воскресенье, 14 октября 2018 г.

Сделан первый качественный снимок кольца у черной дыры

Астрономам впервые удалось получить качественный снимок газопылевой структуры, окружающей активную сверхмассивную черную дыру в центре галактики. Известно, что сверхмассивные черные дыры, находящиеся в центре галактик, "поглощают" все, что приближается к ним достаточно близко, однако возможность наблюдать такие явления в Млечном Пути предоставляется достаточно редко. Теперь же, однако, с помощью радиотелескопа ALMA астрономы смогли заснять очень активную черную дыру в центре спиральной галактики M77, находящейся в 47 млн световых лет от Земли. В центре M77 находится активное галактическое ядро; это означает, что газ и материя постоянно поглощаются центральной черной дырой и излучается интенсивный свет. Такие активные области Вселенной могут помочь ученым понять, как ведут себя галактики и сверхмассивные черные дыры, находящиеся в их центре. Новое открытие было сделано научной группой из Японии; исследователи при помощи радиотелескопа ALMA смогли заснять активное ядро M77. Об исследовании кратко сообщает портал New Atlas.

Специалисты обнаружили небольшую газопылевую структуру, окружающую черную дыру; радиус этого облака, вращающегося вокруг черной дыры, составлял 20 световых лет. Существование данных структур предполагалось на протяжении десятилетий, однако, как отмечают исследователи, впервые качественный снимок такой структуры удалось получить лишь сейчас.

Радиотелескоп ALMA позволяет делать снимки в очень высоком разрешении; при этом, как подчеркивают исследователи, важно было регистрировать микроволновое излучение от молекул цианистого водорода (HCN) и формил-ионов (HCO+). Отмечается, что данные молекулы "светятся" в микроволновом диапазоне лишь при достаточной плотности и, таким образом, позволяют сказать о плотности данного газопылевого облака.

Кроме того, исследователи заметили, что обнаруженная структура вращается несколько хаотично и движение газа не всегда регулируется гравитацией черной дыры. По предположению ученых, это может быть связано с тем, что в прошлом M77 столкнулась с другим объектом, возможно, с небольшой галактикой.

четверг, 4 октября 2018 г.

New simulation sheds light on spiraling supermassive black holes

A new model is bringing scientists a step closer to understanding the kinds of light signals produced when two supermassive black holes, which are millions to billions of times the mass of the Sun, spiral toward a collision. For the first time, a new computer simulation that fully incorporates the physical effects of Einstein's general theory of relativity shows that gas in such systems will glow predominantly in ultraviolet and X-ray light. Just about every galaxy the size of our own Milky Way or larger contains a monster black hole at its center. Observations show galaxy mergers occur frequently in the universe, but so far no one has seen a merger of these giant black holes. "We know galaxies with central supermassive black holes combine all the time in the universe, yet we only see a small fraction of galaxies with two of them near their centers," said Scott Noble, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The pairs we do see aren't emitting strong gravitational-wave signals because they're too far away from each other. Our goal is to identify - with light alone - even closer pairs from which gravitational-wave signals may be detected in the future." Scientists have detected merging stellar-mass black holes - which range from around three to several dozen solar masses - using the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO).

Gravitational waves are space-time ripples traveling at the speed of light. They are created when massive orbiting objects like black holes and neutron stars spiral together and merge.
Supermassive mergers will be much more difficult to find than their stellar-mass cousins. One reason ground-based observatories can't detect gravitational waves from these events is because Earth itself is too noisy, shaking from seismic vibrations and gravitational changes from atmospheric disturbances. The detectors must be in space, like the Laser Interferometer Space Antenna (LISA) led by ESA (the European Space Agency) and planned for launch in the 2030s.

Observatories monitoring sets of rapidly spinning, superdense stars called pulsars may detect gravitational waves from monster mergers. Like lighthouses, pulsars emit regularly timed beams of light that flash in and out of view as they rotate. Gravitational waves could cause slight changes in the timing of those flashes, but so far studies haven't yielded any detections.

But supermassive binaries nearing collision may have one thing stellar-mass binaries lack - a gas-rich environment. Scientists suspect the supernova explosion that creates a stellar black hole also blows away most of the surrounding gas. The black hole consumes what little remains so quickly there isn't much left to glow when the merger happens.

Supermassive binaries, on the other hand, result from galaxy mergers. Each supersized black hole brings along an entourage of gas and dust clouds, stars and planets. Scientists think a galaxy collision propels much of this material toward the central black holes, which consume it on a time scale similar to that needed for the binary to merge. As the black holes near, magnetic and gravitational forces heat the remaining gas, producing light astronomers should be able to see.

