BA: The Crab is still crabby

[bystander]

The Crab is still crabby
Bad Astronomy | 04 Oct 2010

A thousand years ago, and 6500 light years away from Earth, a high mass star exploded. An octillion tons of gas blasted outwards at speeds of thousands of kilometers per second, forming tendrils and wisps as it raced away. At the center of the conflagration, the core of the star had collapsed into an ultradense object called a neutron star. It has the mass of the Sun crammed into a ball only 20 – 30 km (12 – 18 miles) across, and is spinning at a rate of 30 times per second.

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All this happened a long time ago. The debris is what we now call the Crab Nebula, and is one of the best-studied objects in the sky. And that’s a good thing, because even now the Crab is capable of throwing tantrums… and we can see it when it does!

This image is a brand spanking new shot of the heart of the Crab Nebula taken by the Hubble Space Telescope. And by new, I mean it was taken on Saturday, October 2! It’s a bit hard to see what’s going on, so I created an annotated version:

The pulsar is labeled. It’s sitting right at the center of the gas cloud, which extends way beyond the edges of this picture. As the pulsar spins, it emits a fast stream of particles that act like a wind, compressing the gas in the nebula and creating those circles of light. They look elliptical because the whole system is tilted, and you’re seeing it like a DVD held at an angle. From what I can tell, the bottom left is the side toward us, and the upper right is farther away, as if we’re looking down on it.

In mid-September, just a couple of weeks ago, several orbiting observatories noted that there was an increased amount of gamma rays coming from this part of the sky. Gamma rays are the highest energy form of light, and there aren’t many sources in the sky that can create them at all, let alone in quantities that can be seen. The Crab is the brightest continuous gamma-ray source we know, and so it was immediately put on the Most Wanted list.

Hubble was quickly pointed at the pulsar, and that image above was taken. Now note the rectangular bit to the left of the pulsar. Some of that stuff was seen in earlier images, but not this bright! You can compare it to the image on the right, taken in December 1993 by Hubble, which I’ve rotated and scaled to be close to what we see in the new image. The overall structure is very roughly the same, but clearly that gas to the left is brighter in the new image. I suspect that rectangular structure in the new image is actually a ring, the shape distorted by density variations in the gas.

What must have happened [note: I'm conjecturing here and may be way off, but the basic stuff is probably correct] is that something occurred on the pulsar: maybe it had a starquake, or a bit of material fell on it (the crushing gravity of a neutron star guarantees that anything hitting it will be doing so at a large fraction of the speed of light, generating a lot of explosive energy; even a marshmallow hitting at that speed explodes like an atomic bomb!). But something violent happened, and a blast of gamma-rays ensued.

Image Credit: NASA/ESA/Hubble

[neufer]

The Crab is still crabby
Bad Astronomy | 04 Oct 2010

In mid-September, just a couple of weeks ago, several orbiting observatories noted that there was an increased amount of gamma rays coming from this part of the sky. Gamma rays are the highest energy form of light, and there aren’t many sources in the sky that can create them at all, let alone in quantities that can be seen. The Crab is the brightest continuous gamma-ray source we know, and so it was immediately put on the Most Wanted list.

Most Wanted list That seems a bit excessive IMO.

Can't we just call it is a "pulsar of interest?"

[bystander]

Fermi's Large Area Telescope Sees Surprising Flares in Crab Nebula
SLAC National Accelerator Laboratory | 2011 Jan 06

[url=http://hubblesite.org/newscenter/archive/releases/2005/37/][size=110][b]A Giant Hubble Mosaic of the Crab Nebula[/b][/size][/url] [i](Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University))[/i]

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The Crab Nebula, one of our best-known and most stable neighbors in the winter sky, is shocking scientists with a propensity for fireworks—gamma-ray flares set off by the most energetic particles ever traced to a specific astronomical object. The discovery, reported today by scientists working with two orbiting telescopes, is leading researchers to rethink their ideas of how cosmic particles are accelerated.

"We were dumbfounded," said Roger Blandford, who directs the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University. "It's an emblematic object," he said; also known as M1, the Crab Nebula was the first astronomical object catalogued in 1771 by Charles Messier. "It's a big deal historically, and we're making an amazing discovery about it."

