The 1995 Superoutburst of AL Com
The section "Early VSNET Postings!" has been moved to the
later part of this page.
Poster Presented at Keele CV Conference
(The object was selected as "the star of the year" in this
Observation of WZ Sge-type Dwarf Nova AL Comae Berenices
T. Kato (1), D. Nogami (1) , H. Baba (1), K. Matsumoto (2),
J. Arimoto (2), K. Tanabe (2), K. Ishikawa (2)
(1. Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-01,
Japan, 2. Astronomical Institute, Osaka Kyoiku University,
Asahigaoka, Kashiwara, Osaka 582, Japan)
Although presently classified as a dwarf nova,
is peculiar in many respects. The star exhibited three historical
separated by 33 years. The light curves of its ourbursts
resemble a fast nova in that they show (1) rather rapid initial decline,
(2) subsequent slower decline which sometimes lasts 60 to 100 days, and
(3) large (about mag) outburst amplitudes.
From these features, this object had been long believed to be
a recurrent nova.
However, spectroscopic and photometric observations in 1978 outburst
confirmed the object to be a dwarf nova, not a classical nova
(Patterson et al., 1981, and references therein).
Most striking was the discovery of
superhumps which are one of the defining
SU UMa-type dwarf novae.
There are two major competing theories to explain the peculiar
nature of WZ Sge. One is the
mass-transfer burst theory,
which is supported by the discovery of large-amplitude
periodic humps early in its 1978 outburst (Patterson et al. 1981).
The period of the humps was equal to the orbital period, and they were
considered to reflect the hot spot enhanced by the mass-transfer burst,
although the hump maxima occured 0.17 orbital phase prior to the orbital
humps in quiescence.
The other is the extension of
thermal and tidal instability theory
of SU UMa-stars towards the lowest mass-transfer rate (Osaki 1995).
Numerical simulations have shown that thermal instability of the
accretion disk occurs rarely under combination of very low mass-transfer
rate and very low viscosity of the accretion disk during quiescence.
It is shown that a normal outburst in such condition always leads
to the tidal instability to trigger a superoutburst.
We here report new findings and its implications obtained through
the intensive observation of the very recent spectacular outburst of
which has proven to be an almost perfect twin to WZ Sge in many
2. 1995 Superoutburst of AL Com
AL Com was discovered by Rosino during its outburst in 1962. Its
dwarf nova nature was confirmed by subsequent spectroscopy during
outburst, and by discovery of historical outbursts back to 1892.
The existence of a deep dip in the middle of the fading stage was
noticed during this outburst (Bertola 1964).
The next impressive outburst was observed in 1975, when the object
showed a similar long-lasting outburst and possible re-brightening
after the decline to near quiescence.
Its exceptionally large outburst amplitude (about 8 mag) for a dwarf
nova has attracted a number of investigators. For example, Howell and
Szkody (1987) showed a large amplitude variation with a period near
40 min. They suggested from this period that AL Com may belong to
DQ Her-type magnetic
systems or AM CVn-type
systems. Further spectroscopic observation by Mukai (1990) precluded
the latter possibility. More extensive photometry by Abbott et al.
(1992) showed two distinct periodicities of 41 min and 87 -- 90 min,
the latter was suggested to be the orbital period, and the former
the rotation period of the white dwarf. From these periodicities
they concluded that AL Com bears the properties of both an enigmatic
dwarf nova WZ Sge and a unique intermediate polar
After 20 years' dormancy, AL Com was again observed by D. York (AAVSO)
during its rise to an outburst in early April 1995. The news was soon
relayed through Internet, accompanied by a burst of e-mails full of
astonishment. The object reached its historical record of mv=12.2
at maximum, and started fading. We then started time-resolved CCD
photometry during the full course of the outburst. Most of the observations
were done by a 60-cm telescope at
(Kyoto University) and
by a 50-cm telescope at Osaka Kyoiku University. The overall light curve
is given in Fig. 1.
(General V-band light curve of AL Com during the 1995
outburst. Large dots represent CCD observations by us, and small dots
visual observations reported through VSNET. Note the exceptionally
long duration of the outburst interrupted by a deep dip.)