"It's very important to proceed on two tracks," said co-author Manuela Campanelli, director of the Center for Computational Relativity and Gravitation at the Rochester Institute of Technology in New York, who initiated this project nine years ago.

"Modeling these events requires sophisticated computational tools that include all the physical effects produced by two supermassive black holes orbiting each other at a fraction of the speed of light. Knowing what light signals to expect from these events will help modern observations identify them. Modeling and observations will then feed into each other, helping us better understand what is happening at the hearts of most galaxies."

The new simulation shows three orbits of a pair of supermassive black holes only 40 orbits from merging. The models reveal the light emitted at this stage of the process may be dominated by UV light with some high-energy X-rays, similar to what's seen in any galaxy with a well-fed supermassive black hole.

Three regions of light-emitting gas glow as the black holes merge, all connected by streams of hot gas: a large ring encircling the entire system, called the circumbinary disk, and two smaller ones around each black hole, called mini disks. All these objects emit predominantly UV light. When gas flows into a mini disk at a high rate, the disk's UV light interacts with each black hole's corona, a region of high-energy subatomic particles above and below the disk. This interaction produces X-rays. When the accretion rate is lower, UV light dims relative to the X-rays.

Based on the simulation, the researchers expect X-rays emitted by a near-merger will be brighter and more variable than X-rays seen from single supermassive black holes. The pace of the changes links to both the orbital speed of gas located at the inner edge of the circumbinary disk as well as that of the merging black holes.

"The way both black holes deflect light gives rise to complex lensing effects, as seen in the movie when one black hole passes in front of the other," said Stephane d'Ascoli, a doctoral student at Ecole Normale Superieure in Paris and lead author of the paper. "Some exotic features came as a surprise, such as the eyebrow-shaped shadows one black hole occasionally creates near the horizon of the other."

The simulation ran on the National Center for Supercomputing Applications' Blue Waters supercomputer at the University of Illinois at Urbana-Champaign. Modeling three orbits of the system took 46 days on 9,600 computing cores. Campanelli said the collaboration was recently awarded additional time on Blue Waters to continue developing their models.

The original simulation estimated gas temperatures. The team plans to refine their code to model how changing parameters of the system, like temperature, distance, total mass and accretion rate, will affect the emitted light. They're interested in seeing what happens to gas traveling between the two black holes as well as modeling longer time spans.

"We need to find signals in the light from supermassive black hole binaries distinctive enough that astronomers can find these rare systems among the throng of bright single supermassive black holes," said co-author Julian Krolik, an astrophysicist at Johns Hopkins University in Baltimore.

"If we can do that, we might be able to discover merging supermassive black holes before they're seen by a space-based gravitational-wave observatory."

среда, 3 октября 2018 г.

Black holes ruled out as universe's missing dark matter

For one brief shining moment after the 2015 detection of gravitational waves from colliding black holes, astronomers held out hope that the universe's mysterious dark matter might consist of a plenitude of black holes sprinkled throughout the universe. University of California, Berkeley, physicists have dashed those hopes. Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe. Primordial black holes could only have been created within the first milliseconds of the Big Bang as regions of the universe with a concentrated mass tens or hundreds of times that of the sun collapsed into objects a hundred kilometers across. The results suggest that none of the universe's dark matter consists of heavy black holes, or any similar object, including massive compact halo objects, so-called MACHOs. Dark matter is one of astronomy's most embarrassing conundrums: despite comprising 84.5 percent of the matter in the universe, no one can find it. Proposed dark matter candidates span nearly 90 orders of magnitude in mass, from ultralight particles like axions to MACHOs.

Several theorists have proposed scenarios in which there are multiple types of dark matter. But if dark matter consists of several unrelated components, each would require a different explanation for its origin, which makes the models very complex.

"I can imagine it being two types of black holes, very heavy and very light ones, or black holes and new particles. But in that case one of the components is orders of magnitude heavier than the other, and they need to be produced in comparable abundance. We would be going from something astrophysical to something that is truly microscopic, perhaps even the lightest thing in the universe, and that would be very difficult to explain," said lead author Miguel Zumalacarregui, a Marie Curie Global Fellow at the Berkeley Center for Cosmological Physics.

An as-yet unpublished reanalysis by the same team using an updated list of 1,048 supernovas cuts the limit in half, to a maximum of about 23 percent, further slamming the door on the dark matter-black hole proposal.

