Detecting Isolated Black Holes through Astrometric Microlensing
A significant fraction of the mass of an old stellar population should be in the form of non-luminous, isolated black holes (BHs). Yet there has never been an unambiguous detection of a solitary BH---not surprisingly, since the expected accretion rate from the ISM is extremely low, and thus they emit essentially no radiation.
The only technique available to detect such isolated BHs is astrometric microlensing---the relativistic deflection of light from background stars. HST is the only instrument currently capable of detecting such tiny deflections.
We have underway a multi-year program of HST high-precision astrometry of long-duration microlensing events in the Galactic bulge, using the WFC3 camera. Our aim is the first detection of stellar-mass black holes, by monitoring five optimally selected events. Our program has met with success, with clear detections of the deflections in two events, indicative of non-luminous massive lenses. However, the 25 orbits allocated to this project for Cycles 17, 18, and 19 have proven to be insufficient to complete the program. We had to abandon 3 of our initial targets part way through the monitoring, due to problems with the associated ground-based photometry. We selected instead three other high-priority targets, which we are monitoring. However, we will need 6 more orbits in Cycle 20 to complete the original objectives of the program.
We will continue our observations of long-duration events which do not show any light contribution from the lens (continuation of 11707/12322/12670). We will take most images in a single filter, but also take a small number of images in a different filter to check the color dependenacy, if any, of the astrometric shift.
We will try to take as many images as possible in a single ORIENT for better astrometri accuracy. However, maintaining a single ORIENT may not be possible for all the observations. Since almost all of our targets are close to the ecliptic, we plan to use the same orient for nearly half of the year, and then flip it by 180 deg.
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