\documentstyle[twoside, epsf]{article}

\input ibvs2.sty
\begin{document}

\IBVShead{4652}{2 December 1998}

\IBVStitle{ Flares Discovered on 1RXS J220111+281849}

\IBVSauth{R. Greimel and R.M. Robb}

\IBVSinsto{Climenhaga Observatory, Dept.\ of Physics and Astronomy, 
University of Victoria, Victoria, BC, Canada, V8W 3P6, 
Internet: robb@uvic.ca, greimel@uvastro.phys.uvic.ca}

\IBVSabs{1RXS J220111+281849 is a M4V spotted star with a rotational}
\IBVSabs{period of about 0.448 days.  Six flares were observed.}
\SIMBADobj{G188-38}
\IBVStyp{ flare star }
\IBVSkey{stars: activity}

\begintext

The star 1RXS J220111+281849 (Voges et al.\ 1997) = GSC\,2215\_1629
(Jenkner et al.\ 1990) = G\ 188-38 (Giclas et al.\ 1980) = HIP 108706
(ESA 1997) was found to have significant X-ray emission in a survey by
the ROSAT satellite (Bade et al.\ 1998).  It was classified as an M4
star by Reid (1995) with an apparent magnitude of V = 11.99 by Weis
(1987), (B$-$V) = 1.63 (Harrington et al.\ 1993) and a distance of
$8.96\pm.25$~pc (ESA 1997).  As part of a search for radial velocity
variations of nearby K and M dwarfs, Delfosse et al.\ (1998) measured
its $v \sin i$ to be 29.6 km\,sec$^{-1}$ and observed hydrogen
lines to be in emission.

\IBVSfig{8.5cm}{4652-f1.ps}{Finder chart labeled with the GSC numbers.}

Plotted in Figure 1 is the field of stars observed with the automated
0.5-m telescope and reduced in a fashion identical to that described in
Robb et al.\ (1997).  Tabulated in Table 1 are the stars'
identification numbers, coordinates (J2000) and magnitudes from the
Hubble Space Telescope Guide Star Catalog (GSC) (Jenkner et
al.\ 1990).  Differential $\Delta$R magnitudes are calculated in the
sense of the target star minus GSC\,2215\_1776.  Brightness variations
during a night were measured by the standard deviation of the
differential magnitudes, which ranged from 0\fmm 005 for bright stars
on a good night to 0\fmm 050 for the faint stars on poor nights.

\begin{table}
\caption{Stars observed in the field of GSC\,2215\_1629}
\begin{center}
\begin{tabular}{ccccc}
\hline
GSC No. & R.A. & Dec. & GSC & $\Delta$R \\
  & J2000&J2000& Mag.& Mag. \\
\hline
1629& $22\hr01\mm13\sec$&$+28\deg$18\arcm24\arcs& 11.6 &$0.307\pm.015$\\ 
1776& $22\hr01\mm41\sec$&$+28\deg$17\arcm51\arcs& 10.7 & -\\
1602& $22\hr01\mm35\sec$&$+28\deg$20\arcm02\arcs& 12.5 &$1.780\pm.004$\\
1587& $22\hr01\mm31\sec$&$+28\deg$21\arcm50\arcs& 14.0 &$3.372\pm.007$\\
1779& $22\hr01\mm23\sec$&$+28\deg$17\arcm08\arcs& 13.8 &$3.222\pm.003$\\
1719& $22\hr01\mm15\sec$&$+28\deg$18\arcm31\arcs& 13.8 &$3.145\pm.016$\\
1683& $22\hr01\mm09\sec$&$+28\deg$18\arcm58\arcs& 14.2 &$3.786\pm.013$\\
1572& $22\hr01\mm43\sec$&$+28\deg$21\arcm45\arcs& 13.3 &$2.615\pm.005$\\
\hline
\end{tabular}
\end{center}
\end{table}

The run mean of the thirteen nightly means and their standard deviation,
which is a measure of the night to night variations, are given in Table 1
as $\Delta R$ and its uncertainty.  The high precision of these data can
be seen from the standard deviation of the $\Delta R$ of GSC\,2215\_1779
minus GSC\,2215\_1776, which is 0\fmm003 and shows that these two stars
are constant at this level of precision. The fainter stars have the
expected larger standard deviation. The star GSC\,2215\_1629 had obvious
flares of up to 0\fmm3 during four nights and also slight changes of about
0\fmm020 on an hourly time scale. 

