Virtual Star Party – January 5, 2014: Jupiter in Opposition and 6 Telescopes!

Hosts: Fraser Cain and Scott Lewis

Astronomers:
David Dickinson in Florida
Michael Phillips in North Carolina
Bill McLaughlin in Oregon
Gary Gonella in California
Paul Stewart in New Zealand
Shahrin Ahmad in Malaysia
Stuart Foreman in San Francisco
Thad Szabo in California
Continue reading “Virtual Star Party – January 5, 2014: Jupiter in Opposition and 6 Telescopes!”

Argon – The First Noble Gas Molecules Discovered In Space

Messier 1 Hubble Image: Credit - NASA, ESA, J. Hester and A. Loll (Arizona State University)

There are only six of them: radon, helium, neon, krypton, xenon and the first molecules to be discovered in space – argon. They are all odorless, colorless, monatomic gases with very low chemical reactivity. So where did a team of astronomers using ESA’s Herschel Space Observatory make their rather unusual discovery? Try Messier 1… The “Crab” Nebula!

In a study led by Professor Mike Barlow (UCL Department of Physics & Astronomy), a UCL research team was taking measurements of cold gas and dust regions of this famous supernova remnant in infrared light when they stumbled upon the chemical signature of argon hydrogen ions. By observing in longer wavelengths of light than can be detected by the human eye, the scientists gave credence to current theories of how argon occurs naturally.

“We were doing a survey of the dust in several bright supernova remnants using Herschel, one of which was the Crab Nebula. Discovering argon hydride ions here was unexpected because you don’t expect an atom like argon, a noble gas, to form molecules, and you wouldn’t expect to find them in the harsh environment of a supernova remnant,” said Barlow.

When it comes to a star, they are hot and ignite the visible spectrum. Cold objects like nebular dust are better seen in infrared, but there’s only one problem – Earth’s atmosphere interferes with the detection of that end of the electromagnetic spectrum. Even though we can see nebulae in visible light, what shows is the product of hot, excited gases, not the cold and dusty regions. These invisible regions are the specialty of Herschel’s SPIRE instruments. They map the dust in far-infrared with their spectroscopic observations. In this instance, the researchers were somewhat astounded when they found some very unusual data which required time to fully understand.

“Looking at infrared spectra is useful as it gives us the signatures of molecules, in particular their rotational signatures,” Barlow said. “Where you have, for instance, two atoms joined together, they rotate around their shared center of mass. The speed at which they can spin comes out at very specific, quantized, frequencies, which we can detect in the form of infrared light with our telescope.”

According to the news release, elements can exist in varying forms known as isotopes. These have different numbers of neutrons in the atomic nuclei. When it comes to properties, isotopes can be somewhat alike to each other, but they have different masses. Because of this, the rotational speed is dependent on which isotopes are present in a molecule. “The light coming from certain regions of the Crab Nebula showed extremely strong and unexplained peaks in intensity around 618 gigahertz and 1235 GHz.” By comparing data of known properties of different molecules, the science team came to the conclusion the mystery emission was the product of spinning molecular ions of argon hydride. What’s more, it could be isolated. The only argon isotope which could spin like that was argon-36! It would appear the energy released from the central neutron star in the Crab Nebula ionized the argon, which then combined with hydrogen molecules to form the molecular ion ArH+.

Professor Bruce Swinyard (UCL Department of Physics & Astronomy and Rutherford Appleton Laboratory), a member of the team, added: “Our discovery was unexpected in another way — because normally when you find a new molecule in space, its signature is weak and you have to work hard to find it. In this case it just jumped out of our spectra.”

Is this instance of argon-36 in a supernova remnant natural? You bet. Even though the discovery was the first of its kind, it is doubtless not the last time it will be detected. Now astronomers can solidify their theories of how argon forms. Current predictions allow for argon-36 and no argon-40 to also be part of supernova structure. However, here on Earth, argon-40 is a dominant isotope, one which is created through the radioactive decay of potassium in rocks.

