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\begin{document}

\IBVShead{5620}{30 March 2005}

\IBVStitletl{A New Bright U G\lowercase{em} Variable Identified with the}{X-Ray Source 1RXS J053234.9+624755}

\IBVSauth{Bernhard, K.$^{1,6}$; Lloyd, C.$^{2}$; Berthold, T.$^{3,6}$; Kriebel, W.$^{4,6}$; Renz, W.$^{5,6}$ }

\IBVSinst{A--4030 Linz, Austria; e-mail: {\tt klaus.bernhard@liwest.at}}
\IBVSinst{Space Science \& Technology Department, Rutherford Appleton 
Laboratory, Chilton, Didcot, \\  \indent Oxon.\ OX11 0QX, UK; 
e-mail: {\tt cl@astro1.bnsc.rl.ac.uk}}
\IBVSinst{Sternwarte Sonneberg, Sternwartestr. 32, D-96515 Sonneberg,
Germany; e-mail: {\tt tb@4pisysteme.de }}
\IBVSinst{D--84072 Osterwaal Post Au; e-mail: {\tt kriebel-au@t-online.de}}
\IBVSinst{D--76227 Karlsruhe Durlach; e-mail: {\tt w\_renz@onlinehome.de}}
\IBVSinst{Bundesdeutsche Arbeitsge\-mein\-schaft f\"{u}r Ver\"{a}nderliche 
Sterne e.V. (BAV), Munsterdamm 90,\\ \indent D--12169~Berlin, Germany}

\SIMBADobjAliasJ2000{1RXS J053234.9+624755}{2MASS J05323386+6247520}{05 32 33.87 +6247 52.1}

\IBVStyp{U Gem}
\IBVSkey{U Gem - X-ray identification - outburst time scale - photometry}

\IBVSabs{A new U Gem variable has been discovered in the ROTSE1 database and}
\IBVSabs{identified with the ROSAT X-ray source 1RXSJ053234.9+624755. Previous}
\IBVSabs{outbursts have been identified on archival plates leading to a}
\IBVSabs{recurrence time scale of 133.6+/-0.4 days. The star}
\IBVSabs{normally lies at mag 16 but during outburst can reach mag 11.}


\begintext

During a programme of optical identification of X-ray sources from the
ROSAT all-sky survey bright source catalogue (Voges et al., 1999) with
variable stars in the ROTSE1 database (Wo\'{z}niak et al., 2004) it was
found that the uncatalogued variable at 05\hr32\mm32\fsec68
+62\deg47\arcm54\farcs7 was coincident the X-ray source 1RXS
J053234.9+624755. The ROTSE1 light curve is available from the Northern
Sky Variability Survey (NSVS) website (see reference Wo\'{z}niak et al.,
2004) and is shown in Figure 1. ROTSE1 magnitudes are broadly equivalent
to R. The data show two, or including the poor quality points, three
large outbursts from magnitude $R\sim$ 16.0 to 12.9. The last outburst is
particularly well covered by the ROTSE1 data and is shown in more detail
in Figure 2. This, and the rapid rise time of the second outburst argues
against any other type of large variation suggesting that this star is a
cataclysmic variable (CV) of the U Gem type with a recurrence time scale
of $\sim 133$ days. The duration of the outbursts is very short, about 
4 days in total, and the initial decline rate, $T_1 = 1.0$ days.

\begin{table}[!b]\centering
\normalsize
\caption{Photometry of the two sources close to the ROTSE1 position}
\vskip 2pt
\begin{tabular}{l|cccccc|cccccc} \hline
 & & & & East & & & & & & West \\
Source    & B & R & I & J & H & K & B & R & I & J & H & K  \\ \hline
GSC 2.2   & 16.0 & 15.8 & & & & & 16.0 & 15.1 \\
USNO A2.0 & 16.3 & 16.4 & & & & & 15.8 & 15.1 \\
USNO B1.0 & 16.8 & 16.3 & 15.7 & & & & 16.4 & 15.4 & 14.8\\
	  & 16.5 & 16.0 & & & & & 16.2 & 15.5 \\
2MASS     & & & & 15.18 & 15.02 & 14.30 & & & & 14.15 & 14.06 & 13.83 \\
\hline
\end{tabular}
\end{table}

