CfA: Explosion Illuminates Invisible Galaxy in the Dark Ages

[bystander]

Explosion Illuminates Invisible Galaxy in the Dark Ages
Harvard Smithsonian Center for Astrophysics | 2013 Aug 06

[imghover=http://www.cfa.harvard.edu/image_archiv ... /lores.jpg]http://www.cfa.harvard.edu/image_archiv ... /lores.jpg[/imghover][hr][/hr]Before light from the gamma-ray burst arrives at the Earth for astronomers to study, it
passes through interstellar gas in its host galaxy (top left), and intergalactic gas between
the distant galaxy and us (bottom right). This gas filters the light by absorbing some colors
and leaves a signature on the light that can be seen in its spectrum. This "signature" allows
scientists to characterize the gamma-ray burst, its environment, and the material between
us and the distant galaxy. (Credit: Gemini Observatory/AURA, artwork by Lynette Cook)[/i][hr][/hr]

More than 12 billion years ago a star exploded, ripping itself apart and blasting its remains outward in twin jets at nearly the speed of light. At its death it glowed so brightly that it outshone its entire galaxy by a million times. This brilliant flash traveled across space for 12.7 billion years to a planet that hadn't even existed at the time of the explosion - our Earth. By analyzing this light, astronomers learned about a galaxy that was otherwise too small, faint and far away for even the Hubble Space Telescope to see.

"This star lived at a very interesting time, the so-called dark ages just a billion years after the Big Bang," says lead author Ryan Chornock of the Harvard-Smithsonian Center for Astrophysics (CfA).

"In a sense, we're forensic scientists investigating the death of a star and the life of a galaxy in the earliest phases of cosmic time," he adds.

The star announced its death with a flash of gamma rays, an event known as a gamma-ray burst (GRB). GRB 130606A was classified as a long GRB since the burst lasted for more than four minutes. It was detected by NASA's Swift spacecraft on June 6th. Chornock and his team quickly organized follow-up observations by the MMT Telescope in Arizona and the Gemini North telescope in Hawaii.

"We were able to get right on target in a matter of hours," Chornock says. "That speed was crucial in detecting and studying the afterglow."

A GRB afterglow occurs when jets from the burst slam into surrounding gas, sweeping that material up like a snowplow, heating it, and causing it to glow. As the afterglow's light travels through the dead star's host galaxy, it passes through clouds of interstellar gas. Chemical elements within those clouds absorb light at certain wavelengths, leaving "fingerprints." By splitting the light into a rainbow spectrum, astronomers can study those fingerprints and learn what gases the distant galaxy contained.

All chemical elements heavier than hydrogen, helium, and lithium had to be created by stars. As a result those heavy elements, which astronomers collectively call "metals," took time to accumulate. Life could not have existed in the early universe because the elements of life, including carbon and oxygen, did not exist.

Chornock and his colleagues found that the GRB galaxy contained only about one-tenth of the metals in our solar system. Theory suggests that although rocky planets might have been able to form, life probably could not thrive yet.

"At the time this star died, the universe was still getting ready for life. It didn't have life yet, but was building the required elements," says Chornock.

At a redshift of 5.9, or a distance of 12.7 billion light-years, GRB 130606A is one of the most distant gamma-ray bursts ever found.

"In the future we will be able to find and exploit even more distant GRBs with the planned Giant Magellan Telescope," says Edo Berger of the CfA, a co-author on the publication.

GRB 130606A as a Probe of the Intergalactic Medium and the Interstellar Medium
in a Star-forming Galaxy in the First Gyr After the Big Bang - Ryan Chornock et al

• arXiv.org > astro-ph > arXiv:1306.3949 > 07 Jun 2013 (v1), 16 Jul 2013 (v2)

GRB Lights Up Ancient Hidden Galaxy
Universe Today | Tammy Plotner | 2013 Aug 07

[neufer]

Scientists have studied gamma-ray bursts, which are triggered by the collapse of massive stars, for three decades. Now, one is forcing them to reconsider what they thought they knew.