Noble gas research will continue to be a focus of scientists at UCL. As an amazing coincidence, argon, along with other noble gases, was discovered at UCL by William Ramsay at the end of the 19th century! I wonder what he would have thought had he known just how very far those discoveries would take us?

Original Story Source: University College London (UCL) Press Release

Astrophoto: Widefield, Narrowband View of the Crab Nebula by Nick Howes

Crab Nebula in a widefield, narrowband image. Credit: Nick Howes

This gorgeous shot of the Crab Nebula, or M1, by astronomer Nick Howes shows the famous nebula in a different light than the usual full spectrum views we’ve seen from the likes of the Hubble Space Telescope. Narrowband filters are designed to capture specific wavelengths of light, and since the Crab Nebula is emitting its own light rather than reflecting light from another source, it is a perfect candidate for imaging in narrow, or a limited part of the spectrum.

This 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).

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Weekly SkyWatcher’s Forecast: February 19-25, 2012

Messier 41 - Credit: NOAO/AURA/NSF

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Greetings, fellow SkyWatchers! It’s going to be an awesome week as we watch the planets – Mars, Saturn, Jupiter, Venus and Mercury – dance along the ecliptic plane. You don’t even need a telescope for this show! But that’s not all. We’ll take a look at a wealth of bright star clusters, challenging studies and lots more. I’ll see you in the back yard…

Sunday, February 19 – Today is the birthday of Nicolas Copernicus. Born in 1473, he was the creator of the modern solar system model which illustrated the retrograde motion of the outer planets. Considering this was well over 530 years ago, and in a rather “unenlightened” time, his revolutionary thinking about what we now consider natural is astounding.

Have you been observing retrograde motion while keeping track of Mars? Good for you! You may have also noticed that Mars has dimmed slightly over the last few weeks. Right now it’s around -1.0. Keep track of its many faces!

While we still have dark skies on our side, let’s head for a handful of difficult nebulae in a region just west of Gamma Monocerotis. For binoculars, check out the region around Gamma, it is rich in stars and very colorful! You are looking at the very outer edge of the Orion spiral arm of our galaxy. For small scopes, have a look at Gamma itself – it’s a triple system that we’ll be back to study. For larger scopes? It’s Herschel hunting time…

NGC 2183 (Right Ascension: 6 : 10.8 – Declination: -06 : 13 ) and NGC 2185 (Right Ascension: 6 : 11.1 – Declination: -06 : 13 ) will be the first you encounter as you move west of Gamma. Although they are faint, just remember they are nothing more than a cloud of dust illuminated by faint stars on the edge of the galactic realm. The stars that formed inside provided the light source for these wispy objects and at their edges lay in intergalactic space.

To the southwest is the weaker NGC 2182 (Right Ascension: 6 : 09.5 – Declination: -06 : 20), which will appear as nothing more than a faint star with an even fainter halo about it, with NGC 2170 (Right Ascension: 6 : 07.5 – Declination: -06 : 24) more strongly represented in an otherwise difficult field. While the views of these objects might seem vaguely disappointing, you must remember that not everything is as bright and colorful as seen in a photograph. Just knowing that you are looking at the collapse of a giant molecular cloud that’s 2400 light-years away is pretty impressive!

Monday, February 20 – Today in history celebrates the Mir space station launch in 1986. Mir (Russian for “peace”) was home to both cosmonauts and astronauts as it housed 28 long duration crews during its 15 years of service. To date it is one of the longest running space stations and a triumph for mankind. Spasiba! Today in 1962, John Glenn was onboard Friendship 7 and became the first American to orbit the Earth. As Colonel Glenn looked out the window, he reported seeing “fireflies” glittering outside his Mercury space capsule. Let’s see if we can find some…

The open cluster M41 (Right Ascension: 6 : 46.0 – Declination: -20 : 44) in Canis Major is just a quick drift south of the brightest star in the northern sky – Sirius. Even the smallest scopes and binoculars will reveal this rich group of mixed magnitude stars and fill the imagination with strange notions of reality. Through larger scopes, many faint groupings emerge as the star count rises to well over 100 members. Several stars of color – orange in particular – are also seen along with a number of doubles.

