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\IBVShead{4311}{21 March 1996}

\title{On the peculiar  flickering activity of   HR\,2492\footnotemark
\footnotetext{Based on observations obtained at
the European Southern Observatory, La Silla, Chile}}



\begintext

Be stars are known to be variable  with periods of the order of one day
and  with  amplitudes  of a few percent in  light and radial velocity.
Baade (1984) mentions HR\,2492 = HD\,48917  = 10 CMa  as an example
displaying prominent short-period  spectroscopic variations ($P =
1\fd36$). Associated periodic photometric variations with
 $P = 1\fd35$  were  reported by  Dachs
\& Lemmer (1989) and also by Balona et al. (1992),  who obtained a
period twice as much, viz.  $P = 2\fd63$. 


Photometric variability on time scales between a few days and several
years with typical amplitudes of 0\fm1--0\fm2 in $uvby$ are reported
by Sterken et al.  (1996). Two types of variability are seen, viz.  a
quasi-regular oscillation with  $P= 87\fd9 $ (increasing  amplitude
towards redder wavelengths), and a superimposed very-long-term trend
that, if it is periodic, has a cycle length of  the order of several
years.  Moreover, random short-term variability is added on the
87\fd9 oscillation.

All measurements of HR\,2492 discussed here were obtained by one
observer in a single observing run using a stable instrumental
configuration (Danish 50cm telescope at La Silla) in the framework of
the Long-term photometry of variables project (LTPV, Sterken 1993). The
measurements were obtained differentially using two nearby comparison
stars (see Table~1 for the details). Each measurement consisted of one
uninterrupted integration of 40 s (simultaneously in the $u,v,b$ and
$y$ bands); for the sky background one single 25-s integration was
secured for the program star and for both comparison stars (sequence
APB). The night when the flare was observed occurred close to full
moon, however, all sky background measurements have very consistent
values and the deviating point can not readily  be attributed to
spurious light coming from the moon.  HR\,2492 has the highest count
rate of the three stars, still it displayed the highest internal variance
 throughout the whole observing run, indicating the
presence of steady high-frequency variations in the photon flow that
could be characterised by flickering with (very occasional) flaring. It
would be interesting to undertake high-speed photometric measurements
of this star.

Figure 1 shows a differential $b-y$, $c_1$ diagram for
HR\,2492 and three other Be stars investigated by Sterken et al.
(1996).  Except for HR\,2492, the variations in $b-y$ and $c_1$ are
moderate (but not small, see the scale in $c_1$). The variations of
both indices are huge in the case of HR\,2492, with a most interesting
single excursion in $b-y$ on HJD\,2446425.66, where the $b-y$ index
suddenly becomes 0\fm2 redder. This effect is solely due to a spike
that is visible  in the $y$ band  as a sudden brightening of 0\fm13;
this jump can also be seen in the non-differential data in the
instrumental system (Figure~2), and is---according to
the low mean errors on the successive integrations---without any doubt
of non-instrumental origin (note that evident observational
mistakes---such as centering errors or observing in cloudy
weather---tend to diminish light in all passbands rather than produce
spikes).  We\break
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\centerline{2}
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\begin{center}
Figure 1. Differential $b-y$, $c_1$ diagram for  HR\,2492  and three
other Be stars (indices relatively to their respective comparison
stars) with similar  records of observation (left). $uvby$ differential
magnitudes of HR\,2492 around JD\,2446425 causing the outlying $b-y$
data point; tick marks on the magnitude axis are 0\fm05 apart (right)
\large
\end{center}
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\centerline{\epsffile{4311-f2.ps}}
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\normalsize
\centerline{Figure 2. $y$ count rates of  HR\,2492 and comparison stars HR\,2415 and HR\,2497}
\large

