[vsnet-alert 2758] BZ UMa bright outburst!!

[Taichi Kato <tkato>]

BZ UMa bright outburst!!

Dear Colleagues,

   As predicted by Watanabe-san (cf. vsnet-future 3
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-future/msg00003.html),
Eddy Muyllaert reports that the SU UMa-type candidate BZ UMa is undergoing
a bright "Watanabe outburst".  The rise to outburst was caught by Jose Ripero
on March 8.

  YYMMDD(UT)   mag  observer
  990227.621  <132  (T. Watanabe)
  990227.631  <128  (H. Maehara)
  990227.756  <129  (H. Itoh)
  990227.847  <138  (E. Muyllaert)
  990301.646  <128  (H. Maehara)
  990303.544  <121  (T. Watanabe)
  990303.851  <141  (E. Muyllaert)
  990303.971  <137  (G. Poyner)
  990305.681  <128  (H. Maehara)
  990306.146   159  (G. Hanson)
  990307.133  <157  (G. Hanson)
  990308.944   141  (J. Ripero)
  990309.787   104  (E. Muyllaert)
  990309.846   104  (E. Muyllaert)

   The present brightness is the brightest since 1994.  In 1996 May, the
object underwent the second brightest outburst (G. Poyner, mv=10.6), which
was a well-followed normal outburst.  On 1997 Jan. 8, Gene Hanson followed
a dramatic rapid rise precisely (this was also a normal outburst, mv=11.4
at maximum).

   Although there may be no further need to stress on the importance of
BZ UMa and the potential implication of the current bright outburst,
I'd like to cite some previos vsnet-alert articles, starting from vsnet-alert
332, for convenience.

   The full article can be seen at:

  http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/DNe/bzuma.html

Regards,
Taichi Kato

===========================================================================

Comment by F. Ringwald

It would be highly desirable to obtain time-resolved photometry of this
dwarf nova to search for superhumps. BZ UMa has an orbital period of 97.8
minutes (below the period gap), and has outbursts only rarely, perhaps
once per year or less (Jurcevic J. S., Honeycutt R. K., Schlegel E. M.,
Webbink R. F., 1994, PASP, 106, 481; Ringwald F. A., Thorstensen J. R.,
Hamwey R. M., 1994, MNRAS, in press). Its rare outbursts all seem to be of
extreme amplitude and duration, and may well be superoutbursts: BZ UMa is
probably a low-luminosity SU UMa star, and may only have superoutbursts.
It is therefore likely to have superhumps. 

Finding charts are given by Downes R. A., Shara M. M., 1993, PASP, 105, 127
and by Bruch A., Fischer F.-J., Wilmsen U., 1987, A&AS, 70, 481.

Coordinates are:
08 49 52.5   +57 00 04  (Epoch 1950.0)
08 53 44.1   +57 48 41  (Epoch 2000.0)

It is currently up late, after midnight at most Northern Hemisphere sites. 
I have telescope time next week at Kitt Peak; a multi-longitude study
could make resolving period aliases easier. 

Comment by T. Kato

  It may not be the case for BZ UMa, at least this object is not a very
ordinary SU UMa star.  This star shows infrequent bright normal outburst;
a representative one in 1992.  The following data are taken from IAUCs.

  1992  10/23.963UT, [13.7(Poyner); 26.052, 10.7 (Poyner);
        26.979, 11.7 (P.Schmeer, Germany); 27.810, 12.4(W.Worraker, Didcot);
        28.721, 13.2 (Worraker).

  We have additional two sets of CCD photometry of similar short outbursts.
From these records, we may conclude that the normal outbursts of this
dwarf nova are more abundant than superoutbursts.

  We have no further information on 1992 outburst, but if the identification
of a superoutburst by P. Schmeer is correct, superoutbursts of this star may
be _fainter_ than the normal outbursts.  Such behavior is sometimes observed
in TOADs
(Tremendous Outburst Amplitude Dwarf Novae), but the amplitude of
BZ UMa is less than five magnitudes (typical quiescent magnitude V=15.5).
How can one explain all these facts?