Blandford was part of a KIPAC team led by scientists Rolf Buehler and Stefan Funk that used observations from the Large Area Telescope, one of two primary instruments aboard NASA’s Fermi Gamma-ray Space Telescope, to confirm one flare and discover another. Their report was posted online today in Science Express alongside a report from the Italian orbiting telescope Astro-rivelatore Gamma a Immagini LEggero, or AGILE, which also detected gamma-ray flares in the Crab Nebula.

The Crab Nebula, and the rapidly spinning neutron star that powers it, are the remnants of a supernova explosion documented by Chinese and Middle Eastern astronomers in 1054. After shedding much of its outer gases and dust, the dying star collapsed into a pulsar, a super-dense, rapidly spinning ball of neutrons that emits a pulse of radiation every 33 milliseconds, like clockwork. ...

Crab nebula outbursts shock astronomers
Science Daily | Ron Cowen | 2011 Jan 01

[url=http://hubblesite.org/newscenter/archive/releases/2005/37][size=110][b]Crab Nebula: A Dead Star Creates Celestial Havoc[/b][/size][/url] [i]Credit: X-ray: NASA/CXC/ASU/J.Hester et al. (Chandra); Optical: NASA/ESA/ASU/J.Hester & A.Loll (Hubble); Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz (Spitzer)[/i]

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Astronomers consider the Crab nebula one of the steadiest sources of high-energy radiation in the universe. Radiation from the supernova remnant is believed to be so constant that astronomers use it as a standard candle with which to measure the energetic radiation of other astronomical sources.

That’s why researchers are astounded that two spacecraft recently recorded giant gamma-ray hiccups from the Crab, the remnants of a stellar explosion 6,500 light-years from Earth that was observed by humans in 1054. The intensity of the Crab’s gamma-ray radiation suddenly became two to three times stronger for three days beginning September 19, scientists with the Italian Space Agency’s AGILE telescope reported in a September 22 Astronomical Telegram, an e-mail communication. Researchers with Fermi’s Gamma-ray Space Telescope found an even larger increase over roughly the same time period, they reported in a telegram on the following day. Both teams also announced they had found evidence of previous flares — the AGILE telescope recorded an outburst in the fall of 2007 while the Fermi team spotted one in February 2009.

The suspected source of the energetic flares, along with steadier radiation emanating from the nebula, is blizzards of electrons spat out by the Crab’s pulsar — the rapidly rotating, exploded cinder of a star that lies at the very center of the Crab nebula. But figuring out exactly how the electrons got revved up to energies of at least 10¹⁵ electron volts — the most energetic charged particles ever associated with a distinct astrophysical object — for so short a time has astronomers at the biannual Texas Symposium on Relativistic Astrophysics, held this year in Heidelberg, Germany, scratching their heads and searching for new models. ...

Mystery flares betray hidden force within Crab Nebula
New Scientist | Rachel Courtland | 2011 Jan 06

[url=http://chandra.harvard.edu/photo/2009/crab/][size=110][b]The Crab Nebula: A Cosmic Icon[/b][/size][/url] [i](Credit : X-ray: NASA/CXC/SAO/F.Seward (Chandra); Optical: NASA/ESA/ASU/J.Hester & A.Loll (Hubble); Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz (Spitzer))[/i]

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Forget the Large Hadron Collider: an interstellar dust cloud called the Crab nebula has been identified as the most powerful known particle accelerator in the universe. But exactly how it boosts particles to record-breaking speeds is a mystery. The finding also adds an extra complication for astronomers who use the Crab to calibrate their instruments.

Two orbiting telescopes have revealed that the nebula, which sits some 6500 light years from Earth, releases brief, bright flares of gamma-rays, a pair of papers released today report. These flares are most likely produced by electrons that have been whipped up to record-breaking speeds.

These are "the highest particle energies ever associated with a single source", says Elisa Bernardini of the DESY research centre in Germany in an accompanying paper.

The discovery of the flares comes as a surprise. The Crab nebula is used as a "standard candle" to calibrate astronomical instruments because for most of the time it gives off a steady stream of gamma rays and other wavelengths.

It is made up of the gas and dust left over from a stellar explosion observed by Chinese astronomers in AD 1054, and the constant brightness is thought to be due to the pulsar, or dead spinning star, at the nebula's centre, which sends electrons slamming into the nebula. Magnetic fields help confine this electron stream to the nebula, but also slow down the electrons, causing them to emit light. ...