First noted telescopically by Giovanni Batista Hodierna in the mid-1500s, ancient texts indicate that Aristotle saw this naked-eye cluster some 1800 years earlier. Like other Hodierna discoveries, M41 was included on Messier’s list – along with even brighter clusters of antiquity such as Praesepe in Cancer and the Pleiades in Taurus. Open cluster M41 is located 2300 light years away and recedes from us at 34km/sec – about the speed Venus moves around the Sun. M41 is a mature cluster, around 200 million years old and 25 light years in diameter. Remember M41… Fireflies in night skies.

Tuesday, February 21 – Tonight is New Moon! Tonight let’s take a journey just a breath above Zeta Tauri and spend some quality time with a pulsar embedded in the most famous supernova remnant of all. Factually, we know the Crab Nebula to be the remains of an exploded star recorded by the Chinese in 1054. We know it to be a rapid expanding cloud of gas moving outward at a rate of 1,000 km per second, just as we understand there is a pulsar in the center. We also know it as first recorded by John Bevis in 1758, and then later cataloged as the beginning Messier object – penned by Charles himself some 27 years later to avoid confusion while searching for comets. We see it revealed beautifully in timed exposure photographs, its glory captured forever through the eye of the camera — but have you ever really taken the time to truly study M1 (Right Ascension: 5 : 34.5 – Declination: +22 : 01)? Then you just may surprise yourself…

In a small telescope, M1 might seem to be a disappointment – but do not just glance at it and move on. There is a very strange quality to the light which reaches your eye, even though initially it may just appear as a vague, misty patch. Allow your eyes to adjust and M1 will appear to have “living” qualities – a sense of movement in something that should be motionless. The “Crab” holds true to many other spectroscopic studies. The concept of differing light waves crossing over one another and canceling each other out – with each trough and crest revealing differing details to the eye – is never more apparent than during study. To observe M1 is to at one moment see a “cloud” of nebulosity, the next a broad ribbon or filament, and at another a dark patch. When skies are stable you may see an embedded star, and it is possible to see six such stars.

Many observers have the ability to see spectral qualities, but they need to be developed. From ionization to polarization – our eye and brain are capable of seeing to the edge of infra-red and ultra-violet. Even a novice can see the effects of magnetism in the solar “Wilson Effect.” But what of the spinning neutron star at M1’s heart? We’ve known since 1969 that M1 produces a “visual” pulsar effect. About once every five minutes, changes occurring in the neutron star’s pulsation affect the amount of polarization, causing the light waves to sweep around like a giant “cosmic lighthouse” and flash across our eyes. M1 is much more than just another Messier. Capture it tonight!!

Wednesday, February 22 – Today in 1966, Soviet space mission Kosmos 110 was launched. Its crew was canine, Veterok (Little Wind) Ugolyok (Little Piece of Coal); both history making dogs. The flight lasted 22 days and held the record for living creatures in orbit until 1974 – when Skylab 2 carried its three-man crew for 28 days.

Since we’ve studied the “death” of a star, why not take the time tonight to discover the “birth” of one? Our journey will start by identifying Aldeberan (Alpha Tauri) and move northwest to bright Epsilon. Hop 1.8 degrees west and slightly to the north for an incredibly unusual variable star – T Tauri.

Discovered by J.R. Hind in October 1852, T Tauri and its accompanying nebula, NGC 1555 (Right Ascension: 4 : 22.9 – Declination: +19 : 32), set the stage for discovery with a pre-main sequence variable star. Hind reported the nebula, but also noted that no catalog listed such an object in that position. His observations also included a 10th magnitude uncharted star and he surmised that the star in question was a variable. On each count Hind was right, and both were followed by astronomers for several years until they began to fade in 1861. By 1868, neither could be seen and it wasn’t until 1890 that the pair was re-discovered by E.E. Barnard and S.W. Burnham. Five years later? They vanished again.

