsch, F.-J., & Myers, G. 2018, AJ, 156, 150

VSOLJ
c/o Keiichi Saijo National Science Museum, Ueno-Park, Tokyo Japan
Editor Seiichiro Kiyota
e-mail: skiyotax@gmail.com