1
No.105Jan. 2023

ISSN 0917-2211

Variable Star Bulletin
LS And: WZ Sge-type outburst ﬁrst time since the 1971 discovery
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 2023 Jan. 09
Abstract
LS And was a transient discovered in 1971 in the M 31 region and it has been argued whether it could
be an intergalactic nova or a dwarf nova. Using the Zwicky Transient Facility (ZTF) data, I found that theobject underwent the second known outburst in 2022 April. The behavior was that of a WZ Sge-type dwarf
nova with a long fading tail and the light curves of the 1971 and 2022 outbursts matched very well. The
light curves suggest that LS And is a typical WZ Sge-type dwarf nova near (but before reaching) the period
minimum of cataclysmic variables. The true observed peak of the 1971 outburst was likely 12.2 mag. The
outburst parameters were similar to those of other WZ Sge-type dwarf novae. The fading tail lasts more thana year and the object is still currently on this tail. There was a hint of 0.5-mag temporary brightening on thefading tail and the object appears still active after the outburst.

LS And was discovered by van den Bergh et al. (1973) in the region of M 31 (named “m” in their paper).
van den Bergh et al. (1973) stated that the object was visible only on a blue and on a yellow plate taken in
immediate succession on 1971 August 26. van den Bergh et al. (1973) suggested that the variable might beeither a supernova or a ﬂare star. Although van den Bergh et al. (1973) did not give the brightness of this object,
it was estimated to be 12.5 from their ﬁgure by Romano (1977).
Sharov (1973) examined plates taken in the Crimean Station of Sternberg Astronomical Institute and Latvian
Radio Astrophysical Observatory. Sharov (1973) succeeded in obtaining one observation near the maximum andthe light curve of the fading part. Sharov (1973) noted the presence of a star of 21–22 mag on Palomar Observatory
Sky Survey (POSS). Based on the large amplitude exceeding 8 mag, rapid fading (0.2 mag d−1 ) in the early fading
part and the very slow (less than 0.001 mag d−1 ) fading rate in the late fading part, Sharov (1973) stated thatthe star was unlikely a supernova or a ﬂare star. The light curve, however, did not resemble those of typicalnovae or dwarf novae and Sharov (1973) suggested that it might be a very distant nova (i.e. intergalactic nova)
if it was indeed a nova.
Romano (1977) examined Asiago plates and presented a rough light curve of the outburst (probably unawareof the work by Sharov 1973). Romano (1977) indicated that the variable was at the limit of visibility (∼20.5 mag)on POSS and that color was almost white. Romano (1977) excluded a ﬂare star based on the light curve andalso a supernova based on the absence of a galaxy near the star. Romano (1977) concluded that this object isprobably a dwarf nova of UV Per type.1
Following Romano (1977), Meinunger (1977) studied Sonneberg plates (probably also unaware of the workby Sharov 1973) and constructed a light curve. Meinunger (1977) concluded that the star was clearly a fast
nova and could not be a supernova due to the absence of a galaxy near the star. Meinunger (1977) excluded along-period dwarf nova (like UV Per) based on the facts: (1) the amplitude was larger than 8 mag [Meinunger(1977) even suggested that the object on POSS was a unrelated one], (2) the decline after the maximum was too
1 UV Per was considered to be the prototype of dwarf novae with large-amplitude and rare outbursts at that time (cf. Petit 1960).
WZ Sge was considered as a recurrent nova and the concept of WZ Sge-type dwarf novae was not present. See Kato (2015) for amodern review of WZ Sge-type dwarf novae.

