COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3642

                                             Konkoly Observatory
                                             Budapest
                                             18 July 1991

                                             HU ISSN 0374-0676


     Clues for periodic energy releases during a stellar flare


 Introduction:

 Observations of flares on the Sun have shown the very tight evolution of the
white light, gamma-ray, X-ray and microwave emissions. This highlights the direct
link between the locations of white light emission and energy deposition in the
atmosphere. By analogy to the solar case, one may expect the stellar white
light flare emission to be a meaningful tracer of the energy release and
deposition processes. Notably, observations by Beskin et al. (1989) have
shown that the continuum emission is essentially thermal in character.

On the other hand, observations of quasi-periodic changes in the light curves
may provide an interesting set of information and constraints, so as to model
the energy release processes and magnetic field topology (e.g. Tajima et
al., 1987). This seems an interesting opportunity in the stellar case since no
spatial resolution is yet possible. Clues for quasi-periodicities were
discovered by Rodono (1974), Roizman and Kabichev (1985), Chugainov (1987) and
Zalinian and Tovmassian (1987). Here we report on evidence for periodic
brightness variations during a white light flare on Wolf 424 AB (dM5.5e).

 Observations:

The observations were acquired at the European Southern Observatory with the 1m
telescope and its standard photometric equipment. This flare was observed only
in the U-band with a time resolution of about 4s. Weather conditions were
excellent at the time of the observations. 

We show an enlargement of the flare in Figure 1. The flare reached 2.50 mag 
at maximum. The complete light curve may be found in Houdebine (1990). 

The analysis of the strip chart and the recorded light
curves allowed us to identify 12 "bursts" occurring during this flare (see
Figure 1). The first five bursts display very sharp enhancements and decays.
The luminosity increase reached a value of 0.46 magnitude per second for the
fourth event, which suggests that we are dealing with rather compact sources.

We listed in Table 1 the Universal Time, approximate duration and fluxes of
each event, and the time delay Delta t between two successive events. Assuming
that two and one undetected events occurred respectively between the bursts No
8 and No 9, and the bursts No 9 and No 10, we inferred the evolution of Delta t
as given in Figure 2. The figure highlights the periodic character of these
bursts, and further shows a rapid decrease in the period - i.e. increase in
the bursts frequency - during the preflare phase and a rather constant value
later.

                                  [FIGURE 1]

 fig. 1: Flare light curve showing the identification of the flare bursts.

                                  [FIGURE 2]

 fig. 2: Evolution of the time delays between two successive bursts.
        Circles indicate measurements using the curve in fig. 1, whereas
        stars relate to measurements from the strip chart recording (see
        Houdebine, 1990).

                                   [FIGURE 3]

 fig. 3: Flare U-band curve and burst flux variations. One notes that
        the burst's maximum is reached at the very beginning of the slow
        U-band rise.


     Table 1: Parameters of the "bursts".

No.  UT         Duration  Delta t+  Delta t++   Flux
                 (s)        (s)     (s)       Arbitrary unit
1     01:26:40   6          0        0          8.5
2     01:27:24   5          44       44.4       5.1
3     01:27:55   8          31       31.5       8.8
4     01:28:24   8          29       27        54.7
5     01:28:45   8          21       21        31.3
6     01:29:07   -          22       25.5      15.3
7     01:29:35   4          28       -         26.1
8     01:29:55   4          20       -          4.7
9     01:31:06   9          71       -         11.2
10    01:31:50   9          44       -         15.1
11    01:32:12   12         22       -          7.8
12    01:32:33   12         22       -          8.3
+ Time delay from previous burst (data)
++ Time delay from previous burst (strip chart)

The burst fluxes are superimposed on the global light curve in figure 3. The
former increased sharply immediately before the flare onset that we define as
the start of the slowly evolving component. We believe that we observed
evidence for a periodic energy release phenomenon during a stellar white light
flare.

Owing to the very high frequency of white light flares on flare stars,
we expect that observations with a time resolution better than 3s in a given
band will allow a statistical investigation of these events of great interest.


                      E.R. HOUDEBINE
                      DUNSINK OBSERVATORY
                      DUBLIN INSTITUTE FOR ADVANCED STUDIES
                      CASTLEKNOCK, DUBLIN 15, IRELAND.

References:

BESKIN, G.M. ET AL.: 1989, IAU Coll. 104 on Solar and Stellar Flares,
     Poster Papers, Catania  Obs. Spe. Pub., Eds. B.M. Haisch and M. Rodono, 95 
CHUGAINOV, P.F.: 1987, Iz. Krym. Astro. Obs., 76, 54. [BIBCODE 1987IzKry..76...54C ]
HOUDEBINE, E.R.: 1990, PhD Thesis of the Universite d'Orsay - Paris XI, 91400
     Orsay, France.
NAKAJIMA, H., KOSUGI, T., KAI, K., EOME, S.: 1983, Nature, 305, 292. [BIBCODE 1983Natur.305..292N ]
RODONO, M.: 1974, Astron. and Astrophys., 210, 303. [BIBCODE 1974A&A....32..337R ]
ROIZMAN, G.S., KABICHEV, G.I.: 1985, Astron. Zh., 62, 1095. [BIBCODE 1985AZh....62.1095R ]
TAJIMA, T., SAKAI, J., NAKAJIMA, H., KOSUGI, T., BRUNEL, F., KUNDU, M. R.: 1987,
     Astrophys. J. 321,1031 [BIBCODE 1987ApJ...321.1031T ]
ZALINIAN, V.P., TOVMASSIAN, H.M.: 1987, IAU Comm. 27, I.B.V.S., No. 2992.