[Aavso-photometry] Nova Cyg 2005 (V2361 CYG) - Dramatic fade

[Richard Miles]

On Monday, February 28, 2005 4:50 PM
Arne Henden wrote:

> Results from this morning with the 1.0m, Using Richard's calibration:
>   JD2453430.0286  V  17.48  0.03
>   JD2453430.0348  Ic 13.23  0.03

Many thanks, Arne, for securing accurate photometry at this stage in the 
fade.
Clearly the V-I color index is at +4.25, not significantly different to my 
last measure, so my extrapolation to even higher index was a wrong 
assumption.

Looking at the data now, this nova does appear to be a record breaker.

A useful nova to compare it with is V1500 CYG (Nova Cygni 1975).
This was a remarkable event some 30 years ago and was very thoroughly 
studied given that it reached V=1.8 and had an abolute mag. at maximum close 
to -10.0.  A good summary of the photometry made during the first 2 year's 
observations is given in, P.Tempesti, Astron. Nachrichten 300, H.1, 51 
(1979). See:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979AN....300...51T&db_key=AST

V1500 CYG was a very fast nova with a t3 of around 4 days, which is close 
to, if not the fastest known.
Line broadening when near maximum indicated velocities of 3600 km/s, again 
extremely fast and compares closely with V2361 CYG reported to be 3200 km/s, 
again indicative of a highly energetic nova.

The latest datapoint from Arne corresponds to a 7.5 mag fade in just 18 
days.
For V1500 CYG, the same degree of fading took 57 days, so it is clear that 
this characteristic of V2361 CYG is highly unusual.

V1500 exhibited an exponential fade rate starting 3 days after maximum 
described as:
 m = 2.5 + 3.9*log t (days)
V2361 is now well into it's exponential decline, starting about 4-5 days 
after maximum, for which a corresponding expression is currently:
 V = 9.1 + 7.8*log t (days)
i.e. the exponent is twice the value in the case of this new nova although 
more data are needed to get an accurate 'decay rate'.

The V-I color is interesting.  I have determined V-Ij from published data 
and compared the two novae:
Time after max.,   V1500,   V2361,  Delta
0,   +0.6,   +2.4,   1.8
3,   +1.6,   +2.6,   1.0
9,   +1.9,   +4.1,   2.2
15,   +2.0,   +4.2,   2.2
Thus there does appear to be an additional reddening in V-I amounting to 
about 2.0 mag presumably due to interstellar reddening.  Both novae are very 
close to the galactic plane (latitude of 0.0 deg for V1500 and 3.6 deg for 
V2361) so this makes sense.

Given these similarities it does now look as though this object will not 
brighten again especially since such behaviour tends to be associated with 
slow novae.  Unfortunately the eventual nova remnant is likely to be very 
faint indeed in which case it may not be possible to fully characterise, for 
example by looking for periodicity in the lightcurve (anyone with time on 
the VLT, Keck?).

Richard Miles