1
No.99

Apr. 2022

ISSN 0917-2211

Variable Star Bulletin
Gaia19bxc: possible polar below the period minimum
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan

Received 2022 Apr. 10

Abstract
Gaia19bxc is a transient detected on 2019 May 9 by the Gaia Photometric Science Alerts Team. I analyzed
the past public Zwicky Transient Facility (ZTF) data and found that Gaia19bxc has a period of 0.04473647(3) dand two diﬀerent maxima in one cycle. This object also showed high and low states in the ZTF data. Basedon the high amplitude (2.0 mag) of the short-term variations, short period, almost zero color indices between
the diﬀerent ZTF bands and the absence of a longer period, I classiﬁed it to be a likely polar. There has been
no established polar below the period minimum of cataclysmic variables (CVs) and Gaia19bxc could be theﬁrst such an object. CVs below the period minimum usually have a secondary star with a stripped evolved
core and Gaia19bxc is expected to have a similar secondary. If this is indeed the case, Gaia19bxc could becomea highly magnetized exotic ultracompact binary during its secular evolution.

Gaia19bxc is a transient detected by the Gaia Photometric Science Alerts Team1 . The object was detected at 19.28 mag on 2019 May 9 and was reported as a “variable Gaia source brightens by 2 mag”. The
object stayed bright for months after this according to the Gaia light curve. The object is located at 17h 31m
58.s 468, +27◦ 09′ 36.′′ 12 (J2000.0) has BP =20.570(188), RP =19.726(142) and a parallax ϖ=0.565(772) mas(Gaia Collaboration et al. 2021). The American Association of Variable Stars (AAVSO) Variable Star Index(VSX: Watson et al. 2006) classiﬁed it as a possible dwarf nova (UG:). The Zwicky Transient Facility (ZTF:
Masci et al. 2019) listed it as a suspected variable star (Ofek et al. 2020). I analyzed the light variation using
the ZTF public data2 .
The long-term variation is shown in ﬁgure 1. The object apparently shows high and low states and thevariation is not that of a dwarf nova. A phase dispersion minimization (PDM: Stellingwerf 1978) analysis of the
ZTF data after removing the global trend using locally-weighted polynomial regression (LOWESS: Cleveland1979) yielded a period of 0.04473647(3) d (ﬁgure 2). There were two maxima of diﬀerent amplitudes in one
cycle. The amplitude of the variation was very large, reaching 2.0 mag. The phased light curve of the brightstate is shown in ﬁgure 3. The phased light curve for the intermediate state is shown in ﬁgure 4. The proﬁle
was almost the same between diﬀerent states and the colors between the diﬀerent ZTF bands were almost zero.
The phased light curve of the low state could not be examined due to the small number of observations. The
SDSS colors (Ahumada et al. 2020) at four diﬀerent epochs had u − g of +0.1 to +1.6 and i − z of +0.0 to+0.9. The u − g values seem to be redder than many polars, although individual measurements were apparently
aﬀected by large-amplitude, short-term variations. The large variation was already apparent in the SDSS data
(g=21.56–23.04).
1 <http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia19bxc/>.
2 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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18
ZTF r

ZTF g

ZTF i

19
20
21
22
58200 58400 58600 58800 59000 59200 59400 59600
Figure 1: Long-term ZTF light curve of Gaia19bxc.

