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

 FEB 22.7870     11.30  MXL    CCDI   EASILY DETECTED IN SINGLE I IMAGES    ERR: .06     
 FEB 22.7870    <15.4   MXL    CCDV   BASED ON 29 X 40 SEC STACKED V FRAMES
 FEB 21.7952     14.76  MXL    CCDV   21 X 40SEC STACKED FRAMES USED (6-CM APERTURE) ERR: .15 
 FEB 21.7952     10.64  MXL    CCDI   V-I COLOR INDEX = 4.12 +/- 0.16   ERR: .05