"We are back to the standard discussions. What is dark matter? Indeed, we are running out of good options," said Uros Seljak, a UC Berkeley professor of physics and astronomy and BCCP co-director. "This is a challenge for future generations."

The analysis is detailed in a paper published this week in the journal Physical Review Letters.

Dark matter lensing
Their conclusions are based on the fact that an unseen population of primordial black holes, or any massive compact object, would gravitationally bend and magnify light from distant objects on its way to Earth.

Therefore, gravitational lensing should affect the light from distant Type Ia supernovas. These are the exploding stars that scientists have used as standard brightness sources to measure cosmic distances and document the expansion of the universe.

Zumalacarregui conducted a complex statistical analysis of data on the brightness and distance supernovas catalogued in two compilations - 580 in the Union and 740 in the joint light-curve analysis (JLA) catalogs - and concluded that eight should be brighter by a few tenths of a percent than predicted based on observations of how these supernovas brighten and fade over time. No such brightening has been detected.

Other researchers have performed similar but simpler analyses that yielded inconclusive results. But Zumalacarregui incorporated the precise probability of seeing all magnifications, from small to huge, as well as uncertainties in brightness and distance of each supernova. Even for low-mass black holes - those 1 percent the mass of the sun - there should be some highly magnified distant supernovas, he said, but there are none.

"You cannot see this effect on one supernova, but when you put them all together and do a full Bayesian analysis you start putting very strong constraints on the dark matter, because each supernova counts and you have so many of them," Zumalacarregui said.

The more supernovas included in the analysis, and the farther away they are, the tighter the constraints. Data on 1,048 bright supernovas from the Pantheon catalog provided an even lower upper limit - 23 percent - than the newly published analysis.

Seljak published a paper proposing this type of analysis in the late 1990s, but when interest shifted from looking for big objects, MACHOs, to looking for fundamental particles, in particular weakly interacting massive particles, or WIMPs, follow-up plans fell by the wayside. By then, many experiments had excluded most masses and types of MACHOs, leaving little hope of discovering such objects.

At the time, too, only a small number of distant Type Ia supernovas had been discovered and their distances measured.

Only after the LIGO observations brought up the issue again did Seljak and Zumalacarregui embark on the complicated analysis to determine the limits on dark matter.

"What was intriguing is that the masses of the black holes in the LIGO event were right where black holes had not yet been excluded as dark matter," Seljak said. "That was an interesting coincidence that got everyone excited. But it was a coincidence."

понедельник, 24 сентября 2018 г.

Matter falling into a black hole at 30 percent of the speed of light

A UK team of astronomers report the first detection of matter falling into a black hole at 30% of the speed of light, located in the centre of the billion-light year distant galaxy PG211+143. The team, led by Professor Ken Pounds of the University of Leicester, used data from the European Space Agency's X-ray observatory XMM-Newton to observe the black hole. Their results appear in a new paper in Monthly Notices of the Royal Astronomical Society. Black holes are objects with such strong gravitational fields that not even light travels quickly enough to escape their grasp, hence the description 'black'. They are hugely important in astronomy because they offer the most efficient way of extracting energy from matter. As a direct result, gas in-fall - accretion - onto black holes must be powering the most energetic phenomena in the Universe. The centre of almost every galaxy - like our own Milky Way - contains a so-called supermassive black hole, with masses of millions to billions of times the mass of our Sun. With sufficient matter falling into the hole, these can become extremely luminous, and are seen as a quasar or active galactic nucleus (AGN). However black holes are so compact that gas is almost always rotating too much to fall in directly. Instead it orbits the hole, approaching gradually through an accretion disc - a sequence of circular orbits of decreasing size. As gas spirals inwards, it moves faster and faster and becomes hot and luminous, turning gravitational energy into the radiation that astronomers observe.

The orbit of the gas around the black hole is often assumed to be aligned with the rotation of the black hole, but there is no compelling reason for this to be the case. In fact, the reason we have summer and winter is that the Earth's daily rotation does not line up with its yearly orbit around the Sun.

Until now it has been unclear how misaligned rotation might affect the in-fall of gas. This is particularly relevant to the feeding of supermassive black holes since matter (interstellar gas clouds or even isolated stars) can fall in from any direction.

Using data from XMM-Newton, Prof. Pounds and his collaborators looked at X-ray spectra (where X-rays are dispersed by wavelength) from the galaxy PG211+143. This object lies more than one billion light years away in the direction of the constellation Coma Berenices, and is a Seyfert galaxy, characterised by a very bright AGN resulting from the presence of the massive black hole at its nucleus.

