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TOPIC: Black Holes
Blobrana


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Runaway Black Hole
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A runaway black hole barrelling out of a galaxy at more than two billion miles per hour? Evidence of a quick getaway in the aftermath of a massive intergalactic collision? Thats just what astronomers Erin Bonning of the Paris Observatory and Gregory Shields and Sarah Salviander of The University of Texas at Austin have been searching for. They are presenting the results of their search for supermassive speed demons today at the 210th meeting of the American Astronomical Society in Honolulu, Hawaii.

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Blobrana


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RE: Black Holes
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Many galaxies are fitted with "seat belts" that keep their colossal black holes from flying out into space after violent collisions, a new analysis suggests.
The work may explain why astronomers have so far failed to observe any black holes being hurled out of their host galaxies a phenomenon predicted by gravitational theory.
When two galaxies collide, the supermassive black holes at their cores are thought to merge eventually. According to the general theory of relativity, this clash of black holes should release a powerful burst of gravitational waves, shaking the fabric of space-time. And if the burst is lopsided, it will also give the newly merged black hole a kick, like the recoil from a gun.
Detailed computer simulations performed in the last few months have shown that one in ten of these gravitational-wave bursts should kick the newly merged black hole up to speeds of more than 1000 kilometres per second fast enough to evict the black holes from their galactic homes forever.

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Blobrana


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When two black holes merge, they make waves--powerful gravitational waves. And if the merger spews these waves preferentially in one direction, it can shoot the merged hole the other way at high speed. In the 1 June Physical Review Letters, two teams use computer simulations to show that the recoil speed for a pair of "supermassive" black holes could reach 4000 kilometres per second--ten times faster than previous estimates and fast enough to escape from even the biggest galaxy. Such a "black hole rocket" would be nearly invisible, but in some cases, according to a study in an upcoming issue of Physical Review Letters, it might be observable if it brings enough material from the galaxy along with it.
Astrophysicists believe there is a supermassive black hole at the core of almost every galaxy, and when a pair of galaxies collides, their central black holes merge, too. The gravitational radiation emitted by the merger is usually in a preferred direction, giving the merged hole a "kick" in the opposite direction. Since the 1960s, astrophysicists have attempted to calculate the velocity of this recoil, but the complexity of the equations for general relativity has made it difficult to model spinning black holes or to deal with the extreme space-time distortions that appear during the final plunge. Computational techniques developed during the past two years allow much more precise simulations, and astrophysicists are now scrambling to calculate how fast the recoil could be.

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Blobrana


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Rogue black holes
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Astronomers are hunting an elusive target: rogue black holes that have been ejected from the centres of their home galaxies. Some doubted that the quarry could be spotted, since a black hole must be gobbling matter from an accretion disk in order for that matter to shine. And if a black hole is ripped from the core of its home galaxy and sent hurling into the outskirts, the thinking goes, then its accretion disk might be left behind.
New calculations by theorist Avi Loeb (Harvard-Smithsonian Centre for Astrophysics) give black hole hunters a reason to hope. Loeb showed that, generically, a black hole ejected from the centre of a galaxy could bring its accretion disk along for the ride and remain visible for millions of years.

"Matter in the disk is swirling around the black hole much faster than the typical black-hole ejection speed. That matter is so tightly bound that it follows the black hole like a herd of sheep around a shepherd" - Avi Loeb .

In the scenario examined by Loeb, two galaxies collide and merge. The spinning, supermassive black holes at the core of each galaxy coalesce, emitting powerful gravitational radiation in a preferred direction. Computer simulations recently demonstrated that the net momentum carried by the radiation gives the remnant black hole a large kick in the opposite direction. The black hole recoils at speeds of up to ten million miles per hour -- fast enough to traverse an entire galaxy in a cosmically short time of only ten million years.

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Blobrana


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RE: Black Holes
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How to survive in a black hole
There's no escape, but how can you maximize your remaining time?
So there you are: you discover that your spaceship has inadvertently slipped across the event horizon of a black hole the boundary beyond which nothing, not even light, can escape the hole's fearsome gravity. The only question is how you can maximize the time you have left. What do you do?

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Blobrana


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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 NASAs Goddard Space Flight Centre (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.
Alternative techniques have previously suggested that the invisible object was a black hole of about 10 solar masses.

"This agreement gives us a lot of confidence that our method works" - Nikolai Shaposhnikov.

"It can help determine a black holes mass when alternative techniques fail" - Lev Titarchuk.

Working independently from Shaposhnikov and Titarchuk, Tod Strohmayer and Richard Mushotzky, also from GSFC, and four colleagues, used Titarchuks technique on XMM data and stumbled upon an Intermediate Mass Black Hole (IMBH)- the existence of which is in theory controversial.
They estimated that an ultraluminous X-ray source in the nearby galaxy, NGC 5408, harbours a black hole with a mass of about 2 000 Suns.

This is one of the best indications to date for an IMBH - Nikolai Shaposhnikov.

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.

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Blobrana


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There's a supermassive black hole (up to a billion times the mass of a usual black hole) in the center of every galaxy.


Credit BBC

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Blobrana


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NGC4051
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 Previous observations had only revealed the average properties of the escaping gas. XMM-Newton has the special ability to watch a single celestial object with several instruments at the same time. With this, the team collected more detailed information about variations in the gas brightness and ionisation state.
The team also saw that the gas was escaping from much closer to the black hole than previously thought. They could determine the fraction of gas that was escaping.

We calculate that between 25 percent of the accreting material is flowing back out - Fabrizio Nicastro, Harvard-Smithsonian Centre for Astrophysics.

 This was less than some astronomers had expected.

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Blobrana


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Warm gas escaping from the clutches of enormous black holes could be the key to a form of intergalactic pollution that made life possible, according to new results from ESAs XMM-Newton space observatory, published today. Black holes are not quite the all-consuming monsters depicted in popular culture.
Until gas crosses the boundary of the black hole known as the Event Horizon, it can escape if heated sufficiently. For decades now, astronomers have watched warm gas from the mightiest black holes flowing away at speeds of 1000-2000 km/s and wondered just how much gas escapes this way. XMM-Newton has now made the most accurate measurements yet of the process.
The international team of astronomers, led by Yair Krongold, Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, targeted a black hole two million times more massive than the Sun at the centre of the active galaxy NGC 4051.

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Blobrana


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Hawking temperatures
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Title: Can Hawking temperatures be negative ?
Authors: Mu-In Park
(revised v3)

It has been widely believed that the Hawking temperature for a black hole is uniquely determined by its metric and positive. But, I find that this does ''not'' seem to be true in the recently discovered black holes which include the exotic black holes and the black holes in the three-dimensional higher curvature gravities. I show that the Hawking temperatures, which are measured by the quantum fields in thermal equilibrium with the black holes, are not the usual Hawking temperature but the new temperatures that have been proposed recently and can be negative. The associated new entropy formulae, which are defined by the first law of thermodynamics, versus the black hole masses show some genuine effects of the black holes which do not occur in the spin systems. Some cosmological implications and physical origin of the discrepancy with the standard analysis are noted also.

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