[BAA Comets] Recent and forthcoming outbursts of Comet 29P/SW1 / Photometry

[Richard Miles]

The following note was sent to the comets-ml mailing list.
If anyone wants more info I can correspond offline.

I ought to warn UK observers that this comet is at almost -30 degrees 
Declination and so would be a difficult target as seen from the UK.

Richard

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A) Recent Outbursts

The last two outbursts of this comet were on:

2014 May 01.32 +/-0.09 reaching magnitude 13.8
2014 May 12.0 +/-0.4 reaching magnitude 14.8

B) Next Outburst

Using a model for the nucleus in which a small number of discrete sources 
exist and following the historical evolution of such sources, one area 
(Source B) appears now to contribute most of the marked outbursts since the 
last two years. The next possible eruption of this source is expected on 
2014 June 26.3 +/-3.9.

The trigger for an eruption is taken to be maximal solar heating of the 
near-surface of what appears to be a very slowly rotating nucleus. The 
source which is the subject of the current prediction appears to be 
relatively complex and so the uncertainty in the predicted outburst time is 
relatively large.

By the model, individual sources may not erupt for many revolutions of the 
nucleus and this can lead to long gaps in time between some consecutive 
outbursts. Sources, if they do exist, therefore erupt in a stochastic 
fashion, some being much less frequent than others. The present prediction 
is based on just 2 revolutions of 29P's nucleus, and if it does take place 
it will further demonstrate that Source B is especially active. The 
probability the outburst will occur is about 40-50% and so if it misses on 
this occasion then we shall have to wait one or more additional revolutions 
of the nucleus before it happens again.

C) Photometry

A request - Analysis of MPC photometric data shows that some observers, such 
as those using FoCAs 
(http://www.astrosurf.com/cometas-obs/ArtSoftUtil/Focas/FocasIng.htm) 
produce nuclear magnitude measurements (designated 'N' or 'm2') that are 
very consistent and of good accuracy. The main reasons are: a consistent 
photometric aperture is used nominally 10"x10" in size (equivalent to an 
aperture diameter of 11 arcsec); the background sky subtraction uses an area 
of background sky some distance from the nucleus where significant light 
from the coma is absent; the R magnitude system is employed.

Software designed to maximise astrometric accuracy (such as Astrometrica) 
often give erroneous photometry - usually too faint if say a 10" diameter 
aperture is used because the concentric background sky is often contaminated 
by the light of the outer coma.

Can observers take steps to ensure more consistency in achieving accurate 
photometry. This may mean doing the astrometry first and the photometry in a 
separate exercise. A circular aperture of radius 5.5" is preferred as 
standard. Your image scale ("/pixel) can make this difficult, and so in this 
case measure using 'n' pixels radius a little below 5.5" in size, and then 
again using 'n+1' pixels radius above 5.5". Unless you are using dedicated 
software (such as FoCAs), it is probably best to just integrate the light in 
terms of total counts. Then determine the background sky to subtract by 
repositioning the same aperture(s) in a part of the frame free from coma, 
faint stars and galaxies. Interpolate the brightness to the nearest tenth of 
a magnitude.

I shalln't go into any more detail here but observers can contact me offline 
for any assistance.

Richard Miles
BAA
arps {at} britastro.org