Scientists are pretty sure black holes are real. Soon they’ll know for sure the biggest problem with trying to detect a black hole is that even the supermassive ones are relatively tiny.
“The largest one in the sky [is] the black hole in the center of the Milky Way,” Dimitrios Psaltis, an astrophysicist at the University of Arizona, writes me in an email. “And taking a picture of it would be equivalent to taking a picture of a DVD on the surface of the moon.”
What’s more, because of their strong gravity, black holes tend to be surrounded by other bright matter that makes it hard to see the object itself.
That’s why, when hunting for black holes, astronomers don’t usually try for direct observation. Instead, they look for evidence of the effects of a black hole’s gravity and radiation.
But an ongoing effort is linking up several radio telescopes around the world in an effort to see a black hole up close for the first time. The historic image is due in 2017.
A supermassive black hole is depicted in this artist’s concept, surrounded by a swirling disk of material falling onto it. Credits: NASA/JPL-Caltech
The baffling and strange behaviors of black holes have become somewhat less mysterious recently, with new observations from NASA’s Explorer missions Swift and the Nuclear Spectroscopic Telescope Array, or NuSTAR. The two space telescopes caught a supermassive black hole in the midst of a giant eruption of X-ray light, helping astronomers address an ongoing puzzle: How do supermassive black holes flare?
The results suggest that supermassive black holes send out beams of X-rays when their surrounding coronas — sources of extremely energetic particles — shoot, or launch, away from the black holes.
“This is the first time we have been able to link the launching of the corona to a flare,” said Dan Wilkins of Saint Mary’s University in Halifax, Canada, lead author of a new paper on the results appearing in the Monthly Notices of the Royal Astronomical Society. “This will help us understand how supermassive black holes power some of the brightest objects in the universe.”
This diagram shows how a shifting feature, called a corona, can create a flare of X-rays around a black hole. The corona (feature represented in purplish colors) gathers inward (left), becoming brighter, before shooting away from the black hole (middle and right). Astronomers don’t know why the coronas shift, but they have learned that this process leads to a brightening of X-ray light that can be observed by telescopes. Credits: NASA/JPL-Caltech
Supermassive black holes don’t give off any light themselves, but they are often encircled by disks of hot, glowing material. The gravity of a black hole pulls swirling gas into it, heating this material and causing it to shine with different types of light. Another source of radiation near a black hole is the corona. Coronas are made up of highly energetic particles that generate X-ray light, but details about their appearance, and how they form, are unclear.
Astronomers think coronas have one of two likely configurations. The “lamppost” model says they are compact sources of light, similar to light bulbs, that sit above and below the black hole, along its rotation axis. The other model proposes that the coronas are spread out more diffusely, either as a larger cloud around the black hole, or as a “sandwich” that envelops the surrounding disk of material like slices of bread. In fact, it’s possible that coronas switch between both the lamppost and sandwich configurations.
The new data support the “lamppost” model — and demonstrate, in the finest detail yet, how the light-bulb-like coronas move. The observations began when Swift, which monitors the sky for cosmic outbursts of X-rays and gamma rays, caught a large flare coming from the supermassive black hole called Markarian 335, or Mrk 335, located 324 million light-years away in the direction of the constellation Pegasus. This supermassive black hole, which sits at the center of a galaxy, was once one of the brightest X-ray sources in the sky.
“Something very strange happened in 2007, when Mrk 335 faded by a factor of 30. What we have found is that it continues to erupt in flares but has not reached the brightness levels and stability seen before,” said Luigi Gallo, the principal investigator for the project at Saint Mary’s University. Another co-author, Dirk Grupe of Morehead State University in Kentucky, has been using Swift to regularly monitor the black hole since 2007.
In September 2014, Swift caught Mrk 335 in a huge flare. Once Gallo found out, he sent a request to the NuSTAR team to quickly follow up on the object as part of a “target of opportunity” program, where the observatory’s previously planned observing schedule is interrupted for important events. Eight days later, NuSTAR set its X-ray eyes on the target, witnessing the final half of the flare event.
After careful scrutiny of the data, the astronomers realized they were seeing the ejection, and eventual collapse, of the black hole’s corona.
“The corona gathered inward at first and then launched upwards like a jet,” said Wilkins. “We still don’t know how jets in black holes form, but it’s an exciting possibility that this black hole’s corona was beginning to form the base of a jet before it collapsed.”
How could the researchers tell the corona moved? The corona gives off X-ray light that has a slightly different spectrum — X-ray “colors” — than the light coming from the disk around the black hole. By analyzing a spectrum of X-ray light from Mrk 335 across a range of wavelengths observed by both Swift and NuSTAR, the researchers could tell that the corona X-ray light had brightened — and that this brightening was due to the motion of the corona.
