BG Geminorum
This campaign is now finished. Many thanks to all the observers who
took some data along the way,
Mark Reynolds 11/09/09.
δ : +27 41 50.7
V_mag ∼ 13
2009A MDM Observations
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 filter). We do not require standard star observations as we have previous observations during which this feld 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 filters 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 R-band.
Current Status
Motivation
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 an factor of 3, as well as being the only known eclipsing black hole binary system in the Galaxy.
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 was in eclipse during these observations. Clearly, this is an unsatisfactory situation.
Here, we plan to obtain optical monitoring observations of BG Gem
throughout its entire 90 day orbit to accurately constrain the orbital
ephemeris
Click here to learn more about
the eclipsing HMXB BG Geminorum.
Previous work
- Benson et al., 2000, ApJ, 119, 890
- An optical study of BG Geminorum: An ellipsoidal binary with an unseen primary star.
- Kenyon et al., 2002, AJ, 124, 1054
- The eclipsing binary BG Geminorum: Improved constraints on the orbit and the structure of the accretion disc.
- Elebert et al., 2007, IAUS, 238, 361
- Optical, UV and X-ray analysis of the black hole candidate BG Geminorum