1. Gamma-Ray Flares from the Crab Nebula - Fermi-LAT Collaboration
   • Science 331(6018) 739 (2011 Feb 11) DOI: 10.1126/science.1199705
     arXiv.org > astro-ph > arXiv:1011.3855 > 16 Nov 2010 (v1), 10 Jan 2011 (v3)
2. Discovery of Powerful Gamma-Ray Flares from the Crab Nebula - M Tavani et al
   • Science 331(6018) 736 (2011 Feb 11) DOI: 10.1126/science.1200083
     arXiv.org > astro-ph > arXiv:1101.2311 > 12 Jan 2011
3. On the variability of the GeV and multi-TeV gamma-ray emission from the Crab Nebula - W Bednarek, W Idec
   • arXiv.org > astro-ph > arXiv:1011.4176 > 18 Nov 2010, 15 Feb 2011 (v2)

[neufer]

The Crab is still crabby
Bad Astronomy | 04 Oct 2010

Click to view full size image

This image is a brand spanking new shot of the heart of the Crab Nebula taken by the Hubble Space Telescope. And by new, I mean it was taken on Saturday, October 2! It’s a bit hard to see what’s going on, so I created an annotated version:

The pulsar is labeled. It’s sitting right at the center of the gas cloud, which extends way beyond the edges of this picture. As the pulsar spins, it emits a fast stream of particles that act like a wind, compressing the gas in the nebula and creating those circles of light. They look elliptical because the whole system is tilted, and you’re seeing it like a DVD held at an angle. From what I can tell, the bottom left is the side toward us, and the upper right is farther away, as if we’re looking down on it.

In mid-September, just a couple of weeks ago, several orbiting observatories noted that there was an increased amount of gamma rays coming from this part of the sky. Gamma rays are the highest energy form of light, and there aren’t many sources in the sky that can create them at all, let alone in quantities that can be seen. The Crab is the brightest continuous gamma-ray source we know, and so it was immediately put on the Most Wanted list.

Hubble was quickly pointed at the pulsar, and that image above was taken. Now note the rectangular bit to the left of the pulsar. Some of that stuff was seen in earlier images, but not this bright! You can compare it to the image on the right, taken in December 1993 by Hubble, which I’ve rotated and scaled to be close to what we see in the new image. The overall structure is very roughly the same, but clearly that gas to the left is brighter in the new image. I suspect that rectangular structure in the new image is actually a ring, the shape distorted by density variations in the gas.

What must have happened [note: I'm conjecturing here and may be way off, but the basic stuff is probably correct] is that something occurred on the pulsar: maybe it had a starquake, or a bit of material fell on it (the crushing gravity of a neutron star guarantees that anything hitting it will be doing so at a large fraction of the speed of light, generating a lot of explosive energy; even a marshmallow hitting at that speed explodes like an atomic bomb!). But something violent happened, and a blast of gamma-rays ensued.

<<Mr. Eugene H. Krabs is a character on the television cartoon series SpongeBob SquarePants. He is voiced by Clancy Brown. Mr. Krabs is the owner and founder of the Krusty Krab restaurant and his rival Sheldon Plankton has a struggling restaurant called the Chum Bucket across the road from his. The character is seen in most episodes of the show, as well as in movies, video games, and other media based on the series.

Mr. Krabs lives in a rusty anchor with his 16-year old daughter Pearl. Pearl is constantly embarrassed by her dad and his cheap antics, leading them to show some distance with each other. Mr. Krabs is extremely rich, but is extremely cheap when it comes to losing even one cent. In fact, he considers money to be living beings, and in more recent episodes, he berates other persons when they lose money. In the Episode "Grandpappy the Pirate", he revealed to his grandfather that he was not a pirate, causing his grandfather great disappointment and anger until he found out about Mr. Krabs cheap and high prices for the food in the Krusty Krab. Mr. Krabs is also seen with a pet worm named Mr. Doodles. Mr. Doodles has been seen in only a few episodes, but is usually seen walking with Mr. Krabs.>>

[bystander]

Crab Nebula activity keeps confounding
Science News | Ron Cowen | 2011 May 11

Latest outbursts send theorists scuttling for an explanation

The latest and greatest outbursts from the Crab Nebula — long known for its steady high-energy glow — are challenging theories about how the heavens accelerate charged particles to high energies.

