1

No.84

Nov. 2021

ISSN 0917-2211

Variable Star Bulletin
ASASSN-19ax: SU UMa-type dwarf nova with a long superhump
period and post-superoutburst rebrightenings
Taichi Kato1 , Hiroshi Itoh2 , Seiichiro Kiyota3 , Tonny Vanmunster4 , Tamás Tordai5 ,Yusuke Tampo1 , Naoto Kojiguchi1 , Masaaki Shibata1 , Junpei Ito1 , Roger D. Pickard6 , Sjoerd Dufoer7
1

Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

2

Variable Star Observers League in Japan (VSOLJ), 1001-105 Nishiterakata, Hachioji, Tokyo 192-0153, Japan
3
4
5

VSOLJ, 7-1 Kitahatsutomi, Kamagaya, Chiba 273-0126, Japan

Center for Backyard Astrophysics Belgium, Walhostraat 1A, B-3401 Landen, Belgium

Polaris Observatory, Hungarian Astronomical Association, Laborc utca 2/c, 1037 Budapest, Hungary
6
7

3 The Birches, Shobdon, Leominster, Herefordshire, HR6 9NG, UK

3 Vereniging Voor Sterrenkunde (VVS), Oostmeers 122 C, 8000 Brugge, Belgium
Received 2021 Nov. 01
Abstract

We observed ASASSN-19ax during the long outburst in 2021 September–October. The object has been
conﬁrmed to be an SU UMa-type dwarf nova with a superhump period of 0.1000–0.1001 d. This object
showed two post-superoutburst rebrightenings both in the 2019 and 2021 superoutbursts. These observations
have established that ASASSN-19ax belongs to a group of long-period SU UMa-type dwarf novae which show
multiple rebrightenings. This phenomenon probably arises from premature quenching of the superoutburst
due to the weak 3:1 resonance near the stability border of the resonance, resulting in a considerable amount of
disk mass after the superoutburst. We noted that ASASSN-19ax is very similar to QZ Ser, an SU UMa-type
dwarf nova with an orbital period of 0.08316 d and an anomalously hot, bright secondary star, in that bothobjects showed multiple post-superoutburst rebrightenings at least once and that they are bright in quiescence.
We expect that the core of the secondary in ASASSN-19ax may be evolved as in QZ Ser.

1

Introduction

ASASSN-19ax is a dwarf nova discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee
et al. 2014) at g=14.84 on 2019 January 12.1 The ASAS-SN team described this object as “red star outburst,
matches to PS1 G=16.1, previous outburst in CRTS”. Using ASAS-SN Sky Patrol data (Shappee et al. 2014,
Kochanek et al. 2017),2 T.K. noticed that the 2019 January outburst already started on 2018 December 31(peaking at g=12.9) and that this outburst looked like a superoutburst with a dip and rebrightening (vsnetalert 22936).3 Despite the red color, this object was suspected to be an SU UMa-type dwarf nova (vsnet-alert
22936). Although time-resolved observations started on 2019 January 15, the object was already fading and nosuperhump-like signal was recorded.
1 <http://www.astronomy.ohio-state.edu/ assassin/transients.html>.
2 <https://asas-sn.osu.edu/>.
3 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/22936>.

2
Another outburst was detected by Eddy Muyllaert on 2019 September 5 (originally reported in cvnetoutburst, and cited in vsnet-alert 23543).4 Although time-resolved observations were initiated, this outburst
faded rapidly within 5 d of the initial detection. No periodic modulations were detected and this outburst shouldhave been a normal outburst.
Later on, we used Public Data Release of the Zwicky Transient Facility (Masci et al. 2019) observations5and conﬁrmed that the 2019 January outburst was followed by two rebrightenings (ﬁgure 1b).
Although multiple rebrightenings are usually seen in WZ Sge-type dwarf novae (Kato 2015), ASASSN-14ho
showed four rebrightening despite its long orbital period (Porb ) of 0.24315(10) d (Gasque et al. 2019). Kato
(2020) suggested that ASASSN-14ho is an SU UMa star above the period gap mimicking a WZ Sge-type dwarf
nova. There are conﬁrmed instances of long-Porb SU UMa-type dwarf novae with multiple rebrightenings (e.g.
ASASSN-18aan: Wakamatsu et al. 2021; QZ Ser: VSNET unpublished, see later in this paper; Mis V1448:
N. Kojiguchi et al. in preparation). We therefore considered that ASASSN-19ax should be another long-Porb SU
UMa star mimicking a WZ Sge-type dwarf nova.
On 2021 September 10, Eddy Muyllaert detected a bright (mv =12.8) outburst of this object. Considering
the background stated above, we launched a campaign via VSNET Collaboration (Kato et al. 2004) in vsnet-alert
26254.6 The log of observations (including the 2019 data) is given in table 1.

