1No.104

Jan. 2023

ISSN 0917-2211

Variable Star Bulletin
Very low state in PY Per in 2022
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 2023 Jan. 02
Abstract
Using VSNET, VSOLJ, ASAS-SN and ATLAS observations, I found that the Z Cam star PY Per spent along, faint low state reaching 19.1 mag at least between 2022 June and November. No dwarf nova outburst
was recorded during this interval. TESS data during this low state showed two maxima in one orbital cycle
and can be interpreted as an ellipsoidal modulation arising from the secondary. These observations suggestthat the mass-transfer almost stopped during this low state and strengthen the identiﬁcation of PY Per as a
VY Scl star. PY Per had shown an unusual outburst resembling an SU UMa-type superoutburst less thanhalf a year before (Kato 2022, arXiv:2204.12056) and these phenomena may have been physically related.

In Kato (2022), I reported the detection of an outburst (2021 December–2022 January) resembling anSU UMa-type superoutburst in the Z Cam star PY Per with an orbital period of 0.15468(5) d (Kato 2022;
Taylor and Thorstensen 1996). Kato (2022) also detected faint states in observations by the American Associationof Variable Stars (AAVSO), and showed that this object is also an VY Scl-type cataclysmic variable (CV) [seee.g., Warner (1995) for CVs in general and their subtypes]. VY Scl-type faint states are sometimes associatedwith Z Cam stars, most notably the 1996–1997 totally unexpected long-lasting fading in RX And (Sion et al.
2001; Kato et al. 2002; Schreiber et al. 2002; Kato 2004).
After the solar conjunction following the unusual outburst mentioned above, I noticed that PY Per was inlow state without any outburst in Variable Star Observers League in Japan (VSOLJ) and VSNET (Kato et al.
2004) observations (vsnet-alert 27029.1 ). This has been conﬁrmed by using the All-Sky Automated Survey for
Supernovae (ASAS-SN) Sky Patrol (Shappee et al. 2014; Kochanek et al. 2017) data. These observations startedin 2022 June (ASAS-SN) and 2022 July (VSOLJ). Using the data by the Asteroid Terrestrial-impact Last AlertSystem (ATLAS: Tonry et al. 2018; Heinze et al. 2018; Smith et al. 2020) Forced Photometry (Shingles et al.
2021)2 , I further conﬁrmed that the 2022 fading episode was the deepest (reaching 19.1 mag) and longest (morethan 150 d) ever recorded in this object (ﬁgure 1).
For a comparison, a light curve of the preceding seasons is given in ﬁgure 2. In the 2019–2020 season(before BJD 2458950), the object showed frequent low-amplitude outbursts as was typical for a Z Cam star. Thebehavior was similar to this at least between 2016 and 2019. In the 2020–2021 season (BJD 2459030–2459290),
the quiescence became fainter (near 18.0 mag) and longer and brighter outbursts in addition to smaller ones
became more prominent than in the previous season. Although the quiescent brightness (18.0 mag) was almost
as faint as the VY Scl-type low state mentioned in Kato (2022), the object deﬁnitely showed dwarf nova-type
outbursts. The light curve in 2020–2021 probably illustrated the behavior when the mass-transfer rate decreased.
Compared to the 2020–2021 season, the mass-transfer rate probably returned normal in the 2021–2022 season(left part of ﬁgure 1). There was, however, an interval lacking outbursts (BJD 2459525–2459568), but not as
faint as the 2020–2021 quiescence, preceding the unusual outburst resembling an SU UMa-type superoutburst.
Accumulation of matter in the disk during this interval may have caused the unusual outburst. Although this1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27029>
2 The ATLAS Forced Photometry is available at <https://fallingstar-data.com/forcedphot/>.

2unusual outburst may have been physically related to the very faint low state in 2022, observations were impossible
due to the solar conjunction and how this very faint state started remains a mystery. Note that the 1996–1997
low state in RX And was preceded by an unusually long standstill. These rare phenomena might have been
physically related and the case would also be suspected in PY Per.
I analyzed Transiting Exoplanet Survey Satellite (TESS) observations obtained in 2022.3 The full light-curveis available at the Mikulski Archive for Space Telescope (MAST4 ). The TESS observations started on 2022
October 28 (BJD 2459882) and ended on 2022 November 26 (BJD 2459910). PY Per started rising in the TESS
data on the ﬁnal two days. Since most of the TESS data were obtained when PY Per was in deep low state andsince the object has nearby (unrelated) contaminating stars, I did not attempt to extract the ﬂux of PY Per but
used the ﬂux combined with contaminating stars. Using the data before BJD 2459908.6 (object in low state),I could detect the orbital period and modulations (ﬁgure 3). The period was determined to be 0.15453(2) dwith the Phase Dispersion Minimization (PDM, Stellingwerf 1978) method after removing long-term trends by
locally-weighted polynomial regression (LOWESS: Cleveland 1979). The errors of periods by the PDM method
were estimated by the methods of Fernie (1989) and Kato et al. (2010). Although the obtained period is inagreement with that obtained in high state in Kato (2022), the orbital proﬁle is very diﬀerent. In high state,
there was a single peak in one orbit [ﬁgure 3 in Kato (2022)], while the current observations clearly show twomaxima in one orbit. This feature most likely represents an ellipsoidal variation of the secondary and TESS
photometry supports the very weak (or no) contribution from the accretion disk. These observations support
that the mass accretion almost stopped in this very low state in PY Per and strengthen the identiﬁcation of thisobject as a VY Scl star.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ASAS-SN,ATLAS and TESS teams for making their data available to the public. I am grateful to VSOLJ and VSNET
observers for reporting observations and to Naoto Kojiguchi for providing a script for downloading TESS data.
The contributors from VSNET and VSOLJ in 2022 were Pavol A. Dubovsky, Masao Funada, Hiroshi Itoh, Eddy
Muyllaert, Yutaka Maeda, Masayuki Moriyama and Gary Poyner.
This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.
The Asteroid Terrestrial-impact Last Alert System (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 the survey 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 Institute of Astrophysics (MAS), Chile.

List of objects in this paper
RX And, Z Cam, PY Per, VY Scl, SU UMa

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312141618
ATF o20 59400

ATF c59500

ASN gC59600

vis59700

59800

59900

Figure 1: 2021–2022 light curve of PY Per using ATLAS forced photometry (ATF), ASAS-SN (ASN), VSOLJand VSNET (C for CCD close to visual and vis for visual) observations. The left part of this ﬁgure corresponds
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41214

16

18
ATF o2058700

ATF c58800

ASN g

C5890059000

vis59100

59200

59300

Figure 2: Light curve of PY Per in the 2019–2021 season. The symbols are the same as in ﬁgure 1. In the2019–2020 season (before BJD 2458950), the object showed frequent low-amplitude outbursts as was typical for aZ Cam star. In the 2020–2021 season (BJD 2459030–2459290), the quiescence became fainter (near 18.0 mag) andlonger and brighter outbursts in addition to smaller ones became more prominent than in the previous season.

5

θ
1.00

0.99

0.98

P=0.15453
0.148

0.150

0.152

0.154

0.156

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0.160

0.162

(d)
−0.04

−0.02

0.00

0.02

−0.5

0.0

0.5

1.0

1.5

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VSOLJ
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Editor Seeichiro Kiyota
e-mail:skiyotax@gmail.com