Wednesday, May 16, 2007

Weighing Blackholes

Artists Impression of Black Hole. Credits: NASA

ESA's XMM-Newton has helped to find evidence for the existence of controversial Intermediate Mass Black Holes. Scientists used a new, recently proven method for determining the mass of black holes.

Nikolai Shaposhnikov and Lev Titarchuk, at NASA’s Goddard Space Flight Center (GSFC), have used the technique to determine the mass of the black hole, Cygnus X-1, located in the constellation Cygnus (the Swan) approximately 10 000 light years away in our Galaxy, the Milky Way.

The elegant technique, first suggested by Titarchuk in 1998, shows that Cygnus X-1, part of a binary system, contains 8.7 solar masses, with a margin of error of only 0.8 solar masses. Cygnus X-1 was one of the first compelling black hole candidates to emerge in the early 1970s. The system consists of a blue supergiant and a massive but invisible companion.

Credits: NASA / Honeywell Max-Q Digital Group / Dana Berry

Artist’s impression of a binary system akin to Cygnus X-1. It consists of a blue supergiant star (right) and a black hole. The black hole is surrounded by a gaseous accretion disk that is fed by the star. Some black holes emit jets along the polar axis, as shown here.

The existence of IMBHs is controversial because there is no widely accepted mechanism for how they could form. But they would fill in a huge gap between black holes such as Cygnus X-1 - which form from collapsing massive stars and contain perhaps 5 to 20 solar masses - and the 'monsters' (up to thousand million solar masses) that lurk in the cores of large galaxies.

Titarchuk’s method takes advantage of a relationship between a black hole and its surrounding accretion disk. Gas orbiting in these disks eventually spirals into the black hole. When a black hole’s accretion rate increases to a high level, material piles up near the black hole in a hot region that Titarchuk likens to a traffic jam.

Titarchuk has shown that the distance from the black hole where this congestion occurs scales directly with the mass of the black hole. The more massive the black hole, the farther this congestion occurs and the longer the orbital period.

In his model, hot gas piling up in the congestion region is linked to observations of X-ray intensity variations that repeat on a nearly, but not perfectly, periodic basis. These Quasi-Periodic Oscillations (QPOs) are observed in many black hole systems. The QPOs are accompanied by simple, predictable changes in the system’s spectrum as the surrounding gas heats and cools in response to the changing accretion rate.

New technique for ‘weighing’ black holes ESA Press Release
Echoes from darkness by Louise Riofrio
A Ring of Dark Matter from Centauri Dreams
Hubble finds Ring of Dark Matter? Hubble Press Release

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