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The large majority of the accreting supermassive black holes in the Universe are obscured by large columns of gas and dust. The location and evolution of this obscuring material have been the subject of intense research in the past decades, and are still highly debated. A decrease in the covering factor of the circumnuclear material with increasing accretion rates has been found by studies carried out across the electromagnetic spectrum. The origin of this trend has been suggested to be driven either by the increase in the inner radius of the obscuring material with incident luminosity due to the sublimation of dust; by the gravitational potential of the black hole; by radiative feedback; or by the interplay between outflows and inflows. However, the lack of a large, unbiased and complete sample of accreting black holes, with reliable information on gas column density, luminosity and mass, has left the main physical mechanism regulating obscuration unclear. Using a systematic multi-wavelength survey of hard X-ray-selected black holes, here we show that radiation pressure on dusty gas is indeed the main physical mechanism regulating the distribution of the circumnuclear material. Our results imply that the bulk of the obscuring dust and gas in these objects is located within the sphere of influence of the black hole (i.e., a few to tens of parsecs), and that it can be swept away even at low radiative output rates. The main physical driver of the differences between obscured and unobscured accreting black holes is therefore their mass-normalized accretion rate., Comment: To appear in the 28 September 2017 issue of Nature. This is the authors' version of the work

Stellar-mass black holes are discovered in X-ray emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses >10 solar masses, which is consistent with the fact that the most massive stellar black holes known so all have masses within 1 sigma of 10 solar masses. Here we report a mass of 15.65 +/- 1.45 solar masses for the black hole in the recently discovered system M33 X-7, which is located in the nearby galaxy Messier 33 (M33) and is the only known black hole that is in an eclipsing binary. In order to produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45 day orbit about its 70.0 +/- 6.9 solar mass companion, there must have been a ``common envelope'' phase of evolution in which a significant amount of mass was lost from the system. We find the common envelope phase could not have occured in M33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars., To appear in Nature October 18, 2007. Four figures (one color figure degraded). Differs slightly from published version. Supplementary Information follows in a separate posting

Intermediate mass black holes play a critical role in understanding the evolutionary connection between stellar mass and super-massive black holes. However, to date the existence of these species of black holes remains ambiguous and their formation process is therefore unknown. It has been long suspected that black holes with masses $10^{2}-10^{4}M_{\odot}$ should form and reside in dense stellar systems. Therefore, dedicated observational campaigns have targeted globular cluster for many decades searching for signatures of these elusive objects. All candidates found in these targeted searches appear radio dim and do not have the X-ray to radio flux ratio predicted by the fundamental plane for accreting black holes. Based on the lack of an electromagnetic counterpart upper limits of $2060 M_{\odot}$ and $470 M_{\odot}$ have been placed on the mass of a putative black hole in 47 Tucanae (NGC 104) from radio and X-ray observations respectively. Here we show there is evidence for a central black hole in 47 Tuc with a mass of M$_{\bullet}\sim2300 M_{\odot}$$_{-850}^{+1500}$ when the dynamical state of the globular cluster is probed with pulsars. The existence of an intermediate mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate and therefore contrary to expectations is gas starved. This intermediate mass black hole might be a member of electromagnetically invisible population of black holes that are the elusive seeds leading to the formation of supermassive black holes in galaxies., Published in Nature, Corrigendum added

Hundreds of stellar-mass black holes likely form in a typical globular star cluster, with all but one predicted to be ejected through dynamical interactions. Some observational support for this idea is provided by the lack of X-ray-emitting binary stars comprising one black hole and one other star ("black-hole/X-ray binaries") in Milky Way globular clusters, even though many neutron-star/X-ray binaries are known. Although a few black holes have been seen in globular clusters around other galaxies, the masses of these cannot be determined, and some may be intermediate-mass black holes that form through exotic mechanisms. Here we report the presence of two flat-spectrum radio sources in the Milky Way globular cluster M22, and we argue that these objects are black holes of stellar mass (each ~ 10-20 times more massive than the Sun) that are accreting matter. We find a high ratio of radio-to-X-ray flux for these black holes, consistent with the larger predicted masses of black holes in globular clusters compared to those outside. The identification of two black holes in one cluster shows that the ejection of black holes is not as efficient as predicted by most models, and we argue that M22 may contain a total population of ~ 5-100 black holes. The large core radius of M22 could arise from heating produced by the black holes., Comment: Nature, in press for October 4 issue

Magnetar's pulsations confirm that the Galaxy's central black hole generates a strong magnetic field.

Many galaxies are thought to have supermassive black holes at their centres-more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +/- 1.5) x 10(6) solar masses (M(*)). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.

