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Finding Hidden Black Holes

The super-massive black holes thought to lurk in nearby galaxies present us with a problem. They should suck in surrounding gas and dust to produce x-rays, and it has been the assumption that black holes hidden by such materials, also known as ‘Compton-thick objects,’ are responsible for much of the overall x-ray background. Yet an x-ray census using data from Integral, ESA’s orbiting International Gamma Ray Astrophysics Laboratory, showed that a mere 15 percent of black hole galaxies detected were of the hidden Compton-thick variety.

And later work at NASA (GSFC) and the Integral Science Data Centre (Geneva), using two years of Integral data, shows an even smaller fraction. So where do the x-rays come from? “Naturally, it is difficult to find something we know is hiding well and which has eluded detection so far,” says Volker Beckmann (NASA GSFC, and lead author of an upcoming paper on the subject). “Integral is a telescope that should see nearby hidden black holes, but we have come up short.”

A hidden black hole

Image: This artist’s impression shows the thick dust torus that astronomers believe surrounds many supermassive black holes and their accretion discs. When the torus is seen edge-on as in this case, much of the light emitted by the accretion disc is blocked, creating a “hidden” black hole. Credit: ESA / V. Beckmann (NASA-GSFC).

The implication is that if hidden black holes really are the source of most of the x-ray background radiation, they must be located in the distant universe. That could be the outcome if the super-massive black holes near us have all had time to consume the gas and dust that once surrounded them, leaving them with little material to feast on. After all, the x-rays used in these observations are produced by the heating of such material as it falls into the black hole, and a lack of gas and dust would make the black hole much harder to detect.

Larger surveys are ahead, hoping to track the evolution of black holes going back into earlier epochs. Such surveys (one is planned using data from the Swift spacecraft) may show whether this revised theory is viable, or whether nearby black holes are simply hidden much more deeply than previously believed.

You can find the Integral survey results in Bassani et al., “Integral IBIS Extragalactic survey: Active Galactic Nuclei Selected at 20-100 keV,” The Astrophysical Journal 636 (10 January 2006), pp. L65-L68. Also see Beckmann et al., “The Hard X-ray 20-40keV AGN Luminosity Function,” scheduled for The Astrophysical Journal (pre-print available here).

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  • ljk February 1, 2007, 15:26

    What if black holes are actually gravastars…

    General Relativity and Quantum Cosmology

    abstract gr-qc/0701154

    From: Avery Broderick [view email]

    Date: Mon, 29 Jan 2007 03:04:28 GMT (64kb)

    Where are all the gravastars? Limits upon the gravastar model from accreting black holes

    Authors: Avery E. Broderick (1), Ramesh Narayan (1) ((1) Harvard-Smithsonian Center for Astrophysics)

    Comments: 11 pages, 4 figures

    Journal-ref: Classical and Quantum Gravity, Volume 24, Issue 3, pp. 659-666 (2007)

    The gravastar model, which postulates a strongly correlated thin shell of anisotropic matter surrounding a region of anti-de Sitter space, has been proposed as an alternative to black holes. We discuss constraints that present-day observations of well-known black hole candidates place on this model. We focus upon two black hole candidates known to have extraordinarily low luminosities: the supermassive black hole in the Galactic Center, Sagittarius A*, and the stellar-mass black hole, XTE J1118+480. We find that the length scale for modifications of the type discussed in Chapline et al. (2003) must be sub-Planckian.


  • ljk June 19, 2007, 11:05

    Singular Sources of Energy in Stars and Planets

    Authors: B.E. Zhilyaev

    (Submitted on 17 Jun 2007)

    Abstract: If primordial low-mass black holes (PBH) exist in the Universe than many of stars and planetary bodies appear to be infected by them. This is also true in regard to the Sun and likely Jupiter and Saturn. The availability of even the very low-mass inner relativistic reactor may lead to essential changes in evolution scenario of a celestial body on its lifetime scale. Black holes in stellar interior may be found either in consequence of captures process or incorporation during the formation of a star from interstellar clouds. Surprisingly that in the equilibrium state a PBH growth is a long-lived process with e-folding rise time of billion years. One can envision a PBH orbiting inside the Sun. Our considerations showed that the PBH experiences very little friction in passing through the stellar matter. If the BH mass is above 10^{-5}M_{sun} the major contribution to the luminosity comes from the relativistic gravitational reactor. In such a case a star evolves towards the Eddington limit. This should lead to considerable expansion of a star and a global stability loss.