Only last year, scientists were astonished to find that the nebula — a giant cloud 6,500 light-years from Earth with the spinning cinder of an exploded star at its center — had spat out gamma-ray flares that fluctuated on time scales of only a few days (SN: 1/1/11, p. 11).

Last month, however, the nebula outdid itself, says Rolf Buehler of the SLAC National Accelerator Laboratory in Menlo Park, Calif. The Crab hurled gamma-ray flares, more energetic and five times brighter than any previously recorded, that fluctuated over just one to three hours. Buehler announced the findings, based on observations with the Fermi Gamma-ray Space Telescope, on May 11 in Rome at the annual Fermi symposium. A second team observed the April fireworks with AGILE, another orbiting telescope.

“These recently discovered flares indeed present a new set of challenges and highlight our ignorance of how this fascinating [object] works,” comments theorist Dmitri Uzdensky of the University of Colorado at Boulder.

Earlier outbursts had already suggested that the Crab is producing gamma rays by accelerating electrons and positrons to energies around a quadrillion, or 10¹⁵, electron-volts — about 100 times higher than the maximum energy of protons at the Large Hadron Collider, the world’s most powerful atom smasher. Because no signal can travel a distance greater than light can in a given period of time, the rapid variation of the April flares indicates that the charged particles were revved up within a tiny region of the vast Crab no bigger than the solar system, Buehler notes.

“The main question is how particles can be accelerated to such high energies so fast,” he says. The observations may require a new, separate population of energetic charged particles, similar to those that produce the strongest X-ray bursts associated with solar flares, says Jonathan Arons of the University of California, Berkeley.

Although the Crab’s gamma-ray flares are believed to be generated by electrons or positrons gyrating around magnetic fields, the energy of the observed radiation is several times higher than such a process can normally produce, says theoretical astrophysicist Mitchell Begelman.

In a theoretical model proposed by a University of Colorado team of Begelman, Uzdensky and Benoît Cerutti, the charged particles are accelerated near the nebula's center where magnetic fields are violently rearranged, unleashing enormous amounts of energy in the presence of a strong electric field. Charged particles get sucked into the region, and the field enables them to form a higher-energy beam — and generate higher-energy radiation — than they normally would. The researchers posted their work online at arXiv.org on May 5. Although the study is based only on the older flares, the overall picture remains the same for the April outbursts, Uzdensky says.

Regions where magnetic fields are violently rearranged generate turbulence and undulations, which may result in wiggles in the beam of energetic charged particles and the gamma rays it produces. The fluctuations may occur simply because a wiggling beam could be seen only during the short time it swept across Earth’s line of sight.

Reconnection-Powered Linear Accelerator and Gamma-Ray Flares in the Crab Nebula - DA Uzdensky, B Cerutti, MC Begelman

• arXiv.org > astro-ph > arXiv:1105.0942 > 04 May 2011

[owlice]

The movie here from Chandra: http://chandra.harvard.edu/photo/2011/crab/

Too cool for words!!!

[luigi]

Thank you!!!
And my jaw is? Where?

[owlice]

On the floor, next to mine!

[bystander]

Fermi Spots 'Superflares' in the Crab Nebula
NASA Fermi | CXC | 2011 May 11

Click to play embedded YouTube video.

[attachment=1]Crab-superflare.jpg[/attachment]

[attachment=0]Crab_flare.jpg[/attachment]

The famous Crab Nebula supernova remnant has erupted in an enormous flare five times more powerful than any flare previously seen from the object. On April 12, NASA's Fermi Gamma-ray Space Telescope first detected the outburst, which lasted six days.

The nebula is the wreckage of an exploded star that emitted light which reached Earth in the year 1054. It is located 6,500 light-years away in the constellation Taurus. At the heart of an expanding gas cloud lies what is left of the original star's core, a superdense neutron star that spins 30 times a second. With each rotation, the star swings intense beams of radiation toward Earth, creating the pulsed emission characteristic of spinning neutron stars (also known as pulsars).

Apart from these pulses, astrophysicists believed the Crab Nebula was a virtually constant source of high-energy radiation. But in January, scientists associated with several orbiting observatories, including NASA's Fermi, Swift and Rossi X-ray Timing Explorer, reported long-term brightness changes at X-ray energies.