[b][color=#0000FF]This image shows an artist's rendering on how a gamma-ray burst occurs with a massive star collapsing and creating a black hole, beaming out focused light and radiation bursts. Astronomers in April 2013 saw the biggest and brightest cosmic explosion ever witnessed, a large gamma-ray burst.[/color][/b]

---

By Pete Spotts, Staff writer, November 21, 2013

<<An exploded star some 3.8 billion light-years away is forcing scientists to overhaul much of what they thought they knew about gamma-ray bursts – intense blasts of radiation triggered, in this case, by a star tens of times more massive than the sun that exhausted its nuclear fuel, exploded, then collapsed to form a black hole.

Last April, gamma rays from the blast struck detectors in gamma-ray observatories orbiting Earth, triggering a frenzy of space- and ground-based observations. Many of them fly in the face of explanations researchers have developed during the past 30 years for the processes driving the evolution of a burst from flash to fade out, according to four research papers appearing Friday in the journal Science.

“Some of our theories are just going down the drain,” said Charles Dermer, an astrophysicist at the Los Alamos National Laboratory in New Mexico and a member of one of the teams reporting on their observations of the burst, known as GRB 130427A.

The event, dubbed a long-duration gamma-ray burst (GRB), is typically seen in the distant, early universe, Dr. Dermer said during a briefing Thursday. This one was much closer. And while typical long-duration bursts last from a few seconds to a few minutes, GRB 130427A put on its display for 20 hours.

The event's duration, relatively close proximity, and the range of observatories in space and on the ground that could monitor it at a range of wavelengths has provided scientists with an unprecedented opportunity to explore the workings of one of the more extreme ends a star can inflict on itself. The encouraging news: Traits seen in the gamma-ray emissions from initial burst through the afterglow compare favorably to the traits seen in the behavior of gamma rays in more-distant, long-duration bursts. “This suggests that GRBs in the contemporary universe and in the early, distant universe share a common central engine,” Dermer said.

In October, a team led by Dong Xu, an astrophysicist at the University of Copehagen, found evidence for a stellar explosion, or supernova, at GRB 130427A's location. The evidence pointed to a type of supernova that involves a star with at least 20 to 30 times the sun's mass. Such stars are so large than when they finally collapse, they form black holes. In the process of collapsing, the black hole sends jets of electrons and ionized gas spiraling out along along magnetic fields forming near the black hole's poles. These jets punch their way through the turbulent, expanding layers of material the star shed in the explosion and its prelude. And they collide with the interstellar medium – the dust and gas between stars. Collisions inside and outside the expanding layer can generate gamma rays, which tend to be focused in the direction the jets point. This makes the object an extremely bright gamma-ray beacon, if briefly, when the viewing angle is just right.

Indeed, GRB 130427A “topped the charts” in the amount of gamma-ray photons it released, the energy levels some of those photons achieved, total explosion energy, and its gamma-ray brilliance, added Paul Hertz, who heads NASA's astrophysics division in Washington. At visible wavelengths, the burst was the second brightest GRB researchers have seen. That made possible the detailed measurements that have left researchers scratching their chins. For instance, ordinarily, one might expect the gamma-ray photons with the highest energy to appear immediately following the star's explosion, researchers say. But with GRB 130427A, some of the highest energy photons, including the new record-holder, appeared hours after the blast. “This is hard to explain with our current models,” Dermer said. In addition, gamma rays and emissions at visible wavelengths brightened and dimmed in tandem, quite unexpected because theory suggested they come from different regions of the expanding shells of material and thus should have peaked and dimmed at different times. Finally, theorists had posited different mechanisms for generating gamma rays and X-rays that are part of the light show a long-duration gamma-ray burst puts on. The result should have been a fadeout for the two forms of light punctuated by periods where emissions were interrupted. Instead, the two dimmed smoothly.

