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\IBVShead{5739}{8 December 2006}

\IBVStitle{Discovery of 19 New Historical Nova Candidates in M31}

\IBVSauth{Henze, M.$^1$; Meusinger, H.$^1$; Pietsch, W.$^2$}

\IBVSinst{Th\"uringer Landessternwarte Tautenburg, D-07778 Tautenburg, Germany}

\IBVSinst{Max Planck Institute for Extraterrestrial Physics, D-85748 Garching, Germany}

\NEDobj{M 31}

\begintext

We have conducted a systematic search for historical novae in M31 on digitized archival
plates. A comprehensive description of the data material, the method, and the results will be
given in a separate paper (Henze et al., 2007). Here we present a brief summary of the attempt
and announce, as the most important result, 19 new nova candidates.

The M31 field is the most frequently observed field in the archive of the Tautenburg Schmidt
telescope (134/200/400).  Our search is based upon 306 selected plates in the $UBV$ bands
taken in the years 1960 to 1996. A single plate covers an unvignetted field of $3\fdeg3 \times
3\fdeg3$ with a plate scale of 51.4 arcsec/mm.  The limits of the $B$ plates are typically in
the range $B_{\rm lim} = 19\mm\ldots 21\mm$. Although the majority of these plates
were not taken as a part of a systematic survey, they constitute a valuable observational
material suited to search for bright variables in our neighbour galaxy.

All plates have been digitized with the Tautenburg Plate Scanner (Brunzendorf \& Meusinger,
1999) and were reduced using the software package {\it Source Extractor} (Bertin \&
Arnouts, 1996).  For the astrometric and photometric calibration of one selected reference
plate per filter band we used the USNO-B1.0 catalogue (Monet et al., 2003) and the Local Group
Survey catalogue (Massey et al., 2006), respectively.  Special care was taken to consider the
strongly fluctuating background surface brightness.  All plates of the same filter band were
transformed into the system of the corresponding reference plate which results in an overall
astrometric uncertainty of $\sim 0.5$ arcsec and a relative photometric uncertainty of
0.2--0.3 mag. The absolute photometric uncertainties on the reference plates are about 0.5 mag
over the magnitude interval 16--20 mag. Finally, the data set for every single plate was
cross-correlated with the data sets from all other plates to create two catalogues:  (a) the
multi-detection table of $\sim 3 \times 10^5$ objects detected on at least two plates of the
same colour and ($B$) the single-detection table of $\sim 1.1 \times 10^6$ objects detected on
only one plate. Since we decided to use a low detection limit for the object detection, in
order to reach a high completeness at faint magnitudes, the tables are substantially
contaminated by noise detections. This has to be considered for the selection of novae
candidates: single-detections were used only to {\it confirm} multi-detections or
single-detections in other filter bands. For the multi-detection objects light curves were
created and searched for typical nova features.

\newpage

Typical features of nova light curves were modeled using novae in M31 which were previously discovered on  
Tautenburg plates by  Moffat (1967) and B\"orngen (1968):

\begin{itemize}

\item {\bf Short time span of observability:} Due to the distance of M31
and the plate limit of $\sim 20\mm$, novae have a typical time of
observability of 20--30 days. The parameter value applied for the search
was 50 days ($U,V$) and 70 days ($B$) respectively.
\item {\bf Prominent peak:} A nova light curve should show a significant
peak which must be brighter than the plate limit and be outside the
$1\sigma$ error range of the modified light curve {\it without} the peak.
\item {\bf Singular event:} Classical novae do not recur on a timescale
less than 100 years. Therefore every nova event in our data base should be
unique. We also searched for recurrent novae, namely such that show
repeated outbursts on a timescale less than 100 years, but we did not find
any.
\end{itemize}

Every promising candidate was individually checked on the original plates to decide whether it
could be a nova or not.  The spatial distribution of the 19 objects classified as formerly
unknown nova candidates is shown in Fig.~1.  The mapped area is a cutout from
the field of the astrometric reference plate corresponding to the area containing the new
candidates. The key data are summarized in Table~1.  Another 32 previously
catalogued novae were established by our program. This is the reason why the consecutive
numbering in Table~1 starts with 33. The full set of data will be provided in
Henze et al.\ (2007).

