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Quasistars
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'Quasistars' may harbour hidden black holes
The biggest black holes in the universe might have grown within the bellies of giant stars, a new study suggests. If these hole-bearing "quasistars" exist, then they might be bright enough to see from across the universe.
Quasistars are one attempt to explain the existence of supermassive black holes, which astronomers have detected at the hearts of most large galaxies, and whose origin is still unknown.
Smaller black holes are easier to account for a massive star's core can sometimes collapse into a black hole with around 10 times the mass of the Sun. But their big brothers can be a billion times as massive.
It is possible that the smaller siblings can grow that big by eating stars and gas or by colliding with each other and merging. But they would have to grow up very quickly in cosmic terms, because some supermassive black holes were already around just a few hundred million years after the big bang.
Mitchell Begelman and colleagues at the University of Colorado in Boulder, US, have worked out how the big holes might have gotten a head start in life.

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RE: blackhole spectra
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A University of Utah scientist has helped answer one of the great astrophysics questions of the last 100 years.
Miguel Mostafa, an assistant professor of physics, was among hundreds of collaborating scientists who discovered that ultra high energy cosmic rays, the most energetic particles in the universe, likely come from black holes.

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For decades, large populations of active black holes have been considered missing.
These supermassive black holes produce highly energetic structures, called quasars, which consist of doughnut-shaped clouds of gas and dust that surround and feed the budding black holes.

 The findings are the first direct evidence that most, if not all, massive galaxies in the distant universe spend their youths building monstrous black holes at their cores - Co-author David Alexander, in the Department of Physics at Durham University.

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Astronomers have unmasked hundreds of black holes hiding deep inside dusty galaxies billions of light-years away.
The massive, growing black holes, discovered by NASA's Spitzer and Chandra space telescopes, represent a large fraction of a long-sought missing population. Their discovery implies there are hundreds of millions of additional black holes growing in our young universe, more than doubling the total amount known at that distance.

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Black Holes in Feeding Frenzy
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Two UH astronomers using the Hubble Space Telescope believe they have identified what makes at least some quasars shine: the black hole at the centre of a massive galaxy with little gas of its own is gobbling up material from a colliding gas-rich galaxy. The merging of two galaxies has long been thought to be an efficient way of driving gas deeply into a galaxy to feed the central black hole, but there was only indirect evidence for such a mechanism until now.

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Supermassive black holes
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Frenetic growth of supermassive black holes in the early universe

Linhua Jiang (Steward Observatory, University of Arizona) has led a United States/Germany team of astronomers in the study of some of the most distant and youngest known quasars. The six distant quasars they observed are at redshifts ranging between z = 5.8 to 6.3 and correspond to a period when the universe was only about one billion years old. Using the Gemini Near Infrared Spectrograph (GNIRS) at Gemini South and the Near-Infrared Imager (NIRI) at Gemini North, the team found these very young quasars to be already super-enriched in heavy elements. The quasars are also powered by extremely massive black holes. The Jiang et al. results cast a new light on the assembly of black holes and the chemical enrichment of the universe less than one billion years after the Big Bang.

early universe_4

Quasars are the most luminous and most energetic objects in the universe. The stellar-like objects are surmised to be powered by radiation from matter accreting onto supermassive black holes at the centre of host galaxies in the process of forming. Dense gas in the region surrounding the black hole moves at high velocities. It gives rise to a broad line region (BLR) in spectra that can be used as a diagnostic of several properties of the gas itself and of the central black hole.

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GRO J1655-40
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Title: Low/Hard State Spectra of GRO J1655-40 Observed with Suzaku
Authors: Hiromitsu Takahashi, Yasushi Fukazawa, Tsunefumi Mizuno, Ayumi Hirasawa, Shunji Kitamoto, Keisuke Sudoh, Takayuki Ogita, Aya Kubota, Kazuo Makishima, Takeshi Itoh, Arvind N. Parmar, Ken Ebisawa, Sachindra Naik, Tadayasu Dotani, Motohide Kokubun, Kousuke Ohnuki, Tadayuki Takahashi, Tahir Yaqoob, Lorella Angelini, Yoshihiro Ueda, Kazutaka Yamaoka, Taro Kotani, Nobuyuki Kawai, Masaaki Namiki, Takayoshi Kohmura, Hitoshi Negoro

