* Astronomy

Scientists have found new evidence that black holes are performing the disappearing acts for which they are known.

A team from MIT and Harvard has found that a certain type of X-ray explosion common on neutron stars is never seen around their black hole cousins, as if the gas that fuels these explosions has vanished into a void.
This is strong evidence, the team said, for the existence of a theoretical border around a black hole called an event horizon, a point from beyond which nothing, not even light, can escape.

Ron Remillard of the MIT Kavli Institute in Cambridge, Mass., led the analysis and is discussing his team's result today at a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C. His colleagues are Dacheng Lin of MIT and Randall Cooper and Ramesh Narayan of the Harvard-Smithsonian Centre for Astrophysics in Cambridge.
The scientists studied a complete sample of transient X-ray sources detected with NASA's Rossi X-ray Timing Explorer during the last nine years. They detected 135 X-ray bursts from the 13 sources believed to be neutron stars, but none from the 18 suspected black holes.
Gas released by a nearby star can accumulate on the hard surface of a neutron star, and it will eventually erupt in a thermonuclear explosion. The more massive compact objects in this study suspected of being black holes appeared to have no surface. Gas falling toward the black hole seems to disappear.

"Event horizons are invisible by definition, so it seems impossible to prove their existence. Yet by looking at dense objects that pull in gas, we can infer whether that gas crashes and accumulates onto a hard surface or just quietly vanishes. For the group of suspected black holes we studied, there is a complete absence of surface explosions called X-ray bursts" - Ron Remillard

A black hole forms when a very massive star runs out of fuel. Without energy to support its mass, the star implodes. If the star is more than 25 times more massive than our sun, the core will collapse to a point of infinite density with no surface. Within a boundary of about 50 miles from the black hole centre, gravity is so strong that not even light can escape its pull. This boundary is the theoretical event horizon.
Stars of about 10 to 25 solar masses will collapse into compact spheres about 10 miles across, called neutron stars. These objects have a hard surface and no event horizon.

Black holes and their neutron star cousins are sometimes located in binary systems, orbiting a relatively normal star companion. Gas from these stars, lured by strong gravity, can flow toward the compact object periodically. This process, called accretion, releases large amounts of energy, predominantly in the form of X-rays.
Gas can accumulate on a neutron star surface, and when conditions are ripe, the gas will ignite in a thermonuclear explosion that is visible as a one-minute event called a Type I X-ray burst. The suspected black holes -- that is, the more massive types of compact objects in this study -- behave as if they have no surface and are located behind event horizons.
The idea of using the absence of X-ray bursts to confirm the presence of event horizons in black holes was proposed in 2002 by Harvard's Narayan and Jeremy Heyl of the University of British Columbia in Vancouver.

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A research group at Cambridge think that the universe might once have been packed full of tiny black holes. Dr Martin Haehnelt, a researcher in the group led by Astronomer Royal Martin Rees, will present new evidence to support this controversial idea at the Institute of Physics conference Physics 2005 in Warwick.

Most cosmologists believe that supermassive black holes grew up in big galaxies, accumulating mass as time went on. But there is increasing evidence for a different view - that small black holes grew independently and merged to produce the giants which exist today.

Haehnelt points to evidence from recent studies of the cosmic microwave background (CMB).
This radiation, sometimes called `the echo of the big bang` has been travelling unaltered through space since the universe was just 400,000 years old. At that moment the universe cooled through a critical point, letting CMB radiation travel freely for the first time - as though a cosmic fog had lifted.

But new evidence shows that 10 to 15 percent of this radiation has been scattered since then. This indicates a re-warming of the universe which nobody had expected.
This could indicate an era in which small black holes were commonplace.

"Matter accreting around a black hole heats up. and this heating could be a sign that small black holes were widespread in the Universe at that time"- Dr Martin Haehnelt.

If small black holes merged to form the supermassive variety found at the centres of galaxies, there could be telltale evidence. Such a merger begins with two black holes going into orbit around each other, spiralling ever closer together. In the cataclysmic blast of energy when they finally merge, any asymmetry can send the resulting black hole flying off into space.

"If this happened, we might find the occasional galaxy with its central supermassive black hole missing" - Dr Martin Haehnelt
The evidence is by no means conclusive. Until it is, the CMB results will remain a source of heated debate.

Dr Martin Haehnelt is a Reader in Cosmology and Astrophysics at the Institute of Astronomy in the University of Cambridge.

source

Title: Black Holes in Astrophysics
Authors: Ramesh Narayan

Comments: To appear in a forthcoming Special Focus Issue on "Spacetime 100 Years Later" published by the New Journal of Physics



This article reviews the current status of black hole astrophysics, focusing on topics of interest to a physics audience.
Astronomers have discovered dozens of compact objects with masses greater than 3 solar masses, the likely maximum mass of a neutron star.
These objects are identified as black hole candidates. Some of the candidates have masses of 5 to 20 solar masses and are found in X-ray binaries, while the rest have masses from a million to a billion solar masses and are found in galactic nuclei.
A variety of methods are being tried to estimate the spin parameters of the candidate black holes. There is strong circumstantial evidence that many of the objects have event horizons.
Recent MHD simulations of magnetized plasma accreting on rotating black holes seem to hint that relativistic jets may be produced by a magnetic analogue of the Penrose process.

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