T Tauri is the prototype of this particular class of variable stars and is itself totally unpredictable. In a period as short as a few weeks, it might move from magnitude 9 to 13 and other times remain constant for months on end. It is about equal to our own Sun in temperature and mass – and its spectral signature is very similar to Sol’s chromosphere – but the resemblance ends there. T Tauri is a star in the initial stages of birth!

T Tauri are all pre-main sequence and are considered “proto-stars”. In other words, they continuously contract and expand, shedding some of their mantle of gas and dust. This gas and dust is caught by the star’s rotation and spun into an accretion disc – which might be more properly referred to as a proto-planetary disc. By the time the jets have finished spewing and the material is pulled back to the star by gravity, the proto-star will have cooled enough to have reached main sequence and the pressure may have allowed planetoids to form from the accreted material.

Thursday, February 23 – If you have an open western horizon, then be out at twilight! Right now the speedy inner planet – Mercury – will make a brief appearance. Depending on your time zone, you might also spot a very young Moon just above it! For curiosity seekers, you can also find asteroid Vesta to the south of the Moon, along with planet Uranus to the south-east. How cool is that?!

In 1987, Ian Shelton made an astonishing visual discovery – SN 1987a. This was the brightest supernova in 383 years. More importantly, before it occurred, a blue star of roughly 20 solar masses was already known to exist in that same location within the Large Magellanic Cloud. Catalogued as Sanduleak -69-202, that star is now gone. With available data on the star, astronomers were able to get a “before and after” look at one of the most extraordinary events in the universe! Tonight, let’s have a look at a similar event known as “Tycho’s Supernova.”

Located northwest of Kappa Cassiopeia, SN1572 appeared so bright in that year that it could be seen with the unaided eye for six months. Since its appearance was contrary to Ptolemaic theory, this change in the night sky now supported Copernicus’ views and heliocentric theory gained credence. We now recognize it as a strong radio source, but can it still be seen? There is a remnant left of this supernova, and it is challenging even with a large telescope. Look for thin, faint filaments that form an incomplete ring around 8 arc minutes across.

Friday, February 24 – Tonight the slender first crescent of the Moon makes its presence known on the western horizon. Before it sets, take a moment to look at it with binoculars. The beginnings of Mare Crisium will show to the northeast quadrant, but look just a bit further south for the dark, irregular blotch of Mare Undarum – the Sea of Waves. On its southern edge, and to lunar east, look for the small Mare Smythii – the “Sea of Sir William Henry Smyth.” Further south of this pair and at the northern edge of Fecunditatis is Mare Spumans – the “Foaming Sea.” All three of these are elevated lakes of aluminous basalt belonging to the Crisium basin.

For telescope users, wait until the Moon has set and return to Beta Monocerotis and head about a fingerwidth northeast for an open cluster challenge – NGC 2250 (Right Ascension: 6 : 32.8 – Declination: -05 : 02). This vague collection of stars presents itself to the average telescope as about 10 or so members that form no real asterism and makes one wonder if it is indeed a cluster. So odd is this one, that a lot of star charts don’t even list it!

Today in 1968, during a radar search survey, the first pulsar was discovered by Jocelyn Bell. The co-directors of the project, Antony Hewish and Martin Ryle, matched these observations to a model of a rotating neutron star, winning them the 1974 Physics Nobel Prize and proving a theory of J. Robert Oppenheimer from 30 years earlier.

Would you like to get a look at a region of the sky that contains a pulsar? Then wait until the Moon has well westered and look for guidestar Alpha Monocerotis to the south and bright Procyon to its north. By using the distance between these two stars as the base of an imaginary triangle, you’ll find pulsar PSR 0820+02 at the apex of your triangle pointed east.

