[Aavso-photometry] Information on N CYG 05 (V2361 Cyg)

[Aaron Price]

 The following is reposted with permission from Richard Miles
(BAAVSS). It has some very interesting background and thoughts on this
object.

Possible Nova Cygni 2005 (V2361 Cyg)

This object was discovered photographically (Kodak T-Max 400) by Hideo
Nishimura on Feb 10.85 and estimated at V=9.7 (IAU Circular 8483).
Shawn Dvorak obtained an accurate CCDV mag a day or so later and myself and
Robert James have subsequently made CCDV observations since.
I have also determined V-I colors, which has shown that the object was quite
red initially (V-I = +2.4) but that it has continued to redden as the
brightness has declined.  In my view it is not clear what the nature of this
object is, since it is not exhibiting typical nova behaviour.

The object fits the category of being a 'very fast' nova, in that it has
declined 3.0 mags in 9 days (t3 value).  The fastest known nova has been
claimed to be V1493 Aql (Nova Aql 1999 No.1) for which the t3 value was
estimated at 5 days or so (
http://arxiv.org/PS_cache/astro-ph/pdf/0003/0003156.pdf ).  Typically fast
novae exhibit t3 values of 10-20 days and rise 12-16 mags to maximum in less
than 3 days.  They also tend not to show increased reddening with time due
to dust shell accumulation.

So far, the form of the lightcurve of V2361 Cyg is unlike typical very fast
novae, which peak in brightness for a day or so and then begin a rapid
decline, which then tends to slow (concave-shaped lightcurve).  This object
has remained relatively constant brightness for the first 3 days but has
undergone an accelerating decline in brightness (convex-shaped lightcurve)
peaking at about 1.1 mag/day decline, which is very fast indeed.

Given, the red color, and taking account of the precise CCDV photometry
(Dvorak, Miles) the initial maximum brightness is estimated as V=10.1.  Two
unfiltered CCD observations have been announced by Wakuda (9.3) and Kadota
(8.9) but these will have strongly over-estimated the brightness.  The
initial I magnitude is estimated to be 7.7.  The following lists the
precision photometry currently available (JD 2450000+):

JD,  V Mag,  V-I Color,  Comments

3412.349,  10.1,  -,  estimated mag at discovery
3413.964,  10.132,  -,  Dvorak observation
3415.275,  10.278,  2.44
3416.288,  10.404,  2.46
3417.284,  10.677,  2.57
3420.293,  12.013,  3.14
3422.286,  13.70,  3.88
3423.295,  14.76,  4.12
3424.287,  15.8,  4.45, estimated V mag from measured I mag and extrapolated
V-I curve

Further observations are urgently required both photometrically using
filters (V priority) and spectroscopically to understand what is happening
here.  One can speculate.  For example, the nova at maximum is relatively
faint for a galactic nova and indicates that it is relatively distant (> 20
Kpc ?).  However, if the reddening and rapid fade is due to the appearance
of dust at a very early stage in the outburst, the object may be unusually
reddened and therefore nearer.  If the dust shell hypothesis is correct then
we can also expect the object to reach a very deep minimum and then
brighten.  As to how bright the object reaches is yet to be seen but it
could possibly reach say V=13-14 if the initial outburst was significantly
attenuated and the nova is not so distant.  Large telescopes will probably
be required to follow the object in the V band at minimum.  However,
observation through I filters will be an alternative since at minimum the
V-I color will probably exceed +5 or +6.

How exotic this object turns out to be is yet to be seen.  For example, it
may be a binary object (like DQ Her and U Sco) in which case short-period
oscillations in brightness and recurrent events may take place from which
its nature can be better discerned.  We shall see.

Richard Miles
Golden Hill Observatory, Dorset, UK




-- 
Aaron