2.1 Early Stage of Outburst
Within two days of the outburst maximum, periodic modulation was
discovered independently from many observatories. The first night of
the Ouda data showed
doubly humped modulation
(Fig. 2, Apr. 7).
(Time-resolved light curve obtained at the earliest stage
of the outburst. Low-amplitude, doubly-peaked periodic modulation is
This feature showed a continuous decaly until Apr. 12.
A period analysis
using PDM (Phase Dispersion Minimization) gives a very stable period
of 0.05666 +/- 0.00002 day (Fig. 3).
(Folded light curve of the early stage modulation. We
interpret the best estimate of the period (0.05666 day) as the orbital
one (this modulation therefore may be called "orbital superhumps").
Very similar features have been also observed in WZ Sge and HV Vir.)
This period was later confirmed
to be exactly twice (within the error of each estimates) of 41-min
periodicity observed during quiescence.
From this perfect agreement and later discovery of typical superhumps
with a significantly longer period, we have identified this period
as the orbital period. A similar feature was discovered in
which was assumed to originate from the enhanced hot spot
(Patterson et al. 1981). The suggested orbital period differs from
that of WZ Sge only by 2 sec!.
2.2 Appearance of Superhumps
After the decay of initial periodic modulation, a new distinct feature
appeared. The Ouda data on Apr. 17 showed a large amplitude modulation
(Time-resolved light curve obtained 12 days after the
onset of the outburst. Large-amplitude hump features characterstic
to superhumps are evident.)
whose profile was very characteristic to superhumps in usual
SU UMa-type dwarf novae. A period analysis yielded a period of
0.05722 +/- 0.00010
day, which is 1.0 +/- 0.2% longer than the
in the early stage of outburst. The superhump profile is shown in Fig. 5.
(Folded light curve of superhumps. The superhump period
(0.05722 day) is longer than the suggested orbital period by 1.0 \%.
These observations first firmly established the SU UMa-type nature of
This observation first firmly established the SU UMa-type nature of
AL Com. After the development of superhumps, the object entered a stage
of more gradual decline. The outburst amplitude and historical behavior,
the course of the outburst, orbital and superhump periods, and the time
lag before development of superhumps already made us convincing that
AL Com is a perfect twin of WZ Sge.
2.3 The "Dip"
The "dip" is one of the unique characterstics of the outburst light
curve of WZ Sge. The dips occur in the middle stage of the outburst,
and are characterized by a short (a few days) excursion to the faint
state, although still enough brigter than usual quiescence. There seems
to be some evidence that such dips are rather common among SU UMa-type
dwarf novae with very large outburst amplitudes. The same feature was
naturally sought in the case of AL Com. The dip actually occurred as
expected (Fig. 1).
Although our observations were unfortunately hindered by the unfavorable
weather, the latter half of the main dip was clearly visualized.
Fllowing the dip, AL Com brightened rather slowly to the second maximum,
then started a rapid fading. During this stage, we could no longer find
any evidence of superhumps or orbital humps. After this
"second dip", the object showed a recovery which led to a
2.4 The Plateau Stage
After recovery from dip episodes, the time-resolved light curve of
AL Com became flat again. However, within a week from recovery, the
object again started to show a low-amplitude modulation. An analysis
gives a period consistent with the superhump period observed earlier
(Fig. 6, Osaka Kyoiku University data).
(Folded light curve during the "plateau" phase, after
the dip. The best period agrees with the superhump period within the
error of the estimate. From the fact that the superhumps growed after
the dip, we interpret the "plateau" phase as a new superoutburst.)
This finding first clearly
demonstrate that "after-the-dip" behavior traces the usual course of
2.5 The Final Decline
After the plateau phase with superhumps, AL Com started to show
rather irregular variability with a time-scale of hours to a day.
The superhumps seemed to have completely disappeared (Fig. 7).
(Time-resolved light curve just before the final decline.
The superhumps have completely disappeared.)
Soon after this stage, the object showed a precipitous decline.
The quiescent magnitude was not yet reached during our observations.