The Galactic black-hole binary GRO J1655-40 was observed with Suzaku on 2005 September 22--23, for a net exposure of 35 ks with the X-ray Imaging Spectrometer (XIS) and 20 ks with the Hard X-ray Detector (HXD). The source was detected over a broad and continuous energy range of 0.7--300 keV, with an intensity of ~50 mCrab at 20 keV. At a distance of 3.2 kpc, the 0.7--300 keV luminosity is  ~ 5.1 x 10^{36} erg s^{-1} (~ 0.7 % of the Eddington luminosity for a 6  solar mass black hole). The source was in a typical low/hard state, exhibiting a power-law shaped continuum with a photon index of  ~ 1.6. During the observation, the source intensity gradually decreased by 25% at energies above  ~ 3 keV, and by 35% below 2 keV. This, together with the soft X-ray spectra taken with the XIS, suggests the presence of an independent soft component that can be represented by emission from a cool (~ 0.2 keV) disk. The hard X-ray spectra obtained with the HXD reveal a high-energy spectral cutoff, with an e-folding energy of  ~ 200 keV. Since the spectral photon index above 10 keV is harder by  ~ 0.4 than that observed in the softer energy band, and the e-folding energy is higher than those of typical reflection humps, the entire 0.7--300 keV spectrum cannot be reproduced by a single thermal Comptonisation model, even considering reflection effects. Instead, the spectrum (except the soft excess) can be successfully explained by invoking two thermal-Comptonisation components with different y-parameters. In contrast to the high/soft state spectra of this object in which narrow iron absorption lines are detected with equivalent widths of 60--100 eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 25 eV.

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Supermassive black holes
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Recent calculations indicate that when two galaxies, and the supermassive black holes that lie at their centres, merge, these galactic `marriages` frequently produce gravitational forces strong enough to kick the new combined black hole right out of its merged galaxy. However, so far, none of the many `empty nest` galaxies predicted by such calculations have been found.
Now researchers at the University of Maryland say merged black holes probably are kicked out far less often than predicted because torques from the vast accretion disk of parental galactic material that spins around and feeds merging black holes act to align their spins in a way that reduces the kick force.
In findings presented today during a press briefing at the American Astronomical Society's meeting in Honolulu Hawaii, Maryland astronomers Tamara Bogdanovic and Christopher Reynolds propose that in the majority of gas-rich galactic mergers, torques from gas accretion align the spins of supermassive black holes and their orbital axis with a large-scale gas disk. This mechanism, they say, helps explain the ubiquity of black holes at the centre of galaxies despite the potentially large kicks from gravitational radiation recoil.

While we expect a black hole ejection to be uncommon in the aftermath of gas-rich mergers, it is still possible that it may happen, especially in merging galaxies that are relatively gas-poor. Future observations of such gas-poor mergers may point to a class of massive galaxies without a central supermassive black hole - Tamara Bogdanovic .

What we have uncovered here is a remarkable interaction between the galactic scale gas disk and the comparatively tiny black holes - Christopher Reynolds .

It has recently been shown using relativistic calculations that when galaxies -- and the supermassive black holes at their centre -- merge, a large pulse of gravitational radiation is produced. This radiation pulse, essentially ripples in the fabric of space itself, can kick the final black hole up to speeds of 3000 km/s. But these large kicks require the black holes to be rapidly spinning, and for the spin axes to be oppositely directed and tilted over into the plane of the orbit. The difficulty this poses, say Bogdanovic and Reynolds is that the escape speed from most galaxies is less that 1000 km/s. Therefore, if large recoil speeds are typical, one might expect that many galaxies that have undergone major mergers would be without a black hole.
Current estimates are that over the past 6 billion years, about a half of all galaxies have undergone mergers. And yet, in contradiction to what the models predict, all observed galaxies with bulges indicative of mergers appear to have central supermassive black holes.

 It therefore seems that there is an astrophysical avoidance of the types of supermassive black hole coalescences that would lead to kicks beyond galactic escape speeds, write Bogdanovic, Reynolds and fellow Maryland astronomer Coleman Miller in a paper (to be published in the June 1 2007 issue of Astrophysical Journal Letters) explaining their proposal.

The lower than expected kicks can be explained if (1) the spins of merging black holes are all small, (2) there is a large difference in mass between most merging black holes or (3) the spins tend to align with each other and be oriented perpendicular to the orbit of the pair of black holes.
Observations do not favour the low spin mechanism and evidence suggests strongly that coalescence of comparable-mass black holes (which would enable high kick out speeds) should be common.

The most likely solution therefore seems to be that astrophysical processes tend to align the spins of supermassive black holes with the orbital axis, write Bogdanovic, Reynolds and Miller.

This work was supported by the University of Maryland, College Park -Astronomy Centre for Theory and Computation Prize Fellowship program and by funding from the National Science Foundation.

Source University of Maryland

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Black holes' weight
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We know they are heavy, but how exactly do you weigh a black hole? The usual way to estimate the mass of black holes orbiting stars is to look at how the star wobbles under the influence of the black hole's motion. But this method is imprecise, says Nikolai Shaposhnikov, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Instead, Shaposhnikov and Lev Titarchuk, at George Mason University in Fairfax, Virginia, thought about the disc of material ripped from the companion star that forms around such black holes. As more matter falls in, the inner part of the disc becomes congested, "like when five lines of traffic have to merge into one", Shaposhnikov says. This clogged material oscillates, producing X-ray pulses at a rate that is related to the black hole's mass.
The pair measured such pulses using NASA's Rossi X-ray Timing Explorer (RXTE) satellite and found the black hole Cygnus X-1 has 8.7 times the mass of our sun - reducing the estimated error of previous measurements tenfold.

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