Discovering an Unseen Black Hole

Paradoxically black holes, which emit no light because of their strong gravitational fields, are regularly discovered via the bright X-ray emission that arises when material accretes onto a torus around a black hole, heating the torus to high temperatures. Thousands of X-ray emitting black holes are known, ranging in size from supermassive monsters at the hearts of galaxies (like the recently imaged shadow of the giant in M87 with nearly a billion solar-masses) to smaller, stellar-mass-size black holes. The remarkable measurements of gravitational wave emission from merging dense objects, including black holes, illustrate how non-electromagnetic radiation can also be used to spot black holes. Astronomers, however, think that many and perhaps even most black holes are currently neither accreting material nor in the final stages of merging. How, then, to spot them since they do not radiate?

The first stellar-mass black hole that is not emitting any detectable radiation has now very likely been discovered by a team of scientists including CfA astronomers Dave Latham, Allyson Bieryla, Gilbert Esquerdo, Perry Berlind, and Michale Calkins. Their method was to hunt for a star whose wobble implied that it was orbiting with a massive, unseen binary companion star, probably a black hole. This velocity wobble method is the same as is used in the search for planets around other stars, a technique that CfA astronomers helped to pioneer.

The star 2MASSJ05215658+4359220 (its long name incorporates its coordinates in the sky) is a giant star located in the direction of the constellation Auriga and which is about twelve thousand light-years away. The team discovered that the star was wobbling with a period of about eighty-three days with a velocity variation of about eighty km/sec. The peculiar behavior of the star was first noticed using infrared spectroscopy from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and that was followed up by searching for light (versus velocity) variations using the All-Sky Automated Survey for Supernovae (ASAS-SN) instrument and then with more precise spectroscopy using the Tillinghast Reflector Echelle Spectrograph (TRES) at SAO's Fred L. Whipple Observatory.

The combination of datasets from these campaigns enabled the scientists to determine the mass of the star as being about 3.2 solar-masses; the mass of its non-accreting, unseen companion has a lower mass limit (because the inclination of the orbit is not known) that is about the same, about 3.3 solar-masses . This size black hole is itself an important discovery since the size falls in the so-called compact object “mass gap” between more commonly known neutron stars and more typical sized stellar-mass black holes. The unseen object might in principle be a neutron star, buts since its mass is larger than the highest known neutron star mass (2.0 solar-masses), and since the size is compatible with some models of black hole mass sizes, the team argues that the unseen object is probably a black hole, the first one to be detected without its being directly seen.

"A Noninteracting Low-Mass Black Hole–Giant Star Binary System," Todd A. Thompson, Christopher S. Kochanek, Krzysztof Z. Stanek, Carles Badenes, Richard S. Post, Tharindu Jayasinghe, David W. Latham, Allyson Bieryla, Gilbert A. Esquerdo, Perry Berlind, Michael L. Calkins, Jamie Tayar, Lennart Lindegren, Jennifer A. Johnson, Thomas W.-S. Holoien, Katie Auchett, Kevin Covey, Science 366, 637, 2019.