Astronomers observe the first radiation belt seen outside of our solar system

Astronomers have described the first radiation belt observed outside our solar system, using a coordinated array of 39 radio dishes from Hawaii to Germany to obtain high-resolution images. The images of persistent, intense radio emissions from an ultracool dwarf reveal the presence of a cloud of high-energy electrons trapped in the object's powerful magnetic field, forming a double-lobed structure analogous to radio images of Jupiter's radiation belts.

"We are actually imaging the magnetosphere of our target by observing the radio-emitting plasma -- its radiation belt -- in the magnetosphere. That has never been done before for something the size of a gas giant planet outside of our solar system," said Melodie Kao, a postdoctoral fellow at UC Santa Cruz and first author of a paper on the new findings published May 15in Nature.

Strong magnetic fields form a "magnetic bubble" around a planet called a magnetosphere, which can trap and accelerate particles to near the speed of light. All the planets in our solar system that have such magnetic fields, including Earth, as well as Jupiter and the other giant planets, have radiation belts consisting of these high-energy charged particles trapped by the planet's magnetic field.

Earth's radiation belts, known as the Van Allen belts, are large donut-shaped zones of high-energy particles captured from solar winds by the magnetic field. Most of the particles in Jupiter's belts are from volcanoes on its moon Io. If you could put them side by side, the radiation belt that Kao and her team have imaged would be 10 million times brighter than Jupiter's.

Particles deflected by the magnetic field toward the poles generate auroras ("northern lights") when they interact with the atmosphere, and Kao's team also obtained the first image capable of differentiating between the location of an object's aurora and its radiation belts outside our solar system.

The ultracool dwarf imaged in this study straddles the boundary between low-mass stars and massive brown dwarfs. "While the formation of stars and planets can be different, the physics inside of them can be very similar in that mushy part of the mass continuum connecting low-mass stars to brown dwarfs and gas giant planets," Kao explained.

Characterizing the strength and shape of the magnetic fields of this class of objects is largely uncharted terrain, she said. Using their theoretical understanding of these systems and numerical models, planetary scientists can predict the strength and shape of a planet's magnetic field, but they haven't had a good way to easily test those predictions.

"Auroras can be used to measure the strength of the magnetic field, but not the shape. We designed this experiment to showcase a method for assessing the shapes of magnetic fields on brown dwarfs and eventually exoplanets," Kao said.

The strength and shape of the magnetic field can be an important factor in determining a planet's habitability. "When we're thinking about the habitability of exoplanets, the role of their magnetic fields in maintaining a stable environment is something to consider in addition to things like the atmosphere and climate," Kao said.

To generate a magnetic field, a planet's interior must be hot enough to have electrically conducting fluids, which in the case of Earth is the molten iron in its core. In Jupiter, the conducting fluid is hydrogen under so much pressure it becomes metallic. Metallic hydrogen probably also generates magnetic fields in brown dwarfs, Kao said, while in the interiors of stars the conducting fluid is ionized hydrogen.

The ultracool dwarf known as LSR J1835+3259 was the only object Kao felt confident would yield the high-quality data needed to resolve its radiation belts.

"Now that we've established that this particular kind of steady-state, low-level radio emission traces radiation belts in the large-scale magnetic fields of these objects, when we see that kind of emission from brown dwarfs -- and eventually from gas giant exoplanets -- we can more confidently say they probably have a big magnetic field, even if our telescope isn't big enough to see the shape of it," Kao said, adding that she is looking forward to when the Next Generation Very Large Array, currently being planned by the National Radio Astronomy Observatory (NRAO), can image many more extrasolar radiation belts.

"This is a critical first step in finding many more such objects and honing our skills to search for smaller and smaller magnetospheres, eventually enabling us to study those of potentially habitable, Earth-size planets," said coauthor Evgenya Shkolnik at Arizona State University, who has been studying the magnetic fields and habitability of planets for many years.

The team used the High Sensitivity Array, consisting of 39 radio dishes coordinated by the NRAO in the United States and the Effelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany.

"By combining radio dishes from across the world, we can make incredibly high-resolution images to see things no one has ever seen before. Our image is comparable to reading the top row of an eye chart in California while standing in Washington, D.C.," said coauthor Jackie Villadsen at Bucknell University.

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Astronomers capture black hole eruption spanning 16 times the full Moon in the sky

Astronomers have produced the most comprehensive image of radio emission from the nearest actively feeding supermassive black hole to Earth.

The emission is powered by a central black hole in the galaxy Centaurus A, about 12 million light years away.

As the black hole feeds on in-falling gas, it ejects material at near light-speed, causing 'radio bubbles' to grow over hundreds of millions of years.

When viewed from Earth, the eruption from Centaurus A now extends eight degrees across the sky -- the length of 16 full Moons laid side by side.