2

Table 1: Observations of the 1971 outburst of LS And.
JD∗
mag† source‡
JD∗mag† source‡
JD∗
mag†
179
[19.0
3
223.497 18.30
2
292
[19.0
183.468 [20.02
224.511 18.56
2
294
[19.0
183.508 [13.65
225.541 18.30
2
296
[19.0
235
18.5
3
298
[19.0
187.479 12.7:
5
187.508 11.7:
5
236.248 18.80
2
300
19.0:
190
12.5
1
237.261 18.83
2
302
19.0
238.405 18.83
2
304
19.0
191
13.8*
4
191.504 13.60
2
239.392 18.83
2
305.304 18.83
240
18.7
3
308
19.0
193
14.0*
4
193.476 14.1:
5240.407 18.83
2
320
19.0:
193.507 14.1:
5
242
18.7
3
324
19.0
245
18.5
3
332
[19.0
195.492 14.5::
5
195.515 14.5::
5245.339 19.02
335.238 18.8:
208
15.85
4
246.254 18.83
2
353.24
19:
249
18.5
3
570.392 19.2:
209
14.9
3
209.359 15.80
2
249.276 [18.82
575.408 19.2:
210
16.25
4
252.434 18.8:
2655.286 [18.3
210.499 15.98
2
254.519 [18.32
681.291 19.0
212
16.4
3
263
18.5:
3
682.168 [19.2
213.486 17.54
2
266.367 18.8
2
684.233 19.4
214
17.6*
4
268.427 18.83
2
685.201 [19.2
215
17.8*
4
271
19.0
3
688.219 19.4
217
18.0*
4
276
[19.0
3
983
20.0:
217.367 18.30
2
276.284 18.83
2
987
20.0:
220.358 18.33
2
277.396 18.8:
2
2105
20:
221.545 18.38
2
278.308 18.9
2
222.4
18.43
2
280
[19.0
3
∗
JD−2441000.
†[ upper limits. : uncertain. * eye estimate from the published ﬁgure.
‡
1: van den Bergh et al. (1973), 2: Sharov (1973), 3: Romano (1977),
4: Meinunger (1977), 5: Sharov and Karimova (1978).

source‡
3
3
3
3
3
3
3
2
3
3
3
3
2
2
2
2
2
2
2
2
2
2
5
5
5

fast and (3) no further outbursts were observed. Meinunger (1977) suggested that this object was probably avery bright nova in the halo of M 31.
Sharov and Karimova (1978) and his colleagues examined materials and found new records during the outburst close to the maximum in the collection of Odessa Observatory. Precise astrometry of the outbursting object
using the materials at Latvian Radio Astrophysical Observatory indicated the identity with the object on POSS.
Based on the large (9 mag) amplitude, exceeding those of dwarf novae, Sharov and Karimova (1978) consideredthat the object should be regarded as a fast nova despite its small amplitude for a nova. Sharov and Karimova(1978) also remarked that the supposed nova did not follow the maximum magnitude relation with decline timefor M 31 novae (Sharov 1989), and suggested that either the relation was broken or the object was an intergalactic
nova 100–150 kpc from the Sun. This classiﬁcation by Sharov and Karimova (1978) was adopted in Duerbeck(1987) and LS And was classiﬁed as a fast nova in General catalogue of variable stars (GCVS: Kholopov et al.
1985). In GCVS version 4.2 for extragalatic variables, LS And was also given a name M31V0002 probably
reﬂecting the possibility of an object in M 31.
Although most professional astronomers considered or treated LS And as a nova (Downes and Shara 1993;
Szkody 1994; Collazzi et al. 2009; Evans et al. 2014; Özdönmez et al. 2018), and some suspected to be an Xray nova (Rosenbush 1999) or a recurrent nova (Duerbeck 1988; Pagnotta and Schaefer 2014), I may have been
the ﬁrst to become conﬁdent that this should be a large-amplitude dwarf nova after knowing this object in
the freshly published work by Duerbeck (1987). A part of the atmosphere in the late 1980s among amateur

3

12

vdB73

S73

R77

M77

S78

14

16

18

20
41180

41200

41220

41240

Figure 1:
Light curve of the 1971 outburst of LS And using the data in table 1. The sources are
vdB73 (van den Bergh et al. 1973), S73 (Sharov 1973), R77 (Romano 1977), M77 (Meinunger 1977) and S78(Sharov and Karimova 1978). The “v” symbols represent upper limits.

astronomers was already told in Kato (2022a). Visual monitoring of LS And for a new outburst already started
in 1987 by VSOLJ members, and then by observers around the world. Although results have not been fruitfulfor decades [now exceeding 6000 observations without detecting an outburst in the American Association of
Variable Stars (AAVSO)2 ; I myself had more than 200 non-detection visual observations when I was an amateur
astronomer], I consistently considered LS And as a candidate WZ Sge star (Kato et al. 2001, 2002). I expectedthat the Gaia satellite would clarify the nature of LS And, but there was no parallax information in Gaia DR2(Gaia Collaboration et al. 2018). The blue color (Gaia B − R=+0.25) and a large proper motion were, however,suﬃcient to convince me of the dwarf nova-type nature. The parallax in Gaia EDR3 (Gaia Collaboration et al.
2021) was not conclusive, probably due to the faintness of this object. The color in Gaia EDR3 was even bluer(B − R=−0.06).
The “moment” arrived like lightening when I was examining light curves obtained by the Zwicky Transient
Facility (ZTF: Masci et al. 2019)3 . It was when I started examining light curves of recent ZTF data. As usual,I was looking at the table of dwarf novae listed in alphabetical order, and almost unconsciously typed LS And(as a matter of fact, I already did not pay special attention to this object regularly since I knew that it had
been well monitored by amateur observers and considered that no missed outburst would be expected in the
ZTF data). The reason why I specially selected LS And was unknown, but the light curve on the display was afamiliar one of a WZ Sge star. I initially considered that I entered a name of a diﬀerent well-known WZ Sge star(almost unconsciously as a routine work), but realized that it was “LS And”. Unthinkable! I initially could not
believe my eyes, but it was indeed LS And and I almost automatically issued vsnet-alert 272674 , even without
suﬃcient patience for waiting the result of a query to the All-Sky Automated Survey for Supernovae (ASAS-SN)
Sky Patrol data (Shappee et al. 2014; Kochanek et al. 2017). My emotion at that time may have been similar toa situation when I encountered a rare bird which I could not believe (cf. Kato 2022a). Birders will agree.
In the world of birders, it must have become the busiest moment after any discovery — one needs to locate
2 <http://www.aavso.org/data-download>.
3 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>.
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27267>.