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\centerline{\epsffile{4311-f3.ps}}
\normalsize
\begin{center}
Figure 3. 17\,500 K blackbody energy distribution for HR\,2492. Filled circles
are from (dereddened) {\it ubvy} data, open circles represent 
{\it UBVB$_1$B$_2$V$_1$}G photometry (see text)
\end{center}
\large
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\centerline{3}
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\noindent
find additional support for our conclusion from  an
investigation of the amount of variance  seen in the counts of
successive 1--second integrations recorded during observations.  That
variance, as a rule, is a very good indicator of the goodness-of-sky
quality and should be of the same order of magnitude for the comparison
stars as for the program star, provided that the   sky transparency is
stable and that the photon noise  is similar for all stars involved.
In the case of HR\,2492 this internal dispersion  amounts to more than
twice the overall mean error obtained from comparison-star data
covering more than 8 years.  Note that the peculiar measurement on 
HJD\,2446425.66 is not listed  in Manfroid et al. (1991) because it was
removed by an automatic filtering procedure checking for occasional
outliers in the data.


The interpretation of the observed phenomenon is not straightforward,
especially because the flare has been seen in one passband only. 
One could, for example,  compare the observed  0\fm2 excursion in $b-y$
with the strong and abrupt reddening $\delta(b-y) = 0\fm7$ of 28~CMa
reported by Mennickent et al.  (1994), though that case is different,
since the flare was related to a strong fading. An example of an
equally unexplainable but opposite
situation  is given by Ventura et al. (1995), who observed a strong
$U$-band flare (with associated micro-activity) in the light output of
the dM3.5e star V\,1054Oph, whereas no flare in $V$ was seen.


Using $E_{B-V}= 0.236$ derived from Geneva colours, we dereddened  our
mean $uvby$ data (Table~1) and the Geneva photometry
from Rufener (1988).  Figure~3 shows the results,
together with a blackbody energy distribution corresponding to 17500~K
(Waters et al. 1987).  There is no sign of the presence of a red
companion, and unpublished $JHKLM$ photometry collected by one of us
(C.S.) confirms that in the near-infrared region there is no excess
radiation of the sort expected from a cool companion.  Waters et al.
(1987) discussed the infrared excesses at 12, 25 and 60~$\mu$m of a
total of 59 Be stars. For 57 stars in this sample---among which
HR\,2492---the infrared excesses can be completely attributed to a
circumstellar equatorial disk.  

In spite of the absence of a red companion HR\,2492 could be a binary
containing a close compact secondary (white dwarf or neutron star).
 This companion should be surrounded 
by a classical accretion disk which always acts as an effective reprocessor
of energy from short to longer wavelengths. Therefore, the observed 
flare could be caused by a reprocessed X-ray burst into optical
wavelengths  (through the procedure of absorption of part of
the infalling X-rays and consequent heating with emission of most of
its energy beyond $\lambda$540--560nm, the $y$ passband).
Transient X-ray radiation is liberated whenever potential
energy is released in an strong accidental accretion event onto the
surface of a compact companion.

\vskip 2mm
\normalsize
Table 1. Program  (P) and comparison stars (A, B): average  $
y(V),\ b-y,\ m_1,\ c_1$ magnitudes and their overall standard deviations $\sigma$
based on the non-differential nightly mean values for each star. $N$
denotes the total number of observations of each  star. 
The results are based solely on data belonging to System~7 taken during
a time interval of more than 8 years (Sterken et al., 1993, see also
Sterken, 1993) 
\large
\vskip 1mm
\begin{center} 
%\begin{flushleft}
\begin{tabular}{llllllrllllll} 
\hline 
\multicolumn{1}{c}{LTPV}             &
\multicolumn{1}{c}{HR}        &
\multicolumn{1}{c}{MK}           &
\multicolumn{1}{c}{$ y(V)$}      &
\multicolumn{1}{c}{$ b-y$}      &
\multicolumn{1}{c}{$ m_1$}      &
\multicolumn{1}{c}{$ c_1$}      &
\multicolumn{1}{c}{$N$}           &
\multicolumn{1}{c}{$\sigma_{y}$} &
\multicolumn{1}{c}{$\sigma_{b-y}$} &
\multicolumn{1}{c}{$\sigma_{m_1}$} &
\multicolumn{1}{c}{$\sigma_{c_1}$} &
\multicolumn{1}{c}{}             \\
\hline 
P4004 &  2492  & B2III(e)  & 5.235 & $-$0.008 & 0.040 & $-$0.039 & 139 
&  .051 &   .016 &  .013 &   .025      \\
A4004 &  2415  & B8V          & 5.626 & $-$0.031 & 0.115 &  0.871 & 226 
&  .007 &   .003 &  .005 &   .006      \\
B4004 &  2497  & B8IV         & 6.537 & $-$0.046 & 0.121 &  0.492 & 192 
&  .008 &   .003 &  .004 &   .007      \\ 
\hline
%\end{flushleft}
\end{tabular}
\end{center}


