1
No.116

May. 2023

Variable Star Bulletin
The 2022 active state of the AM CVn star NSV 1440
Taichi Kato1 , Rod Stubbings2
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp
2 Tetoora Observatory, 2643 Warragul-Korumburra Road, Tetoora Road, Victoria 3821, Australia
stubbo@dcsi.net.au
Received 2023 May 04
Abstract
We found an active state lasting for ∼200 d in the AM CVn star NSV 1440 in 2022. During this state,
the object reached a magnitude of 16.5, 2.0–2.5 mag above quiescence, and showed a number of superposed
normal outbursts. Such an active state was probably brought either by an enhanced mass-transfer fromthe secondary or increased quiescent viscosity of the accretion disk. These possibilities are expected to be
distinguished by an observation of the interval to the next superoutburst. We also found that the brightness
and the course toward the end of the event were similar to the post-superoutburst fading tail in 2021. The
mechanism producing the 2022 active state and post-superoutburst fading tails in AM CVn stars may be thesame, and the present ﬁnding is expected to clarify the nature of these still poorly understood fading tails in
AM CVn stars, and potentially of the corresponding phenomenon in hydrogen-rich WZ Sge stars. We also
note that the faint, long “superoutbursts” in long-period AM CVn stars claimed in the past were not true
outbursts powered by disk instability, but were more likely phenomena similar to the 2022 active state in NSV
1440.

NSV 1440 was discovered as a variable star (BV 1025) with a photographic range of 12.6 to fainter than
15.0 (Knigge and Bauernfeind 1967). The variability of this object was not studied further and was listed as asuspected variable in Kukarkin et al. (1982). In 2015, the All-Sky Automated Survey for Supernovae (ASASSN, Shappee et al. 2014) detected the transient ASASSN-15sz (K. Stanek, vsnet-alert 19288)1 , which was soon
identiﬁed with NSV 1440 (D. Denisenko, vsnet-alert 19289)2 . Subsequent time-resolved photometry immediately
detected short-period variations (vsnet-alert 192913 and vsnet-alert 192934 ), followed by rapid fading (vsnetalert 19298)5 . These variations were soon identiﬁed as early superhumps and ordinary superhumps [see e.g.,
Kato (2015) for the nomenclature of superhumps in WZ Sge stars], and the object was suggested to be the ﬁrst
AM CVn star showing both early and ordinary superhumps (vsnet-alert 19304)6 . This object showed multiple
rebrightenings (starting with the one reported in vsnet-alert 19328)7 , which has been now identiﬁed as one of
the characteristic features of AM CVn-type superoutbursts (see e.g., Kato and Kojiguchi 2021). Another very
similar superoutburst with multiple rebrightenings occurred in 2017 and Isogai et al. (2019) reported on these two
superoutbursts. Isogai et al. (2019) measured the periods of early and ordinary superhumps to be 0.0252329(49) d
and 0.025679(20) d, respectively. The former is expected to be very close to the orbital period (Porb ). Isogai
et al. (2019) determined a mass ratio of q=0.045(2).
Not much work was done since Isogai et al. (2019) for this object, one of the reasons probably being the object
near the south celestial pole, which is inaccessible from northern observers. Although Pichardo Marcano et al.
1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19288>.
2 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19289>.
3 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19291>.
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19293>.
5 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19298>.
6 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19304>.
7 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19328>.

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Table 1: Superoutbursts in NSV 1440.
Year
2015
2017
2019
2021

