1No.97

Mar. 2022

ISSN 0917-2211

Variable Star Bulletin
Gaia22ayj: outburst from a deeply eclipsing 9.36-min binary?
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan

Received 2022 Mar. 24

Abstract
Gaia22ayj (=ZTF19aagmvuk) was detected as a transient on 2022 March 3 both by the Gaia satellite
and the Zwicky Transient Facility (ZTF). I analyzed the past public ZTF data and found that Gaia22ayj
showed coherent large-amplitude double-wave variations with a period of 0.00649910257(13) d = 9.36 min.
The period can be either from the spin period of the white dwarf or the orbital period of the binary. I consider
the latter possibility more likely based on the light curve resembling an eclipsing binary and on the stabilityof the proﬁle. The presence of an outburst lasting at least ∼1 d suggests that this system has an accretion
disk. If Gaia22ayj is indeed an eclipsing binary with a period of 9.36 min, this is the shortest orbital period
ever measured in eclipsing accreting binaries and is the shortest record of a system with a dwarf nova-type
outburst. The presence of an outburst is unusual for an AM CVn system with this orbital period and thisobject might be in the turn-on phase of the mass-transfer. If the ultrashort orbital period of Gaia22ayj is
conﬁrmed, this object would an ideal target to detect period variations within several years to conﬁrm its
evolutionary state and to identify the future evolutionary consequence.

Gaia22ayj is a transient detected by the Gaia Photometric Science Alerts Team1 . The object was detected
at 17.01 mag on 2022 March 3. Five Gaia observations between March 3.112 and 4.038 UT showed signiﬁcantshort-term variations between 16.55 and 17.01. The object had been registered as a variable ZTF19aagmvuk2
by the Zwicky Transient Facility (ZTF: Masci et al. 2019). The same outburst was detected in outburst by
ZTF and the Transient Alert Broker Lasair (Smith et al. 2019) reported r=16.09 on March 3.399 UT. The Gaia
EDR3 position, magnitudes and parallax 08h 25m 26.s 528, −22◦ 32′ 12.′′ 34 (J2000.0), BP =19.570, RP =18.461
and ϖ=0.43(22) mas, respectively (Gaia Collaboration et al. 2021).
Using the ZTF public data3 , I found coherent short-period variations in time-series ZTF observations on
2019 Jan. 10 (T. Kato, vsnet-alert 266744 ; ﬁgure 2). Phase dispersion minimization (PDM: Stellingwerf 1978)
analysis yielded a period of 0.006500(7) d with two diﬀerent maxima and minima in one cycle (ﬁgure 3). I have
conﬁrmed that this period can express the entire ZTF light curve as well. The orbital period was reﬁned using theMarkov-Chain Monte Carlo (MCMC)-based method introduced in Kato et al. (2010). The resultant ephemeris
isMin I (BJD) = 2458699.17927(1) + 0.00649910257(13)E.
(1)
The phase-folded light curve of the entire ZTF data (excluding the Lasair data) is shown in ﬁgure 4. This period
express all the observations very well.
1 <http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia22ayj/>.
2 <https://lasair.roe.ac.uk/object/ZTF19aagmvuk/>.
3 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>.
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26674>.

2

16ZTF r

ZTF g

ZTF Lasair (r)outburst
182058200 58400 58600 58800 59000 59200 59400 59600 59800
Figure 1: Long-term ZTF light curve of Gaia22ayj. An outburst was detected on 2022 March 3.

181920210.86

0.88

0.90

0.92

0.94

Figure 2: ZTF light curve of Gaia22ayj on 2019 Jan. 10. Periodic short-period variations are clearly seen.

3

θ
1.0

0.8

0.6

0.4

0.2

P=0.00650

0.00.0055

0.00600.00650.00700.00751.0

1.5(d)

−101

−0.5

0.0

0.5

Figure 3: PDM analysis of Gaia22ayj using the ZTF data on 2019 Jan. 10. (Upper): PDM analysis. (Lower):mean proﬁle. The epoch and period were from equation (1).

417

ZTF r

18

1920

ZTF g21

P=0.0064991026 d
−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Figure 4: The phase-folded light curve of the entire ZTF data. The epoch and period were from equation (1).

