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\IBVShead{5720}{2 August 2006}

\IBVStitletl{Found a Nova in M31: The True Optical Counterpart}{ of the M31 Supersoft X-ray Source 191}

\IBVSauth{Smirnova, O.; Alksnis, A.}

\IBVSinsto{Institute of Astronomy, University of Latvia,
Raina bulv.\ 19, Riga, LV-1586, Latvia;\\ \indent e-mail: o.smirnova@inbox.lv}

\SIMBADobj{[PFH2005] 191}
\NEDobj{M 31}

\begintext

In this note we report the discovery of a nova in M31 (NGC 224) which
turned out to be the true optical counterpart of the supersoft X-ray
source 191 of the M31 XMM Newton survey catalogue instead of the Nova
1992-01 proposed by Pietsch et al.\ (2005a). The nova was found by one of
us (O.S.) on scanned archival photoplates taken in October and November
of 2001 for search for novae in M31 with the Schmidt telescope
(80/120/240-cm) at the Baldone Astrophysical Observatory of the Institute
of Astronomy, University of Latvia.

The coordinates of the nova were obtained using the Aladin Sky Atlas 
(CDS) image astrometric calibration tool with respect to the positions
of field stars from the $BVRI$ catalogue of M31 (Magnier et al., 1992).
The resulting nova position derived from 7 scanned plates is the following:
$$
{\rm R.A.} = 00\hr41\mm54\fsec26,\qquad
{\rm Decl.} = +41\deg07\arcm23\farcs9\qquad
{\rm (equinox~2000.0),}
$$
with 1-sigma error of 0\farcs2. It is located 564\farcs4 West and 524\farcs6 South of
the center of M31; a finding chart is given in Figure~1. No record of this object was found in any searches of
the papers or WWW pages devoted to novae in M31.

\IBVSfig{9.7cm}{5720-f1.ps}{The scan of the photoplate taken on October
15, 2001 with the discovered nova (marked with an arrow). The white and black crosses show positions of X-ray source 191 and Nova 1992-01 respectively}
\IBVSfigKey{5720-f1.ps}{[PFH2005] 191}{finding chart}

\vspace{-12pt}
Times of the middle of exposures in Julian days and $B$-magnitudes of the
nova based on the secondary standard stars from the $BVRI$
catalogue of M31 (Magnier et al., 1992) are given in Table~1. The light curve of the nova is presented in Figure~2.

\begin{table}[h]
\begin{center}
{\normalsize Table 1}\\
\smallskip
\begin{tabular}{cc|cc}
\hline
   JD          &    $B$  &  JD            &   $B$    \\
2452000 +      &     mag & 2452000 +      &     mag \\
\hline
            151.472        &   $>19.5$ & 204.250    &   17.7 \\
            196.262        &    16.6 &   207.390    &   18.0 \\
            198.256        &    16.8 &   208.267    &   18.2 \\
            199.282        &    16.8 &   226.208    &   18.5: \\
            203.234        &    17.3 &   228.238    & $>19.6$ \\
\hline
\end{tabular}
\end{center}
\end{table}

The available photometric data for the nova do not allow to determine the
time and the value of the maximum brightness exactly. However, the $dB/dt$
parameter can be estimated from the general slope of the light curve
between the brightest observation, when $B = 16.6$ and the observation
closest to 2 magnitudes fainter than the maximum. Excluding an uncertain
measurement we estimate $dB/dt = 0.13$ m/d equivalent to the rate of decline $t_2
\sim 15$ days, which corresponds to the fast novae according to the
classification by Payne-Gaposchkin (1957). The mean maximum magnitude for
novae with similar rate of decline is $B = 16.5$, according to the relation
between the rate of decline and the magnitude at maximum for M31 novae
obtained by Capaccioli et al.\ (1989).  It could indicate that the first
observation of the nova was about one day past its maximum light.

Comparing the nova position with those of X-ray sources in the catalogue
of XMM-Newton survey of M31 (Pietsch et al., 2005b) we found that 
source 191 is located at a distance of 0\farcs9 East and 0\farcs1 South from 
the nova. This X-ray source is classified as supersoft as other known
X-ray sources identified with novae. The 1-sigma error of X-ray source
position is 0\farcs88, including systematic error. Within error its position
coincides with the position of the nova.

