1No.81

Sep. 2021

ISSN 0917-2211

Variable Star Bulletin
Orbital and spin periods of the candidate white dwarf pulsar
ASASSN-V J205543.90+240033.5
Taichi Kato1 , Franz-Josef Hambsch2,3,4 , Elena P. Pavlenko5 , Aleksei A. Sosnovskij5
1

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

23Groupe Européen d’Observations Stellaires (GEOS), 23 Parc de Levesville, 28300 Bailleau l’Evêque, France

Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV), Munsterdamm 90, 12169 Berlin, Germany
45

Vereniging Voor Sterrenkunde (VVS), Oostmeers 122 C, 8000 Brugge, Belgium

Federal State Budget Scientiﬁc Institution “Crimean Astrophysical Observatory of RAS”,
Nauchny, 298409, Republic of Crimea

Received 2021 Sep. 09

Abstract
ASASSN-V J205543.90+240033.5 has been suggested to be a white dwarf pulsar by Kato (2021, arXiv:2108.09060).
We obtained time-resolved photometry and identiﬁed the orbital and spin periods to be 0.523490(1) d and0.00678591(1) d = 9.77 min, respectively. These values strengthen the similarity of this object with AR Sco.
We estimated that the strength of the spin pulse is 3.6 times smaller than in AR Sco.

1Introduction

ASASSN-V J205543.90+240033.5 is a variable object detected by the All-Sky Automated Survey for Supernovae(ASAS-SN, Shappee et al. 2014, Kochanek et al. 2017). This object was also independently listed as a candidate
RR Lyr star by Sesar et al. (2017) using the Panoramic Survey Telescope and Rapid Response System (PanSTARRS1, Chambers et al. 2016). It was also independently listed as a variable star ATO J313.9329+24.0092 byHeinze et al. (2018) using the Asteroid Terrestrial-impact Last Alert System (ATLAS, Tonry et al. 2018) data.
In Kato (2021), one of the authors (TK) reported superimposed long and short periods and suggested that thisobject could be a white dwarf pulsar similar to AR Sco (Marsh et al. 2016; Stiller et al. 2018).

2Observation and true orbital period

Following this suggestion, FJH reported time-resolved photometry on seven consecutive night between 2021
August 24 and 30 using a 50-cm telescope located in San Pedro de Atacama (Chile). The total number ofobservations was 979 and the time resolution was 96 s. EP and AS also supplied a light curve using the 2.6-meter
reﬂector (Shajn Telescope, ZTSh) in Crimea. EP suggested that the orbital period would be longer than the0.2 d one suggested by TK in vsnet-alert 26186.1
1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26186 >.

2161718

P=0.523490 d−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 1: Phase-folded variations of ASASSN-V J205543.90+240033.5 in the ZTF data. Filled circles and squares
represent ZTF r and g observation, respectively. The epoch was chosen as BJD 2458773.24.
We reanalyzed the data from Public Data Release 6 of the Zwicky Transient Facility (Masci et al., 2019)
observations2 and found that the true orbital period is 0.523490(1) d (ﬁrst announced in vsnet-alert 262063 ),not 10.803(2) d as reported in Kato (2021). The period and error were determined by the Phase Dispersion
Minimization (PDM; Stellingwerf 1978) method and the methods in Fernie (1989) and Kato et al. (2010). Figure
1 shows the phase-folded light curve using the true orbital period. The light curve is almost perfectly sinusoidal
and is interpreted as the reﬂection-type variation caused by strong irradiation from the white dwarf. This period
is conﬁrmed to express all the observations by FJH (ﬁgure 2). The initially obtained period of 10.803 d was analias of the 0.523490 d period resulting from the intervals of the nightly ZTF observations.

3Spin period

ASASSN-V J205543.90+240033.5 showed short-period coherent variation in two ZTF time-resolved runs (Kato,2021). The periods on the two nights were 0.00675(3) d and 0.00682(2) d. The pulse proﬁle using FJH data isshown in ﬁgure 3. Almost sinusoidal 0.06-mag variations were clearly detected. The best period using this data
set is 0.0067882(7) d, although there remains a possibility of aliases (such as 0.006742 d). Using this value, wehave been able to identify the spin period using the ZTF data (ﬁgure 4). The resultant period is 0.00678591(1) d= 9.77 min.