\IBVSfig{8cm}{5620-f1.eps}{The ROTSE1 data showing the combined light
of the two stars as discussed in the text. At quiescence the CV is below
the detection threshold. The flagged data are shown as open squares.}
\IBVSfigKey{5620-f1.eps}{1RXSJ053234.9+624755}{light curve}

The ROTSE1 position lies almost exactly between two stars in the GSC 2.2,
USNO and 2MASS catalogues, so its identification is not immediately
obvious. The two stars are separated by 17\arcs\ and the preceding
(west) component is the brighter, by approximately one magnitude. The
resolution element of the ROTSE1 survey is $\sim$ 14\arcs\ so these stars
are on the resolution limit.

The available photometry of these two stars has been collected in Table 1
and suggests that the brighter component is always above the ROTSE1
threshold of $R \sim 15.5$ while the fainter component is (usually) below
it. The ROTSE1 position is derived from a combination of the two sources,
the western component during CV quiescence, and the CV dominated
position during outburst. All the photometric and astrometric evidence
strongly suggests that the CV is the eastern component of this pair at
05\hr32\mm33\fsec87 +62\deg47\arcm52\farcs1 and this is confirmed by
recent observations.

In Figure 1 there are several measurements apparently showing the CV at
quiescence but these are probably entirely due to the brighter companion.
Even the brighter measurements will still have a significant contribution
from the companion, and the CV will only begin to dominate at magnitudes
above $R \sim$ 14.5.

The colours of the CV are not well determined in the optical, but yield
an average $B-R=0.3$ which is consistent with this type of object.
However, the 2MASS colours (Cutri et al., 2003), $J-H=0.15$ and
$H-K=0.72$ suggest a very red object at the longest wavelengths, which
contrasts with the blue object seen in the optical. This combination of
colours has been seen before in CVs (Hoard et al., 2002). The 2MASS
colours do not suggest any particular type of CV; most have colours
similar to main-sequence stars and all types have objects with anomalous
colours. It is obviously a recurrent object with a relatively short cycle
and the statistics suggest weakly that it is a U Gem star rather than a
magnetic CV.

\IBVSfig{8cm}{5620-f2.eps}{The detail of the last outburst from the
ROTSE1 data.}
\IBVSfigKey{5620-f2.eps}{1RXSJ053234.9+624755}{light curve}

\begin{table}[!b]
\begin{center}
\normalsize
\caption{Outbursts of 1RXSJ053234.9+624755}
\vskip 2pt
\begin{tabular}{ccccl} \hline
HJD & mag & Cycle & $O-C$(d) & Source \& remarks \\ \hline
2447975.312 & 11.6~ & ~0& ~$-$5.8 & Sonneberg plate pv \\
2448650.344 & 12.4~ & ~5& ~+1.2 & Sonneberg plate pv \\ 
2449058.306 & 13.7~ & ~8& ~+8.4 & Sonneberg plate pg \\
2450097.427 & 13.6~ & 16& $-$21.3 & Sonneberg plate pg \\
2451332.672 & 12.79 & 25& +11.5 & ROTSE *flag 8192 \\
2451463.772 & 13.06 & 26 &~+9.0 & ROTSE \\
2451597.676 & 12.89 & 27&~+9.3 & ROTSE \\
2453446.309 & 12.5~ & 41&$-$12.4 & Pietz, Kriebel, Schmeer \& Renz\\
\hline
\end{tabular}
\end{center}
* flag 8192: High scatter of the corrections across the map larger than
0.1 mag
\end{table}

The X-ray source 1RXS J053234.9+624755 from the ROSAT all-sky survey
bright source catalogue (Voges et al., 1999) lies close by at
05\hr32\mm34\fsec90 +62\deg47\arcm55\farcs5 with a nominal uncertainty of
8\arcs. The X-ray source lies 8\arcs\ from the CV, within the
error ellipse, and 24\arcs\ ($3\sigma$) from the companion. The
source was observed by the ROSAT PSPC with a count rate of
$0.260\pm0.0270/s$ so assuming an optical magnitude, $V=16.3$ this leads
to $F_x/F_{opt} = 0.30$. For CVs in general $F_x/F_{opt}$ is notoriously
variable but this value is in the middle of the range. When combined
with the hardness ratios, $HR1=0.15\pm0.10$ and $HR2=0.16\pm0.14$ this
object lies on the lower edge of the group with higher $F_x/F_{opt}$
ratios. In the hardness ratio plane this object lies in the central, more
neutral group of CVs, as opposed to the `hard' group or with the small number
of very soft sources (see Motch et al., 1998).