It was captured using the Murchison Widefield Array (MWA) telescope in outback Western Australia.

The research was published today in the journal Nature Astronomy.

Lead author Dr Benjamin McKinley, from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), said the image reveals spectacular new details of the radio emission from the galaxy.

"These radio waves come from material being sucked into the supermassive black hole in the middle of the galaxy," he said.

"It forms a disc around the black hole, and as the matter gets ripped apart going close to the black hole, powerful jets form on either side of the disc, ejecting most of the material back out into space, to distances of probably more than a million light years.

"Previous radio observations could not handle the extreme brightness of the jets and details of the larger area surrounding the galaxy were distorted, but our new image overcomes these limitations."

Centaurus A is the closest radio galaxy to our own Milky Way.

"We can learn a lot from Centaurus A in particular, just because it is so close and we can see it in such detail," Dr McKinley said.

"Not just at radio wavelengths, but at all other wavelengths of light as well.

"In this research we've been able to combine the radio observations with optical and x-ray data, to help us better understand the physics of these supermassive black holes."

Astrophysicist Dr Massimo Gaspari, from Italy's National Institute for Astrophysics, said the study corroborated a novel theory known as 'Chaotic Cold Accretion' (CCA), which is emerging in different fields.

"In this model, clouds of cold gas condense in the galactic halo and rain down onto the central regions, feeding the supermassive black hole," he said.

"Triggered by this rain, the black hole vigorously reacts by launching energy back via radio jets that inflate the spectacular lobes we see in the MWA image. This study is one of the first to probe in such detail the multiphase CCA 'weather' over the full range of scales," Dr Gaspari concluded.

Dr McKinley said the galaxy appears brighter in the centre where it is more active and there is a lot of energy.

"Then it's fainter as you go out because the energy's been lost and things have settled down," he said.

"But there are interesting features where charged particles have re-accelerated and are interacting with strong magnetic fields."

MWA director Professor Steven Tingay said the research was possible because of the telescope's extremely wide field-of-view, superb radio-quiet location, and excellent sensitivity.

"The MWA is a precursor for the Square Kilometre Array (SKA) -- a global initiative to build the world's largest radio telescopes in Western Australia and South Africa," he said.

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Astronomers detect possible radio emission from exoplanet

 

Illustration of a 'hot Jupiter' exoplanet orbiting nearby star.

By monitoring the cosmos with a radio telescope array, an international team of scientists has detected radio bursts emanating from the constellation Boötes -- that could be the first radio emission collected from a planet beyond our solar system.

The team, led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zarka of the Observatoire de Paris -- Paris Sciences et Lettres University and Jean-Mathias Griessmeier of the Université d'Orléans will publish their findings in the forthcoming research section of Astronomy & Astrophysics, on Dec. 16.

"We present one of the first hints of detecting an exoplanet in the radio realm," Turner said. "The signal is from the Tau Boötes system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself. From the strength and polarization of the radio signal and the planet's magnetic field, it is compatible with theoretical predictions."

Among the co-authors is Turner's postdoctoral advisor Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences, and a professor of astronomy.

"If confirmed through follow-up observations," Jayawardhana said, "this radio detection opens up a new window on exoplanets, giving us a novel way to examine alien worlds that are tens of light-years away."

Using the Low Frequency Array (LOFAR), a radio telescope in the Netherlands, Turner and his colleagues uncovered emission bursts from a star-system hosting a so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. The group also observed other potential exoplanetary radio-emission candidates in the 55 Cancri (in the constellation Cancer) and Upsilon Andromedae systems. Only the Tau Boötes exoplanet system -- about 51 light-years away -- exhibited a significant radio signature, a unique potential window on the planet's magnetic field.

Observing an exoplanet's magnetic field helps astronomers decipher a planet's interior and atmospheric properties, as well as the physics of star-planet interactions, said Turner, a member of Cornell's Carl Sagan Institute.

Earth's magnetic field protects it from solar wind dangers, keeping the planet habitable. "The magnetic field of Earth-like exoplanets may contribute to their possible habitability," Turner said, "by shielding their own atmospheres from solar wind and cosmic rays, and protecting the planet from atmospheric loss."

Two years ago, Turner and his colleagues examined the radio emission signature of Jupiter and scaled those emissions to mimic the possible signatures from a distant Jupiter-like exoplanet. Those results became the template for searching radio emission from exoplanets 40 to 100 light-years away.

After poring over nearly 100-hours of radio observations, the researchers were able to find the expected hot Jupiter signature in Tau Boötes. "We learned from our own Jupiter what this kind of detection looks like. We went searching for it and we found it," Turner said.

The signature, though, is weak. "There remains some uncertainty that the detected radio signal is from the planet. The need for follow-up observations is critical," he said.

Turner and his team have already begun a campaign using multiple radio telescopes to follow up on the signal from Tau Boötes.

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