1
No.106

Feb. 2023

ISSN 0917-2211

Variable Star Bulletin
SU UMa-type supercycle in the IW And-type dwarf nova BO Cet
above the period gap
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 2023 Feb. 05
Abstract
BO Cet is renowned cataclysmic variable having an orbital period above the period gap (0.139835 d) andshowing both features of a Z Cam/IW And star and an SU UMa star. Using the Asteroid Terrestrial-impact
Last Alert System (ATLAS) forced photometry and the All-Sky Automated Survey for Supernovae (ASASSN) Sky Patrol data, I found that BO Cet underwent a superoutburst in 2022 October–November after aseries of short, normal outbursts with increasing amplitudes. This sequence of outbursts (supercycle) is what
is seen in many SU UMa stars and this observation strengthened the suggestion that the accumulating mass
and angular momentum in the disk during repeated normal outbursts caused a superoutburst even in theunusual system BO Cet. The outburst just preceding the superoutburst bore characteristics of an IW Andtype standstill. This phenomenon reinforces the suggestion that the terminal outburst in IW And stars occurs
when the disk radius reaches a certain limit. I consider that this outburst was a failed superoutburst, during
which the disk reached the radius of the 3:1 resonance but the outburst faded before superhumps developed.
In BO Cet with a mass ratio on the borderline of the stability of the 3:1 resonance, there may have been
a competition between the eﬀects of tidal truncation and the 3:1 resonance as the disk radius grew and thelatter won in the current case. This ﬁnding in BO Cet might suggest that IW And-type and SU UMa-type
phenomena are more strongly physically related than have been thought.

1

Introduction

Major categories of cataclysmic variables (CVs) include novalike (NL) variables and dwarf novae (DNe). In
NL-type variables, the accretion disk is thermally stable due to the high mass-transfer rate from the secondary.
In DNe, the disk is thermally unstable and shows outbursts. [For general information of CVs, see e.g., Warner(1995)]. DNe are subdivided (mainly) into SS Cyg stars, Z Cam stars and SU UMa stars. Z Cam stars show
standstills (equivalent to NL-type thermally stable states) and SU UMa stars show superoutbursts in addition to
ordinary outbursts in SS Cyg stars. During superoutbursts, superhumps with periods a few percent longer thanthe orbital periods are observed, and they are considered as the deﬁning characteristics of SU UMa stars (see e.g.,
Osaki 1996). These superhumps are considered to be the result of the 3:1 resonance in the disk (Whitehurst 1988;
Hirose and Osaki 1990; Lubow 1991). Only systems with relatively small mass ratios (q = M2 /M1 ) are considered
to hold a large disk reaching the radius of the 3:1 resonance and this is considered to determine whether a certain
system can show an SU UMa-type phenomenon. The borderline of q for SU UMa stars is usually consideredsomewhere between 0.25 (Whitehurst 1988) and 0.33 (Murray et al. 2000).
Recently, some of Z Cam stars are known to show atypical behavior (see also Hameury and Lasota 2014),
often characterized by (1) slowly rising standstills, sometimes with damping oscillations, are terminated bybrightening, (2) the sequence is often very regular with almost a constant recurrence time and (3) deep dips are
sometimes seen following brightening (Kato 2019). These objects are called IW And stars [Note that not allitems are always seen in all IW And stars. The item (1) can be considered as the deﬁning characteristic]. There2
is a suggestion that the IW And-type phenomenon is a manifestation of a limit cycle reﬂecting the increase of
the angular momentum of the disk during a sequence of small outbursts in the outer part of the disk (Kimura
et al. 2020a,b). This picture is very similar to SU UMa stars having two types of cycles (outburst cycle andsupercycle) (Osaki 1989, 1996) and led to a suggestion that tidal truncation might work in IW And stars just as
the 3:1 resonance works in SU UMa stars [subsection 4.1 in Kimura et al. (2020a) and Kato et al. (2021)].
BO Cet is the ﬁrst object showing both characteristics of SU UMa stars and Z Cam/IW And stars (Katoet al. 2021) [currently two more (but much fainter) objects are known: MGAB-V349 (vsnet-alert 26832)1 andZTF J181732.64+101954.5 (vsnet-alert 26776)2 ] and this object is expected to provide a clue in understanding
the relation between these two classes of DNe.