AL Com stayed at about V=18.6 at least for some days after the rapid
The present observations clearly show that AL Com is a genuine
member of SU UMa-type dwarf novae, and that this object is almost
a twin of an enigmatic dwarf nova WZ Sge in many respects.
We here try to explain several peculiar features common to these
objects (hereafter "WZ Sge-type dwarf novae", cf. Bailey 1979).
3.1 Origin of Early Stage Modulation
As described in Chap. 2.1, we consider the period of this modulation
has the same period with the orbital period. In the case of WZ Sge,
the singly humped profile suggested the hot spot as the origin.
However, in the case of AL Com (and HV Vir
in our study), the
profile of this modulation is doubly peaked, and sometimes dominated
by dip features rather than humps.
This finding seems to preclude the
hot spot as the source of light.
A possibility of reprocessed light
by the secondary is also precluded, because this interpretation should
predict a sinusoidal light variation. We therefore suspect that this
modulation originates from the accretion disk itself (either by changing
energy output from the disk or the effect of geometrical obscuration).
In either cases, we have no need to think of enhanced mass-transfer
as was assumed in explaining WZ Sge-type phenomenon. One attractive
possibility is that this modulation represents "immature" superhumps.
Numerical simulations have shown that during the growing stage of the
tidal instability, the period of superhumps tends to be close to the
orbital period. It is, however, a mystery why such growing stages are
observed only in WZ Sge-type dwarf novae.
Fully grown superhumps were observed in AL Com. The good seasonal
condition allowed us unambiguously determine the period. The superhump
period of AL Com is 1.0% longer than the suggested orbital period.
This value of superhump excess is the second smallest among SU UMa-type
dwarf novae (the smallest: WZ Sgr 0.8% *1). Since the superhump excess is
known to correlate with the binary mass ratio, the mass ratio of AL Com
should be the second extreme one among established SU UMa-type dwarf novae.
A small difference from WZ Sge in this respect may be a cause of small
difference in the outburst behavior between these two systems.
(*1: Note that this value has been later found to be incorrect.
cf. vsnet-alert 6285)
3.3 The Dip
We don't have enough observational evidence about the origin of the
dip. However, the reported rate of decline of AL Com from reported
observations (mostly from visual observers of AAVSO) suggests that
the dip may be produced by a thermal transition wave starting from the
outer accretion disk. The magnitudes during the dips were well above
quiescence in both AL Com and WZ Sge.
This implies that the inner part of the accretion disk remained hot
even during the dip. The recovery from the dip was rather slow.
This implies that the recovery represents the "inside-out" type
propagation of the thermal transition wave. Nonexistence of orbital
humps during this stage strongly supports that the recovery from the
dip was not powered by a mass-transfer
but by the thermal instability of the
3.4 The Plateau Phase
After the dip, AL Com showed a short-living maximum followed by a
rapid decline. After this "secondary dip" the star entered a new
stage of the "plateau phase". The same features can be traced in the
light curve of WZ Sge during 1978 outburst. As already described
in Chap. 2.4, the superhumps growed during this phase. We can therefore
identify this stage as a start of a new superoutburst. A short-living
maximum after the main dip may represent a normal outburst which
triggered this superoutburst.
3.5 The Final Decline
The final decline can be again considered to represent the propagation
of a thermal transition wave through the accretion disk. The disappearance
of superhumps a few days before this decline implies that the cessation
of the tidal instability is responsible for the decline. After the
decline, AL Com stayed about 2 mag brighter than the reported quiescence.
Although the observational materials are limited due to its faintness,
it is likely that some inner part of the accretion disk remained hot
even after the final decline. This mechanism may be responsible for the
long fading tail of WZ Sge, which still remains poorly understood.
A similar phenomenon was also observed after the main (or super-) outburst
of GRO J0422+32,
a USXT (ultrasoft X-ray transient) which bears a number
of similarities with WZ Sge-type dwarf novae.
Measurement of the contribution of the accretion disk and the hot spot
to the observed light may be a clue in understanding this poorly understood
unique feature of WZ Sge-type dwarf novae and USXTs.