Month
NovemberAugustFebruary
August

Day
21
21
16
30

BJD
2457350245798724585312459457

Peak magnitude
V =12.9
mvis =13.0
mvis =13.0
mvis =12.9

Interval (d)
–
637
544
926

(2021) referred to the presence of Transiting Exoplanet Survey Satellite (TESS) (Ricker et al. 2015)8 observationsof this object, nothing particular was discussed. This is understandable, since (currently) public TESS data only
recorded quiescent parts and we could not detect a periodic signal corresponding to Porb .
One of the authors (RS) noticed a number of short outbursts in 2022 by visual observations. Using ASAS-SN
observations and Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) forced photometry(Shingles et al. 2021), we found that this object was indeed brighter its ordinary quiescence [BP =18.44 and
RP =18.42 according to Gaia Collaboration et al. (2022)] and the even reached g=16.5 between these outbursts.
The long-term light curves are shown in ﬁgures 1 and 2. Only positive detections are shown and all other
ASAS-SN observations were upper limits. Although there are segments with only a few points, a missed longoutburst is excluded from these ASAS-SN upper-limit observations. Since the end of 2021, ATLAS observations
became available and the variation of the quiescent level and occurrence of short outbursts are clearly shownin the third and fourth panels of ﬁgure 2. An enlargement of the 2022 active state is shown in ﬁgure 3. We
must note, however, isolated ASAS-SN points near quiescence of these ﬁgures should be regarded as upper limits
rather than real values since they were incidentally positively detected among other upper-limit observations due
to random errors. A phenomenon similar to the 2022 active state was not present at least between 2015 and 2021
based on observations by the ASAS-SN and RS.
Superoutbursts shown in these ﬁgures are summarized in table 1. The dates are given for optical peaks. The
2019 superoutburst was preceded by a precursor outburst. All these outbursts had a short initial segment (4–6 d)followed by complex rebrightenings (both long and short ones depending on the superoutburst) and a fading
tail (see Kato and Kojiguchi 2021), which is commonly seen after AM CVn-type superoutbursts. Among thesesuperoutbursts, the 2021 one appears to have been the most powerful one in that it had the longest duration (6 d)of the initial segment. This is probably a consequence of the long quiescent interval before this superoutburst.
A larger mass should have been accumulated in the disk before this superoutburst.
After this 2021 superoutburst, the quiescent level strongly varied, associated by frequent occurrence of short
outbursts as initially reported by RS. There are two possibilities for this phenomenon (and possibly a combinationof them):• The mass transfer-rate (Ṁtr ) varied and outbursts occurred frequently when Ṁtr was high.
• The quiescent viscosity varied and outbursts occurred frequently when the viscosity was high, if there was
suﬃcient mass in the disk.
Both possibilities appear to naturally explain the observed phenomenon. AM CVn can have variable Ṁ as in
the recent example of a standstill in CR Boo (Kato et al. 2023) and the variation in Ṁtr could to be a viable
explanation. The variation in the quiescent viscosity also appears to be promising, and has been proposed to
explain post-superoutburst rebrightenings in hydrogen-rich systems (WZ Sge stars) (Osaki et al. 2001; Meyerand Meyer-Hofmeister 2015). The same explanation would apply to a helium disk, and since the helium diskrequires a higher temperature to partially ionize (Tsugawa and Osaki 1997; Kotko et al. 2012), a small change
in the quiescent viscosity would produce a more prominent eﬀect than in hydrogen-rich dwarf novae. Thesepossibilities may be distinguished by observing the next superoutburst. If Ṁtr indeed increased, the mass of thedisk increased more rapidly than in ordinary quiescence and the next superoutburst is expected to occur earlier
than expected or to be a powerful one. If the quiescent viscosity increased and Ṁtr remained the same, the massin the disk should have been drained more quickly than in ordinary quiescence and the next superoutburst is
expected to occur later. It is a pity that this object does not show orbital modulations even in TESS data andit was impossible to detect a possible enhancement of the hot spot resulting from increased Ṁtr .
8 <https://tess.mit.edu/observations/>. The full light-curve is available at the Mikulski Archive for Space Telescope (MAST,<http://archive.stsci.edu/>).

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12
14
16
18
57150
12

ASN V

ASN g

ATLAS oATLAS cvisual

57250

57300

57350

57400

57450

57200

57500

14
16
18
57550

57600

57650

57700

57750

57800

57850

12
14
16
18
57900

57950

58000

58050

58100

58150

58200

12
14
16
18
58250

58300

58350

58400

58450

58500

58550

58600

Figure 1: Light curve of NSV 1440 in 2015–2019. ASN and “visual” refer to ASAS-SN observations and visualobservations by RS, respectively. Only positive detections are shown and all other ASAS-SN observations were
upper limits. No other major outburst was missed.