\IBVSfig{10cm}{5739-f1.ps}{Distribution of the new 19 Tautenburg nova candidates over the
galaxy M31. Black dots indicate the objects detected on the reference plate. The outer spiral
arms of the galaxy are clearly recognizable by their overabundance of detected objects. Big
filled squares mark the new nova candidates}
\IBVSfigKey{5739-f1.ps}{M 31}{finding chart}

\newpage

\begin{table}[ht] 
\centering
\caption{Basic data for the new nova candidates: identification number (1), right ascension
and declination for J2000 (2,3), magnitude of the detected maximum and filter band (4), Julian
date (5), and year of the outburst (6).}
\medskip
\begin{tabular}{rrrrrr}\hline
	ID & $\alpha$ [\deg] & $\delta$ [\deg] & mag & JD & year\\ 
	(1)&  (2)                &     (3)             &  (4)&(5) & (6)\\ \hline
	33 & 11.90747 & 41.75843 & 19.4 ($B$) & 2437913 & 1962\\ 
	34 & 10.01956 & 40.62433 & 18.5 ($V$) & 2438373 & 1963\\ 
        35 & 11.43947 & 41.75058 & 17.7 ($U$) & 2439417 & 1966\\ 
	36 & 9.33152 & 40.52856  & 18.9 ($B$) & 2440917 & 1970\\ 
        37 & 10.38148 & 40.87432 & 17.7 ($B$) & 2441328 & 1972\\ 
	38 & 9.81823 & 40.52356 & 18.9 ($B$) & 2441680 & 1972\\
	39 & 10.36907 & 40.88704 & 18.7 ($V$)  & 2442741 & 1975\\ 
  	40 & 10.44820 &  40.95410 & 18.2 ($U$) & 2442775 & 1975\\
        41 & 11.47816 & 40.92837 & 19.3 ($B$) & 2444194 & 1979\\
 	42 & 10.14823 & 41.32396 & 17.8 ($U$) & 2444490 & 1980\\
	43 & 10.47874 & 41.01253 & 18.1 ($U$) & 2444490 & 1980\\ 
	44 & 10.74837 & 41.28688 & 18.0 ($U$) &  2444490 & 1980\\ 
	45 & 11.08057 & 41.60674 & 19.4 ($B$) & 2445940 & 1984\\ 
        46 & 11.51205 & 41.73243 & 18.9 ($U$) & 2446299 & 1985\\
	47 & 11.58009 & 41.97954 & 18.5 ($U$) & 2446299 & 1985\\
        48 & 12.51510 & 41.42756 & 18.8 ($U$) & 2446299 & 1985\\
        49 & 10.43303 & 41.07269 & 16.9 ($B$) & 2448893 & 1992\\
	50 & 10.76761 & 40.40784 & 17.5 ($B$)  & 2450316 & 1996\\
	51 & 11.54533 & 41.61147 & 19.4 ($B$)  & 2450317 & 1996\\
\hline
\end{tabular}
\end{table}


Finally, we would like to emphasize that the good astrometric accuracy of this ``new
historical'' novae makes them suitable for the correlation with previously found ones in order
to search for recurrent novae. With the only exception of nova~39, none of our new nova
candidates could be identified on POSS~II plates, in the SIMBAD database, or in the GCVS
(Artyukhina et al., 1995). The position of nova~39 coincides, with a position difference of
1 arcsec, with the nova number 32 in Table~4 of Baade \& Arp (1964) discovered between the
years 1945--1949. Therefore, nova~39 is a good candidate for a recurrent nova with repeated
outbursts on a timescale less than 100 years. Unfortunately, Baade \& Arp do not report the
epoch of their observation and thus the recurrence time can be estimated only roughly to
26--30 years. Additional information on the actual epoch of the Baade \& Arp nova~32 / Table~4
would be useful. Because the 1975 outburst of nova~39 has not been reported so far, we list
it as a new nova candidate, even though it is probably a recurrent nova.

{\bf Acknowledgments:} This research has made use of the SIMBAD database and of the Aladin sky
atlas which are operated at CDS, Strasbourg, France, and the General catalogue of Variable
Stars Volume V Extragalactic Variable Stars (GCVS Vol.~V) which is operated at Sternberg
Astronomical Institute, Moscow, Russia.


\references

Artyukhina, N.M., Durlevich, O.V., Frolov, M.S., et al., 1995, {\it
General catalogue of Variable Stars, 4th ed., vol.~V. Extragalactic
Variable Stars}, ``Kosmosinform'', Moscow

Baade, W., Arp, H., 1964, {\it ApJ}, {\bf 139}, 1027

Bertin, E., Arnouts, S., 1996, {\it A\&ASS}, {\bf 117}, 393

B\"orngen, F., 1968, {\it Mitteilungen des Karl-Schwarzschild-Observatoriums Tautenburg},
{\bf 40}

Brunzendorf, J., Meusinger, H., 1999, {\it ACHA}, {\bf 6}, 55

Henze, M., Meusinger, H., Pietsch, W., 2007, in preparation

Massey, P., Olsen, K.A.G., Hodge, P.W., 2006, {\it AJ}, {\bf 131}, 2478

Moffat, A.F.J., 1967, {\it AJ}, {\bf 72}, 1356

Monet, D.G., Levine, S.E., Casian, B., et al., 2003, {\it AJ}, {\bf 125}, 984

\endreferences

\vfill

\IBVSerratum{5701}

The star listed as V2028 Cyg in IBVS 5701 should be V2088 Cyg.

\bigskip

Geir Klingenberg

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