The persitent "superhump" periodicity observed in
HV Vir, another
object very similar to WZ Sge, during the same stage by Leibowitz et al.
(1994) supports a significant contribution to the light from the accretion
disk this stage. [Although this modulation was originally ascribed to an
orbital modulation (Leibowitz et al. 1994), our new analysis strongly
favors the persistent superhumps.]
Abbott T. M. C., Robinson E. L., Hill G. J., Haswell C. A. 1992, ApJ 399, 680
Bailey J., 1979, MNRAS 189, 41P
Bertola F. 1964, Ann d'Astrophys. 27, 298
Howell S. B., Szkody P. 1987, PASP 100, 224
Leibowitz E. M., Mendelson H., Bruch A., Duerbeck H. W., Seitter W. C.,
Richter G. A. 1994, ApJ 421, 771
Mukai K. et al. 1990, MNRAS 245, 385
Osaki Y. 1995, PASJ 47, 47
Patterson J., McGraw J. T., Coleman L., Africano J. L. 1981, ApJ 248, 1067
Early VSNET Postings! (titles only)
103 04/05 firstname.lastname@example.org AL Com
104 04/05 email@example.com AL COM in outburst!?
105 04/05 firstname.lastname@example.org Rare Outburst of AL Com
106 04/06 email@example.com Re: AL Com
108 04/05 firstname.lastname@example.org AL Com
109 04/06 email@example.com AL Com periodic waves
110 04/07 firstname.lastname@example.org AL COM FINDER CHARTS -- IMAGE AND POSTSCRIPT FIL
111 04/07 email@example.com AL COM position
112 04/08 firstname.lastname@example.org detection of "early superhumps" in AL Com
113 04/08 email@example.com AL Com: times of "early superhump" minima
114 04/08 firstname.lastname@example.org Rare Superoutburst of AL Com
115 04/09 email@example.com AL Com: "early" superhumps have decayed
116 04/09 firstname.lastname@example.org Nightly averaged magnitudes of AL Com
117 04/10 sdi00707@niftyser AL Com started to fade??
118 04/14 email@example.com revised period of AL Com
119 04/17 firstname.lastname@example.org AL Com light curve to date
120 04/18 email@example.com the submitted article on AL Com
121 04/18 firstname.lastname@example.org AL Com entering a new stage
123 04/26 email@example.com AL Com
125 05/03 firstname.lastname@example.org Re: Rare Outburst of V795 Cyg (& AL Com)
126 05/03 email@example.com AL Com "dips"
128 05/04 firstname.lastname@example.org Re: AL Com "dips"
133 05/05 email@example.com AL Com brighter! (Pietz)
134 05/06 firstname.lastname@example.org Confirmation of AL Com rebrightening
135 05/05 email@example.com AL Com periods
136 05/06 firstname.lastname@example.org AL Com on Mar. 5
138 05/07 email@example.com AL Com on May 6
139 05/07 firstname.lastname@example.org AL COM and others
141 05/18 email@example.com large-amplitude variation in AL Com
142 05/19 firstname.lastname@example.org rapid decline of AL Com
143 05/20 email@example.com Superoutburst of AL Com terminated?
145 05/23 firstname.lastname@example.org AL Com faded
Related systems (WZ Sge-type dwarf novae) observed by the
VSNET Collaboration team
Initial letter on WZ Sge 2001 by the VSNET Collaboration (Ishioka et al.)
Initial letter on WZ Sge 2001 by the VSNET Collaboration (Ishioka et al.)
HV Vir (2001)
RZ Leo (2000-2001)
RZ Leo paper (PASJ)
RZ Leo paper (PASJ) (PDF)
AL Com (2001)
EG Cnc (1996-1997)
HV Vir (1992) paper (PASJ)
HV Vir (1992) paper (PASJ) (PDF)
HV Vir (2002) paper (PASJ)
HV Vir (2002) paper (PASJ) (PDF)
UZ Boo (1994)
V592 Her (1998)
V592 Her paper (PASJ)
V592 Her paper (PASJ) (PDF)
SU UMa-type dwarf novae in general
VSNET light curve (requires Java)
VSNET data search
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