Coronas can move very fast. The corona associated with Mrk 335, according to the scientists, was traveling at about 20 percent the speed of light. When this happens, and the corona launches in our direction, its light is brightened in an effect called relativistic Doppler boosting.
Putting this all together, the results show that the X-ray flare from this black hole was caused by the ejected corona.
“The nature of the energetic source of X-rays we call the corona is mysterious, but now with the ability to see dramatic changes like this we are getting clues about its size and structure,” said Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology in Pasadena, who was not affiliated with the study.
Many other black hole brainteasers remain. For example, astronomers want to understand what causes the ejection of the corona in the first place.
NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory in Pasadena, California, for NASA’s Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive. JPL is managed by Caltech for NASA.
Stephen Hawkingannounced during at the KTH Royal Institute of Technology in Stockholm, in Sweden, that he has solved how you couldescape ablack hole.
The presentation was made at the Hawking Radiation conference, which was co-hosted by the theoretical physics institute, Nordita, and the University of North Carolina, on the campus of KTH Royal Institute of Technology in Stockholm.
“I propose that the information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon. The message of this lecture is that black holes ain’t as black as they are painted. They are not the eternal prisons they were once thought. Things can get out of a black hole both on the outside and possibly come out in another universe.”
Supermassive black holes, often with masses billions of times that of the Sun, are located at the heart of almost all galaxies in the Universe. These black holes can accrete huge amounts of matter in the form of a surrounding disc. While most of this matter is fed into the black hole, some can escape moments before capture and be flung out into space at close to the speed of light as part of a jet of plasma. How this happens is not well understood, although it is thought that strong magnetic fields, acting very close to the event horizon, play a crucial part in this process, helping the matter to escape from the gaping jaws of darkness.
Up to now only weak magnetic fields far from black holes — several light-years away — had been probed. In this study, however, astronomers from Chalmers University of Technology and Onsala Space Observatory in Sweden have now used ALMA to detect signals directly related to a strong magnetic field very close to the event horizon of the supermassive black hole in a distant galaxy named PKS 1830-211. This magnetic field is located precisely at the place where matter is suddenly boosted away from the black hole in the form of a jet.
The team measured the strength of the magnetic field by studying the way in which light was polarised, as it moved away from the black hole.
“Polarisation is an important property of light and is much used in daily life, for example in sun glasses or 3D glasses at the cinema,” says Ivan Marti-Vidal, lead author of this work. “When produced naturally, polarisation can be used to measure magnetic fields, since light changes its polarisation when it travels through a magnetised medium. In this case, the light that we detected with ALMA had been travelling through material very close to the black hole, a place full of highly magnetised plasma.”
The astronomers applied a new analysis technique that they had developed to the ALMA data and found that the direction of polarisation of the radiation coming from the centre of PKS 1830-211 had rotated. These are the shortest wavelengths ever used in this kind of study, which allow the regions very close to the central black hole to be probed.
“We have found clear signals of polarisation rotation that are hundreds of times higher than the highest ever found in the Universe,” says Sebastien Muller, co-author of the paper. “Our discovery is a giant leap in terms of observing frequency, thanks to the use of ALMA, and in terms of distance to the black hole where the magnetic field has been probed — of the order of only a few light-days from the event horizon. These results, and future studies, will help us understand what is really going on in the immediate vicinity of supermassive black holes.”
 Much weaker magnetic fields have been detected in the vicinity of the relatively inactive supermassive black hole at the centre of the Milky Way. Recent observations have also revealed weak magnetic fields in the active galaxy NGC 1275, which were detected at millimetre wavelengths.
 Magnetic fields introduce Faraday rotation, which makes the polarisation rotate in different ways at different wavelengths. The way in which this rotation depends on the wavelength tells us about the magnetic field in the region.
 The ALMA observations were at an effective wavelength of about 0.3 millimetres, earlier investigations were at much longer radio wavelengths. Only light of millimetre wavelengths can escape from the region very close to the black hole, longer wavelength radiation is absorbed.
This research was presented in a paper entitled “A strong magnetic field in the jet base of a supermassive black hole” to appear in Science on 17 April 2015
The team is composed of I. Martí-Vidal (Onsala Space Observatory and Department of Earth and Space Sciences, Chalmers University of Technology, Sweden), S. Muller (Onsala Space Observatory and Department of Earth and Space Sciences, Chalmers University of Technology, Sweden), W. Vlemmings (Department of Earth and Space Sciences and Onsala Space Observatory, Chalmers University of Technology, Sweden), C. Horellou (Department of Earth and Space Sciences, Chalmers University of Technology, Sweden) and S. Aalto (Department of Earth and Space Sciences, Chalmers University of Technology, Sweden).
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the US National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.