"The Crab Nebula hosts high-energy variability that we're only now fully appreciating," said Rolf Buehler, a member of the Fermi Large Area Telescope (LAT) team at the Kavli Institute for Particle Astrophysics and Cosmology, a facility jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University.

Since 2009, Fermi and the Italian Space Agency's AGILE satellite have detected several short-lived gamma-ray flares at energies greater than 100 million electron volts (eV) -- hundreds of times higher than the nebula's observed X-ray variations. For comparison, visible light has energies between 2 and 3 eV.

On April 12, Fermi's LAT, and later AGILE, detected a flare that grew about 30 times more energetic than the nebula's normal gamma-ray output and about five times more powerful than previous outbursts. On April 16, an even brighter flare erupted, but within a couple of days, the unusual activity completely faded out.

"These superflares are the most intense outbursts we've seen to date, and they are all extremely puzzling events," said Alice Harding at NASA's Goddard Space Flight Center in Greenbelt, Md. "We think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that's happening remains a mystery."

The Crab's high-energy emissions are thought to be the result of physical processes that tap into the neutron star's rapid spin. Theorists generally agree the flares must arise within about one-third of a light-year from the neutron star, but efforts to locate them more precisely have proven unsuccessful so far.

Since September 2010, NASA's Chandra X-ray Observatory routinely has monitored the nebula in an effort to identify X-ray emission associated with the outbursts. When Fermi scientists alerted astronomers to the onset of a new flare, Martin Weisskopf and Allyn Tennant at NASA's Marshall Space Flight Center in Huntsville, Ala., triggered a set of pre-planned observations using Chandra.

"Thanks to the Fermi alert, we were fortunate that our planned observations actually occurred when the flares were brightest in gamma rays," Weisskopf said. "Despite Chandra's excellent resolution, we detected no obvious changes in the X-ray structures in the nebula and surrounding the pulsar that could be clearly associated with the flare."

Scientists think the flares occur as the intense magnetic field near the pulsar undergoes sudden restructuring. Such changes can accelerate particles like electrons to velocities near the speed of light. As these high-speed electrons interact with the magnetic field, they emit gamma rays.

To account for the observed emission, scientists say the electrons must have energies 100 times greater than can be achieved in any particle accelerator on Earth. This makes them the highest-energy electrons known to be associated with any galactic source. Based on the rise and fall of gamma rays during the April outbursts, scientists estimate that the size of the emitting region must be comparable in size to the solar system.

CXC: Crab Nebula: The Crab in Action & The Case of The Dog That Did Not Bark (2011 May 11)

[attachment=2]crab_montage.jpg[/attachment]

Click to play embedded YouTube video.

Click to play embedded YouTube video.

A new movie from NASA's Chandra X-ray Observatory shows a sequence of Chandra images of the Crab Nebula, taken over an interval of seven months. Dramatic variations are seen, including the expansion of a ring of X-ray emission around the pulsar (white dot near center) and changes in the knots within this ring.

However, arguably the most striking result of these observations is the variations that were not observed, or in analogy with a famous Sherlock Holmes story¹, this could be a case where the fact that the dog that did not bark helps to solve a mystery.

The pulsar at the center of the Crab Nebula is a neutron star that spins around about 30 times a second. It was created from a supernova explosion in our galaxy that was observed by astronomers in China and other countries in the year 1054.

As the young pulsar slows down, large amounts of energy are injected into its surroundings. In particular, a high-speed wind of matter and anti-matter particles plows into the surrounding nebula, creating a shock wave that forms the expanding ring seen in the movie. Jets from the poles of the pulsar spew X-ray emitting matter and antimatter particles in a direction perpendicular to the ring.

The goal of these latest Chandra observations was to pinpoint the location of remarkable gamma-ray flares observed by NASA's Fermi Gamma Ray Observatory and Italy's AGILE Satellite. A strong gamma-ray flare was observed from the Crab in September 2010, followed by an even stronger series of "superflares" in April 2011. The gamma-ray observatories were not able to locate the source of the flares within the nebula, but it was hoped that Chandra, with its high-resolution images, would.

Chandra began observing the Crab on monthly intervals beginning six days after the discovery of the gamma-ray flare in September 2010. This established a baseline of seven images of the nebula before the superflare was seen just last month.

