From astrobites:Through the Lenses of Black Holes

Astrobites bloc

Source: Through the Lenses of Black Holes

Title: A Search for Stellar-Mass Black Holes via Astrometric Microlensing
Author: J. R. Lu, E. Sinukoff, E. O. Ofek, A. Udalski, S. Kozlowski
First Author’s Institution: Institute for Astronomy, University of Hawai’i
Status: Accepted for publication in ApJ

When high-mass (>= 8 Msun) stars end their lives in blinding explosions known as core-collapse supernovae, they can rip through the fabric of space-time and create black holes with similar masses, known as stellar-mass black holes. These vermin black holes dwarf in comparison to their big brothers, supermassive black holes that typically have masses of 10⁶-10⁹ Msun. However, as vermin usually do, they massively outnumber supermassive black holes. It is estimated that 10⁸-10⁹ stellar mass black holes are crawling around our own Milky Way, but we’ve only caught sight of a few dozens of them.

As black holes don’t emit light, we can only infer their presence from their effects on nearby objects. All stellar mass black holes detected so far reside in binary systems, where they actively accrete from their companions. Radiation is emitted as matter from the companion falls onto the accretion disk of the black hole. Isolated black holes don’t have any companions, so they can only accrete from the surrounding diffuse interstellar medium, producing a very weak signal. That is why isolated black holes, which make up the bulk of the black hole population, have long escaped our discovery. Perhaps, until now.

The authors of today’s paper turned the intense gravity of black holes against themselves. While isolated black holes do not produce detectable emission, their gravity can bend and focus light from background objects. This bending and focusing of light through gravity is known as gravitational lensing. Astronomers categorize gravitational lensing based on the source and degree of lensing: strong lensing (lensing by a galaxy or a galaxy cluster producing giant arcs or multiple images), weak lensing (lensing by a galaxy or a galaxy cluster where signals are weaker and detected statistically), and microlensing (lensing by a star or planet). During microlensing, as the lens approaches the star, the star will brighten momentarily as more and more light is being focused, up until maximum magnification at closest approach, after which the star gradually fades as the lens leaves. This effect is known as photometric microlensing (see this astrobite). Check out this microlensing simulation, courtesy of Professor Scott Gaudi at The Ohio State University: the star (orange) is located at the origin, the lens (open red circle) is moving to the right, the gray regions trace out the lensed images (blue) as the lens passes by the star, while the green circle is the Einstein radius. The Einsten radius is the radius of the annular image when the observer, the lens, and the star are perfectly aligned.

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