2

Results and Discussion

The initial observations on 2021 September 16 by S.K. already suggested the presence of superhumps (vsnet-alert
26256).7 T.V. reported the detection of superhumps with a period of 0.1015(24) d (vsnet-alert 26260).8 This
period was reﬁned by using observations of three observers to 0.10012(2) d (vsnet-alert 26263).9
After removing the global trend of the outburst by using locally-weighted polynomial regression (LOWESS,
Cleveland 1979), the times of superhumps maxima were determined by the template ﬁtting method as describedin Kato et al. (2009). The resultant times of superhump maxima are given in table 2. There appears to be aphase jump between E=59 and E=70. This epoch corresponds to the termination of the initial superoutburst
and this phase jump was most likely caused by transition to (traditional) late superhumps10 (cf. Haefner et al.
1979; Vogt 1983; van der Woerd et al. 1988) arising from the hot spot.
The superhump periods were determined by Phase Dispersion Minimization (PDM; Stellingwerf 1978)
method after removing the global trend using LOWESS. The errors were estimated by the methods of Fernie (1989) and Kato et al. (2010). The proﬁles and periods were almost identical during the superoutburst(period = 0.10013(2) d, ﬁgure 2) and the post-superoutburst phase (period = 0.10002(4) d, ﬁgure 3) despite aphase jump between them. Superhumps became below the detection limit after BJD 2459485.3. Based on ourobservations and ASAS-SN data, two post-superoutburst rebrightenings occurred on BJD 2459486 and 2459492.5(ﬁgure 1c). Superhumps disappeared after the ﬁrst rebrightening. No orbital signal was detected in the ZTF
data in quiescence.
These observations have established that ASASSN-19ax belongs to a group of long-Porb SU UMa-type dwarf
novae which show multiple rebrightenings. This phenomenon probably arises from premature quenching of thesuperoutburst due to the weak 3:1 resonance near the stability border of this resonance, resulting in a considerable
amount of disk mass after the superoutburst (Kato 2020). The parameters for these objects are summarized intable 3.
The quiescent MV = +7.6 (Gaia Collaboration et al. 2021) is much brighter than MV = +12 expected fromthe orbital period (Knigge 2006). This implies that the secondary has an evolved core as in QZ Ser, an SU
UMa-type dwarf nova with a 2-hour orbital period and an anomalously hot, bright secondary star (Thorstensen
et al. 2002). QZ Ser also showed two post-superoutburst rebrightenings after the 2013 March-April superoutburst
and one rebrightening in 2020 March-April superoutburst (Kahle 2020, although rebrightenings are not shown).
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/23543>.
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>.
6 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26254>.
7 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26256>.
8 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26260>.
9 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26263>.
10 Many classical examples of late superhumps in short-P
orb systems, however, turned out to be stage C superhumps as describedin Kato et al. (2009).

3

12

a

ZTF g

ZTF r
ASAS−SN (g)

14

16

58200

58400

58600

58800

12

59000

59200

c

b

59400

Time−series CCD
Snapshot CCD
Visual

14

16

70

75

80

85

90

95

100

105

65

70

75

80

85

90

95

100

Figure 1: Light curve of ASASSN-19ax. a: Long-term light curve based on ASAS-SN and ZTF data. The ﬁrst
and last outbursts were superoutburst and two outbursts between them were normal outbursts. b: Superoutburst
in 2019 January. Vertical green ticks represent rebrightenings. c: Superoutburst in 2021 September. Time-series
CCD observations (this work, binned to 0.1 d), snapshot CCD and visual observations from VSNET and VSOLJ
were plotted. Vertical green ticks represent rebrightenings.

