Gamma-ray bursts (GRB): Afterglow discovery surprises scientists

Measurements of polarized light in the afterglow of GRB 120308A by the Liverpool Telescope and its RINGO2 instrument indicate the presence of a large-scale stable magnetic field linked with a young black hole, as shown in this illustration. 

Credit: NASA's Goddard Space Flight Center /S. Wiessinger

Research from an international team of scientists led by the University of Leicester has discovered for the first time that one of the most powerful events in our universe, Gamma-Ray Bursts (GRB), behave differently than previously thought.

The study, published in the prestigious scientific journal Nature, uses evidence from observation of a GRB to rule out most of the existing theoretical predictions concerning the afterglow of the explosions.

Klaas Wiersema

For Dr Klaas Wiersema, of the University of Leicester's Department of Physics and Astronomy, it was handy that he was up in the middle of the night tending to his three-year-old son which is when he got the alert that a GRB had occurred.

He said: "When a suitable GRB is detected by a satellite, I get a text message on my phone, and then I have to very quickly tell the observatory in Chile exactly which observations I want them to take, and how.

"This is usually a rather stressed and frantic few hours of working, as fast as possible, on my laptop throughout our night-time, and I remember very well that my son, who was three at the time, was up a lot that night too, so I kept on running back and forth between my laptop, my phone to call the observatory in Chile, and my son's cot!"

The effort was worth it- and has led to scientific findings that will change theoretical understandings of the afterglows of GRBs.

Dr Wiersema explains: "About once per day, a short, very bright flash of gamma-rays (the most energetic form of light) is detected by satellites. These flashes are called gamma-ray bursts (GRBs), and take place in galaxies far away, when a massive star collapses at the end of its life.

"These GRBs are followed by a so-called "afterglow", slowly fading emission that can be seen at all wavelengths (including visible light), for a few days to weeks."

"We know that the afterglow emission is formed by a shockwave, moving at very high velocities, in which electrons are being accelerated to tremendous energies."

"These fast moving electrons then produce the afterglow light that we detect.

When a massive star dies it explodes as a supernova. 

The core of the star collapses into a black hole, and in care cases a jet is formed along the rotation axis of the newly formed black hole. 

Processes in this jet emits gamma radiation, which we observe as a so-called gamma-ray burst. 

Typically gamma-ray bursts last a few minutes. 

When the jet hits material surrounding the dying star an afterglow is formed. 

New observations of the degree of polarisation of the afterglow light has shown that the afterglow behaves differently than expected 

Credit: NASA

"However, how this acceleration process actually works is very hard to study on Earth in laboratories, or using computer simulations."

"What we do, is study the polarised light of the afterglow using large optical telescopes, and special filters, that work much like the filters in Polaroid sunglasses."

Gamma-ray burst 121024A, as seen on the day of burst by ESO's Very Large Telescope (VLT) in Chile. Only a week later the source had faded completely. 

Credit: Dr Klaas Wiersema, University of Leicester, UK and Dr Peter Curran, ICRAR.

Dr Wiersema says it is important to remember that light is a wave, when light is linearly polarised, it means that the wave vibrations lie in a plane; and when light is circularly polarised, it means that that this plane rotates on the sky.

He added: "Different theories for electron acceleration and light emission within the afterglow all predict different levels of linear polarisation, but theories all agreed that there should be no circular polarisation in visible light."

Peter Curran, ICRAR

"This is where we come in: we decided to test this by carefully measuring both the linear and circular polarisation of one afterglow, of GRB 121024A, detected by the Swift satellite."

"Using the ESO Very Large Telescope (VLT) in Chile, we measured both the linear and circular polarisation of an afterglow with high accuracy."

"Much to our surprise we clearly detected circular polarisation, while theories predicted we should not see any at all."

"We believe that the most likely explanation is that the exact way in which electrons are accelerated within the afterglow shockwave is different from what we always thought."

"It is a very nice example of observations ruling out most of the existing theoretical predictions – exactly why observers like me are in this game!

More information: Paper: Circular polarisation in the optical afterglow of GRB 121024A, Nature, DOI: 10.1038/nature13237

Exoplanet formation surprise: Partially Filled Disk Gap

A team of researchers has discovered evidence that an extrasolar planet may be forming quite far from its star-- about twice the distance Pluto is from our Sun.