The researchers found the spectra to be strongly red-shifted, showing the observed matter to be falling into the black hole at the enormous speed of 30 per cent of the speed of light, or around 100,000 kilometres per second. The gas has almost no rotation around the hole, and is detected extremely close to it in astronomical terms, at a distance of only 20 times the hole's size (its event horizon, the boundary of the region where escape is no longer possible).

The observation agrees closely with recent theoretical work, also at Leicester and using the UK's Dirac supercomputer facility simulating the 'tearing' of misaligned accretion discs. This work has shown that rings of gas can break off and collide with each other, cancelling out their rotation and leaving gas to fall directly towards the black hole.

Prof. Pounds, from the University of Leicester's Department of Physics and Astronomy, said: "The galaxy we were observing with XMM-Newton has a 40 million solar mass black hole which is very bright and evidently well fed. Indeed some 15 years ago we detected a powerful wind indicating the hole was being over-fed. While such winds are now found in many active galaxies, PG1211+143 has now yielded another 'first', with the detection of matter plunging directly into the hole itself."

He continues: "We were able to follow an Earth-sized clump of matter for about a day, as it was pulled towards the black hole, accelerating to a third of the velocity of light before being swallowed up by the hole."

A further implication of the new research is that 'chaotic accretion' from misaligned discs is likely to be common for supermassive black holes. Such black holes would then spin quite slowly, being able to accept far more gas and grow their masses more rapidly than generally believed, providing an explanation for why black holes which formed in the early Universe quickly gained very large masses.

понедельник, 30 июля 2018 г.

First Successful Test of General Relativity Near Supermassive Black Hole

Obscured by thick clouds of absorbing dust, the closest supermassive black hole to the Earth lies 26,000 light-years away at the centre of the Milky Way. This gravitational monster, which has a mass four million times that of the Sun, is surrounded by a small group of stars orbiting around it at high speed.This extreme environment - the strongest gravitational field in our galaxy - makes it the perfect place to explore gravitational physics, and particularly to test Einstein's general theory of relativity. New infrared observations from the exquisitely sensitive GRAVITY, SINFONI and NACO instruments, developed under the lead of the Max Planck Institute for Extraterrestrial Physics (MPE), have now allowed astronomers to follow one of these stars, called S2, as it passed very close to the black hole during May 2018. At the closest point this star was at a distance of less than 20 billion kilometres from the black hole and moving at a speed in excess of 25 million kilometres per hour - almost three percent of the speed of light.

The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.

These extremely precise measurements were made by an international team led by Reinhard Genzel (MPE) in Garching, Germany, in conjunction with collaborators around the world, at the Paris Observatory-PSL, the Universite Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA (Centro de Astro?sica e Gravitacao) and ESO. The observations are the culmination of a 26-year series of ever-more-precise observations of the centre of the Milky Way using ESO instruments.

"This is the second time that we have observed the close passage of S2 around the black hole in our galactic centre. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution," explains Genzel. "We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects."

The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein's general theory of relativity. This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the motion of a star around a supermassive black hole.

The team used SINFONI to measure the velocity of S2 towards and away from Earth and the GRAVITY instrument in the VLT Interferometer (VLTI) to make extraordinarily precise measurements of the changing position of S2 in order to define the shape of its orbit. GRAVITY creates such sharp images that it can reveal the motion of the star from night to night as it passes close to the black hole - 26,000 light-years from Earth.

"Our first observations of S2 with GRAVITY, about two years ago, already showed that we would have the ideal black hole laboratory," adds Frank Eisenhauer (MPE), principal investigator of GRAVITY and the SINFONI spectrograph.

"During the close passage, we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2."

More than one hundred years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more - in a much more extreme laboratory than he could have possibly imagined!

"Due to the extremely strong gravitational field we expect to see the effects of general relativity - but only if we can look close enough," says Stefan Gillessen, "This is why we needed to push the technology. With SINFONI we can measure the radial velocity of stars very accurately and GRAVITY gives us extremely sharp images and accurate positions."

Continuing observations are expected to reveal another relativistic effect very soon - a small rotation of the star's orbit, known as Schwarzschild precession - as S2 moves away from the black hole.

Xavier Barcons, ESO's Director General, concludes: "ESO has worked with Reinhard Genzel and his team and collaborators in the ESO Member States for over a quarter of a century. It was a huge challenge to develop the uniquely powerful instruments needed to make these very delicate measurements and to deploy them at the VLT in Paranal. The discovery announced is the very exciting result of a remarkable partnership."