4

12

ZTF r

ZTF g

ATLAS oATLAS c

ASN g

14

16

18

20
59700

59720

59740

59760

Figure 2: Light curve of the 2022 outburst of LS And using ZTF, ATLAS and ASAS-SN data. There were no
upper limit observations before the initial detection.

12

vdB73

S73

R77

M77

S78

2022

14

16

18

20
41180

41200

41220

41240

Figure 3: Comparison of light curves of the 1971 and 2022 outburst of LS And. The symbols for the 1971
observations are the same as in ﬁgure 1. The 2022 data (ZTF r magnitudes) were shifted by 18503 d.

5

12

ZTF r

ZTF g

ATLAS oATLAS c

ASN g

14

16

18

20

22

59500

59600

59700

59800

59900

Figure 4: Long-term light curve of the 2022 outburst of LS And. The symbols are the same as in ﬁgure 2.

the bird and take images or recordings suﬃcient for a proof of the existence of a rare bird. The case for thedetection of the 2022 outburst of LS And was diﬀerent. There was no special care for preserving the data shownon the display, and I went to the library (fortunately very close) to search the light curve of the 1971 outburst,
which still stayed deep in my memory even after decades.
So it’s time to return to science. In table 1, I summarized photometric data for the 1971 outburst. The
magnitudes were all photographic (equivalent to B). Magnitudes with * were estimated by my eyes from theﬁgure in Meinunger (1977), which are probably correct to ±1 d and ±0.1 mag. The magnitude for JD=190 was
similarly estimated from a published ﬁgure by Romano (1977). Meinunger (1977) claimed that the object wasestimated too bright by Romano (1977). The light curve drawn from these data is presented in ﬁgure 1. This
is not much diﬀerent from the one published in Sharov and Karimova (1978), but is worth presenting here since
Sharov and Karimova (1978) is diﬃcult to reach.
The 2022 light curve is shown in ﬁgure 2. It is very clear that the 1971 and 2022 light curves are very
similar: plateau-type fading lasting for ∼20 d followed by rapid decline and subsequent slow fading. They are
typical WZ Sge-type outbursts without rebrightening (type D superoutburst in Kato 2015). It is also well-known
that the same WZ Sge star tends to repeat the same type of rebrightening (Kato 2015) and LS And is also
the case. Although the mechanism of rebrightening(s) is not yet well understood, empirical relationship showsthat WZ Sge stars without rebrightening are mostly objects near the period minimum of cataclysmic variables,
but before reaching it (ﬁgure 17 in Kato 2015). The orbital period of LS And is thus expected to be within
0.053–0.060 d. The fading rate of the plateau phase (BJD 2459696–2459714.5) was 0.089(1) mag d−1 , whichcorresponds to log td =1.05, a typical value for a WZ Sge star without rebrightening and not resembling a period
bouncer (see ﬁgure 87 in Kato et al. 2014). A comparison between the 1971 and 2022 outbursts is shown in ﬁgure
3 (from now on, I treat all photometric bands in visual wavelengths almost identical with V , which is a good
approximation for a WZ Sge star in outburst). These outbursts were almost exactly the same and the interval ofthese two outburst was 18503 d (=50.66 yr). This comparison suggests that the 2022 outburst would not havestarted before JD 2459682 (2022 April 12). Deﬁnitely a sigh! (particularly for amateur observers) considering
the almost no evening visibility of this object in mid-April.
People may wonder if these outburst could be those of an SU UMa star rather than a WZ Sge star, and how
I can be conﬁdent about the classiﬁcation without observation of early superhumps (cf. Kato 2015). I show along-term light curve of the 2022 outburst in ﬁgure 2. The object was brighter by 1.5 mag after the outburst. The
post-outburst phenomenon is a long fading tail, which is characteristic to a WZ Sge-type outburst and not seen