4

Table 1: Log of observations of ASASSN-19ax
Start∗
58499.2738
58500.2393
58501.2928
58732.5664
58734.1642
58736.5648
58740.1789
59474.0042
59475.3303
59477.0463
59477.0582
59477.3102
59478.0761
59479.9487
59481.0249
59483.9989
59484.0416
59485.0089
59486.9912
59489.0201
59489.9769
59490.9299
59491.9420
59492.9612
59495.9646
59497.9578
59498.9098
59501.9186
59501.9596
59502.9103
59502.9591
59504.9101
59507.9012
59510.8743
59511.0784∗

End∗58499.3387
58500.3326
58501.4191
58732.6240
58734.2567
58736.6851
58740.3169
59474.1054
59475.6309
59477.3156
59477.2289
59477.4182
59478.2785
59480.0281
59481.2709
59484.2493
59484.2599
59485.2052
59487.3250
59489.3104
59490.2410
59491.3262
59492.3263
59493.2458
59496.2602
59498.2561
59499.0653
59502.3293
59502.2411
59503.3120
59503.1697
59505.1489
59508.0267
59511.0001
59511.1653

mag†
14.26215.002
15.27613.28914.43615.62015.44013.19213.43713.65013.547
13.572
13.84514.60515.11814.981
15.16115.03414.92015.26615.28915.26014.54714.53515.33115.34215.36315.294
1.662
15.3151.637
15.38815.46815.4301.572

error‡
0.002
0.004
0.003
0.002
0.002
0.009
0.006
0.005
0.004
0.003
0.003
0.006
0.003
0.007
0.004
0.004
0.005
0.004
0.007
0.007
0.005
0.004
0.007
0.003
0.003
0.002
0.007
0.004
0.001
0.003
0.001
0.007
0.006
0.003
0.003

N§
61
87
138
83
219
162
233
207
382
390
339
142
389
119
383
412
222
389
264
260
260
666
495
349
503
466
235
331
778
281
580
147
209
226
188

obs∥
Van
RPc
Van
Trt
Ioh
DFS
Ioh
Kis
Van
Ioh
Kis
Trt
Ioh
Kis
Ioh
Kis
Ioh
Kis
Ioh
Ioh
Ioh
Ioh
Ioh
Ioh
Ioh
Ioh
Ioh
Ioh
KU1
Ioh
KU1
Ioh
Ioh
Ioh
KU1

band#
C
V
C
V
C
V
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C

JD−2400000.
Mean magnitude.
‡
1σ of the mean magnitude.
§
Number of observations.
∥Observer’s code: DFS (S. Dufoer), Ioh (H. Itoh), KU1 (Kyoto U.,
campus obs.), Kis (S. Kiyota), RPc (R. Pickard), Trt (T. Tordai),
Van (T. Vanmunster)#
Filter. “C” means unﬁltered.
†

5

θ
1.0

0.8

0.6

P=0.10013
0.4
0.090

0.095

0.100

0.105

0.110

1.0

1.5

(d)

−0.10

−0.05

0.00

0.05

−0.5

0.0

0.5

Figure 2: Superhump proﬁle of ASASSN-19ax during the 2021 superoutburst (before BJD 2459481). (Upper):
PDM analysis. We analyzed 100 samples which randomly contain 50% of observations, and performed the PDM
analysis for these samples. The bootstrap result is shown as a form of 90% conﬁdence intervals in the resultant
PDM θ statistics. (Lower): Phase-averaged proﬁle.

6

θ

1.0

0.9

0.8

P=0.10002
0.090

0.095

0.100

0.105

0.110

1.0

1.5

(d)

−0.10

−0.05

0.00

0.05

0.10

−0.5

0.0

0.5

Figure 3: Superhump proﬁle of ASASSN-19ax after the 2021 superoutburst (BJD 2459481–2459486). (Upper):
PDM analysis. (Lower): Phase-averaged proﬁle.

