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\IBVShead{5750}{22 January 2007}

\IBVStitletl{A New Long-Period U G\lowercase{em} Variable Identified with the}{ X-Ray Source 1RXS J224342.3+305526}

\IBVSauth{Bernhard, K.$^{1,7}$; Lloyd, C.$^{2}$; Boyd, D.$^{3,8}$; Pietz, J.$^{4,7}$; Jones, J.L.$^{5,9}$; Renz, W.$^{6,7}$}

\IBVSinst{A--4030 Linz, Austria; e-mail: {\tt klaus.bernhard@liwest.at}}

\IBVSinst{Department of Physics and Astronomy, Open University, Milton Keynes 
MK7 6AA, UK; \\ \indent e-mail: {\tt C.Lloyd@open.ac.uk}}

\IBVSinst{5 Silver Lane, West Challow Oxon OX12 9TX UK;
e-mail: {\tt drsboyd@dsl.pipex.com}}

\IBVSinst{D--50374 Erftstadt, Rostocker Str.\ 62, Germany; e-mail: {\tt j.pietz@arcor.de}}

\IBVSinst{3190 Douglas Circle, Lake Oswego, OR 97035, USA:
e-mail: {\tt nt7t@comcast.net}}

\IBVSinst{D--76227 Karlsruhe Durlach, Germany; 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}

\IBVSinst{BAA, Variable Star Section, Burlington
House, Piccadilly, London W1J 0DU, UK}

\IBVSinst{AAVSO, 25 Birch Street, Cambridge, MA 02138, USA}

\SIMBADobjJ2000{1RXSJ224342.3+305526}{22 43 40.71 +30 55 20.1}

\IBVSecomment{= 2MASS 22434070+3055200}

\IBVStyp{ U Gem }
\IBVSkey{U Gem - X-ray identification - orbital hump - photometry}
\IBVSabs{A new long-period U Gem variable has been discovered in the ROTSE1}
\IBVSabs{database and identified with the ROSAT X-ray source 1RXSJ224342.3}
\IBVSabs{+305526. The star shows sinusoidal outbursts of nearly 3 mag on a time}
\IBVSabs{scale of 18 days on which are superimposed an orbital double hump with}
\IBVSabs{variations of up to 0.6 mag with a period of 0.422 days.}

\begintext

During a programme of optical identification of X-ray sources the uncatalogued
variable, NSVS 8915780 at 22\hr43\mm40\fsec7 +30\deg55\arcm22\arcs\ in the
ROTSE1 database (Wo\'{z}niak et al., 2004), has been found to be coincident the
X-ray source 1RXS J224342.3+305526 from the ROSAT all-sky survey faint source
catalogue (Voges et al., 1999). The separation between the two sources is
22\arcs, which is consistent with the uncertainty of 19\arcs\ in the position of
the X-ray source. The star is also identified as GSC 02736-01067 and is
catalogued by 2MASS at 22\hr43\mm40\fsec71 +30\deg55\arcm20\farcs1 (2000).

The ROTSE1 light curve is shown in Figure 1 and is available from the Northern
Sky Variability Survey (NSVS) website (see reference Wo\'{z}niak et al., 2004).
The data show a cyclical variation between $R\sim 16.0$ and 13.5, with a period
$\sim 16$ days. However, the data are better fitted by a period of twice this
value, with alternate maxima having slightly different magnitudes. The large
amplitude and short time scale, and the possible association with a X-ray source
suggest that this is a U Gem type cataclysmic variable (CV). The 2MASS colours
of $J-H=0.09\pm 0.02$ and $H-K=0.06\pm 0.02$ (Cutri et al., 2003) suggest a star
with a spectral type of mid-to-late A{}. While in general the $IR$ colours of
CVs tend to match later-type main sequence stars, these colours are consistent
with the bluest CVs seen in the 2MASS data (Hoard et al., 2002). The optical
colours from the USNO-B1.0 (Monet et al., 2003) of $b-r \sim 0.6$ although
approximate, are consistent with this. Although these are not particularly blue
they are again consistent with those of CVs. The pattern of variability is
similar to that seen in several well-observed U Gem stars, e.g., AH Her, RX And,
HL CMa, SY Cnc, CN Ori and Z Cam. All vary in a relatively periodic way on time
scales of $\sim 20$ days with amplitudes of 2--3 magnitudes. All of these are
UGZ stars, possibly indicating that the new variable also belongs to this class.

\newpage

\IBVSfig{7cm}{5750-f1.eps}{All the ROTSE1 data showing the 16 day outburst cycle
with the 32 day period fitted. Flagged (suspect) data, open circles;  unflagged
data, filled circles}
\IBVSfigKey{5750-f1.eps}{1RXSJ224342.3+305526}{light curve}

\IBVSfig{7cm}{5750-f2.eps}{The recent data with different observers shown
as different symbols. Small offsets have been applied to each data set as
part of the fitting process}
\IBVSfigKey{5750-f2.eps}{1RXSJ224342.3+305526}{light curve}

The X-ray source was observed by the ROSAT PSPC with a count rate of
$0.0195\pm0.00771$~cts/s so assuming optical magnitudes $V=16$ and $V=13$ this
leads to $F_{\rm X}/F_{\rm opt} = -0.9$ and $-2.1$ respectively. Despite this
uncertainty the $F_{\rm X}/F_{\rm opt}$ ratio is consistent with the less X-ray
bright grouping of CVs. The hardness ratios are poorly defined with
HR$1=0.55\pm0.40$ and HR$2=0.78\pm0.46$ and these are consistent with both of
the main groupings of CVs in the hardness ratio plane (see Motch et al., 1998).

