Galactic Bulge Survey

I'm in Chile right now on an observing trip, which means a fair amount of free time while the telescope operator does all the real work, so I thought I should make a post about what I'm doing here in the first place.

We're using the Mosaic II instrument on the Blanco 4m telescope at the Cerro Tololo NOAO site in Chile to look for optical counterparts to X-ray sources in the galactic bulge. The X-ray data was taken with Chandra, going to a depth of 10^33 ergs/s (maybe 10^32 ergs/s...I don't have the proposal with me...objects in quiescence anyway). Chandra found 1700 X-ray sources in the bulge. By current population estimates, about 400 of these should be Low Mass X-ray Binaries (LMXBs), a healthy number of which should be at least partially eclipsing systems.

The reasons for looking in the bulge instead of the more populated galactic center are two-fold. First, it's less populated; crowding is less of a problem. Always nice to be able to tell sources apart. The Chandra data has 1-2 arcsecond resolution, so we have a good idea of where we should be looking. Second, there is less dust and therefore less extinction. Some might say, "Well, why not use infrared." There are groups doing just that, but instrumentation is better in the optical than in the infrared, so our data *should* be higher quality.

We have 45 overlapping fields which we are observing 2.5 times a night, on average (maybe a little less because of the occasional system crash), so we end up with 3-4 images of each candidate companion each night. We have 8 nights to observe, and so far we are 5/5 with having good weather. We are looking primarily in the range of magnitude 20-23 stars.

What, you rightly may be asking, is the point of all this? Over the course of the trip, we should have 20-30 images of each star at various times over a week, which should be ample to identify periods of sources in the range of hours to days.

With so many added LMXBs to the current population, we can start to do real population statistics, which allows us to test different models of binary evolution. By getting so many more eclipsing systems, we can also get good mass estimates on a number of neutron stars, thereby providing a more solid observational limit on the neutron star equation of state. We also have a good chance of discovering the first galactic eclipsing black hole system.