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12
14
16
18
58650

58700

58750

58800

58850

58900

58950

12
14
16
18
59000

59050

59100

59150

59200

59250

59300

12
14
16
18
59350

59400

59450

59500

59550

59600

59650

59700

12
14
16
18
59700

59750

59800

59850

59900

59950

60000

60050

Figure 2: Light curve of NSV 1440 in 2019–2023. The symbols are the same as in ﬁgure 1.

5

14
15
16
17
18

59820

59840

59860

59880

59900

59920

59940

Figure 3: Enlargement of the central part of the 2022 active state in NSV 1440. The symbols are the same as
in ﬁgure 1.

It may be worth noting that this active state in 2022 was very similar in brightness and duration to thepost-superoutburst fading tail of the 2021 superoutburst. The mechanism of fading tails of AM CVn stars is still
poorly understood. In hydrogen-rich WZ Sge stars, very long-duration (reaching a few years) fading tails havebeen considered to arise from the cooling white dwarf (Szkody et al. 1998; Godon et al. 2004; Long et al. 2004;
Piro et al. 2005; Godon et al. 2006). This explanation, however, cannot be directly applicable to fading tails of
AM CVn-type superoutbursts at least for two reasons: (1) the mass accumulated or accreted in AM CVn-type
dwarf novae is much smaller and (2) the fading tail ends abruptly (see the third panel of ﬁgure 2), in contrast
to smooth fading expected for a cooling white dwarf. The same feature of abrupt fading was also present in
the 2022 active phase: a sudden drop was present around BJD 2459955. These features suggest rapid switching
between high and low states, either by variable Ṁtr or quiescent viscosity. If the mechanism of the 2022 active
phase is clariﬁed, it will also help understanding the yet unsolved mechanism of the fading tail and associatedrebrightenings in AM CVn stars (see e.g., Kato and Kojiguchi 2021; Rivera Sandoval et al. 2022). This mightalso provide a clue in understanding the exceptional hydrogen-rich dwarf nova V3101 Cyg with a long-lasting
post-superoutburst phase with increased outburst activity (Tampo et al. 2020; Hameury and Lasota 2021).
We should make a comment on long-lasting faint “superoutbursts” claimed in the past. Rivera Sandoval
et al. (2020) reported a faint “superoutburst” lasting more than a year in SDSS J080710.33+485259.6 with aphotometric period of 53.3(3) min (Kupfer et al. 2019). This “superoutburst” was very unusual in its morphology
and the peak brightness (18.2 mag) was only ∼2 mag above the quiescence. Rivera Sandoval et al. (2021) reported
a similar 0.5–1 mag (according to their paper; the amplitude was 1.5 mag according to the ATLAS data) longlasting brightening in SDSS J113732.32+405458.3 with Porb of 59.6±2.7 min (Carter et al. 2014). Based on these
two “outbursts”, Rivera Sandoval et al. (2022) suggested the presence of two types of superoutbursts in long-Porb
AM CVn stars. One type has a large amplitude and a short duration, and it follows the tendency expected fromthe disk-instability model. This type is what we referred to superoutbursts here. The other type with a small
amplitude and a long duration appears to be similar to the active state of NSV 1440 in 2022, although short
outburst on them were absent in these two objects claimed in the past. The peak MV of the long “outburst” in
SDSS J113732.32+405458.3 was +10.9 using the Gaia parallax (Gaia Collaboration et al. 2022) and the ATLAS
data. This value is close to the peak MV = +8.9 in the active state of NSV 1440 in 2022. The long “outburst”
in SDSS J113732.32+405458.3 was probably a phenomenon similar to the active state in NSV 1440 rather thana true outburst considering the presence of genuine superoutbursts in NSV 1440 with much larger amplitudes.
Although the parallax of SDSS J080710+485259 was not determined well, the peak MV is expected to be similar
to that of SDSS J113732.32+405458.3 considering the similar amplitude. These active states should not be called
outbursts or superoutbursts, and they should not be discussed as outbursts in the context of disk instability.

6

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The authors are grateful to the ATLAS
and ASAS-SN teams for making their data available to the public. We are also grateful to Naoto Kojiguchi for
helping downloading TESS data.
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
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.

List of objects in this paper
CR Boo, AM CVn, V3101 Cyg, WZ Sge, NSV 1440, ASASSN-15sz, BV 1025, SDSS J080710.33+485259.6,
SDSS J113732.32+405458.3

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