(cba:news) [Fwd: Request for Observing BG Gem.]

Joe Patterson jop at astro.columbia.edu
Thu Jan 29 15:48:17 EST 2009

HI CBAers,

This looks like a good CBA project.  Mark wants coverage of BG Gem over 
a 90 day period.  It's bright enough for us, and he merely needs (I 
think) differential photometry with respect to that marked comparison 
star.  Of course, he doesn't need a full time series, since it's a 90 
day period... but get enough points that he can assess the random error 
of the observation.  BTW it seems to be a filters-only project.

Among the CVs, keep firing away at T Leo = QZ Vir!


Dear Fellow MDM Observer

I have been granted time for a program of photometric observations of the
black hole candidate BG Geminorum at the MDM 1.3m. The aim of these
observations is to significantly improve the poorly constrained
ephemeris. The orbital period of this binary system is approximately 90
days, as such observations will be required over a number of months.

As BG Gem is bright at optical wavelengths (V  13), these observations
will be short (30s x 4 in each lter). We do not require standard star
observations as we have previous observations during which this eld was
calibrated. In total, the estimated time per night (acquisation +
exposure) should be no more than 10  15 mins.

We request imaging in the V, R & I-band lters every night, or as close to
this as possible (Feb 1st  May 1st ). In the event that observations are
possible with only a single filter, we prioritise observations in the

See my BG Gem webpage


for background information, nightly observing window, charts and the quasi
real time lightcurve.

BG Gem, V mag :  13
= 06 03 30.81,  = +27 41 50.7

Filter                                 Integration Time     # Exposures
V                                      30                       4
R  --  highest priority                30                       4
I                                      30                       4

I'll be observing at the MDM myself on a number of occasions over the
coming semesters and will be more than happy to return the favour at some
stage in the future.


Mark Reynolds

markrey at umich.edu
+734 764 4160

BACKGROUND INFORMATION (this info will also be on the website)

Title : Updating the Ephemeris of BG Geminorum

BG Geminorum is a long period eclipsing binary system (Porb  90 days);
currently the nature of the primary is uncertain. Recent observational
evidence indicate that the primary star is a black hole. If so, BG
Geminorum would be the black hole binary system with the largest orbital
period, by a factor of 3, as well as being the only known eclipsing black
hole binary system in the Galaxy.  Here, we propose optical monitoring
observations with the MDM 1.3m, which will contrain the orbital ephemeris.

Based on the mass function, mass ratio (q  M2 /M1 ) and the inclination
angle, in combination with the observed maximum emission line velocities
in the UV & optical spectra, the primary star in BG Gem is either (i)
B-type star (ii) a stellar mass back hole.

The ephemeris for BG Gem is currently poorly contrained. The current error
on the time of eclipse large, e.g. our NOT spectra do not display high
velocity emission lines; however, due to the large uncertainties in the
time of eclipse, it is possible that the black hole in eclipse during
these observations. Clearly, this is an unsatisfactory situation.

Here, we propose to obtain optical monitoring observations of BG Gem
throughout its entire 90 day to accurately constrain the binary orbit.
Plus...... other relevant science case info and images, i.e. previous
spectra are available at the web site.


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