The results, published in the journal Science, are based on new radio observations tracking a star as it gets torn apart by a black hole. Such violent events yield a burst of light which is produced as the bits and pieces of the star fall into the black hole. For the first time, the researchers were able to show that this burst of light is followed by a radio signal from the matter that was able to escape the black hole by travelling away in a jetted outflow at nearly the speed of light.
The discovery of the jet was made possible by a rapid observational response after the stellar disruption (known as ASAS-SN-14li) was announced in December 2014. The radio data was taken by the by the 4 PI SKY team at Oxford, using the Arcminute Microkelvin Imager Large Array located in Cambridge.
“Previous efforts to find evidence for these jets, including my own, were late to the game,” said Sjoert van Velzen of Johns Hopkins University, the study’s lead author. Co-author Nicholas Stone added that “even after they got to the game, these earlier attempts were observing from the bleachers, while we were the first to get front row seats.”
In this branch of astronomy, the ‘front row’ means a distance of 300 million light years, while previous observations were based on events at occurring least three times as far away.
Jets are often observed in association with black holes, but their launch mechanism remains a major unsolved problem in astrophysics. Most supermassive black hole are fed a steady diet of gas, leading to jets that live for millions of years and change little on a human timescale. However, the newly discovered jet behaved very differently: the observations show that following a brief injection of energy, it produced short but spectacular radio fireworks.
The observed jet was anticipated by the so-called scale-invariant model of accretion, also known as the Matryoshka-doll theory of astrophysics. It predicts that all compact astrophysical objects (white dwarfs, neutron stars, or black holes) that accrete matter behave and look the same after a simple correction based on solely the mass of the object. In other words, the larger Matryoshka doll (a supermassive black hole) is just a scaled-up version of the smaller doll (a neutron star). Since neutron stars consistently produce radio-emitting jets when they are supplied with a large amount of gas, the theory predicts that supermassive black holes should do the same when they swallow a star.
“I always liked the elegant nature of the scale-invariant theory, but previous observations never found evidence for the new type of jet it predicted,” said van Velzen. “Our new findings suggest that this new type of jet could indeed be common and previous observations were simply not sensitive enough to detect them.”
“These jets are a unique tool for probing supermassive black holes,” said co-author Dr Morgan Fraser of Cambridge’s Institute of Astronomy. “While black holes themselves do not emit light, by observing how a star is torn apart as it falls in we can indirectly study the sleeping monster at the heart of a galaxy.”
The study hypothesises that every stellar disruption leads to a radio flare similar to the one just discovered. Ongoing surveys such as the Gaia Alerts project, led by the University of Cambridge will find many more of these rare events.
“Gaia has exceptionally sharp eyes, and is ideally suited to find events like this, which occur in the very centres of galaxies,” said co-author Dr Heather Campbell, also from Cambridge’s Institute of Astronomy. “Finding more of these rare events may further our understanding of the processes that allow black holes to launch such spectacular outflows.”
Van Velzen, S. et. al. ‘A radio jet from the optical and X-ray bright stellar tidal disruption flare ASASSN-14li.’ Science (2015). DOI: 10.1126/science.aad1182
Adapted from a Johns Hopkins press release.
An international team of astrophysicists, including researchers from the University of Cambridge, has observed a new way for gas to escape the gravitational pull of a supermassive black hole.These jets are a unique tool for probing supermassive black holesMorgan FraserNASA/Goddard Space Flight Center/CI Lab Detail from animation of a black hole devouring a star
The text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.
Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way
Nature 527, 7579 (2015). doi:10.1038/nature15747
Authors: L. M. Howes, A. R. Casey, M. Asplund, S. C. Keller, D. Yong, D. M. Nataf, R. Poleski, K. Lind, C. Kobayashi, C. I. Owen, M. Ness, M. S. Bessell, G. S. Da Costa, B. P. Schmidt, P. Tisserand, A. Udalski, M. K. Szymański, I. Soszyński, G. Pietrzyński, K. Ulaczyk, Ł. Wyrzykowski, P. Pietrukowicz, J. Skowron, S. Kozłowski & P. Mróz
The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium (‘metals’) have been found in the outer regions (‘halo’) of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions (‘bulges’) of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1–2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.
Ubiquitous time variability of integrated stellar populations
Nature 527, 7579 (2015). doi:10.1038/nature15731
Authors: Charlie Conroy, Pieter G. van Dokkum & Jieun Choi
Long-period variable stars arise in the final stages of the asymptotic giant branch phase of stellar evolution. They have periods of up to about 1,000 days and amplitudes that can exceed a factor of three in the I-band flux. These stars pulsate predominantly in their fundamental mode, which is a function of mass and radius, and so the pulsation periods are sensitive to the age of the underlying stellar population. The overall number of long-period variables in a population is directly related to their lifetimes, which is difficult to predict from first principles because of uncertainties associated with stellar mass-loss and convective mixing. The time variability of these stars has not previously been taken into account when modelling the spectral energy distributions of galaxies. Here we construct time-dependent stellar population models that include the effects of long-period variable stars, and report the ubiquitous detection of this expected ‘pixel shimmer’ in the massive metal-rich galaxy M87. The pixel light curves display a variety of behaviours. The observed variation of 0.1 to 1 per cent is very well matched to the predictions of our models. The data provide a strong constraint on the properties of variable stars in an old and metal-rich stellar population, and we infer that the lifetime of long-period variables in M87 is shorter by approximately 30 per cent compared to predictions from the latest stellar evolution models.
Planetary science: The Moon's tilt for gold
Nature 527, 7579 (2015). doi:10.1038/527455a
Authors: Robin Canup
The Moon's current orbit is at odds with theories predicting that its early orbit was in Earth's equatorial plane. Simulations now suggest that its orbit was tilted by gravitational interactions with a few large bodies. See Letter p.492
Collisionless encounters and the origin of the lunar inclination
Nature 527, 7579 (2015). doi:10.1038/nature16137
Authors: Kaveh Pahlevan & Alessandro Morbidelli
The Moon is generally thought to have formed from the debris ejected by the impact of a planet-sized object with the proto-Earth towards the end of planetary accretion. Models of the impact process predict that the lunar material was disaggregated into a circumplanetary disk and that lunar accretion subsequently placed the Moon in a near-equatorial orbit. Forward integration of the lunar orbit from this initial state predicts a modern inclination at least an order of magnitude smaller than the lunar value—a long-standing discrepancy known as the lunar inclination problem. Here we show that the modern lunar orbit provides a sensitive record of gravitational interactions with Earth-crossing planetesimals that were not yet accreted at the time of the Moon-forming event. The currently observed lunar orbit can naturally be reproduced via interaction with a small quantity of mass (corresponding to 0.0075–0.015 Earth masses eventually accreted to the Earth) carried by a few bodies, consistent with the constraints and models of late accretion. Although the encounter process has a stochastic element, the observed value of the lunar inclination is among the most likely outcomes for a wide range of parameters. The excitation of the lunar orbit is most readily reproduced via collisionless encounters of planetesimals with the Earth–Moon system with strong dissipation of tidal energy on the early Earth. This mechanism obviates the need for previously proposed (but idealized) excitation mechanisms, places the Moon-forming event in the context of the formation of Earth, and constrains the pristineness of the dynamical state of the Earth–Moon system.
Planetary science: Martian moon will break apart
Nature 527, 7579 (2015). doi:10.1038/527413e
Phobos, one of Mars's two moons, will disintegrate some 20 million to 40 million years from now, and its particles will form the only planetary ring in the inner Solar System.Benjamin Black and Tushar Mittal of the University of California, Berkeley, made these predictions