The resultant period of 0.0064991026 d = 9.36 min is very short. Although this could be the spin period of abinary containing a white dwarf, such as in the white dwarf pulsar AR Sco (Marsh et al. 2016; Stiller et al.
2018) and candidate white dwarf pulsars ZTF J185139.81+171430.3 = ZTF18abnbzvx (Kato and Kojiguchi
2021; Klingler and Pavlov 2021; Pavlenko and Sosnovskij 2021), ASASSN-V J205543.90+240033.5 (Kato 2021;
Kato et al. 2021; Wagner et al. 2021), and more recently GLEAM-X J162759.5−523504.3 (Katz 2022). The proﬁle of variations in Gaia22ayj and the constancy of the proﬁle, however, do not look similar to those of whitedwarf pulsars.
The phase-folded light curve rather suggests a β Lyr-type or W UMa-type eclipsing binary with diﬀerent
minima. The slightly brighter second maximum is suggestive of the O’Connell eﬀect (O’Connell 1951). The
only such short-period (9.36 min) binaries known to us are AM CVn stars composed of white dwarfs [For areview of AM CVn stars see e.g. Solheim (2010)]. Among AM CVn stars, two short-period systems HM Cnc(5.36 min) and V407 Vul (9.48 min) are considered to be direct impact accretors without forming an accretion
disk (Marsh and Steeghs 2002; Ramsay et al. 2002). The third shortest system ES Cet (10.3 min) has recently
been shown to have an accretion disk (Bąkowska et al. 2021). Aside from these accreting systems, an eclipsingbinary with a period of 6.91 min (ZTF J153932.16+502738.8) is also known (Burdge et al. 2019).
If Gaia22ayj is indeed an accreting 9.36-min eclipsing binary, this object is the shortest period known suchan object (the record up to now was ES Cet). The presence of eclipses in ES Cet was conﬁrmed only by rotational
disturbance in the proﬁles of emission lines (Bąkowska et al. 2021), and not yet by photometry. Gaia22ayj would
be very important case due to the very large amplitudes of photometric variations. Following the interpretationof an eclipsing binary, the large depths of the eclipses require that the system is seen almost edge-on and thebrightness of the two components do not diﬀer greatly. A simpliﬁed calculation suggests that a luminosity
ratio of ∼0.4 could reproduce the observed diﬀerence in depths of the minima. With this orbital period, theDoppler beaming is expected to be an order of 1%(=0.01 mag) (Zucker et al. 2007) and would be detectable byhigh-precision photometry.
The presence of an outburst lasting nearly 1 d in Gaia22ayj suggests the presence of instability in the accretion
disk, rather than a short-lived ﬂare or an accretion event as seen in in intermediate polars. The detection inX-rays by the Swift X-ray telescope (1SWXRT J082526.4−223212: D’Elia et al. 2013) is compatible with anaccreting binary. The presence of an accretion disk makes Gaia22ayj similar to ES Cet. The disks in short-period
AM CVn stars, like ES Cet, are considered to be thermally stable due to the high mass-accretion rate considering

5
the standard evolutionary path in which the mass transfer is driven by gravitational wave radiation and themass-losing white dwarf ﬁlls the Roche lobe (Tsugawa and Osaki 1997; Kotko et al. 2012). The case of Gaia22ayj
appears to be diﬀerent. If Gaia22ayj is indeed an AM CVn-like object, it might be in the turn-on phase of themass-transfer before reaching the period minimum (Deloye et al. 2007; Solheim 2010).

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF
team for making their data available to the public. We are grateful to Naoto Kojiguchi for helping downloading
the ZTF data. This research has made use of the AAVSO Variable Star Index and NASA’s Astrophysics DataSystem.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory aspart 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).
We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts).

List of objects in this paper
ES Cet, HM Cnc, AM CVn, β Lyr, W UMa, V407 Vul, 1SWXRT J082526.4−223212, ASASSN-V J205543.90+240033.5,
Gaia22ayj, GLEAM-X J162759.5−523504.3, ZTF J153932.16+502738.8, ZTF J185139.81+171430.3, ZTF18abnbzvx,
ZTF19aagmvuk

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e-mail: skiyotax@gmail.com