When Fermi scientists saw that more flaring activity was beginning in April 2011, a pre-planned set of five Chandra observations was initiated. Two of these observations were made when strong gamma-ray flares occurred, but no clear evidence was seen for correlated flares in the Chandra images. The movie shows the April observations in "slow motion" to focus on the time when the gamma-ray superflares occurred. The movie shows three loops through the sequence of images, along with a timeline near the bottom.

Despite the lack of a "barking dog" in the X-ray data, these observations, as in the Sherlock Holmes story¹, will help scientists to home in on an explanation of the gamma-ray flares. The Chandra data provide strong constraints on the behavior, at relatively low energies, of the particles that have been accelerated to produce the gamma-ray flares. Another possible explanation follows if the gamma-ray flaring occurred in regions very close to the pulsar. Then they would have been missed by Chandra, because the Crab pulsar is so bright that the detectors are in essence "overexposed" so variations from that region cannot be observed. Note that in the movie an artificial source of constant brightness is included to show the position of the pulsar.

[list=1][*] "Is there any point to which you would wish to draw my attention?"
"To the curious incident of the dog in the night-time."
"The dog did nothing in the night-time."
"That was the curious incident."

The story by Sir Arthur Conan Doyle is "Silver Blaze"[/list]

Crab Nebula Erupts in a Superflare
Universe Today | Nancy Atkinson | 2011 May 11

Recent eruptions in the Crab Nebula mystify astronomers
Smithsonian Institute | Research Online | 2011 May 12

[neufer]

Click to play embedded YouTube video.

Click to play embedded YouTube video.

[Ann]

Wow, that's a cool "double-view" of a hot object, neufer. Talk about a beating heart.

As for the down-to-Earth carab, although it may be a far rarer object than the Carb Nebula from a cosmic point of view, I'll still leave it for others to admire in videos.

Ann

[bystander]

Crab Pulsar Dazzles Astronomers with Gamma-Ray Beams
Center for Astrophysics | 2011 Oct 06

[i]Artist's concept of the Crab Nebula pulsar. (Credit: David A. Aguilar (CfA))[/i]

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A thousand years ago, a brilliant beacon of light blazed in the sky, shining brightly enough to be seen even in daytime for almost a month. Native American and Chinese observers recorded the eye-catching event. We now know that they witnessed an exploding star, which left behind a gaseous remnant known as the Crab Nebula.

The same object that dazzled skygazers in 1054 C.E. continues to dazzle astronomers today by pumping out radiation at higher energies than anyone expected. Researchers have detected pulses of gamma rays with energies exceeding 100 billion electron-volts (100 GeV) -- a million times more energetic than medical X-rays and 100 billion times more than visible light.

"If you asked theorists a year ago whether we would see gamma-ray pulses this energetic, almost all of them would have said, 'No.' There's just no theory that can account for what we've found," said corresponding author Martin Schroedter of the Harvard-Smithsonian Center for Astrophysics (CfA).

The gamma rays come from an extreme object at the Crab Nebula's center known as a pulsar. A pulsar is a spinning neutron star -- the collapsed core of a massive star. Although only a few miles across, a neutron star is so dense that it weighs more than the Sun.

Rotating about 30 times a second, the Crab pulsar generates beams of radiation from its spinning magnetic field. The beams sweep around like a lighthouse beacon because they're not aligned with the star's rotation axis. So although the beams are steady, they're detected on Earth as rapid pulses of radiation.

The discovery was reported by an international team of scientists in a paper in the October 7 issue of Science. Corresponding author Nepomuk Otte, a postdoctoral researcher at the University of California, Santa Cruz, said that some researchers had told him he was crazy to even look for pulsar emission in this energy realm.

"It turns out that being persistent and stubborn helps," Otte said. "These results put new constraints on the mechanism for how the gamma-ray emission is generated."

Some possible scenarios to explain the data have been put forward, but it will take more data, or even a next-generation observatory, to really understand the mechanisms behind these gamma-ray pulses.

The gamma-ray pulses were detected by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) -- the most powerful very-high-energy gamma-ray observatory in the Northern Hemisphere. VERITAS is located at the Smithsonian's Whipple Observatory, just south of Tucson, Ariz.

