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Tag Archives: supermassive black hole

In Chapter 15 (the opening photo, p. 382, and Figure 15-15d, p. 395), we discuss the prospective effect of a gas cloud called G2 (“G” for “gas”) that was heading for the center of the Milky Way, perhaps dropping material in to the supermassive black hole known as Sagittarius A* (pronounced A-star) and causing it to flare brightly in x-rays and radio waves, at least. But the prediction for its closest approach is about now, mid-2014, and no brightening has apparently happened. It is still possible that there could be dramatic flaring in the future, but that could be years or decades off.

Credit: ESO

Credit: ESO

Scientists at the Max-Planck Institute for Extraterrestrial Physics in Germany base it on their observations with the European Space Agency’s Very Large Telescope. They suggest that “G2 may be a bright knot in a much more extensive gas streamer.”

Daryl Haggard, who has recently moved to Amherst College from Northwestern University, is lead author of a report of Chandra X-ray Observatory monitoring of “Sgr A*/G2” through six observations in the first half of 2014, including the predicted time of the closest encounter.

These articles describing the situation is available free online, and the main results are discussed by correspondent Ron Cowen in The New York Times for July 22, 2014.

Links: NY Times article by Cowan; the original ApJ article by Oliver Pfuhl, Stefan Gillessen, and a dozen others; Daryl Haggard’s report, from The Astronomer’s Telegram.

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A recent article in Nature describes how a supermassive black hole’s spin has been measured via the gravitational lens of a foreground galaxy, that fortuitously lies along the same line-of-sight. The new measurements have enabled astronomers to find that a supermassive black hole powering a distant quasar has grown through coherent, rather than random, episodes of mass accretion (see Section 17.3, p. 458-459). The following text is a digest of Guido Risalti’s summary in Nature‘s ‘News and views’ section, March 13, 2014.

Supermassive black holes are simple systems. They are characterized by just two quantities, their mass and their spin. Whereas the total amount of accretion and any mergers that a supermassive black hole undergoes are encoded in its mass, how this mass was assembled is encoded in its spin. A few ordered accretion events or mergers of large black holes produce high spins, and short, random accretion processes produce low spins. Measuring these spins is therefore a major goal of extragalactic astronomy: the spins of supermassive black holes hold a key to understanding the evolution of their host galaxies.

But how can we measure the spins? According to Einstein’s general theory of relativity, a black hole’s gravitational field twists space-time around it. Such twisting depends on the black hole’s spin, so measuring the twisting allows the spin to be estimated. The signature of space-time distortion is imprinted on the emission of radiation from regions close to the black hole’s event horizon – the surface beyond which no radiation can escape. The best way to perform such a measurement is to observe X-rays reflected by the disk.

In their study, R. C. Reis and colleagues break new ground by obtaining a spin measurement of a quasar at a distance of more than 6 billion light years from Earth, from a time when the Universe was about half its current age. This remarkable result was possible owing to the exceptional nature of the observed source – a quadruply imaged, gravitationally lensed quasar.

Credit: ACS & NICMOS/ESA/HST/STScI/AURA/NASA

The light from the distant quasar is both magnified and split into four different images by the gravitational field of a foreground elliptical galaxy (the lens) that, by chance, is on the line of sight of the quasar. For this reason, the authors could analyse four ‘copies’ of the X-ray spectrum of the quasar, each with an intensity significantly magnified by the lens. The resulting X-ray spectra have a quality that matches the best that has been obtained for nearby sources, and allowed a robust measurement of the black hole’s spin. As it turns out, the spin is large (close to the highest possible value that theory predicts), suggesting that the black hole acquired its mass through coherent phases of mass accretion.

Links: The Nature article (behind paywall); a widefield view of the lens via U. Michigan press release.

The European Research Council (ERC) has awarded 14 million euros (around $19 million) to a team of European astrophysicists to construct the first accurate image of a black hole. The team will test the predictions of current theories of gravity, including Einstein’s general theory of relativity. The funding is provided in the form of a synergy grant, the largest and most competitive type of grant of the ERC. This is the first time an astrophysics proposal has been awarded such a grant.

The team, led by investigators at the University of Nijmegen, the Max Planck Institute for Radio Astronomy, and Goethe University in Frankfurt, hopes to measure the shadow cast by the event horizon of the black hole in the center of the Milky Way, find new radio pulsars near this black hole, and combine these measurements with advanced computer simulations of the behavior of light and matter around black holes as predicted by theories of gravity. They will combine several telescopes around the globe to peer into the heart of our own galaxy, which hosts a mysterious radio source called Sagittarius A* which is considered to be the central supermassive black hole. (See p. 383 and Section 15.5, p. 391.)

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Credit and © M. Moscibrodzka & H. Falcke, Radboud-Universität Nijmegen

Black holes are notoriously elusive with a gravitational field so large that even light cannot escape their grip. The team plans to make an image of the event horizon – the border around a black hole which light can enter, but not leave.  The scientists want to peer into the heart of our own galaxy, which hosts a mysterious radio source called Sagittarius A*. The object is known to have a mass of around 4 million times the mass of the Sun and is considered to be the central supermassive black hole of the Milky Way.

As gaseous matter is attracted towards the event horizon by the black hole’s gravitational attraction, strong radio emission is produced before the gas disappears. The event horizon should then cast a dark shadow on that bright emission. Given the huge distance to the center of the Milky Way, the shadow is equivalent to the size of an apple on the Moon seen from Earth. By combining high-frequency radio telescopes around the world, in a technique called very long baseline interferometry (VLBI), even such a tiny feature is, in principle, detectable.

In addition, the group wants to use the same radio telescopes to find and measure pulsars around the very same black hole. Pulsars are rapidly spinning neutron stars, which can be used as highly accurate natural clocks in space. While radio pulsars are found throughout the Milky Way, surprisingly none had been found in the center of the Milky Way until very recently.