2

BO Cet

BO Cet was initially discovered as a NL-type CV by R. Remillard (1992) but no details were published. The CV
nature was conﬁrmed by Rodríguez-Gil et al. (2007). The orbital period was initially detected by the membersof the Center for Backyard Astrophysics.3 and was reﬁned by Bruch (2017), placing BO Cet above the period
gap, which appeared to be consistent with the NL-type classiﬁcation.
This object, however, was conﬁrmed to be a dwarf nova by Kato et al. (2021). Using the data by the
VSNET Collaboration (Kato et al. 2004), Kato et al. (2021) clariﬁed that the object is an eclipsing object withan orbital period of 0.139835 d. Furthermore, Kato et al. (2021) showed that the object also showed IW And-type
standstills. The most unexpected ﬁnding by Kato et al. (2021) was that BO Cet showed superhumps during oneof its long outburst. Superhumps had a period 7.8% longer than the orbital period leading to an estimation of
the mass ratio of q=0.31–0.34. Superhumps during a long outburst deﬁnes the SU UMa-type dwarf nova andit was very unusual that an object far above the period gap showed the SU UMa-type phenomenon (but not
unprecedented). It was considered that q of BO Cet is very close to the upper limit of q in which the disk canreach the radius of the 3:1 resonance. BO Cet could have accidentally reached this radius, since there had beenno such a phenomenon recorded in this object despite past observations for more than 10 years, and this might
implicitly suggest such a superoutburst should be rare. This suggestion, however, was disproven only two years
later.

3

Observations and Analysis

Using the All-Sky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data (Shappee et al. 2014; Kochanek
et al. 2017) and the Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) forced photometry(Shingles et al. 2021), I noticed that BO Cet gradually brightened following a bright outburst in 2022 June.
During this brightening phase, small outbursts with increasing amplitudes were recorded (ﬁgure 1 lower panel:
BJD 2459765–2459850). The object stopped fading and brightened (BJD 2459850–2459861) just like an IW Andtype standstill. After reaching a minimum around BJD 2458871, this object underwent a long, bright outburst(ﬁgures 1, 2).
During this long outburst, the increasing scatter in the light curve was suggestive of developing superhumps(ﬁgure 2). A phase dispersion minimization (PDM; Stellingwerf 1978) analysis indeed detected a strong superhump signal (ﬁgure 3). I used the methods of Fernie (1989) and Kato et al. (2010) to determine 1σ errors.
The resultant period of 0.1496(6) d is in good agreement with the value 0.15069(3) d obtained during the 2020
superoutburst (Kato et al. 2021). No signiﬁcant superhump signal was detected after BJD 2459888. This wasprobably due to the decrease in the amplitude of superhumps and the limited number of observations (10 per
day or even less). If a time-resolved photometric campaign had been conducted, the superhump signal may havebeen detected for a longer time.

1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26832>.
2 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26776>.
3 J. Patterson in 2002, <http://cbastro.org/communications/news/messages/0274.html>.

3

13
14
15
16
17
59350

ASN g

59400

59450

ATLAS o

59500

ATLAS c

59550

59600

59650

13
14
15
16
17
59700

59750

59800

59850

59900

59950

60000

Figure 1: Light curve of BO Cet in 2021–2023. ASAS-SN (ASN) and ATLAS observations are used. In the upper
panel, an IW And-type standstill (terminal brightening with sudden fading) was recorded (BJD 2459593–2459610,
2022 January). In the lower panel, an SU UMa-like supercycle was recorded between BJD 2459744 and 2459890:
following a long, bright outburst, a sequence of short repeated outbursts with increasing amplitudes led to along, bright outburst. The long, bright outburst starting on BJD 2459875 was conﬁrmed to be a superoutburst(enlargement of the grey box in ﬁgure 2).

4

14

15

16
59870

59875

59880

59885

59890

59895

Figure 2: Enlargement of the light curve of the 2022 superoutburst. The symbols are the same as in ﬁgure 1.
The scatter in the middle part of the light curve indicates superhumps.

4Discussion

4.1

Supercycle

This observation for the ﬁrst time detected a typical supercycle for an SU UMa star in BO Cet: repeated shortnormal outbursts with increasing amplitudes between two long outbursts. The second long outburst was conﬁrmed
to be a superoutburst. Although the nature of the ﬁrst long outburst was unknown due to the lack of observations,
it could also have been a superoutburst as seen from the shape similar to the second one. Although the presenceof a supercycle is not surprising for an SU UMa star, it was not evident in 2020 when the ﬁrst superoutburst
was detected. This observation strongly supports the idea that the accumulating mass and angular momentum
in the disk during repeated normal outbursts caused the disk to expand to the radius of the 3:1 resonance (Osaki
1989, 1996).