\IBVS2fig{6cm}{5620-f3.eps}{Finding chart from a POSS O plate showing a 20\arcm\ $\times$ 20\arcm\ field around the CV. North is up and east to the left.}{5620-f4.eps}{Finding chart from a POSS E plate showing a 10\arcm\ $\times$ 10\arcm\ field around the CV. The bar is 1 arc minute long.}
\IBVSfigKey{5620-f3.eps}{1RXSJ053234.9+624755}{finding chart}
\IBVSfigKey{5620-f4.eps}{1RXSJ053234.9+624755}{finding chart}

In an effort to identify previous outbursts the Sonneberg Plate Archive
was searched and estimates were made on 123 pg plates taken during JD
2448220 -- 2451601 and 111 pv plates taken during JD 2447803 -- 2451927.
As a result four further outbursts were identified and together with the
ROTSE1 data an outburst ephemeris was derived. Following a call for
observations, which was kindly provided by the CVNET \\ ({\tt
http://home.mindspring.com/~mikesimonsen/cvnet/index.html }) another
outburst was detected on 16 March 2005 independently by four observers.
These were by W.~Kriebel (JD~2453446.309, mag 12.0), P. Schmeer 
(JD~2453446.319, mag 11.9) and W.~Renz (JD~2453446.323, mag 12.0) observing
visually and J. Pietz with an unfiltered CCD~SBIG ST-6 (JD~2453446.309, mag
12.5). All the times of outburst are collected in Table 2 and were fitted
by least squares to derive an outburst ephemeris of,

\[
\begin{array}{r@{}r@{}l}
{\rm HJD}_{\rm Max} = 2447981.1 & {} + 133\fday6 &{} \times {\rm E} \\
\pm 8.0 & \pm 0.4
\end{array}
\]

The most recent observations suggest that this star is 
currently undergoing a superoutburst which is brighter and longer than
seen in the ROTSE1 data. The only previous occasion when the star was
observed this bright was on the first of the Sonneberg outbursts 5471
days earlier. There are several, as yet, unpublished reports of
superhumps with a short period, so this star is a new short-period
UGSU-type dwarf nova and will obviously receive considerable attention
in the future.

\vskip 6pt\noindent
{\it Acknowledgements.} It is a pleasure the acknowledge the use of the
SIMBAD database, operated by the CDS at Strasbourg, France.
This publication makes use of data products from the Two Micron All Sky
Survey.

The authors further thank P. Schmeer and J. Pietz for their observations
and Mike Simonsen and John Greaves for helpful comments.


\references


Cutri R.M. et al., 2003, 2MASS All-Sky Catalog of Point Sources,
University of Massachusetts and 
IPAC/California Institute of Technology

Hoard D.W., Wachter S., Clark L.L., Bowers T.P., 2002, {\it Astrophys.
J.}, {\bf 565}, 511

Motch, C., Guillout, P., Haberl, F., Krautter, J., Pakull, M. W., Pietsh,
W., Reinsch, K., Voges, W., Zickgraf, F.-J., 1998, {\it Astron.
Astrophys. Suppl. Ser.}, {\bf 132}, 341

Voges W., et al.,
1999, {\it Astron. Astrophys.},
{\bf 349}, 389, The ROSAT all-sky survey bright source catalogue.

Wo\'{z}niak, P.R., et al., 2004, {\it Astron. J.}, {\bf 127}, 2436,
Northern Sky Variability Survey: Public Data Release \\ - ROTSE1 light
curve {\tt http://skydot.lanl.gov/nsvs/star.php?num=2224889 }

\endreferences

\end{document}