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\centerline{4}
\vskip 1mm
Note that strong wavelength-dependence of spectral features is not
uncommon in systems with accretion disks. An extreme example is
$\beta$~Lyrae, where a 
disk is present together with---at right angles to it---supplementary
elongated clouds of ionised material, as inferred Nordsieck et al.
(1995) and---independently---by Harmanec et al. (1996).
In the $\beta$ Lyrae system the visual and ultraviolet light emissions are
shifted in polarisation angle, consequently disabling  detection of
flare events in different passbands.
\vskip 2mm

This work was  supported by a grant from the Belgian
Fund for Scientific Research~(NFWO).



\vskip .6cm
\beginauthors  
C. STERKEN$^1$
Faculty of Sciences
University of Brussels (VUB)
Pleinlaan 2
B-1050 Brussels, Belgium
\vskip 20mm
\authorsrightcol
N. VOGT$^2$
Astrophysics Group
Pontificia Universidad Cat\'{o}lica de Chile
Casilla 104, Santiago 22, Chile
\vskip 2mm
R.E. MENNICKENT
Universidad de Concepci\'on
Facultad Cs. F\'{\i}sicas y Matem\'aticas
Departemento de F\'{\i}sica
Casilla 4009, Concepci\'on, Chile
\endauthors
\vskip 6mm

$^1$ Belgian Fund for Scientific Research (NFWO)

$^2$ Presently at Sternwarte Sonneberg, Sternwarten str. 32, D-96515 Sonneberg, 

Germany.

\references

Baade, D., 1984, {\it A\&A}, {\bf 134}, 105

Balona, L.A., Cuypers, J., Marang, F., 1992, {\it A\&AS}, {\bf 92}, 533

Dachs, J., Lemmer, U., 1989, {\it Astronomische Gesellschaft, Abstract Series}, {\bf 3}, 61

Harmanec, P., Morand, F., Bonneau, D., et al.,  1996, {\it A\&A}, in press

Mennickent, R.E., Vogt, N., Sterken, C., 1994, {\it A\&AS}, {\bf 108}, 237 

Manfroid, J., Sterken, C., Bruch, A., et al., 1991,  ESO Sc. Rep., No.\,8

Nordsieck K.H., Fox G.K., Code A.D., et al., 1995, {\it UV/Visible Spectropolarimetry of Interacting Binaries by
WUPPE and University of Wisconsin Pine Bluff Observatory}, a poster
paper presented at the AAS Meeting at Pittsburgh, USA

Rufener, F., 1988, {\it Catalogue of Stars measured in the Geneva  Observatory Photometric system} (fourth edition), Gen\`eve 1988

Sterken, C., 1993, in  {\it Precision Photometry},  D. Kilkenny,       E. Lastovica, J. Menzies (Eds.), South African Astronomical       Observatory, 57

Sterken, C., Vogt, N., Mennickent, R.E., 1996, {\it A\&A} in press

Ventura, R., Peres, G., Pagano, I., Rodon\'o, M., 1995, {\it A\&A}, {\bf 303}, 509

Waters, L.B.F.M. et al.  1987, {\it A\&A}, {\bf 185}, 206


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