1
No.119May. 2023

Variable Star BulletinLong-lasting high state of the high-ﬁeld polar AR UMa
Taichi Kato1
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp
Received 2023 May 23
Abstract
Using ASAS-SN Sky Patrol Photometic Database and Asteroid Terrestrial-impact Last Alert System(ATLAS) data, I found that the high-ﬁeld polar AR UMa entered a long-lasting high state in 2022 October.
This object is renowned for its small duty cycle, and short-lived high states have only been occasionally seen
since the discovery. It appears that the present long-lasting high state is the ﬁrst recorded one at least in thelast 30 years and probably even more. Before entering the current long-lasting high state, this object showed
three short-lived high states, which might have been precursors to the current state. Before these short-lived
high states, the object had been in a low state for 8 years and probably more. I reﬁned the orbital period
to be 0.08050066(1) d. The object is still bright and current phenomenon provides a unique opportunity to
study accretion processes onto a strongly magnetized white dwarf and to study the mechanism of maintaining
the long-lasting high state.

There was an extremely soft X-ray transient 1ES 1113+432 discovered from observations by Imaging Proportional Counter of the Einstein Observatory in 1979 (Remillard et al. 1994). At that time, no optical object
brighter than V =16 within 2 arcmin of the X-ray location was found. The object was high above the Galactic
plane and a classical X-ray nova seemed unlikely. Remillard et al. (1994) obtained optical imaging and founda very blue object within 1.2 arcmin radius of the X-ray error circle. This optical counterpart (V =16.5) had
already been known as a semiregular (SR) variable star (Kholopov et al. 1985; Meinunger and Wenzel 1968),
which was originally discovered by Hoﬀmeister (1963) as an RR Lyr star (SON 7744) with a range of 14.5–16 mag.
Remillard et al. (1994) obtained spectra, determined the orbital period (1.9322 hr = 0.08051 d) and detected“outburst”-like variations on historical plates. Remillard et al. (1994) identiﬁed this object to be a polar (AM Herstar) [see e.g., Cropper (1990) for a review of polars] which spends most of the time in low-accretion states.
Referring to the manuscript of Remillard et al. (1994), Wenzel (1993) re-examined the plates which were used
to give the earlier SR-type classiﬁcation. It turned out diﬃcult to determine the variability type in the 1960s
when the great diversity of cataclysmic variables (CVs) was still unknown. It is also worth noting that AR UMa
was not selected as a CV by ROSAT (Voges et al. 1999), which was/is known to be very eﬃcient in discoveringpolars. AR UMa should have spent too much time in low states to be detected even by ROSAT as a CV. Thisobject was then shown to be the ﬁrst high-ﬁeld magnetic CV with a ﬁeld strengh of ∼230 MG (Schmidt et al.
1996).
While inspecting of ASAS-SN Sky Patrol Photometic Database (ASAS-SN V2.0: Hart et al. 2023; Shappee
et al. 2014), I noticed that AR UMa had entered a long-lasting high state starting from 2022 October. This
brightening had been independently detected by Yutaka Maeda on 2023 February 21 (14.1 mag, unﬁltered CCD),
but apparently escaped detection by visual monitoring. I immediately issued an alert (vsnet-alert 27706)1 on 2023
May 3, when ASAS-SN V2.0 observations had ended on April 26. After conﬁrming that the Asteroid Terrestrialimpact Last Alert System (ATLAS: Tonry et al. 2018) forced photometry (Shingles et al. 2021) recorded thisobject still bright in May, I issued a follow-up notice (vsnet-alert 27708)2 . Campaigns by the VSNET (Kato et al.
2004) and VSOLJ teams (cf. vsnet-alert 27719)3 and by the AAVSO4 were launched. Although time-resolved
1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27706>.
2 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27708>.
3 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27719>.
4 <https://www.aavso.org/aavso-alert-notice-824>.

