20/02/2022
Breaking news. I helped discover a black hole...
From the Royal Astronomical Society of NZ newsletter -
Lone Black Hole Found in the Milky Way
The first detection of a lone black hole in the Milky Way has just been reported. Doubly interesting is that several NZ astronomers, amateur and professional, contributed to the discovery.
Black holes from around three times the mass of the Sun or more have been detected since the 1970s but all of these were orbiting close to another star. They are termed 'stellar-mass' black holes to distinguish them from the monsters at the centres of galaxies. In stellar-mass black holes the gravity of the black hole is pulling gas off the companion star. As the gas spirals into the black hole it is heated by friction as the inner gas circles faster than gas further out. This raises the gas temperature to millions of degrees so it emits x-rays. Thus the first stellar-mass black holes were detected by satellites with x-ray telescopes.
The black holes in binary star systems arise when one of the two stars is big -- nine times the mass of the Sun and bigger -- and explodes as a supernova. The core of the star is crushed to a black hole.
It was always recognised that many (most?) stellar-mass black holes would be made by lone big stars exploding. Since they have no companion star to 'feed' on, they don't make any radiation at all, so are invisible.
This lone black hole was found when it passed in front of a star in 2011. The gravity of the black hole bent the star's light and focused it toward us, a phenomenon called gravitational microlensing. The brightening of the star was detected by telescopes at Mt John and in Chile. From the graph of the brightening and fading of the star (that most of the NZers mentioned contributed to), and follow-up measurements by the Hubble Space Telescope over six years, it was possible to estimate the mass and distance of the black hole. It was found to be around seven times the mass of the Sun and 5000 light years away. It is in the direction of the galactic bulge, the mass of stars around the galactic centre. The follow-up observations showed that the lensing object was dark. Also it was much too massive to be white dwarf or a neutron star.
The observations showed that the black hole was moving at around 45 km/s, faster than surrounding stars. This confirms an idea that collapsing stars can get a little off-centre in the collapse so they get kicked sideways.
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The NZers involved were Phil Yock of Auckland University, Karen Pollard and Michael Albrow of Canterbury University, Tim Nausch and Grant Christie of Auckland Observatory, Jennie McCormick of Farm Cove Observatory in Auckland, John Drummond near Gisborne and Bill Allen near Blenheim. The total author list has ~80 names.
The discovery is written up in
https://www.nature.com/articles/d41586-022-00346-6
A pre-print is at https://arxiv.org/abs/2201.13296
We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t_E ~ 270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2...