The formation of the first massive objects in the infant Universe remains impossible to observe directly and yet it sets the stage for the subsequent evolution of galaxies. While some black holes with masses > billion solar masses? have been detected in luminous quasars less than one billion years after the Big Bang, these individual extreme objects have limited utility in constraining the channels of formation of the earliest black holes. The initial conditions of black hole seed properties are quickly erased during the growth process. From deep, optimally stacked, archival X-ray observations, we measure the amount of black hole growth in z=6-8 galaxies (0.7-1 billion years after the Big Bang). Our results imply that black holes grow in tandem with their hosts throughout cosmic history, starting from the earliest times. We find that most copiously accreting black holes at these epochs are buried in significant amounts of gas and dust that absorb most radiation except for the highest energy X-rays. This suggests that black holes grow significantly more than previously thought during these early bursts, and due to obscuration they do not contribute to the re-ionization of the Universe with their ultraviolet emission., Nature, in press

The standard model of quarks and leptons is used to discuss the signatures of black-hole evaporations. A firm bound on the primordial black hole abundance is obtained from MeV data. It is argued that the MeV bound can be improved by exploiting the new generation of TeV and PeV telescopes.

The detection of two candidate black holes in a dense system of stars in the Milky Way suggests that a larger population of such objects might be lurking in this system. See Letter p.71 The current consensus is that a typical globular star cluster can accommodate just one stellar-mass black hole. Many more will be created, but all but one will be ejected following dynamical interactions. New observations of the Milky Way globular cluster M22, however, reveal the presence of two radio sources in one cluster, with properties typical of accreting black holes each more than ten times the mass of the Sun. This suggests that the ejection of black holes is not as efficient as predicted by most models. And it may not stop there: the authors speculate that there could be a population of tens of black holes in M22, either as single black holes or in binaries where there is no mass transfer taking place.

Collective X-ray emission from distant galaxies reveals a hidden population of supermassive black holes. This finding suggests that galaxies and their central black holes have been coevolving since early cosmic times. See Letter p.356 A study of archival X-ray data from the Chandra X-ray observatory provides a measure of black hole growth at the earliest epochs at which observations are possible, around 600 million years after the Big Bang, at a redshift of z = 6–8. The results imply that black holes grow in tandem with their host galaxies, starting from the earliest times. The most copiously accreting black holes are buried in significant amounts of gas and dust that absorb most radiation except for the highest energy X-rays, and do not contribute to the re-ionization of the Universe.

The means by which supermassive black holes form and grow have remained largely unclear. Numerical simulations show that the collision of massive galaxies can naturally lead to the creation of these objects.

There is only indirect evidence of the existence of black holes. A laboratory demonstration of an X-ray interferometer is a step towards achieving the angular resolution necessary to obtain a direct, X-ray image of a black hole.

The observed growth of galaxies suggests that the black holes thought to lurk at their centres may find each other and merge. A large survey of galaxies has finally netted two black holes in a tight pairing.

IF black holes exist in the contracting phase of a closed universe, they will give rise to a pressure and entropy catastrophe. First, the black holes absorb all the radiation; then their apparent horizons merge, and coalesce with the cosmological apparent horizon. All external observers become internal observers. It is possible that the internal metric of some of the merging black holes will be contracting, and others expanding. I suggest here that the resulting violent inhomogeneities can lead to a reexpansion in a significant portion of the universe. Global reexpansion, prompted by the merging of black holes, may thus begin in a semi-classical rather than fully quantum gravitational era, at densities greater than those at which nucleosynthesis occurs. Surviving black holes and inhomogeneities could initiate the formation of structures such as galaxies in the 'new' universe. The behaviour of such an oscillating universe would differ in detail from cycle to cycle.

Quasars, the oldest known objects in the Universe, are powered by gas falling into black holes at their centres. How black holes formed so early in time has been hard to explain, but a new model might have the answer.

When a star wanders too close to a giant black hole, it can be pulled apart by the black hole's tidal force. One such event offers insight into the properties of both the black hole and the star. Central supermassive black holes in distant galaxies are normally invisible to us, but sometimes their presence becomes evident in the form of flares produced by the tidal disruption of a star being accreted to the black hole. Such events are rare, and often we see only the later stages of the encounter — but here, Gezari et al. report detailed monitoring of an ultraviolet and optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696, which was first seen on 31 May 2010, peaked in July and was over by September. The observed continuum is cooler than expected for a simple accreting debris disk, but the well sampled rise and decline of the light curve follows the predicted mass-accretion rate. The black hole has about two million solar masses and the disrupted star had a helium-rich stellar core, as the authors deduced from the spectroscopic signature of ionized helium from the unbound debris.