The planet lies inside a dusty, gaseous disk around a small red dwarf TW Hydrae, which is only about 55 percent of the mass of the Sun.

The discovery adds to the ever-increasing variety of planetary systems in the Milky Way.

The research is published in the Astrophysical Journal.

This dusty protoplanetary disk is the closest one to us, some 176 light-years away in the constellation Hydra.

The astronomers made Hubble Space Telescope observations over a wide range of wavelengths from visible to near infrared and modeled the color and structure of the disk in a way that has not been done before.

They found a deficit of disk material, or partial gap, at about 80 astronomical units (AU) (1 AU is the Earth/Sun distance).

Their models indicate that the depression is about 20 AUs wide, just slightly wider than necessary for a planet-opening gap and consistent with a planet of between 6 and 28 Earth masses.

The feature is seen at all wavelengths indicating it is structural and not a local compositional difference.

The team believes the evidence is strong for planet formation causing the gap.

This graphic shows a gap in a protoplanetary disk of dust and gas whirling around the nearby red dwarf star TW Hydrae, which resides 176 light-years away in the constellation Hydra, sometimes called the Sea Serpent. 

The gap's presence is best explained as due to the effects of a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow. 

In the left image, astronomers used a masking device on the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer to block out the star's bright light so that the disk's structure could be seen. 

The Hubble observations reveal that the gap, which is 1.9 billion miles wide, is not completely cleared out. The illustration at right shows the gap relative to the star. 

The Hubble observations were taken on June 17, 2005. 

Credit: NASA, ESA,

"TW Hydrae is between 5 and 10 million years old, and should be in the final throes of planet formation before its disk dissipates," remarked coauthor Alycia Weinberger of the Carnegie Institution and principal investigator of the observations.

"It is surprising to find a planet only 5 to 10% of Jupiter's mass forming so far out since planets should form faster closer in. In all planet formation scenarios, it's difficult to make a low-mass planet far away from a low mass star."

The goal of these observations was to understand not only whether planets have formed, but also what conditions can result in planet formation and what chemical constituents are available for new planets.

Carnegie astrophysicist Alan Boss, who works on disk instability models, said "If the mass of this suspected planet is as low as it seems to be, this presents a real puzzle. Theory would say that it cannot exist!"

NASA: Surprise Shrimp Visitor Under Antarctic Ice

Credit: NASA

At a depth of 600 feet beneath the West Antarctic ice sheet, a small shrimp-like creature managed to brighten up an otherwise gray polar day in late November 2009.

This critter is a three-inch long Lyssianasid amphipod found beneath the Ross Ice Shelf, about 12.5 miles away from open water. NASA scientists were using a borehole camera to look back up towards the ice surface when they spotted this pinkish-orange creature swimming beneath the ice.

Surprise! Water (hydroxyl) found on the Moon

Never mind Evian or Perrier the next new thing for the discerning diner may be Moon or Lunar Water.

There appears to be, to the surprise of most planetary scientists, water everywhere on the Moon. Although, how many refreshing drops future astronauts might be able to drink, is not clear.

Data from three spacecraft indicate the widespread presence of water or hydroxyl, a molecule consisting of one hydrogen atom and one oxygen atom as opposed to the two hydrogen and one oxygen atoms that make up a water molecule. The discoveries are being published Thursday on the Web site of the journal Science.

“It’s so startling because it’s so pervasive,” said Lawrence A. Taylor of the University of Tennessee, Knoxville, a co-author of one of the papers that analyzed data from a National Aeronautics and Space Administration instrument aboard India’s Chandrayyan-1 satellite. “It’s like somebody painted the globe.”

For decades, the Moon has been regarded as a completely dry place. The dark side is more than ice cold, but when it passes into sunlight, any ice should have long ago been baked away. The possible exceptions are permanently shadowed craters near the Moon’s poles, and data announced this month by NASA verified the presence of hydrogen in those areas, which would most likely be in the form of water.

If water is somehow more widespread, that could make future settlement of the Moon easier, especially if significant water could be extracted just by heating the soil. Oxygen would also be a key component for breathable air for astronauts, and hydrogen and oxygen can also be used for rocket fuel or power generation.