Astronomers observe very-high-energy gamma rays with ground-based Cherenkov telescopes. These gamma rays, coming from cosmic "particle accelerators," are absorbed in Earth's atmosphere, where they create a short-lived shower of subatomic particles. The Cherenkov telescopes detect the faint, extremely short flashes of blue light that these particles emit (named Cherenkov light) using extremely sensitive cameras. The images can be used to infer the arrival direction and initial energy of the gamma rays.

This technique is used by gamma-ray observatories throughout the world, and was pioneered under the direction of CfA's Trevor Weekes using the 10-meter Cherenkov telescope at Whipple Observatory. The Whipple 10-meter telescope was used to detect the first Galactic and extragalactic sources of very-high-energy gamma rays.

VERITAS continues the tradition of Whipple's 10-meter telescope. It is comprised of an array of four 12-meter-diameter Cherenkov telescopes. VERITAS began full-scale observations in September 2007. The telescopes are used to study the remnants of exploded stars, distant galaxies, powerful gamma-ray bursts, and to search for evidence of mysterious dark matter particles.

Crab Beams Most Energetic Gamma Rays from Pulsar
University of California, Santa Cruz | 2011 Oct 06

Very-High-Energy Gamma Rays from Crab Pulsar
Iowa State University | 2011 Oct 06

Pulsed Radiation from Crab Nebula Wasn't Supposed To Be There
Washington University, St. Louis | 2011 Oct 06

Crab emits light at highest energies ever detected from pulsar
University of California, Los Angeles | 2011 Oct 06

Crab Pulsar Emits Light at Higher Energies Than Expected
National Science Foundation | 2011 Oct 06

Star packs big gamma-ray jolt, researchers discover
University of Delaware | 2011 Oct 07

Detection of Pulsed Gamma Rays Above 100 GeV from the Crab Pulsar - VERITAS Collaboration

• Science 334(6052) 69 (7 Oct 2011) DOI: 10.1126/science.1208192
  arXiv.org > astro-ph > arXiv:1108.3797 > 18 Aug 2011

[bystander]

The High Energy Crab
Smithsonian Astrophysical Observatory
Weekly Science Update | 2011 Oct 28

[i]An image of the Crab Nebula taken by the Hubble Space Telescope. Observations of the Crab in gamma-rays find surprisingly bright emission. (Credit: NASA)[/i]

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The Crab Nebula is the remnant of a supernova. Its precursor star exploded in 1054 AD in an event that was recorded by Chinese and (quite probably) Anasazi Indian astronomers. It is called the Crab Nebula because it has many tentacle-like gas filaments extending outward nearly radially. At the center of the explosion is a pulsar, the spinning, super-dense stellar ash often left behind after a supernova explosion. Supernovae play a critical role in the cosmic ecosystem, because they seed space with the elements needed for life, including carbon, oxygen, and nitrogen. All of these elements are made in the precursor stars as a byproduct of nuclear burning; none were present in the early days of the universe.

Although supernovae have been seen since 1054, pulsars were only detected in 1968 thanks to advances in radio telescopes and the fact that pulsars have strong radio emission. Pulsars are rapidly spinning (the Crab pulsar spins about 30 times a second) and have strong magnetic fields. Charged particles in this environment are whipped around at nearly the speed of light, and as a result they shine at wavelengths from very low energy radio to very high energy gamma rays. Studying this light provides key information about the physical processes underway both in pulsars and supernovae. Surprises from these extreme conditions regularly turn up.

CfA astronomers Nicola Galante, Danny Gibbs, Emmet Thomas, Martin Schroedter, and Trevor Weekes, along with a large team of colleagues, used the VERITAS array of telescopes in Arizona to study the high energy gamma-rays from the Crab Nebula. They made two remarkable discoveries: first, they saw the gamma-rays pulsing, just as radiation at shorter wavelengths does. More significantly, they found that these gamma-rays were extremely intense - much brighter than expected from conventional models of pulsar emission. Writing in the latest issue of Science magazine, the scientists conclude that the gamma-rays cannot come from usual pulsar mechanisms like accelerating charged particles. Instead, they suggest the process whereby light is scattered by fast-moving electrons in the outer reaches of the pulsar environment. The new paper adds new insights to the nature of a pulsar's environment, and is a powerful reminder about the surprises nature still has in store.

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