COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2405

                                    Konkoly Observatory
                                    Budapest
                                    28 September 1983

                                    HU ISSN 0374-0676



             FLARE ACTIVITY OF EPSILON AURIGAE? *




   Epsilon Aurigae has been regularly observed at the Yonsei University
Observatory with the 40-cm and the 60-cm reflectors in the UBV system. The number
of observations in each color, since the observation began in 1982 April, exceeds
one hundred. Normally the observation of Eps Aur each night lasted for about
an hour or so and the rest of the night was shared with other program stars.
Atmospheric extinction for each color was determined by the observation of an
extinction star, i. e., a star chosen to observe throughout the night for the
determination of the given night's extinction coefficients, and thus the
differential extinction was corrected promptly for each night.
   Soon after the termination of the ingress of Eps Aur we preempted several
photometrically excellent nights to monitor Eps Aur for the entire night, with
no other program stars included, using Lambda Aur as a comparison and as the
extinction star for the night. Nine such good nights were available in two
months, January and February, during which Eps Aur went well into its total
eclipse. Eta Aur served as the check star. It is our customary procedure to make
a net deflection vs. time diagram for each color of each star in order to correct
any misread or misrecorded net deflection(star-sky), which could easily be made
by the reader of the chart paper, at the earliest stage of the reduction work
preceding computer processing. Through this reduction an unusually large net
deflection in B was noticed on the diagram made for the Eps Aur observations of
Jan. 21. Among over fifty nights' observations made so far, this Jan. 21 data


*Yonsei University Observatory Contribution No.8.


has been reduced in the instrumental magnitude system and the results are shown
in Figure 1. In this figure the open circles represent the blue light curve of


                               [FIGURE 1]

 Figure 1. Blue and yellow light curves of Eps Aur and the yellow light
 curve of Eta Aur.


Eps Aur and the dots represent the yellow light curves of Eps Aur and Eta Aur.
As shown in the figure, Eps Aur exhibits a sudden brightening by about 0.4m in
B and 0.2m in V above their mean magnitudes. This light variation appears to be
real because the V light curve of Eta Aur shown in the same figure remains
constant throughout the night.
   Since the previous eclipse 27 years ago the eclipse light curve of Eps Aur
is known to be trapezoidal with a depth of 0.8m in the range and neighborhood of
visual light wavelengths. There are small, about 0.1m, irregular variation, on
a time scale of about 100 days in V and B-V in all orbital phases, but the light
variation increases to about 0.2m-0.3m during eclipse. There seems, however, no
report that Eps Aur has ever been intensively observed to search for light
variation shorter than 100 days, say night-to-night or even during a night.
  It may be too early to say that there are flare activities in Eps Aur during
total eclipse. The light change of 0.4m in B is, however, much larger than the
long range variations in V reported by Gyldenkerne (1970). In addition the
brightening lasted for only about 20 minutes, which is comparable to the longer
flare durations of known flare stars. The light variation in V is insignificant,
but we will have to account for the fact that the V measurement was 2 minutes
ahead of the peak brightening time estimated on the B light curve. This report
requires comformation by other Eps Aur campaign participants.





               I.-S. NHA and S. J. LEE

          Yonsei University Observatory, Seoul, Korea





Reference:
Gyldenkerne, k. 1970, Vistas in Astronomy 12, 190. [BIBCODE 1970VA.....12..199G ]