No known type of pulsating variables has such a large amplitude and a short period at the same time. The
ﬂat maximum is not characteristic to a short-period pulsating variable. The ZTF colors (almost zero color indices)suggest a blue object like a cataclysmic variable (CV). The period is then either the spin period of an intermediate
polar (IP) with a very large amplitude as in Romanov V48 (Kato and Romanov 2022), the spin period of awhite dwarf pulsar like AR Sco (Marsh et al. 2016; Stiller et al. 2018) and candidates (Kato and Kojiguchi 2021;
Kato et al. 2021), or the orbital period (Porb ) of a polar. The presence of high and low states is also compatible
with a polar. I could not ﬁnd a longer period (corresponding to Porb for an IP) as in Romanov V48. If the object
is indeed a polar, this is very unusual. The Porb is far shorter than the “period minimum” of CVs (Kolb 1993;
Gänsicke et al. 2009; Knigge et al. 2011; Kato 2022),
There are several CVs below the period minimum. One notable case is OV Boo with Porb of 0.046258 d(Littlefair et al. 2007; Patterson et al. 2008; Uthas et al. 2011; Patterson et al. 2017). OV Boo has a large spacevelocity (against the Solar system) and is considered to be a population II CV (Patterson et al. 2008). It is
possible that OV Boo is on the standard evolutionary sequence of population II CVs. There is no good distanceestimate for Gaia19bxc and a possibility for a population II CV is not readily excluded.
There are several hydrogen-rich CVs below the period minimum. The best known example are V485
Cen with Porb =0.040995 d (Augusteijn et al. 1996), EI Psc with Porb =0.044567 d (Thorstensen et al. 2002)
CRTS J174033.4+414756 with Porb =0.045048 d (Kato et al. 2014) and SBS 1108+574 with Porb =0.03845 d(Littleﬁeld et al. 2013). EI Psc is renowned to have a hot, luminous secondary for this Porb (Thorstensen et al.
2002). These objects are considered to have a secondary star with a stripped evolved core and are also referred toas EI Psc stars. It has been theoretically shown that CVs with Porb shorter than the period minimum (including
AM CVn stars) can be formed if the core of the secondary star starts evolving in the early phase of their evolution
as CVs (see e.g. Nelemans et al. 2010; Podsiadlowski et al. 2003).
The lack of systems below the period minimum among hitherto discovered polars is striking considering thenumber of EI Psc-type objects for non-magnetic CVs [see ﬁgure 3 in Belloni et al. (2020)], although there appears
to be no speciﬁc reason why there is no EI Psc-like object having a strongly magnetized white dwarf. 1RXS
J035410.4−165244 = RBS 490 was suggested to be a polar with a possible Porb of 46 min = 0.032 d by a radialvelocity study (Thorstensen et al. 2006; Harrison and Campbell 2015). The most recent observation suggested
a photometric Porb of 0.0703 d (Joshi et al. 2022). I, however, could not ﬁnd a signiﬁcant period around these
periods in this system using the ZTF data. The He II line is weak for a polar in the spectrum and the ﬁeld
strength appears to be low even if it is indeed a polar (Joshi et al. 2022).
If Gaia19bxc is indeed conﬁrmed to be a polar by polarimetry, X-ray observations and spectroscopy, it will be
the ﬁrst object below the period minimum. Since the distance is poorly known, it is not easy to tell whether this

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θ
1.0

0.8

0.6

0.4

P=0.04474
0.044720

0.044730

0.044740

0.044750

(d)

−1

0

1

−0.5

0.0

0.5

1.0

1.5

Figure 2: PDM analysis of Gaia19bxc using the ZTF data. (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): mean proﬁle. 1σ error bars are shown.

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18

ZTF r

ZTF g

ZTF i

1.0

1.2

19

20

21
−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.4

Figure 3: The light curve of Gaia19bxc in high state (BJD 2458500–2458800) using the ZTF data folded by the0.04473647(3) d The zero epoch was chosen as BJD 2458563.005.

18
ZTF r

ZTF i

1.0

1.2

19

20

21

22
−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.4

Figure 4: The light curve of Gaia19bxc in intermediate (before BJD 2458400). The period and epoch are thesame as in ﬁgure 3.

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object has a secondary with an evolved core from photometric data. Such a secondary, however, might explain
the relatively red u − g color (the u − g color of EI Psc is +0.8). If this possibility is indeed the case, Gaia19bxc
could become a highly magnetized exotic ultracompact binary during its secular evolution.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF
team for making their data available to the public. We are grateful to Naoto Kojiguchi for helping downloading
the ZTF data. This research has made use of the AAVSO Variable Star Index and NASA’s Astrophysics Data
System.
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).
We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/
alerts). This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France (DOI:
10.26093/cds/vizier). The original description of the VizieR service was published in 2000, A&AS 143, 23.

List of objects in this paper
OV Boo, V485 Cen, AM CVn, EI Psc, AR Sco, 1RXS J035410.4−165244, CRTS J174033.4+414756, Gaia19bxc,
RBS 490, Romanov V48, SBS 1108+574

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