Intermediate-mass black hole

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An Intermediate-mass black hole (IMBH) is a black hole whose mass is significantly more than stellar black holes (a few tens of the mass of the Sun) yet far less than supermassive black holes (a few millions of the mass of the Sun).

There is less evidence for their existence than for the other two types. Some ultra-luminous X ray sources (ULXs) in nearby galaxies are suspected to be IMBHs, with masses of a hundred to a thousand solar masses. The ULXs are observed in star forming regions (e.g., in starburst galaxy M82^[1], see external links for beautiful pictures of this galaxy), and are seemingly associated with young star clusters which are also observed in these regions. However only a dynamical mass measurement from the analysis of the optical spectrum of the companion star can unveil the presence of an IMBH as the compact accretor of the ULX.

Additional evidence for the existence of IMBHs can be obtained from observation of gravitational radiation, emitted by the compact remnant that orbits the IMBH (e.g. [1]).

However it is not clear how such a black hole would form. On the one hand, they are too massive to be formed by the collapse of a single star, which is how the stellar black holes are thought to form. On the other hand, their environments lack the extreme conditions—i.e., high density and velocities observed at the centers of galaxies—which seemingly lead to the formation of supermassive black holes. There are two popular formation scenarios for IMBHs. The first, is the merging of stellar mass black holes and other compact objects by means of gravitational radiation. The second one is the runaway collision of massive stars in dense stellar clusters and the collapse of the collision product into an IMBH.

In November 2004 a team of astronomers reported the discovery of the first intermediate-mass black hole in our galaxy, orbiting three light-years from Sagittarius A*. This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars. However, recently, a German research group claimed that the presence of an IMBH near the galactic center is doubtful [2]. This conclusion is based on a dynamical study of a small star cluster in which should reside the suspected intermediate mass black hole. The debate on the real existence of intermediate mass black holes is still open.

More recently, in January 2006 a team led by Prof. Philip Kaaret of the University of Iowa, Iowa City announced the discovery of a quasiperiodic oscillation from an intermediate-mass black hole candidate located using NASA's Rossi X-ray Timing Explorer. The candidate, M82 X-1, is orbited by a red giant star that's losing its atmosphere into it. [3] Neither the existence of the oscillation nor its interpretation as the orbital period of the system are fully accepted by the rest of the scientific community. While the interpretation is quite reasonable, the periodicity is claimed based on only about 4 cycles, meaning that it is quite possible for this to be random variation. If the period is real, it could be either the orbital period, as suggested, or a super-orbital period in the accretion disk, as is seen in many other systems. == See also ==

• Black hole, a general survey

Classification by type:

• Schwarzschild, or still, black hole
• Kerr, or spinning, black hole
• Kerr-Newman and Reissner-Nordström, or charged then spinning, black holes

Classification by mass:

• Micro black hole and extra-dimensional black hole
• Primordial black hole, a hypothetical leftover of the Big Bang
• Stellar black hole, which could either be a static black hole or a rotating black hole
• Intermediate-mass black hole
• Supermassive black hole, which could also either be a static black hole or a rotating black hole

[edit] External links

• Chandra images of starburst galaxy M82
• NASA press release for discovery of IMBHs by Hubble Space Telescope

• A New Breed of Black Holes, by Davide Castelvecchi Sky & Telescope April 2006

[edit] References