[vsnet-alert 5366] possible nova in M31: message from G. Sostero

[Makoto Uemura <uemura@kusastro.kyoto-u.ac.jp>]

vsnet administrator team received the following message from G. Sostero 
which was originally posted to vsnet-alert.  The image attached to the mail 
will be sent later.

Regards,
Makoto Uemura

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Date: Fri, 03 Nov 2000 17:23:34 +0000
From: afamstar <afamstar@libero.it>
Subject: [vsnet-alert 0] RE: Image of DSS zone M31 of suspected Nova

Questo ,Ah(B un messaggio a pi,Ay(B sezioni in formato MIME.
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Hi all,
with reference to the e-mail of mr.Scarmato, I would like to made a few
comments:

1) the image we made on November 1st, was extremely deep (limiting
magnitude almost 19.2), and the constrast/brightness parameters of it
were very exaggereted, in order to extract the faint image of the
possible (fading) Nova. The other two small dots visible in our frame,
nearby the possible Nova could be, in my opinion:
i) or extremely reddened foreground stars with a high R-I color index,
in such a way that we catched it at the threshold limit of our frame,
but they were not well recorded in the POSS plate
ii) or noise of our CCD (very likely, this would be the correct answer).
The problem is that when you try to catch extremely fainth objects, at
the threshold limit of the frame, you are cleary fighting with the noise
of your camera.

Anyway the possible Nova is a real object, and is well visible not only
on our frames, but also on  that's of  F. Ewalt, and in the following
images of K. Korlevic. Is it a real Nova in M31? In my opinion, giving
its photometric behaviour, we have only two possible answers:
i) or a genuine Nova in M31
ii) or a Galactic cataclismic variable in the line of sight of M31
I would prefer the first option, because of the lightcurve, and also
because I consider rather difficult that a cataclismic variable with
peaking magnitude close to 17, was missed in all the previous extended
professional surveys of M31 (Palomar, Asiago, etc.). Anyway only a
spectra would give a clear answer, but unfortunately we haven't it...

2) the faint starlike point indicated with a small arrow in the POSS-R
pictur by mr.Scarmato, is clearly a  stellar source with a small B-R
color index. You can recognize it looking at the enclosed POSS-B plate,
where it is much evident. I consider that we cannot see it in our frames
(as in the frames of F. Ewalt, K. Korlevic and that's of every observer
using a CCD, that has the maximum sensitivity in the R-I band) just
because of a color effect.

Thanks to mr.Scarmato for the useful discussion he prompted.

Best regards,
Giovanni Sostero

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