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photometric data have been reported, I primarily deal with the behavior of AR UMa leading to the current brightstate in this article.
The light curve since 2013 using ATLAS, ASAS-SN (not V2.0) and Zwicky Transient Facility (ZTF: Masci
et al. 2019)5 observations is shown in ﬁgures 1 and 2. Several ASAS-SN observations were omitted as noises
by comparing with observations on the same night. Before this, there had been some observations reported to
VSOLJ and VSNET. There was no evidence for a long-lasting high state in 2009–2013, although observations were
fragmentary. There were short-lived high states in 2007 January to March and in 2008 January, both detected by
Ian Miller by snapshot CCD observations (see also Schmidt et al. 1999 for the light curve). Observations before
them were not ideal and visual observations since 1997 reporting magnitudes of 15.0–16.0 probably referred to
the low state. There was a documented short-lived high state in 1996 December (Schmidt et al. 1999). It appears
that the high state starting from 2022 October is the ﬁrst long one since the discovery. According to Wenzel(1993), low states were predominant and only six high states since 1961 (spanning 32 years) had been detected
almost like dwarf nova outbursts. There may have not been a long-lasting high state even since the discovery ofthis object as a variable star.
There were three short-lived brightening events in 2021 May, 2022 April and likely in 2022 June (ﬁgure 2)
despite the long absence (nearly 8 yr) of a similar event before them. These short-lived events may have beenprecursors to the present high state. Although it has been argued that long-lasting low states already challenge
the theories (Breedt et al. 2012), whether these short-lived brightening events reﬂected gradually increasing masstransfer or such precursors could initiate some sort of positive feedback to maintain the high state may be aninteresting theoretical topic.
The orbital proﬁle during the low states before 2022 October is shown in ﬁgure 3. The ASAS-SN, ATLAS
and ZTF data were combined after removal of the long-term trends by locally-weighted polynomial regression (LOWESS: Cleveland 1979). The period wes determined using the phase dispersion minimization (PDM:
Stellingwerf 1978) method, whose error was estimated by the methods of Fernie (1989); Kato et al. (2010). Strong
ellipsoidal variations are apparent [see Schmidt et al. (1996); Howell et al. (2001)]. The diﬀerence between the
primary and secondary maxima was much smaller in Schmidt et al. (1996) and Howell et al. (2001) because theyused infrared bands. In drawing the phase-averaged light curve, I used the following ephemeris determined fromthese data:
T0 (BJD) = 2458786.470(1) + 0.08050059(2)E.
(1)
This period agrees with the one 0.08050074(12) d by Schmidt et al. (1999) from radial-velocity observations. The
diﬀerence in the zero phases between this and Schmidt et al. (1999) (zero crossing by radial-velocity observations)corresponds to a 0.076 orbit after 103304 orbital cycles. Assuming that the present photometric minimum isequivalent to the zero crossing phase, the improved orbital period is 0.08050066(1) d, which can be considered as
the current best determination of the period of this binary.
The orbital proﬁle during the current high state is shonw in ﬁgure 4. ZTF data for this interval are not
yet publicly available and ATLAS and ASAS-SN data were combined. Although the statistic is worse than the
low-state proﬁle, a bump at phase 0.2–0.3 appeared.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. I am grateful to Naoto Kojiguchi
for helping downloading ASAS-SN Sky Patrol Photometic Database and ZTF data and to ASAS-SN, ATLAS
and ZTF teams for making their data available to the public. I am grateful to Franz-Josef Hambsch, Tamás
Tordai, Tonny Vanmunster, Filipp D. Romanov and Hiroshi Itoh for providing time-resolved CCD photometry.
Although they were not presented in this publication, they were useful in conﬁrming the proﬁle and phasing of
the high-state variation.
This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.
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
5 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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13

ZTF g

ZTF r

14

ATLAS oATLAS c

ASN g

ASN V

15
16
17
56600

56650

56700

56750

56800

56850

56900

13
14
15
16
17
56950

57000

57050

57100

57150

57200

57250

13
14
15
16
17
57300

57350

57400

57450

57500

57550

57600

57650

13
14
15
16
17
57650
13

57700

57750

57800

57850

57900

57950

58000

14
15
16
17
58050

58100

58150

58200

58250

58300

58350

Figure 1: Light curve of AR UMa in 2013–2018. The object was in low state. The symbols used in ﬁgure 2 arealso displayed.

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13
14
15
16
17
58400

58450

58500

58550

58600

58650

58700

13
14
15
16
17
58750

58800

58850

58900

58950

59000

59050

59100

13
14
15
16
17
59150

59200

59250

59300

59350

59400

59450

13
14
15
16
17
59500

59550

59600

59650

59700

59750

59800

13
14
15
16
17
59850

59900

59950

60000

60050

60100

60150

60200

Figure 2: Light curve of AR UMa in 2018–2023. The symbols are the same as in ﬁgure 1. Transient high states
were seen in the third and fourth panels. The object entered a long-lasting high state in the last panel.

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θ
1.0

0.8

0.6

P=0.08050
0.080450.080500.08055

0.08060

1.0

1.5

(d)

−0.1

0.0

0.1

−0.5

0.0

0.5

Figure 3: Orbital proﬁle of AR UMa in low states. (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

θ
1.0

0.9

0.8

P=0.08050
0.080450.080500.08055

0.08060

1.0

1.5

(d)
−0.4

−0.2

0.0

0.2

0.4

−0.5

0.0

0.5

Figure 4: Orbital proﬁle of AR UMa in the current high state. (Upper): PDM analysis. (Lower): Phase plot.

7
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.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as
part 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).

List of objects in this paper
AM Her, RR Lyr, AR UMa, 1ES 1113+432, SON 7744

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license.

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