6in an SU UMa star. The presence of the same phenomenon was also reported after the 1971 outburst (Sharov
1973).5 Before the outburst plateau, there was a phase with more rapid fading (more evident in the 1971 light
curve and only one day in the 2022 one). This feature is commonly seen in WZ Sge-type outbursts and is referred
to as a viscous decay phase. Early superhumps are expected during this phase if the binary has a suﬃcient
inclination (Kato 2015, 2022b).
The peak magnitude probably requires re-examination. Although most literature gives 11.7 mag as the
maximum for LS And, it is evident from table 1 that this magnitude was uncertain (“:” usually means thatthe object is close to the limit of photographic materials or the quality of the photograph is poor) and was thebrighter one of two uncertain observations (11.7 and 12.7 mag) only 40 min apart. It looks more likely that thetrue brightest observation was close to their average (12.2 mag). The outburst amplitude based on this value is
8.8 mag using the ZTF data before the 2022 outburst. The true peak would have been brighter, though, since
there was a 4 d observational gap before the ﬁrst observation of the outburst (but see the discussion below).
As seen from the 2022 observations, the magnitude when ordinary superhumps should appear following theviscous decay phase was 14.3 mag. In ordinary WZ Sge stars, the absolute magnitude (MV ) when ordinary
superhumps appear is +5.4 (for an average inclination of 1 radian) (Kato 2022b). Using this value as thestandard candle, the distance modulus of LS And is estimated to be 8.9. The observed peak (12.2 mag) in1971 corresponds to MV =+3.3. The quiescent magnitude (21.0 mag, ZTF data) corresponds to MV =+12.1.
The diﬀerence (6.7 mag) between quiescent magnitude and the magnitude when ordinary superhumps appear istypical for a (non-period bouncer) WZ Sge star (see ﬁg. 23 in Kato 2015; Tampo et al. 2020). Other properties
of LS And are expected to be similar to those of typical WZ Sge stars.
The detection of the 2022 outburst of LS And brought a some kind of despair to observers who had been
expecting to see a fresh outburst for decades. Could there be a possibility that LS And silently underwent
outbursts more frequently only around solar conjunctions? This was indeed the case of the SU UMa star VY Aqr
located close to the ecliptic. Despite the mean interval of superoutbursts of less than 2 yr, this object was
not recorded in superoutburst between 1994 and 2006, and between 2008 and 2020. It was most likely thatsuperoutbursts in this object occurred around solar conjunctions and were not recorded. Although similar things
may have happened in LS And at least in the past, modern deep observations such as ZTF should have detected
the object during a fading tail if there was a missed superoutburst. There was no indication of such a detection in
the ZTF data since 2018, and the outburst interval should be longer than 5 yr. The fading tail lasted more than ayear (Sharov 1973). Sharov and Karimova (1978) described that the object returned to practically the same level
before the outburst after 5.5 yr, although this description may have assumed a nova-type light curve and could
have overestimated the duration of the fading tail. Considering these values and considering that parameters of
LS And are similar to those of typical WZ Sge stars, the next major outburst would be expected after a decade oreven more [see ﬁgure 5 in Kato (2015) for the distribution of outburst intervals in WZ Sge stars]. By comparing
the recorded peak MV =+3.3 (in 1971) with the statistics of known WZ Sge stars (ﬁgure 10 in Tampo et al. 2020),
it appears that the true peak in 1971 was not missed after a considerable delay (i.e. the object was unlikely tohave reached 11.0 mag even at the true peak). The next superoutburst would also be around 12.2 mag. There
are, however, exceptional objects like V3101 Cyg (Tampo et al. 2020; Hameury and Lasota 2021) and there may
be an unexpected phenomenon even after the outburst. In the post-outburst data of LS And, 0.5 mag brightening
lasting for 10–20 d and starting around JD 2459852 was present (ﬁgure 3). This might suggest that LS And is
still active in the post-superoutburst phase and would be worth observing before it ﬁnally returns to quiescence.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF,ATLAS and ASAS-SN teams for making their data available to the public. I am grateful to VSOLJ, AAVSO andVSNET observers for reporting observations and to Naoto Kojiguchi for helping downloading the ZTF data.
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 Consortium
5 It might be interesting to leave a remark that the ﬁgure in Sharov (1973) dealt with this phenomenon rather than the shape ofthe outburst. Please have a look at his ﬁgure if you have a chance too see this reference.

