Self-lensing Binaries in the Galactic Bulge
| Program ID | 19056 |
|---|---|
| Science Category | Stellar Populations |
| Program Type | Analysis |
| Category | Small |
| Principal Investigator | Jeremy Schnittman |
| PI Institution | NASA Goddard Space Flight Center |
| Co-Investigators |
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| Abstract | There are an estimated 100 million stellar-mass black holes in the Milky Way galaxy. To date, we have only detected about 30 of them with X-ray observations of their accretion flows from binary companions. So where are the rest of these millions and millions of black holes, and an even greater number of neutron stars? They are most likely spread evenly throughout the galaxy, closely following the spatial distribution of main sequence (MS) stars. The trick is to find them. In this proposal we describe a novel technique for finding black holes (BHs) and neutron stars (NSs) in binary systems with MS stars. Specifically, we propose to look for a periodic magnification of the MS star caused by gravitational lensing by the BH or NS companion. The effect is very similar to a planetary transit, but with an increase in flux on the order of 100-1000 ppm lasting a few hours each orbit. Thus Roman is the ideal instrument for this search technique. Because BHs are so much rarer than exoplanets, a very large data set is required, hence the advantage of the Roman galactic bulge survey, monitoring the hundreds of millions of point sources with photometry comparable to TESS, but with a much longer baseline and exquisite angular resolution allowing us to observe crowded fields and resolve sources down to 23 mag. Like MS stars, we expect many black holes and neutron stars to reside in binary systems. For systems with sufficiently large binary separation, the compact object is not likely to be accreting, so the MS star will appear normal in most other respects. For binary periods of ~1-100 d, orbital radial velocities would be on the order of 100 km s^-1. Due to their rarity, spectroscopic identification would be prohibitively time consuming. However, photometric variation in the MS flux due to Doppler boosting is well within Roman's detection capabilities for MS stars with magnitudes brighter than F146 of approximately 19. |