The theoretical edifice GRB 130427A is eroding has been 46 years in the making. Scientists stumbled across the first gamma-ray burst in 1967, when a US satellite designed to detect nuclear-weapons tests in space picked up a burst's emissions. As follow-on versions of the satellite with better sensors were lofted, scientists detected more. By 1973, the data were declassified and published, opening a window on the mysterious phenomenon. It took another 18 years to determine that the bursts they were seeing occurred far outside the Milky Way, where researchers originally thought the bursts were taking place.>>

[neufer]

Click to play embedded YouTube video.

Monster Gamma-Ray Burst Challenges Theories
NEWS by Monica Young, Sky & Telescope

<<In the early-morning hours of April 27th, a burst of high-energy photons set off an alarm on the Fermi Gamma-Ray Space Telescope. The photons came from a star that exploded 3.75 billion years ago in spectacular fashion, sending out a brilliant flash of radiation known as a gamma-ray burst. The star’s demise wasn’t just any supernova. The gamma-ray burst GRB 130427A turned out to be the longest, most energetic such explosion on record. Within four minutes of the alarm, the Swift spacecraft had swung its three telescopes to monitor the explosion with Fermi. An array of robotic telescopes (Rapid Telescopes for Optical Response, or RAPTOR), the CARMA millimeter-wave observatory, and the NuSTAR X-ray telescope also joined in on the action. In the end, the explosion flooded 58 observatories worldwide (on the ground and in space) with its photons. In this week’s Science, four international collaborations dissect this data treasure-trove to find out what made the monster tick. And it turns out the observations show astronomers are missing a piece of the GRB puzzle.

These days, GRBs are common observations, appearing at a rate of about one per day, but GRB 130427A was a rare occurrence. Not only was this burst extremely powerful, it also happened to be nearby. Combined, its brightness and location made it easy for telescopes to collect heaping piles of data. And the burst just kept on giving. Although the main burst lasted just 20 seconds (typical for this kind of “long” GRB), stray gamma rays kept pouring in for another 20 hours, long after the main burst had finished — and much longer than any other such burst. But of all the thousands of gamma-ray photons Fermi collected during that time, it only took two to steal the spotlight. The first appeared 19 seconds after the burst began; the next appeared 3¾ minutes later. And these photons packed a serious punch: 73 and 95 billion electron volts, the highest-energy photons ever recorded from a GRB. There’s just one problem: according to the current model used to explain GRBs, these photons shouldn’t have existed.

In the standard picture, “long” GRBs like 130427A (which last on average about 30 seconds), herald the collapse of very massive stars. As the star’s core implodes to form a black hole, gas just outside that boundary feeds a jet that burns its way through the star’s outer layers. Collisions within the jet produce a brilliant burst of emission, which we’ll see if we happen to be looking down the jet’s nozzle. It takes only seconds for the jet to escape the star, but even then it’s not free. It rams into the surrounding gas cocoon left behind by the dead star’s winds, pushing electrons to nearly the speed of light. As the ultra-speedy particles race around magnetic field lines, they release their pent-up energy as radiation: the burst’s afterglow. This synchrotron radiation explains a lot of the emission we see from GRBs, including everything from gamma rays and X-rays to visible light.

For the most part, this model works pretty well to explain GRB 130427A. But those two high-energy photons pose a problem. No matter how fast they spiral, electrons can’t radiate away 95 billion (or 73 billion, for that matter) electron volts of energy. So what if electrons perform double-duty? Alessandro Maselli (INAF-IASF Palermo, Italy), lead author of one of the Science papers, suggests that the same electrons that created the photons in the first place could pass on an extra punch of energy if they later collide with their progeny. But one study’s coauthor, Charles Dermer (Naval Research Laboratory), says if there are two ways to make photons, we should see a jump in the number created at different energies (say, many more gamma rays than X-rays). Instead, the whole spectrum from visible light to ultra-high-energy gamma rays is smooth, suggesting all the photons come from one mechanism.

The Science papers aren’t lacking for alternative explanations for making these gamma-ray photons: the teams propose everything from snapping magnetic field lines to scorching-hot gas around the forming black hole. The synchrotron model hasn’t been ruled out either, though it’s clear that something extra is needed. But so far, none of the suggestions are testable — yet.>>