\IBVSfig{7cm}{5750-f3.eps}{The phase diagram showing the averaged C and
$V$ data when the system is bright (top; expanded by a factor of 4), in
mid range, and at the minimum of the outburst cycle. The bottom plot show
the orbital variation of $V-I$ at the minimum of the outburst cycle.}
\IBVSfigKey{5750-f3.eps}{1RXSJ224342.3+305526}{folded light curve}

\vspace{-12pt}

Further optical observations have been made between September and December 2005
by Bernhard, using a 20-cm SCT with a Starlight Xpress SX CCD-camera unfiltered
(C[lear]) and in $B, V$ and $R$; Boyd, using a 35-cm SCT with a Starlight Xpress
SXV-H9 CCD-camera in C, $V$ and $I$; Pietz using a 28-cm SCT with an ST-6B CCD
camera unfiltered and Jones using a 28-cm SCT with an ST-7 CCD camera and $V$
filter. All these observations are shown in Figure 2. The filtered observations
have been reduced using a calibration provided by Henden (private communication)  
while the C observations have either been reduced as $V$ or in the natural
system.

\IBVSfig{7cm}{5750-f4.eps}{The $V-I$ data showing the change with $V$}
\IBVSfigKey{5750-f4.eps}{1RXSJ224342.3+305526}{color-magnitude diagram}

The new observations mirror the ROTSE1 data with a slightly longer cycle at 18
days again with alternately bright and faint maxima. The minimum magnitude is
relatively constant from cycle to cycle. The variation is very sinusoidal (e.g.,
AH Her, RX And) and not triangular (SY Cnc) or saw toothed (Z Cam). On a
seasonal time scale the variation also seems to be remarkably repeatable, both
within the ROTSE1 and recent data.

The nightly runs of observations have been subjected to a wavelet analysis and
the results have been used to construct scalegrams that are widely used to
examine flickering in CVs (see Fritz \& Bruch, 1998). The scalegrams show the
behaviour typical of flickering which is usually taken as direct evidence of
accretion processes. By itself this confirms the variable as a CV and
strengthens the identification with the X-ray source.

The new observations also reveal a sinusoidal variation with a period of about 5
hours that is consistent with an orbital hump. While this type of variation is
seen in many CVs the amplitude seen here is particularly large, reaching as much
as 0\fmm6 at minimum of the outburst cycle and reducing to $< 0\fmm04$ at
maximum. The range of variation is entirely consistent with the orbital
variation being diluted as the system brightens. However, differences between
alternate cycles suggest that the system shows a double orbit hump with the
ephemeris of
 $${\rm HJD_{MinI}} = 2453679.90(1) + 0\fday42234(3) \times E$$
 for the data in the middle of the range. The light curve (Figure 3) appears to
migrate to later phases as the system brightens, in particular the primary
minimum and the following maximum. The secondary minimum appears to be
relatively stable in phase but flattened, possibly suggesting a partial eclipse.
The orbital variation in $V-I$ at minimum brightness (also shown in Figure 3)
shows a dramatic increase near secondary minimum, presumably when the cool star
dominates the light curve.

Multi-colour photometry reveals a dramatic
increase in temperature as the object brightens with %$B-V \sim ???$ and 
$V-I \sim 1.0$ at minimum and $B-V \sim 0.05$, $V-R \sim 0.06$ and $V-I
\sim 0.1$ at maximum (Figure 4). 

The system probably contains a relatively massive cool star which dominates at
the minimum of the outburst cycle, and the large orbital variation suggests that
the system is seen at high inclination. The shape of the secondary minimum
possibly hints at a grazing eclipse of the accretion disc by the cool star. The
changing shape of the light curve can probably be explained by changes in the
brightness and distribution of emission from the accretion disc and hot spot as
the outburst progresses.

\medskip
{\bf Acknowledgements.} It is a pleasure the acknowledge Arne Henden for
providing the sequence and many other observers including Peter Frank, 
John Greaves, Gary Poyner, Mike Simonsen for their thoughts and comments.
 
\references

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

Fritz, T., Bruch, A., 1998, {\it Astron.\ Astrophys.}, {\bf 332}, 586 

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

Monet D.G., et al., 2003, The USNO-B1.0 Catalogue, US Naval Observatory

Motch, C., et al., 1998, {\it Astron.\ Astrophys.\ Suppl.\ Ser.}, {\bf 132}, 341

Voges, W., et al., 1999, {\tt http://vizier.u-strasbg.fr/viz-bin/Cat?IX/29}

Wo\'{z}niak, P.R., et al., 2004, {\it Astron.\ J.}, {\bf 127}, 2436
\\{\tt http://skydot.lanl.gov/nsvs/star.php?num=8915780}

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