Archive for April 9, 2019

The Shadow of an Event Horizon

Posted in The Universe and Stuff with tags , , , , , on April 9, 2019 by telescoper

There is a paper on the arXiv written about 5 years ago called Towards the event horizon – the supermassive black hole in the Galactic Center by Falcke and Markoff, the abstract of which reads:

The center of our Galaxy hosts the best constrained supermassive black hole in the universe, Sagittarius A* (Sgr A*). Its mass and distance have been accurately determined from stellar orbits and proper motion studies, respectively, and its high-frequency radio, and highly variable near-infrared and X-ray emission originate from within a few Schwarzschild radii of the event horizon. The theory of general relativity (GR) predicts the appearance of a black hole shadow, which is a lensed image of the event horizon. This shadow can be resolved by very long baseline radio interferometry and test basic predictions of GR and alternatives thereof. In this paper we review our current understanding of the physical properties of Sgr A*, with a particular emphasis on the radio properties, the black hole shadow, and models for the emission and appearance of the source. We argue that the Galactic Center holds enormous potential for experimental tests of black hole accretion and theories of gravitation in their strong limits.

Please note that the black hole in the centre of the giant elliptical galaxy M87 is about 1000 times further away from us than the black hole in the centre of the Milky Way but is also about 1000 times more massive, so its Schwarzschild radius is 1000 times larger. The observational challenge of imaging the event horizon is therefore similar in the two cases.

You may find this useful if, by sheer coincidence, there is some big announcement tomorrow..

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Gravitational Wave Flash!

Posted in The Universe and Stuff with tags , , , , on April 9, 2019 by telescoper

The third observing run for Advanced LIGO – O3 – started on April 1 2019, after 19 months upgrading the detectors. Last night, April 8, saw the first new detection of a candidate gravitational wave source, apparently another black hole binary, dubbed S190408an.

It is anticipated that sources like this will be discovered at a rate of roughly one per week for the (planned) year-long run. Given the likely rate of events the policy of LIGO is now to make data publicly available directly without writing papers first. You can find the data entry for this event here, including this map of its position.

Whether the LIGO Scientific Collaboration will release sufficient data for others to perform a full analysis of the signal remains to be seen, but if the predicted detection rate matches reality, the field is going to move very rapidly from studies of individual events to statistical analysis of large populations. Such is the way of science!