Saturday, February 25 – As the Moon begins its westward journey after sunset in a position much easier to observe. The lunar feature we are looking for is at the north-northeast of the lunar limb and its view is often dependent on libration. What are we seeking? “The Sea of Alexander von Humboldt”…

Mare Humboldtianum can sometimes be hidden from view because it is an extreme feature. Spanning 273 kilometers, the basin in which it is contained extends for an additional 600 kilometers and continues around to the far side of the Moon. The mountain ranges which accompany this basin can sometimes be glimpsed under perfect lighting conditions, but ordinarily are just seen as a lighter area. The mare was formed by lava flow into the impact basin, yet more recent strikes have scarred Humboldtianum. Look for a splash of ejecta from crater Hayn further north, and the huge, 200 kilometer strike of crater Bel’kovich on Humboldtianum’s northeast shore.

When the Moon begins to wester, let’s head for Beta Monocerotis and hop about 3 fingerwidths east for an 8.9 magnitude open cluster that can be spotted with binoculars and is well resolved with a small telescope – NGC 2302 (Right Ascension: 6 : 51.9 – Declination: -07 : 04). This very young stellar cluster resides at the outer edge of the Orion spiral arm. While binoculars will see a handful of stars in a small V-shaped pattern, telescope users should be able to resolve 40 or so fainter members.

Until next week, may all of your journeys be at light speed!

If you enjoy the weekly observing column, then you’ll love the book, The Night Sky Companion 2012 written by Tammy Plotner. This fully illustrated observing guide includes star charts for your favorite objects and much more!

The Crab Gets Cooked With Gamma Rays

X-ray: NASA/CXC/ASU/J. Hester et al.; Optical: NASA/HST/ASU/J. Hester et al.; Radio: NRAO/AUI/NSF Image of the Crab Nebula combines visible light (green) and radio waves (red) emitted by the remnants of a cataclysmic supernova explosion in the year 1054. and the x-ray nebula (blue) created inside the optical nebula by a pulsar (the collapsed core of the massive star destroyed in the explosion). The pulsar, which is the size of a small city, was discovered only in 1969. The optical data are from the Hubble Space Telescope, and the radio emission from the National Radio Astronomy Observatory, and the X-ray data from the Chandra Observatory.

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It’s one of the most famous sights in the night sky… and 957 years ago it was bright enough to be seen during the day. This supernova event was one of the most spectacular of its kind and it still delights, amazes and even surprises astronomers to this day. Think there’s nothing new to know about M1? Then think again…

An international collaboration of astrophysicists, including a group from the Department of Physics in Arts & Sciences at Washington University in St. Louis, has detected pulsed gamma rays coming from the heart of the “Crab”. Apparently the central neutron star is putting off energies that can’t quite be explained. These pulses between range 100 and 400 billion electronvolts (Gigaelectronvolts, or GeV), far higher than 25 GeV, the most energetic radiation recorded. To give you an example, a 400 GeV photon is almost a trillion times more energetic than a light photon.

“This is the first time very-high-energy gamma rays have been detected from a pulsar – a rapidly spinning neutron star about the size of the city of Ames but with a mass greater than that of the Sun,” said Frank Krennrich, an Iowa State professor of physics and astronomy and a co-author of the paper.

We can thank the Arizona based Very Energetic Radiation Imaging Telescope Array System (VERITAS) array of four 12-meter Cherenkov telescopes covered in 350 mirrors for the findings. It is continually monitoring Earth’s atmosphere for the fleeting signals of gamma-ray radiation. However, findings like these on such a well-known object is nearly unprecedented.

“We presented the results at a conference and the entire community was stunned,” says Henric Krawczynski, PhD, professor of physics at Washington University. The WUSTL group led by James H. Buckley, PhD, professor of physics, and Krawczynski is one of six founding members of the VERITAS consortium.

An X-ray image of the Crab Nebula and pulsar. Image by the Chandra X-ray Observatory, NASA/CXC/SAO/F. Seward.

We know the Crab’s story and how its pulsar sweeps around like a lighthouse… But Krennrich said such high energies can’t be explained by the current understanding of pulsars. Not even curvature radiation can be at the root of these gamma-ray emissions.

