1No.117

May. 2023

Variable Star Bulletin
Double outbursts in V544 Her and ASASSN-19yt
Taichi Kato1
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp
Received 2023 May 14
Abstract
In Kato et al. (2019, arXiv:1909.00910), I reported on a double outburst and rebrightenings in 2018 inV544 Her. Such a phenomenon is usually observed in WZ Sge stars which evolved after the period bounce
and the colors of V544 Her in quiescence apparently exclude this possibility. Although this phenomenon wasconsidered to be rare, I detected almost exactly the same one in 2021 using ZTF, ATLAS and ASAS-SN publicdata. I also detected a phenomenon very similar to this in ASASSN-19yt in 2022. The same object showed adiﬀerent type of outburst in 2019 whose morphology looked like that of an SS Cyg star. If ASASSN-19yt isan SU UMa star, the morphology of the 2019 outburst would challenge our knowledge in SU UMa stars. If
this object, or V544 Her, is an SS Cyg star, what causes a double outburst and rebrightenings would become
an unsolved problem in dwarf novae.

Kato et al. (2019) reported a superoutburst of the SU UMa star CS Ind, which showed a long precursoroutburst. The overall behavior looked like a “double outburst”, which is sometimes seen in WZ Sge stars (seee.g., Kato 2015) evolved after the period bounce (Kato et al. 2013; Neustroev et al. 2017; Kimura et al. 2018).
In Kato et al. (2019), V544 Her was introduced as a dwarf nova with a double outburst and the similarity
with CS Ind was discussed. Kato et al. (2019) suggested that this would be a rare phenomenon based on theabsence of a similar outburst in the past data. Using Zwicky Transient Facility (ZTF: Masci et al. 2019)1 andAsteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) forced photometry (Shingles et al.
2021) observations, I noticed that V544 Her showed a double outburst again in 2021, contrary to the expectationin Kato et al. (2019). The overall light curve of the past eight seasons using the ZTF, ATLAS and All-Sky
Automated Survey for Supernovae (ASAS-SN, Shappee et al. 2014) data is shown in ﬁgures 1 and 2. Only
positive detections are shown and all other ASAS-SN observations were upper limits. There are several solitary
ASAS-SN detections (such as the recent BJD 2460074 one), but they were likely noises.
The double outbursts in 2018 and 2021 are shown in detail in ﬁgures 3 and 4, respectively. They werevery similar: ﬁrst long outburst, dip, second long outburst and two short rebrightenings. These outbursts verymuch resemble the double outburst recorded in the SU UMa star CS Ind, as already reported. The red color ofV544 Her [an orbital period over 0.2 d was inferred from SDSS colors (Kato et al. 2012); BP =19.59, RP =18.27(Gaia Collaboration et al. 2022)] in quiescence, however, appears to be inconsistent with an SU UMa star witha short orbital period. All ZTF and ATLAS observations were snapshtot data and it was not possible to makea period analysis to detect possible superhumps. It looks like, however, that no large-amplitude (such as 0.2–0.3 mag) superhumps were present. Although the 2021 outburst was also detected by VSOLJ and VSNET (Katoet al. 2004) members, the detections were late during the initial long outburst and no special attention was paidsince the object already appeared to be fading and since the season was too late to make long time-resolved
photometry.
The ﬁrst outburst in 2016 in ﬁgure 1 might have been a complex one, although the limited quality of theASAS-SN data and still limited coverage by ATLAS made it impossible to examine it in detail. If double outbursts
in this system occur relatively frequently (such as once in three years), we may have a more frequent chance to
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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Figure 1: Light curve of V544 Her in 2016–2020. ASN refers to ASAS-SN observations. Only positive detections
are shown and all other ASAS-SN observations were upper limits.

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Figure 2: Light curve of V544 Her in 2020–2023. The symbols are the same as in ﬁgure 1.

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Figure 3: Double outburst and rebrightenings in V544 Her in 2018. The symbols are the same as in ﬁgure 1.

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Figure 4: Double outburst and rebrightenings in V544 Her in 2021. The symbols are the same as in ﬁgure 1.