7of 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).
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
LS And, VY Aqr, V3101 Cyg, UV Per, WZ Sge, SU UMa, M 31, M31V0002

References
Collazzi, A. C., Schaefer, B. E., Xiao, L., Pagnotta, A., Kroll, P., Löchel, K., & Henden, A. A. (2009) The
behavior of novae light curves before eruption. AJ 138, 1846
Downes, R. A., & Shara, M. M. (1993) A catalog and atlas of cataclysmic variables. PASP 105, 127
Duerbeck, H. W. (1988) V394 CrA – outburst light curves and notes on its position among the recurrent novae.
A&A 197, 148
Duerbeck, H. W. (1987) A reference catalogue and atlas of galactic novae. Space Sci. Rev. 45, 1
Evans, A., Gehrz, R. D., Woodward, C. E., & Helton, L. A. (2014) A WISE view of novae – I. the data. MNRAS
444, 1683
Gaia Collaboration et al. (2018) Gaia Data Release 2. Summary of the contents and survey properties. A&A
616, A1
Gaia Collaboration et al. (2021) Gaia Early Data Release 3. Summary of the contents and survey properties.
A&A 649, A1
Hameury, J.-M., & Lasota, J.-P. (2021) Modelling rebrightenings, reﬂares, and echoes in dwarf nova outbursts.
A&A 650, A114
Kato, T. (2015) WZ Sge-type dwarf novae. PASJ 67, 108
Kato, T. (2022a) Evolution of short-period cataclysmic variables: implications from eclipse modeling and stage
a superhump method (with New Year’s gift). VSOLJ Variable Star Bull. 89, (arXiv:2201.02945)
Kato, T. (2022b) Emerging ordinary superhumps as the standard candle for WZ Sge stars. VSOLJ Variable Star
Bull. 90, (arXiv:2202.02956)
Kato, T. et al. (2014) Survey of period variations of superhumps in SU UMa-type dwarf novae. V: The ﬁfth year
(2012–2013). PASJ 66, 30
Kato, T., Sekine, Y., & Hirata, R. (2001) HV Vir and WZ Sge-type dwarf novae. PASJ 53, 1191
Kato, T., Uemura, M., Matsumoto, K., Kinnunen, T., Garradd, G., Masi, G., & Yamaoka, H. (2002) WZ Sge-typestar V592 Herculis. PASJ 54, 999
Kholopov, P. N. et al. (1985) General Catalogue of Variable Stars, fourth edition (Moscow: Nauka Publishing
House)

8
Kochanek, C. S. et al. (2017) The All-Sky Automated Survey for Supernovae (ASAS-SN) light curve server v1.0.
PASP 129, 104502
Masci, F.-J. et al. (2019) The Zwicky Transient Facility: Data processing, products, and archive. PASP 131,018003Meinunger, L. (1977) A bright nova in the surroundings of the Andromeda Nebula. IBVS 1331, 1Özdönmez, A., Ege, E., Güver, T., & Ak, T. (2018) A new catalogue of Galactic novae: investigation of the
MMRD relation and spatial distribution. MNRAS 476, 4162
Pagnotta, A., & Schaefer, B. E. (2014) Identifying and quantifying recurrent novae masquerading as classicalnovae. ApJ 788, 164
Petit, M. (1960) Catalogue des Étoiles variables du type U Geminorum. Journal des Observateurs 43, 17
Romano, G. (1977) Some variable stars in the ﬁeld of M31. AJ 82, 319
Rosenbush, A. E. (1999) X-ray nova candidates among old classical novae. Astrophysics 42, 270
Shappee, B. J. et al. (2014) The man behind the curtain: X-rays drive the UV through NIR variability in the2013 AGN outburst in NGC 2617. ApJ 788, 48
Sharov, A. S. (1973) Interesting variable star. Astron. Tsirk. 793, 1
Sharov, A. S. (1989) The bright novae in M31 and the relation between peak brightness and rate of decline. Soviet
Astronomy Letters 15, 5
Sharov, A. S., & Karimova, D. K. (1978) New data on an interesting variable star. Astron. Tsirk. 998, 1
Szkody, P. (1994) BVRJK observations of northern hemisphere old novae. AJ 108, 639
Tampo, Y. et al. (2020) First detection of two superoutbursts during the rebrightening phase of a WZ Sge-type
dwarf nova: TCP J21040470+4631129. PASJ 72, 49
van den Bergh, S., Herbst, E., & Pritchet, C. (1973) A search for faint variable objects. AJ 78, 375

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