  In addition, BZ UMa showed a long period without detectable outbursts
(Dr. Mattei of may pick it up from the AAVSO records ..), and at least one
occasion when the system went dwon to B=17.8 probably during this period
(the exact time and reference I don't remember).
Changing mass-trnasfer rate?

Comment by F. Ringwald

There seems to be a lot of misinformation about BZ UMa, some of which I
have propagated myself. First, it turns out that while any outbursts are
rare, BZ UMa does have normal outbursts as well as superoutbursts: see the
long-term lightcurve of Jurcevic et al. 1994, PASP, 106, 481. 

Second, while there were no confirmed outbursts between 1974 and 1990, the
AAVSO lightcurve for this period that I have (Mattei J. A., 1989, private
communication) is sparsely sampled. Many normal or even superoutbursts
could have been missed. 

It is also unclear that superoutbursts do not get as bright as normal
outbursts, since so few outbursts of any type have been observed. Again,
this may be just sparse sampling: more monitoring is definitely needed. 

Last, BZ UMa may sometimes get as faint as B = 17.8, but this has been
seen only once (Kaluzny J., 1986, IAU Circ. No. 4287). Confirmation is
needed, and will require a CCD. 

Still, if this is a superoutburst, it should be searched for superhumps.
So please continue observing it! 

Comment by T. Kato

  I have picked up the paper by Jurcevic et al. and examined the light
curve.  The authors state that all the three observed outbursts have
durations shorter than ten days, and we can fairly firmly conclude that
these outbursts are all "normal" (short) outbursts, and not (at least
typical) superoutbursts.  The bightest one reached at least V=10.2,
which already outranged the catalogued limit.  If one assumes the
superoutbursts are brighter than the normal outbursts by 0.5 or 1 magnitude
in this system (analogy with other SU UMa systems), one should conclude
that superutbursts have historically escaped detection.  I am eager to
know how P. Schmeer has identified the 1990 outburst (there was a confusion
in my previous mail with the 1992 outburst) as a superoutburst, because
we know only two visual positive observations.  If that outburst turns
out to be a normal one, this star may not have shown any superoutbursts
at all.  In this case one may expect a very bright superoutbursts, probably
brighter than V=10; otherwise this system may be classified as a peculiar
DN-like CVs below the period gap, like EX Hya or V4140 Sgr.  Don't you
think existence of high excitation emission lines like He II, C III/N III
may somewhat contradict with the "low luminosity SU UMa-type" picture?

Comment by F. Ringwald

No, because the continuum is very weak from the low mass transfer rate, so
all lines are strong. The equivalent width of H alpha is over 200
Angstroms! He II and CIII/N III are still much weaker than H beta, not
like in a magnetic CV or in an old nova, where it can be comparable or
stronger. But the smoking gun is that in high-S/N red spectra, I detected
the M5.5 +/- 0.5 secondary star (Ringwald F. A., 1993, Ph.D. thesis,
Dartmouth College; also Ringwald F. A., Thorstensen J. R., & Hamwey R. 
M., 1994, MNRAS, in press). At an orbital period of 97.8 minutes, an M5.5
dwarf would fit inside the Roche lobe, so that with an M5.5 dwarf at the
observed dilution of 4.5%, this implies an absolute magnitude in R of 10.7
(+0.7/-0.9). 

Compiled data by J. Pietz

BZ UMa Observations (source: The Astronomer)

1990-7 9: 12.2, 10: 12.4, 11: 12.7, 13: 13.9, 18: 14.5
1991-4 12: 12.0, 13: 13.8, 14: 14.5
1991-10 21: 12.0
1992-10 26: 10.7, 27: 12.4-13.2, 28: 15.0 (FID.afoev)
1993-4 29: 10.6, 30: 11.9
1993-10 24: 11.5, 25-26: 12.4

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