“The pulsar in the center of the nebula had been seen in radio, optical, X-ray and soft gamma-ray wavelengths,” says Matthias Beilicke, PhD, research assistant professor of physics at Washington University. “But we didn’t think it was radiating pulsed emissions above 100 GeV. VERITAS can observe gamma-rays between100 GeV and 30 trillion electronvolts (Teraelectronvolts or TeV).”

Just enough to cook one crab… well done!

Original Story Source: Iowa State University News Release. For Further Reading: Washington University in St. Louis News Release.

Crab Nebula Erupts in a Superflare

NASA's Chandra X-ray Observatory reveals the complex X-ray-emitting central region of the Crab Nebula. This image is 9.8 light-years across. Chandra observations were not compatible with the study of the nebula's X-ray variations. Credit: NASA/CXC/SAO/F. Seward et al.

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From a NASA press release:

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. Several other satellites also made observations, which has astonished astronomers by revealing unexpected changes in X-ray emission the Crab, once thought to be the steadiest high-energy source in the sky.

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.

X-ray data from NASA's Fermi, RXTE, and Swift satellites and the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) confirm that the Crab Nebula's output has declined about 7 percent in two years at energies from 15,000 to 50,000 electron volts. They also show that the Crab has brightened or faded by as much as 3.5 percent a year since 1999. Fermi's Large Area Telescope (LAT) has detected powerful gamma-ray flares (magenta lines) as well. (Image credit: NASA's Goddard Space Flight Center)

“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.

It was also observed by NASA’s Rossi X-Ray Timing Explorer (RXTE) and Swift satellites and the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (INTEGRAL). The results confirm a real intensity decline of about 7 percent at energies between 15,000 to 50,000 eV over two years. They also show that the Crab has brightened and faded by as much as 3.5 percent a year since 1999.

“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.

Crab Nebula Flares

A composite illustration of the AGILE satellite and the Crab Nebula imaged by the Chandra observatory. [Image courtesy of ASI, INAF and NASA]
A composite illustration of the AGILE satellite and the Crab Nebula imaged by the Chandra observatory. [Image courtesy of ASI, INAF and NASA]

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The Crab Nebula is one of the most popular targets for astronomers of all stripes. It is readily viewable in moderate sized amateur telescopes and wows new viewers at star parties when they’re informed they’re looking at the remnant of a supernova that exploded in 1054 AD. The nebula is also a popular target for professional astronomers looking to study physics in the environment of a pulsar. Powered by synchrotron radiation from the pulsar, the nebula glows brightly across numerous wavelengths in a steady manner that is so consistent, that astronomers have used it to calibrate instruments in different portions of the spectrum. The largest regular variation discovered was a mere 3.5% in the X-ray portion of the spectrum.

But on September 22 of 2010, the Italian Space Agency’s AGILE satellite observed a sudden brightening in the nebula in the gamma ray portion of the spectrum. The Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope, which observes the Crab regularly, confirmed this flaring. Strangely, telescopes observing the nebula in other spectral regimes showed no brightening at all. The lone exception was a small knot roughly one arcsecond in diameter seen by the Chandra X-ray telescope which is believed to correspond to the base of a jet emanating from the pulsar.

Many telescopes observed the central pulsar in X-rays as well as radio to attempt to discover if there had been a sudden change in the power source itself that caused the sudden brightening, but no changes were apparent. This suggests that the flare didn’t come directly from the pulsar, but rather from the nebula itself, perhaps as an interaction between the jet and the magnetic field of the nebula causing intense synchrotron radiation. If this is the cause, then the energy of the accelerated electrons is among the highest of any astronomical event. Such a case is of interest to astronomers and physicists because it provides a rare test bed into relativistic physics and particle acceleration theory.

While this event was certainly noteworthy, it was not entirely unique. AGILE detected a previous flare on October 7, 2007 and Fermi’s LAT had discovered another in February 2009. Currently, none of these events have been entirely explained but will likely give astronomers a target for future studies. Based on the amount of coverage the Crab Nebula receives from telescopes, astronomers are no expecting that such flares are a relatively common occurrence, happening about once a year. If so, this will provide an excellent opportunity to study such events with more scrutiny.