4Comparison with AR Sco

With the newly determined orbital and spin periods, the similarity between ASASSN-V J205543.90+240033.5
has become more evident (table 1, the periods for AR Sco were taken from Stiller et al. 2018). As seen from thealmost zero color index in ﬁgure 1, ASASSN-V J205543.90+240033.5 is not very signiﬁcantly reddened. The Gaia
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>.
3 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26206 >.

3

16.0

16.5

17.0

17.5

18.0

P=0.523490 d
18.5

−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 2: Phase-folded light curve of ASASSN-V J205543.90+240033.5 using time-resolved photometry by FJH.
The period and epoch are the same as in ﬁgure 1.
Table 1: Comparison between ASASSN-V J205543.90+240033.5 and AR Sco

Orbital period (d)Spin period (d)ASASSN-V J205543.90+240033.5
0.523490
0.00678591

AR Sco0.14853528
0.00136804584

color GBP − GRP is +0.38 (Gaia Collaboration et al., 2021). Considering that the white dwarf and illuminatedhemisphere of the secondary are the dominant source of light, we consider the unreddened color to be around
0. On the other hand, AR Sco is more strongly reddened. The Gaia color GBP − GRP is +1.38. Assumingthat the unreddened colors of these two object are around 0, we can estimate extinctions by using the formula
A(G) = 1.89E(GBP − GRP ) (Wang and Chen, 2019). We adopted A(G) values for 0.72 and 2.61 for ASASSN-VJ205543.90+240033.5 and AR Sco, respectively. Using Gaia parallaxes (Gaia Collaboration et al., 2021), theunreddened absolute magnitudes (MG ) of these two objects are +5.4 and +7.0, respectively. This indicates that
ASASSN-V J205543.90+240033.5 is intrinsically 4.4 times more luminous than AR Sco, which appears to reﬂect
the diﬀerence in the orbital period. The mean pulse amplitude is 0.7 mag (90%) in AR Sco (ﬁgure 10 in Stilleret al. 2018). Considering that the system is 4.4 times more luminous in ASASSN-V J205543.90+240033.5, thepulse amplitudes (0.06 mag) in the latter corresponds to 25% pulse in AR Sco, which means that the strength of
the spin pulse is 3.6 times smaller than in AR Sco. Whether this diﬀerence reﬂects the diﬀerence in the emission
mechanism needs to be studied by further multiwavelength observations.

Acknowledgments
The author is grateful to Naoto Kojiguchi for supplying a wrapper code for obtaining the ZTF data. The authoris also grateful to Yusuke Tampo for processing the VSNET campaign 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

4

θ

1.02

1.00

0.98

0.96

0.94

0.92

P=0.006788
0.90
0.00600.00650.00700.00750.00801.0

1.5(d)
−0.06

−0.04

−0.02

0.00

0.02

0.040.06

−0.5

0.0

0.5

Figure 3: Pulse proﬁle of ASASSN-V J205543.90+240033.5 using FJH data after removing the orbital variation.
(Upper): PDM analysis. We analyzed 100 samples which randomly contain 50% of observations, and performed
the PDM analysis for these samples. The bootstrap result is shown as a form of 90% conﬁdence intervals in theresultant PDM θ statistics. (Lower): Phase-averaged proﬁle.

5

θ

1.00

0.95

P=0.0067859

0.90

0.006700.00675

0.00680

0.00685

(d)
−0.06

−0.04

−0.02

0.000.02

0.04

0.06

−0.5

0.0

0.5

1.0

1.5

Figure 4: Pulse proﬁle of ASASSN-V J205543.90+240033.5 using the ZTF after removing the orbital variation.
(Upper): PDM analysis. (Lower): Phase-averaged proﬁle.

6part 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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