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Figure 5: Double outburst and rebrightenings in ASASSN-19yt in 2022. This outburst occurred near the endof the season and ZTF did not cover the quiescence before this outburst. There were no observations after BJD
2459700. The symbols are the same as in ﬁgure 1.

determine the nature of a double outburst in this system than previously thought. There have recently been
an increasing number of discoveries of SU UMa stars with long orbital periods [e.g., SDSS J094002.56+274942.0(Kato and Vanmunster 2023); ASASSN-15cm (Kato 2023b) and BO Cet (Kato et al. 2021; Kato 2023a)] andV544 Her may join this group. It would be, however, worth noting that some outbursts in dwarf novae have
yet unexplained dips, such as in MASTER OT J055845.55+391533.4 (Kato et al. 2023) and the phenomenon inV544 Her may not be related to SU UMa-type one.
I also noticed that ASASSN-19yt showed a double outburst and rebrightenings in 2022 very similar toV544 Her (ﬁgure 5). The same object showed a (relatively) long outburst in 2019 (ﬁgure 6). This outburst rose
slowly and the overall symmetric shape resembled that of an SS Cyg star, although the maximum fading rate
was larger than in SS Cyg stars. Tonny Vanmunster obtained a single-night run and did not detect superhumps.
If this object is indeed an SU UMa star, the morphology of the 2019 outburst would challenge our knowledge in
SU UMa stars, and, inversely, if this object is an SS Cyg star, what causes a double outburst and rebrightenings
would become an unsolved problem in dwarf novae. Please also remember that there was also an unusual case ofa long outburst and rebrihgtenings in PY Per (Kato 2022), whose nature has not yet been clariﬁed.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF,ATLAS and ASAS-SN teams for making their data available to the public. I am also grateful to Naoto Kojiguchi
for helping downloading ZTF data and Tonny Vanmunster for providing observations of ASASSN-19yt.
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).
This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.

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Figure 6: Long outburst in ASASSN-19yt in 2019. The symbols are the same as in ﬁgure 1.

The ATLAS project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G,
80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from thesurvey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889,
and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible
through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast,
the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Instituteof Astrophysics (MAS), Chile.

List of objects in this paper
BO Cet, SS Cyg, V544 Her, CS Ind, PY Per, SU UMa, WZ Sge, ASASSN-15cm, ASASSN-19yt,
MASTER OT J055845.55+391533.4

References
Gaia Collaboration et al. (2022) Gaia Data Release 3. Summary of the contents and survey properties. A&A(arXiv:2208.00211)
Kato, T. (2015) WZ Sge-type dwarf novae. PASJ 67, 108
Kato, T. (2022) Z Cam star PY Per in SU UMa state? VSOLJ Variable Star Bull. 100, (arXiv:2204.12056)
Kato, T. (2023a) SU UMa-type supercycle in the IW And-type dwarf nova BO Cet above the period gap. VSOLJ
Variable Star Bull. 106, (arXiv:2302.02593)
Kato, T. (2023b) ASASSN-15cm: an SU UMa star with an orbital period of 5.0 hours. VSOLJ Variable Star
Bull. 109, (arXiv:2302.09713)
Kato, T., Hambsch, F.-J., Monard, B., Nelson, P., Stubbings, R., & Starr, P. (2019) CS Indi: SU UMa-typedwarf nova with long precursor outburst. PASJ 71, L4
Kato, T. et al. (2023) MASTER OT J055845.55+391533.4: SU UMa star with a dip and long rebrightening.
VSOLJ Variable Star Bull. 113, (arXiv:2304.07695)

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Kato, T., Maehara, H., & Uemura, M. (2012) Characterization of dwarf novae using SDSS colors. PASJ 64, 63
Kato, T., Monard, B., Hambsch, F.-J., Kiyota, S., & Maehara, H. (2013) SSS J122221.7−311523: Double
superoutburst in a best candidate period bouncer. PASJ 65, L11
Kato, T. et al. (2021) BO Ceti: Dwarf nova showing both IW And and SU UMa-type features. PASJ 73, 1280
Kato, T., Uemura, M., Ishioka, R., Nogami, D., Kunjaya, C., Baba, H., & Yamaoka, H. (2004) Variable Star
Network: World center for transient object astronomy and variable stars. PASJ 56, S1
Kato, T., & Vanmunster, T. (2023) SDSS J094002.56+274942.0: an SU UMa star with an orbital period of 3.92
hours and an apparently unevolved secondary. VSOLJ Variable Star Bull. 114, (arXiv:2304.13311)
Kimura, M. et al. (2018) ASASSN-16dt and ASASSN-16hg: Promising candidates for a period bouncer. PASJ70, 47
Masci, F.-J. et al. (2019) The Zwicky Transient Facility: Data processing, products, and archive. PASP 131,018003
Neustroev, V. V. et al. (2017) The remarkable outburst of the highly evolved post-period-minimum dwarf nova
SSS J122221.7−311525. MNRAS 467, 597
Shappee, B. J. et al. (2014) The man behind the curtain: X-rays drive the UV through NIR variability in the2013 AGN outburst in NGC 2617. ApJ 788, 48
Shingles, L. et al. (2021) Release of the ATLAS Forced Photometry server for public use. Transient Name Server
AstroNote 7, 1Tonry, J. L. et al. (2018) ATLAS: A High-cadence All-sky Survey System. PASP 130, 064505

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Editor Seiichiro Kiyota
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