4.2

Relation to IW And-type phenomenon

As shown in section 3, the ﬁnal outburst before the superoutburst had characteristics similar to an IW And-type
standstill. This ﬁnding supports the interpretation that terminal brightening of an IW And-type standstill occurs
when the disk radius reaches a certain upper limit (Kimura et al. 2020a; Kato et al. 2021), possibly the radius of
tidal truncation.
The 2022 phenomenon of BO Cet was diﬀerent from an IW And-type standstill in that it showed increasing
amplitudes of dwarf-nova outbursts while damping oscillations are seen in IW And stars. This diﬀerence may
have been due to the diﬀerence in the initial condition when the cycle started following a long outburst: in 2022,
the cycle started when the cool region in the disk in BO Cet was larger than in the IW And state (at the startof the cycle, BO Cet was 0.5 mag fainter than average standstills of the same object).

4.3

Failed superoutburst?

As seen in several SU UMa stars (usually with long orbital periods) near the stability border of the 3:1 resonance,
an outburst just preceding a superoutburst tends to be long and bright. Such an outburst is possibly a failed
superoutburst, in which the duration of the outburst is not long enough for the 3:1 resonance to develop despitethat the radius already reaches the radius of the 3:1 resonance. The case in BO Cet in 2022 might be the same.
This ﬁnal outburst also corresponded to IW And-type terminal brightening. In a case in which the radii of the

5

θ
1.0

0.9

0.8

0.7

P=0.1496
0.6
0.10

0.12

0.14

0.16

0.18

0.20

(d)
−0.4

−0.2

0.0

0.2

−0.5

0.0

0.5

1.0

1.5

Figure 3: Superhumps of BO Cet in 2022 (interval BJD 2459876.8–2459887.9). (Upper): PDM analysis. The
bootstrap result using randomly contain 50% of observations is shown as a form of 90% conﬁdence intervals in
the resultant θ statistics. (Lower): Phase plot.

6
3:1 resonance and tidal truncation are very close, as suggested in BO Cet (Kato et al. 2021), both eﬀects – tidal
truncation and the 3:1 resonance might work at the same time. If the 3:1 resonance is strong enough and wins
the competition, it may develop and eventually cause a superoutburst. If this fails, the behavior would look likean ordinary IW And phenomenon. Although this is not beyond speculation, further observations and theoreticalstudies are needed keeping this possibility in mind.

4.4

Suggested observations

This 2022 very unexpected phenomenon was not detected in real time, but was found during an inspection of
public archives. This was primarily due to the lack of regular monitoring and reporting of this object to VSOLJor VSNET despite its brightness (14–15 mag), well withing reach of many amateur instruments. This probably
came from the old impression of BO Cet – an NL-type star with little variation and a dull target for nightly
monitoring. This star never gained popularity during the history of VSNET: only sporadic observations werereported by observers who are mainly interested in NL-type stars. Please remember that the behavior of BO Cet
completely changed after 2019 and it should deserve more regular attention as for other dwarf novae already on
visual/CCD observers’ menus. Such a change in state in a CV may be a transient one and may not easily be
reproduced, just as we have seen in the case of BK Lyn, which had been known as an NL star but showed atransient ER UMa-type state (Kemp et al. 2012; Patterson et al. 2013; Kato et al. 2013, 2014). This ER UMa-typestate (probably had lasted for a decade) in BK Lyn ended in 2013 and has not repeated again.
Due to the long orbital period (compared to typical SU UMa stars), occasional observations to search forsuperhumps may not be very attracting since it requires relatively long time. If your telescope can point theobject quickly, several snapshots per night would be helpful since a small number of nightly observations by
ATLAS and ASAS-SN detected superhumps, a more organized observations should obtain a better result. Moreintensive time-resolved photometry sessions will clarify the presence or absence of superhumps during particular
outburst – when the object is brighter than 14.0 mag, the result would be promising.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616.
The author is grateful to the ATLAS and ASAS-SN teams for making their data available to the public.
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
IW And, Z Cam, BO Cet, SS Cyg, BK Lyn, SU UMa, MGAB-V349, ZTF J181732.64+101954.5

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Editor Seiichiro Kiyota
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