1No.78

Jul. 2021

ISSN 0917-2211

Variable Star Bulletin
MGAB-V859 and ZTF18abgjsdg: ER UMa-type dwarf novae showing standstills
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. 30

Abstract
Using Public Data Release of Zwicky Transient Facility observations, we found that MGAB-V859 andZTF18abgjsdg are dwarf novae and show both ER UMa-type and Z Cam-type states. There had been only
two dwarf novae showing similar transitions between the ER UMa-type and Z Cam-type states. MGABV859 showed both a transition from the ER UMa-type state to a long standstill in 2019 and a Z Cam-typestate in 2020. During the standstill in 2020, this object faded twice and showed dwarf nova-type variations.
ZTF18abgjsdg usually showed ER UMa-type behavior but standstills were seen in 2018 and 2019. Thesupercycles of these objects during the typical ER UMa-type phase were 55 d and 58 d, respectively. Theseobjects provide additional evidence that some ER UMa stars indeed bridge between dwarf nova and novalikestates as proposed in Kato et al. (2016).

1Introduction

ER UMa stars are a subclass of SU UMa-type dwarf novae having very short (much less than 100 d) regularsupercycles, very short recurrence times of normal outbursts and long duty cycles (Kato and Kunjaya 1995;
Robertson et al. 1995; Patterson et al. 1995; for a review, see Kato et al. 1999). The peculiar behavior of ER
UMa stars are believed to be basically understood by the thermal-tidal instability (TTI) theory (Osaki, 1989)
under the condition of exceptionally high mass-transfer rates (Ṁ ) (Osaki, 1995). In dwarf novae with even higherṀ , the disk can become thermally stable and standstills in Z Cam stars are considered to be high-Ṁ , thermallystable states.
There have been four known objects that showed both SU UMa-type and Z Cam-type or IW And-type(subtype of Z Cam stars) behavior. Two of them were ER UMa stars.
• The ER UMa star RZ LMi showed long-lasting superoutbursts in 2016–2017 and this behavior was considered to be a state bridging between an ER UMa star and a novalike (or Z Cam) star (Kato et al.,2016).
• The novalike star BK Lyn showed a temporary ER UMa-type state in 2011–2012 (Kemp et al. 2012;
Patterson et al. 2013). This state might have already started in 2005–2006 (Kato et al., 2013).
• The SU UMa star NY Ser, which is a system close to an ER UMa star but with supercycles not as
regular as in ER UMa stars, showed standstills in 2018 and superoutbursts starting from standstills wereobserved (Kato et al., 2019). The discovery of superoutbursts starting from standstills provided a clue inunderstanding still poorly understood IW And-type dwarf novae (Kato 2019; Kimura et al. 2020).

2MGAB−V859

ZTF r
ZTF g

171819202158200

58400

58600

58800

59000

59200

59400

Figure 1: ZTF light curve of MGAB-V859. Both ER UMa-type state and Z Cam state were present.
• BO Cet has recently shown both IW And-type and SU UMa-type states (Kato et al., 2021), but this staris unrelated to an ER UMa star.
We found that two new objects MGAB-V859 (19h 50m 07.038s +36◦ 40′ 17.24′′ , Gaia Collaboration et al.
2021, discovered by Gabriel Murawski) and ZTF18abgjsdg (19h 19m 32.396s +31◦ 48′ 09.91′′ , discovered by Förster
et al. 2021) show both ER UMa-type states and Z Cam-type standstills using Public Data Release 6 of the Zwicky
Transient Facility (Masci et al., 2019) observations1 . The peculiar behavior of MGAB-V859 (ER UMa star witha long standstill) had also been suggested by Gabriel Murawski in vsnet-chat 8510 on 2020 October 302

2MGAB-V859

The entire light curve of MGAB-V859 is shown in ﬁgure 1. Both ER UMa-type state and Z Cam-type state
were present. The enlargement of the ER UMa-type state is shown in ﬁgure 2. Short supercycle (55 d), frequent
normal outbursts and the long duty cycle (∼0.5) are characteristic to an ER UMa star. Following the second
superoutburst, this object entered a standstill. Figure 3 shows an enlargement of the Z Cam-type state in 2020.
In contrast to NY Ser (Kato et al., 2019), this object faded from standstills just as in ordinary Z Cam stars.
There was a hint of low-amplitude oscillations with a period of ∼10 d just before the termination of the ﬁrst
standstill. This behavior seems to indicate that the disk was becoming thermally unstable before the terminationof the standstill.
There were time-resolved observations in ZTF between JD 2459345.68 and 2459346.81, 9–10 d after the peakof the superoutburst. A period analysis using Phase Dispersion Minimization (PDM, Stellingwerf 1978) yieldeda possible superhump period of 0.0675(2) d (ﬁgure 4). The 1σ error for the PDM analysis was estimated by themethods of Fernie (1989) and Kato et al. (2010). Since the baseline was very short and superhumps in ER UMa
stars tend to decay rather quickly (cf. Kato et al. 1996), this period needs to be conﬁrmed by independent
observations near the peak of a superoutburst.
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>.
2 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/8510>.

317MGAB−V859

ZTF r
ZTF g

1819202158640

58660

58680

58700

58720

58740

58760

58780

Figure 2: Enlarged ZTF light curve of MGAB-V859. The ER UMa-type state with a long superoutburst followed
by frequent normal outbursts (supercycle 55 d) is seen before JD 2458740. The object smoothly entered astandstill following the second superoutburst.

17ZTF rZTF g

MGAB−V859

1819202159000 59020 59040 59060 59080 59100 59120 59140 59160 59180 59200
Figure 3: Enlarged ZTF light curve of MGAB-V859 during the Z Cam state in 2020. Standstills were terminatedby fading as in ordinary Z Cam stars.

4

θ

1.0

0.9

0.8

P=0.06750
0.055

0.060

0.065

0.070

0.075

0.080

(d)

−0.1

0.0

0.1

−0.5

0.0

0.5

1.0

1.5

Figure 4: Possible superhumps in MGAB-V859 during a superoutburst from the ZTF time-resolved data. (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 the resultant
PDM θ statistics. (Lower): Phase-averaged proﬁle.

517ZTF18abgjsdg

ZTF r
ZTF g18

1920

21
58200

58400

58600

58800

59000

59200

Figure 5: ZTF light curve of ZTF18abgjsdg. Both ER UMa-type state and Z Cam state were present.

3

ZTF18abgjsdg

The case of ZTF18abgjsdg is shown in ﬁgure 5. Although the data were less abundant than in MGAB-V859,
standstills following superoutbursts on JD 2458326 and 2458699 were recorded. The supercycles were relatively
regular (∼58 d) after JD 2458966, but were variable JD 2458720. The variable supercycles were similar to what
were observed in RZ LMi in 2016–2017 (Kato et al., 2016) and probably occurred as a result of changing Ṁ .
Although these two objects are very faint, time-resolved observations are encouraged to establish the superhump periods and to study the relation between superhumps and ER UMa-Z Cam transitions.

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 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).

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