\vfill
\IBVSfig{8.5cm}{4652-f2.ps}{Periodogram for 1RXS J220111+281849 for 1998}

The four nights which showed flares were excluded from the search for 
periodicity and the other nine nights were fit with a single sine
curve of various frequencies.  In Figure 2 we have plotted the RMS
deviation of a point from a sine curve as a function of period.

There is little ambiguity in the determination of the photometric period
of GSC 2215\_1629, which gives the ephemeris:
$$
\hbox{HJD of Maxima} = 2451053.386(50) + 0\fday 448(5) \times E,
$$
where the uncertainties in the final digits are given in 
brackets.

The differential (GSC\,2215\_1629$-$GSC\,2215\_1776) R magnitudes of
the nine nights with no flares, phased at this period, are plotted in
Figure 3 with different symbols for each of the nights. We suspect that
this variation is due to rotation of an asymmetry in the distribution
of spots into and out of our view.  Since the star may rotate
differentially and the spot asymmetry may change, we do not expect the
rotation period of the star to be precisely this value.  Combining our
period with the maximum rotation period, $P/\sin i$ of Delfosse et
al.\ (1998) gives the inclination of $58\pm4\deg$ for the axis of
rotation, assuming a radius of 0.3 $R_\odot$, where the quoted
uncertainty does not include the contribution from differential
rotation.

\IBVSfig{8.5cm}{4652-f3.ps}{R band light curve of 1RXS J220111+281849 for
1998}

During four nights we observed flares of large amplitude which are
plotted in Figure~4.
The Heliocentric Julian Dates ($-$2451000) of the onset of the flares, were
55.7223, 60.8024, 60.9995, 70.7525, 76.7180, and 76.9140, where the
time between individual data points is about 0.0020 days. 
On two of the nights we see a pair of flares 
separated in time by 0.1960 and 0.1971 days, which are
identical considering the spacing of the data.  

The probability of two pairs of flares having the same spacing in time
as chosen from our 1452 data points is extremely small and hints at a
possible periodicity.  The time spacing implies that there are either
two regions on opposite sides of the star which flared or one region
which flared near one limb of the star and then the other.  A search
for periods which superimpose the flares in phase, yields a good fit at
the period of 0.4421 days, which is intriguingly close to the
photometric period, but there were many other periods with residuals
nearly as small.  Obviously more data is needed to give any confidence
in a possible periodicity of the flaring activity.  We eagerly await
next year to confirm or disprove this periodicity and urge observers to
contribute to an international campaign by contacting the authors.

\IBVSfig{8.5cm}{4652-f4.ps}{Differential R measurements of
1RXS J220111+281849 for the nights with flares}

\vskip-28pt

\references

Bade N., Engels D., Voges W., Beckmann V.,  Boller Th.,  Cordis L.,
Dahlem M., Englhauser J., Molthagen K., Nass P., Studt J., Reimers D.
1998, {\it A\&A Sup.}, {\bf 127}, 145

Delfosse X., Forveille T., Perrier C., Mayor M. 1998, {\it A\&A}, 
{\bf 331}, 581

ESA 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200

Giclas H.L., Burnham R.Jr., Thomas N.G. 1980, Lowell Obs.\ Bull., 8:6, 
120

Harrington R.S., Dahn C.C., Kallarakal V.V., Guetter H.H., Riepe B.Y.,
Walker R.L., Pier J.R., Vrba F.J., Luginbuhl C.B., Harris H.C. and Ables
H.D. 1993, {\it AJ}, {\bf 105}, 1571

Jenkner H., Lasker B., Sturch C., McLean B., Shara M., Russell J. 1990,
{\it AJ}, {\bf 99}, 2082

Reid I.N., Hawley S.L. and Gizis J.E. 1995, {\it AJ}, {\bf110}, 1838

Robb R.M., Greimel R., Ouellette J. 1997, IBVS No.\,4504

Voges W., Aschenbach B., Boller Th., Brauninger H., Briel U., Burkert W.,
Dennerl K., Englhauser J., Gruber R., Haberl F., Hartner G., Hasinger G.,
Kurster M., Pfeffermann E., Pietsch W., Predehl P., Rosso C., Schmitt J.
H. M. M., Trumper J., Zimmermann U. 1997, Proceedings of the 179th
Symposium of the International Astronomical Union edited by Brian J.
McLean, Daniel A. Golombek, Jeffrey J. E. Hayes, and Harry E. Payne,
Kluwer Academic Publishers.

Weis E.W. 1987, {\it ApJ}, {\bf93}, 451

\end{document}
