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\IBVShead{5755}{12 February 2007}

\IBVStitletl{Spectroscopic Detection of}{ a Spectacular Flare on DX C\lowercase{nc}}

\IBVSauth{Meusinger, H.$^1$; Scholz, R.-D.$^2$; Jahreiss, H.$^3$}

\IBVSinst{Th\"uringer Landessternwarte Tautenburg, D-07778 Tautenburg,
Germany, e-mail: meus@tls-tautenburg.de}

\IBVSinst{Astrophysikalisches Institut Potsdam, An der Sternwarte 16,
14482 Potsdam, Germany,\\ e-mail: rdscholz@aip.de}

\IBVSinst{Astronomisches Rechen-Institut am Zentrum f\"ur Astronomie der
Universit\"at Heidelberg, M\"onchhofstr.\ 12-14, 69120 Heidelberg,
Germany, e-mail: hartmut@ari.uni-heidelberg.de}

\SIMBADobjAlias{DX Cnc}{GJ 1111}
\IBVStyp{ UV }
\IBVSkey{spectroscopy}

\begintext

We announce the serendipitous spectroscopic detection of a spectacular
flare event on DX Cnc. To our knowledge, this is the first
spectroscopically detected strong flare on this star. DX Cnc, classified
as a UV Ceti star (Samus et al., 2004), is one of the most nearby stars
(GJ1111, LHS248) at a distance of 3.6 pc.  Because of its proximity and
late spectral type (M6.5) it has been used as a spectroscopic comparison
star in various studies (e.g., Basri \& Marcy, 1995;  Teegarden et al.,
2003; Caballero et al., 2006).  In a similar sense we used DX Cnc for the
classification of late-type stars in a systematic search for so far
unidentified candidates for members of the immediate solar neighbourhood
(Scholz \& Meusinger, 2002; Scholz et al., 2005). In this context DX Cnc
has been repeatedly observed with the low-resolution long-slit Nasmyth
spectrograph NASPEC at the Tautenburg 2-m telescope and with the
faint-object spectrograph CAFOS at the 2.2-m telescope on Calar Alto,
Spain. The grisms V200 (Tautenburg) and B400 (Calar Alto) were used
resulting in nominal resolutions (FWHM) of about 12~\AA\ (Tautenburg)
and 30~\AA\ (Calar Alto), respectively. The corresponding wavelength
coverage is 4500 to 9000~\AA\ (Tautenburg) and 3500 to 8000~\AA\
(Calar Alto).

\begin{center}
{Table 1. Table of observations and measured H$\alpha$ equivalent widths}
\vskip 1mm
\begin{tabular}{cccrr}
\hline
year--month--day& J.D. (start) & instrument  & $t_{\rm exp}$ [s] &  
EW(H$\alpha$) [\AA]\\
\hline
2006--09--24  & 2454002.6479 & NASPEC V200 & 300 &  $3.2 \pm 0.2$ \\	 
2006--09--24  & 2454002.6443 & NASPEC V200 &  60 &  $2.7 \pm 0.3$ \\
2006--09--23  & 2454001.6556 & NASPEC V200 & 180 & $86.9 \pm 9.9$ \\
2006--09--23  & 2454001.6529 & NASPEC V200 & 180 & $82.2 \pm 9.9$ \\
2006--09--22  & 2454000.6553 & NASPEC V200 & 180 &  $4.2 \pm 1.0$ \\
2006--09--22  & 2454000.6540 & NASPEC V200 &  60 &  $4.0 \pm 0.6$ \\
2003--03--03  & 2452702.3248 & CAFOS  B400 &  60 &   - $\qquad$ \\
2002--03--18  & 2452352.3622 & CAFOS  B400 & 120 &   - $\qquad$ \\
1999--03--24  & 2451293.3266 & CAFOS  B400 & 120 &   - $\qquad$ \\
\hline
\end{tabular}
\end{center}

\IBVSfig{23.0cm}{5755-f1.ps}{Series of 9 low-resolution spectra of DX Cnc
at different epochs normalized at 7500~\AA{}. The Balmer lines in the two
flare spectra were truncated for lucidity}
\IBVSfigKey{5755-f1.ps}{DX Cnc}{low resolution spectra} 

\IBVSfig{23.0cm}{5755-f2.ps}{Average (a) and difference (b) of the two
normalized flare spectra (relative flux) of DX Cnc from 2006 Sep 23 in the
wavelength range of the Balmer lines.  The scales of the two panels are
different}
\IBVSfigKey{5755-f2.ps}{DX Cnc}{flare spectra}

The flare was detected on two spectra taken at the end of the night of
2006 September 22/23. Actually, the target of these observation was the
star USNO-B1.0 1167-0167382 at a distance of about 10 arcsec from DX Cnc.
Although the spectrograph slit was not positioned on DX Cnc, the stray
light from DX Cnc passing through the long-slit was bright enough to
enable the extraction of useful spectra, however with poor S/N below $\sim
5000$~\AA.  The time-lag between the end of the first exposure and the
beginning of the second exposure was 50~s, hence the two spectra cover a
time interval of 430~s. Unfortunately, no other spectra could be taken in
the same night because of the break of dawn. The series of all available
spectra is shown in Fig.~1. All other spectra of DX Cnc do not show
substantial flare activity. The star was obviously in its quiescence stage
on the spectra observed in the night before the flare as well as on the
spectra from the night after the flare.  The two flare spectra do not
significantly differ. This is most likely explained by the assumption that
the duration of the flare was longer than the time interval covered by the
observations. It appears hence useful to compute an average flare spectrum
with reduced S/N from the two single spectra.  Both the difference
spectrum and the average spectrum are shown in Fig.~2.  In addition to
very strong Balmer lines, HeI emission lines at
$\lambda\lambda$~5876,~6678 and metal lines (Na, Mg) are clearly
identified;  the identification of the lines HeI~$\lambda$~4471 and
HeII~$\lambda$~5412 is not safe.

Weak H$\alpha$ emission is seen in all other Tautenburg spectra.  The
higher Balmer lines, the He lines, and the metal lines, on the other hand,
are usually not seen in emission. For H$\alpha$ we measure an equivalent
width of EW(H$\alpha) = 3\ldots4$~\AA\ in the quiescence stage around the
epoch of the flare, in good agreement with the data found in the
literature (Liebert, 1976; Mart\'{\i}n et al., 1996; Mohanty \& Basri,
2003; Fuhrmeister et al., 2005). With their lower resolution, the Calar
Alto spectra do not allow to measure H$\alpha$ in quiescence. From the
average flare spectrum we derive EW(H$\alpha) = 95 \pm 10$~\AA{}. To
measure the equivalent width of H$\beta$, the continuum was estimated by
fitting a mean spectrum from the quiescence stage which yields EW(H$\beta)
= 580 \pm 10$~\AA{}. For the higher Balmer lines it is not possible to
estimate the continuum from our spectra.

Finally, it is worth mentioning that both flare spectra seem to indicate
an enhanced blue continuum. Such a behaviour has been found for other
late-type stars by e.g., Liebert et al.\ (1999) and Scholz et al.\ (2004).  
However, the quality of our flare spectra is not sufficient for a
clear-cut statement on the continuum variation during the flare of DX Cnc.
 
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\end{document}