According to Pietsch et al.\ (2005a) the X-rays at the position of the
source 191 was first detected during XMM-Newton observations at JD
2452280.5, so 84 days after the nova outburst and then six days later. No
X-ray source was detected at that position during the three XMM-Newton
observations made at the moments corresponding to 476, 290 and 107 days
before the nova outburst. Evidently this source was not yet active also 17
days after the nova outburst, as it was covered by the Chandra HRC1
observation 1912, but not reported in the catalogue by Kaaret (2002).

Pietsch et al.\ (2005a), searching for X-ray counterparts of optical novae,
correlated this source with the Nova 1992-01, reported by Shafter \& Irby
(2001) from two Halpha images, taken in December of 1992 and January of
1993. According to Pietsch et al.\ (2005a) the time separation of 3303 days
between Nova 1992-01 outburst and X-ray source rise is significantly
larger than that for the other 22 M31 and M33 novae in the dataset. Therefore
the authors supposed that the Nova 1992-01 was probably a recurrent nova,
which had a new unobserved outburst about 2001, responsible for the
observed X-rays.

\IBVSfig{9cm}{5720-f2.ps}{The light curve of the nova in M31. Filled circles: confident
measurements; open circle: uncertain measurement; triangles: brightness
upper limits (the triangle with arrow corresponds to JD~2452151.472)}
\IBVSfigKey{5720-f2.ps}{[PFH2005] 191}{light curve}

To try to verify the possibility, that our reported nova and the Nova 1992-01
is the same object, we inspected Baldone observatory archival plates of
M31 taken in 1992. We found the Nova 1992-01 on two 1/10/2001 photographs
and measured its coordinates on both plates in the same way, as was done
for the reported nova, and we got
$$
{\rm R.A.} = 00\hr41\mm53\fsec82,\qquad
{\rm Decl.} = +41\deg07\arcm22\farcs5\qquad
{\rm (equinox~2000.0),}
$$
with 1-sigma error of 0\farcs3.

The position separation between the Nova 1992-01 and our reported nova is
5\farcs2, thus they are different objects.

As the Nova 1992-01 lies 6\farcs1 apart from the X-ray source 191, but our
nova at much smaller distance 0\farcs9, the latest must be considered as an
optical counterpart of the X-ray source. In this case the time separation
between the optical outburst and X-ray rise---84 days falls in the
interval of time separation from 63 d to 170 d observed for four other
novae---optical counterparts of X-ray sources in M31, contrary to the
extraordinarily long 9 years time separation in case of previously assumed
identification with the Nova 1992-01. To the common features of these
short-time separation optical counterparts add the fact that three of them
are fast novae in the same way as our nova.\newpage
\references

Capaccioli, M., della Valle, M., Rosino, L., D'Onofrio, M., 1989, {\it AJ}, 97, 1622

Kaaret, P., 2002, {\it ApJ}, 578, 114

Magnier, E.A., Lewin, W.H.G., van Paradijs, J., Hasinger, G., Jain, A.,
Pietsch, W., Truemper, J., 1992, {\it A\&AS}, 96, 379

Payne-Gaposchkin, C., 1957, The Galactic Novae, North-Holland Publ.\ Comp.,
Amsterdam, p.\ 24

Pietsch, W., Fliri, J., Freyberg, M.J., Greiner, J., Haberl, F., Riffeser
A., Sala, G., 2005a, {\it A\&A}, 442, 879

Pietsch, W., Freyberg, M., Haberl, F., 2005b, {\it A\&A}, 434, 483

Shafter, A.W., Irby, B.K., 2001, {\it ApJ}, 563, 749

\endreferences

\bigskip
\bigskip

%From: IBVS 5737
{\bf Corrigendum for IBVS 5720} In the paper by Smirnova \& Alksnis (2006), 
third paragraph from the end, instead of 1/10/2001 should be 1/10/1992.
Our thanks are due to W. Pietsch for pointing out this error.


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