7Table 2: Superhump maxima of ASASSN-19ax
E
max∗
error
O − C†
N‡
0 59474.1147
0.00320.010576
13 59475.4055
0.0004−0.0027148
14 59475.5158
0.00520.007380
15 59475.6048
0.0012−0.004154
30 59477.1103
0.0005−0.0031300
31 59477.2088
0.0005−0.0049
221
32 59477.3122
0.0010−0.0018106
33 59477.4115
0.0008−0.002869
40 59478.1158
0.0006−0.0007145
41 59478.2101
0.0016−0.0067
146
59 59480.0031
0.0032−0.0192
102
70 59481.1384
0.00180.0128118
71 59481.2451
0.00120.0191154
99 59484.0330
0.0019−0.0015158
100 59484.1382
0.00090.0033200
101 59484.2342
0.0016−0.0009188
109 59485.0387
0.00130.0011
130
111 59485.2325
0.0086−0.0057
34∗
BJD−2400000.
†
Against max = 2459474.1042 + 0.100306E.
‡
Number of points used to determine the maximum.

Light curves of QZ Ser are given in ﬁgure 4 for a comparison. The similarity between ASASSN-19ax and QZ
Ser appears to be striking. With the bright (r=15.0) quiescence, detailed spectroscopy of ASASSN-19ax is very
promising. There remains, however, a possibility that this r=15.0 object is not the secondary of the ASASSN19ax binary. The Gaia parallax of this object gives MV = +5.3 for the maximum of ASASSN-19ax, which is notinconsistent with that of a dwarf nova in outburst, and the r=15.0 object is unlikely a background, unrelated
star.
Although the 2021 September superoutburst was visually detected early, time-series CCD observations
started 5 d later and we probably missed the growing stage of superhumps (stage A superhumps). During
the next superoutburst, observations of the early stage will be very important since the period of stage A superhumps is vital for determining the mass ratio (Kato and Osaki 2013), which might be anomalous considering thesimilarity with QZ Ser.

Acknowledgments
This work was supported by JSPS KAKENHI Grants Number 21K03616 (T.K.) and Number 21J22351 (Y.T.).
The authors are grateful to observers of VSNET and VSOLJ who supplied vital data. Yasuyuki Wakamatsu
helped us in compiling data reported to VSNET.
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).
ASAS-SN is supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio
State University and NSF grant AST-1515927.

8

Table 3: Long-Porb (suspected) SU UMa-type dwarf novae with multiple rebrightenings
ObjectOrbitalperiod (d)0.083161(1)Superhump
period (d)∗0.08557(13)Number of short
rebrightenings
1–2–

0.13225(1)20.149454(3)–0.15821(4)0.2275(3)2
5ASASSN-14ho†0.24315(10)–OGLE-BLG-DN-0174†–
0.14474(4)‡OGLE-BLG-DN-0595†–
0.0972(1)‡ASASSN-19ax–
0.10013(2)∗For stage B superhumps when available.
†
Suspected SU UMa star.
‡
Requires conﬁrmation (cf. Kato et al. 2017).

4
3
2
2

QZ SerOT J002656.6+284933 =CSS101212:002657+284933
ASASSN-18aan
Mis V1448

References
Thorstensen et al. (2002),
vsnet-alert 15533
Kato et al. (2017)
Wakamatsu et al. (2021)
vsnet-alert 24912, N. Kojiguchi
et al. in preparation
Kato (2020)
Mróz et al. (2015)
Mróz et al. (2015)
This work

ASAS−SN (V)Snapshot CCD
Visual

12
14
16

2003
18

56360

56370

56380

56390

56400

56410

56420

12
14
16

2020
18

58930

58940

58950

58960

Figure 4: Light curves of superoutbursts of QZ Ser. (Upper): Superoutburst in 2013. ASAS-SN V data, visual
and CCD observations reported to VSNET were used. Two post-superoutburst rebrightenings were present.
(Lower): Superoutburst in 2020. The data were from VSNET Collaboration. The data were binned to 0.1 d. A
post-superoutburst rebrightening is apparent.

9References
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10

VSOLJ
c/o Keiichi Saijo National Science Museum, Ueno-Park, Tokyo Japan
Editor Seiichiro Kiyota
e-mail: skiyotax@gmail.com