1No.80

Aug. 2021

ISSN 0917-2211

Variable Star Bulletin
ASASSN-V J205543.90+240033.5: another white dwarf pulsar?
Taichi Kato1
1Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

Received 2021 Aug 20Abstract
I found that ASASSN-V J205543.90+240033.5 shows large-amplitude (1.2–1.4 mag) nearly sinusoidal variations with a period of 10.803(2) d and very short period variations with a period of 0.0068 d using Public
Data Release of Zwicky Transient Facility observations. The only known object that shows a similar combination of nearly sinusoidal reﬂection variations and very short period, large-amplitude variations is the unique
white dwarf pulsar AR Sco. ASASSN-V J205543.90+240033.5 appears to be very similar to AR Sco andobservations at various wavelengths are desired.

ASASSN-V J205543.90+240033.5 is variable object detected by the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee et al. 2014, Kochanek et al. 2017) and was cataloged as and eclipsing binary (typeEB) with a period of 1.046984 d.1 This object was also independently listed as a candidate RR Lyr star by Sesaret al. (2017) using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1, Chambers et al.
2016). It was also independently listed as a variable star ATO J313.9329+24.0092 by Heinze et al. (2018) usingthe Asteroid Terrestrial-impact Last Alert System (ATLAS, Tonry et al. 2018) data.
I used Public Data Release 6 of the Zwicky Transient Facility (Masci et al., 2019) observations2 and foundthat this object showed large-amplitude (1.2–1.4 mag) nearly sinusoidal variations with a period of 10.803(2) dand very short period variations during time-resolved runs on two nights in the ZTF data (ﬁgures 1, 2).
The object is very blue as can be seen as the almost zero color index in ﬁgures 1, 2. The large distancemodulus (11.1 mag) based on the Gaia parallax (Gaia Collaboration et al., 2021) indicates that the object isintrinsically faint. These features suggest that this 10.803-d day variation is caused by reﬂection by the secondary
star in a binary containing a hot component.
The short-term variations are directly visible to the eye in the ZTF time-resolved photometry (ﬁgure 3).
Phase Dispersion Minimization (PDM; Stellingwerf 1978) analysis after de-trending the data yielded periods of0.00675(3) d for the BJD 2458661 run (ﬁgure 4) and 0.00682(2) d for the BJD 2458802 run (ﬁgure 5). The errorsof the periods were determined by the methods of Fernie (1989) and Kato et al. (2010). These periods are fairlyin agreement and I consider that they represent the spin period of the white dwarf.
This combination of the large-amplitude reﬂection variation and the high-amplitude spin pulse is seenin the white dwarf pulsar AR Sco (Marsh et al. 2016; Stiller et al. 2018). We recently reported that ZTF
J185139.81+171430.3 = ZTF18abnbzvx shows high-amplitude spin variations (Kato and Kojiguchi, 2021) andsuggested it to be a possible white dwarf pulsar, but the large-amplitude reﬂection variation is missing in ZTF
J185139.81+171430.3. The present observations indicate ASASSN-V J205543.90+240033.5 looks more similar toAR Sco than ZTF J185139.81+171430.3 and observations in various wavelengths to search for the evidence fornon-thermal emissions as in AR Sco.
1 Variable Stars Database (AVSD) <http://asas-sn.osu.edu/variables> on 23-Oct-2020) (22-Oct-2019 version).
2 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>.

2ASASSN−V J205543.90+240033.5

ZTF r
ZTF g

161718

58200

58400

58600

58800

59000

59200

59400

Figure 1: Long-term variations of ASASSN-V J205543.90+240033.5 in the ZTF data.

161718P=10.803 d−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 2: Phase-folded variations of ASASSN-V J205543.90+240033.5 in the ZTF data. The symbols are thesame as in ﬁgure 1. The epoch was chosen as BJD 2458773.10.

317.0
17.2
17.4
17.6
17.8
0.86

0.88

0.900.92

0.60

0.62

0.64

0.66

16.4
16.6
16.8
17.0
17.2

Figure 3: Short-term variations of ASASSN-V J205543.90+240033.5 in the ZTF data.

Acknowledgments
The author is grateful to Naoto Kojiguchi for supplying a wrapper code for obtaining the ZTF data.
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).

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Kato, T., & Kojiguchi, N. 2021, VSOLJ Variable Star Bull., 77 (arXiv:2107.09913)
Kato, T., et al. 2010, PASJ, 62, 1525
Kochanek, C. S., et al. 2017, PASP, 129, 104502
Marsh, T. R., et al. 2016, Nature, 537, 374

4θ
1.0

0.8

0.6

0.4

P=0.00675
0.003

0.004

0.005

0.006

0.007

0.008

0.0090.010

0.011

(d)
−0.10

−0.05

0.00

0.05

0.10

−0.5

0.0

0.5

1.0

1.5

Figure 4: Pulse proﬁle of ASASSN-V J205543.90+240033.5 on BJD 2458661. (Upper): PDM analysis. We
analyzed 100 samples which randomly contain 50% of observations, and performed the PDM analysis for thesesamples. The bootstrap result is shown as a form of 90% conﬁdence intervals in the resultant PDM θ statistics.
(Lower): Phase-averaged proﬁle.

5θ

1.0

0.8

0.6

P=0.00682
0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

0.011

(d)
−0.10

−0.05

0.00

0.05

0.10

−0.5

0.0

0.5

1.0

Figure 5: Pulse proﬁle of ASASSN-V J205543.90+240033.5 on BJD 2458802.

1.5

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