    Microscopic PBHs can exist in the interior of planetary bodies too. To produce the required excess of thermal energy on Jupiter and Saturn the masses of PBH captured are assumed to be reached of 4 10^{19} and 7 10^{18} g, respectively. These microscopic objects are comparable to the hydrogen atom in size. One can envision even a planet with the PBH acting as the self-sufficient source of heating. Such a planet does not need a sun to maintain animal life on its surface. This may last eons.

    Comments: 7 pages

    Subjects: Astrophysics (astro-ph)

    Journal reference: KFNTS 4 (2003) 211-216

    Cite as: arXiv:0706.2504v1 [astro-ph]

    Submission history

    From: Boris Zhilyaev E [view email]

    [v1] Sun, 17 Jun 2007 20:12:07 GMT (10kb)


  • ljk June 26, 2007, 15:17

    How to tell a gravastar from a black hole

    Authors: Cecilia B. M. H. Chirenti, Luciano Rezzolla

    (Submitted on 11 Jun 2007 (v1), last revised 22 Jun 2007 (this version, v2))

    Abstract: Gravastars have been recently proposed as potential alternatives to explain the astrophysical phenomenology traditionally associated to black holes, raising the question of whether the two objects can be distinguished at all. Leaving aside the debate about the processes that would lead to the formation of a gravastar and the astronomical evidence in their support, we here address two basic questions: Is a gravastar stable against generic perturbations? If stable, can an observer distinguish it from a black hole of the same mass?

    To answer these questions we construct a general class of gravastars and determine the conditions they must satisfy in order to exist as equilibrium solutions of the Einstein equations. For such models we perform a systematic stability analysis against axial-perturbations, computing the real and imaginary parts of the eigenfrequencies. Overall, we find that gravastars are stable to axial perturbations, but also that their quasi-normal modes differ from those of a black hole of the same mass and thus can be used to discern, beyond dispute, a gravastar from a black hole.

    Comments: 16 pages, 13 figures, minor improvement

    Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph)

    Cite as: arXiv:0706.1513v2 [gr-qc]

    Submission history

    From: Cecilia Chirenti [view email]

    [v1] Mon, 11 Jun 2007 16:35:46 GMT (78kb)

    [v2] Fri, 22 Jun 2007 17:43:53 GMT (78kb)


  • oscar November 26, 2007, 20:31

    how are black holes formed

  • Chris Kelley December 15, 2007, 19:44

    When a star with a mass greater than the Chandrasekhar limit goes supernova, the remnant collapses under it’s own gravity. Pretty much anyway.

  • ljk February 5, 2008, 10:34

    Passage of small black hole through the Earth. Is it detectable?

    Authors: I.B. Khriplovich, A.A. Pomeransky, N. Produit, G. Yu. Ruban

    (Submitted on 30 Jan 2008)

    Abstract: We examine the energy losses of a small black hole passing through the Earth, and in particular, the excitations created in the frequency range accessible to modern acoustic detectors. The dominating contributions to the effect are due to the coherent sound radiation of the Cherenkov type and to the conversion of black hole radiation into sound waves.

    Comments: Concise version of arXiv:0710.3438 with technical calculations omitted; discussion of possible underwater detection added

    Subjects: High Energy Physics – Experiment (hep-ex); Astrophysics (astro-ph); Geophysics (physics.geo-ph)

    Cite as: arXiv:0801.4623v1 [hep-ex]

    Submission history

    From: Andrei Pomeransky [view email]

    [v1] Wed, 30 Jan 2008 09:04:10 GMT (6kb)


  • vishank September 28, 2008, 9:17

    i have a question!!!

    what happens if any black hole just passes far near our solar system?