Most theoretical discussions of active galactic nuclei (including quasars) attribute their energy production either to an accreting black hole or to a precursor stage—for instance a dense star cluster or a supermassive star—whose inevitable end point is a massive black hole1. We explore here the possibility that some active nuclei may contain two massive black holes in orbit about each other. This hypothesis suggests a new interpretation for the observed bending2 and apparent precession3 of radio jets emerging from these objects and may indeed be verified through detection of the direct consequences of orbital motion.

ONE of the most exciting predictions of the general theory of relativity is that a star of more than about 1.4 M⊙ may undergo gravitational collapse to produce a black hole when it has exhausted its nuclear fuel. It would therefore be a very important confirmation of the theory if such holes could be detected. Unfortunately their very small size (a few km) makes it virtually impossible to observe them directly, but their existence could be inferred from the motion of other objects in their gravitational field. The natural place to look for black holes therefore is in binary star systems in which only one object is observed (single line binaries). Thorne and Trimble1 gave a list of all single line spectroscopic binaries in Batten's2 catalogue in which the mass of the unseen object was estimated to be 1.4 M⊙ or over. In most of these binaries the unseen secondary was less massive than the primary observed star and therefore could have been a normal star which is not observed in comparison with its brighter companion. In a smaller number of cases the secondary was heavier than the primary, but in each of these cases Trimble and Thorne were able to suggest a possible reason why it might not have been observed. They concluded therefore that while some of these single line binaries might contain black holes there was no compelling reason for believing that any of them did.

ANY star which has exhausted all of its nuclear fuel and which cannot eject sufficient matter so that its mass becomes less than roughly two solar masses, must undergo gravitational collapse to become a black hole. Thus one usually associates black holes with masses of stellar proportions. But it is possible that during the initial chaotic conditions of the Universe, black holes of much smaller mass1,2, down to 10−5 g, may have been formed. Here we suggest an experiment by which one may detect or set meaningful upper limits to the number density of black holes of small mass (hereinafter referred to as BHSM).

Gibbons and Hawking1 have suggested that a number of spectroscopic binaries contain black holes. The formation of a black hole in a binary system would be accompanied by a sudden loss of mass from the system, and they believe that this mass loss could convert an initially circular orbit into an appreciably elliptical one. Only masses greater than 1.4 M⊙ can collapse into black holes, and normal double-line spectroscopic binaries cannot contain black holes, so Gibbons and Hawking investigate those single-line spectroscopic binaries listed in the Sixth Catalogue2 for which they estimate that each component has a mass greater than 1.4 M⊙. There are only seven such systems, three of which have orbital eccentricities greater than 0.08. On the other hand, there are 55 double-line systems in which both components are more massive than 1.4 M⊙, and eight of these have orbital eccentricities greater than 0.08. Comparing the relative numbers of eccentric and circular orbits in these two rather small samples, Gibbons and Hawking conclude that there is an excessive number of eccentric orbits among single-line spectroscopic binaries, and that the formation of black holes in some of the systems may have produced this excess. Specifically, they suggest that HD 176318, HD 184035 (201 G Sgr), and HD 194495 may contain black holes.

SINCE Einstein applied his general theory of relativity to study the structure of the universe as a whole1, cosmologists have wondered if the universe is geometrically closed or open. Neither theory nor observation has been able to settle this question unambiguously. Several authors have hoped that the universe may after all be a closed, yet unbounded, system. This would solve many problems regarding the nature and origin of the universe, and would fit many of the observations of distant sources made at radio, optical and other wavelengths2. Here I demonstrate that the universe may not only be a closed structure (as perceived by its inhabitants at the present epoch) but may also be a black hole, confined to a localized region of space which cannot expand without limit.

THE discovery1 of the superheavy elements 116, 124, and 126 raises the question of where these elements are likely to have been formed. The majority of the post-iron-peak nuclei are thought to have been produced in conditions of explosive nucleosynthesis (the r-process), particularly in conventional supernova explosions. The ability of the r-process to produce superheavy elements is, however, very uncertain2. The conditions necessary for superheavy element synthesis (β-decays occurring sufficiently slow that the n γ γ n equilibrium is not disturbed) are difficult to realise in astrophysical situations. The n-process (J. B. Blake and D. N. Schramm, unpublished) requires less extreme conditions (the β decays are important) and may occur more often. The majority of the elements normally attributed to the r-process may have been synthesised in this way. Neutron-induced fission causes both processes to terminate at nuclei with high proton numbers, Z, but the n-process may allow it to reach the higher Z value.