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Dark matter search narrows

Fornax dwarf galaxy

This faint smattering of stars is actually a small galaxy. Scientists have been unable to spot evidence of certain kinds of dark matter particles within this galaxy and nine others.

STUDIES DECADES AGO OF THE ROTATION of galaxies, and of the movement of groups of galaxies, led scientists to conclude that the universe contained more matter than could be detected in the normal ways.

Being unseen at visible wavelengths, and with its nature unknown, the putative matter was dubbed “dark matter“, and according to popular models it comprises over 80 per cent of all the matter in the universe.

In the early years of investigation into this strange phenomenon, two broad candidates emerged—MACHOs and WIMPS.

MACHOs were hypothetical “massive compact halo objects”, ie. large bodies such as dim stars, black holes or large free-floating planets that would inhabit the outer or “halo” regions of a galaxy. WIMPs are hypothetical “weakly interacting massive particles”, ie. sub-atomic particles that could pervade space but not interact much with normal forms of matter.

Artist's impression of NASA’s Fermi Gamma-ray Space Telescope

The research used two-years of data collected by NASA’s Fermi Gamma-ray Space Telescope (artist's impression).

Research programmes failed to find evidence of MACHOs, so dark matter investigations now focus on WIMPs.

WIMPs could take many forms—perhaps as one or more of the familiar particles, such as neutrinos, or maybe as-yet-unknown particles.

In new research using two-years of data from NASA’s Fermi Gamma-ray Space Telescope, a team that includes astrophysicist Jennifer Siegal-Gaskins (Caltech) has been able to rule out certain kinds of WIMPs.

According to some models, when two WIMPs collide, they can annihilate each other and produce a burst of gamma rays with specific wavelengths. Such energy bursts would be detectable with Fermi.

The scientists studied 10 small galaxies that circle our Milky Way galaxy, looking for telltale gamma ray signs of WIMP collisions within them. They didn’t spot any.

This negative result will help scientists by eliminating particular kinds of WIMPs from the field of candidates, and will enable them to focus on searches for other kinds.

More information: New Insights on Dark Matter

Story by Jonathan Nally. Images courtesy NASA / Sonoma State University / Aurore Simonnet / ESO / Digital Sky Survey 2.

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Unknown objects at the limits

NASA’s FERMI GAMMA-RAY TELESCOPE is finding hundreds of new objects at the very edge of the electromagnetic spectrum. Many of them have one thing in common—astronomers have no idea what they are. This short video from NASA explains what it’s all about.

Adapted from information issued by NASA.

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The Case of the Cosmic Crab

A NEW MOVIE FROM NASA’S Chandra X-ray Observatory shows a sequence of images of the Crab Nebula, taken over an interval of seven months and showing dramatic variations.

The Crab Nebula is one of the most famous objects in the sky. It is the remnant cloud from a supernova (exploding star) that was seen by astronomers in China and other countries in the year 1054.

At the centre of the nebula is a pulsar, a rapidly spinning neutron star. It has a mass greater than our Sun but is only tens of kilometres wide, and is spinning at the rate of 30 times per second.

The pulsar’s spin is gradually slowing down, and as it does so large amounts of energy are injected into its surroundings. In particular, a high-speed wind of matter and anti-matter particles ploughs into the surrounding nebula, creating a shock wave that forms the expanding ‘ring’ seen in the movie.

In addition, ‘jets’ shooting out from the poles of the pulsar spew X-ray emitting matter and antimatter particles in a direction at right angles to the ring.

The goal of the latest Chandra observations was to pinpoint the location of remarkable flares spotted by NASA’s Fermi Gamma Ray Observatory satellite and Italy’s AGILE satellite.

A strong gamma-ray flare was detected from the Crab in September 2010, followed by an even stronger series of “superflares” in April 2011. The gamma-ray satellites 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.

Scientists have put together a short sequence of the images taken by Chandra, showing the remarkable changes in the nebula:

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 before the superflare was seen last month.

What was unexpected, though, was that nothing significant showed up in the Chandra observations as compared with the Fermi observations. Astronomers are now trying to figure out why that is so.

One possible explanation is that the gamma-ray flares picked up by Fermi happened very close to the pulsar, in which case 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.)

Adapted from information issued by CXC. Crab Nebula image courtesy (X-ray) NASA / CXC / SAO / F. Seward; (optical) NASA / ESA / ASU / J. Hester & A. Loll; (infrared) NASA / JPL-Caltech / Univ. Minn. / R. Gehrz.

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Taking the pulse of the universe

Artist's impression of a pulsar

Artist's impression of a pulsar, a rapidly spinning neutron star that emits streams of radio waves and sometimes gamma rays.

  • Pulsars are small, spinning neutron stars
  • They emit radio waves or gamma rays, and sometimes both
  • Parkes radio telescope working jointly with NASA space telescope

USING THE PARKES radio telescope, CSIRO astronomers are working closely with NASA to unlock one of astronomy’s great enigmas—the science behind pulsars.

The team are using the world-class facilities at Parkes, in combination with NASA’ s Fermi Gamma-Ray Space Telescope, to understand how these small, spinning stars make their beams of radiation.

The project has tracked down 25 ultra-fast ‘millisecond’ pulsars in just two years—the same number discovered in the previous 20 years.

“This has been a hugely productive collaboration, and it is generating unprecedented returns for physics and astronomy,” said the leader of the Parkes observations, CSIRO’s Dr Simon Johnston.

The study of pulsars demands highly advanced scientific infrastructure and expertise.

Pulsars emit beams of radio waves, gamma waves, or both. Sensitive radio telescopes such as the CSIRO facility at Parkes can detect the radio waves as they sweep across the Earth.

But gamma rays—which carry billions of times more energy than the light our eyes can see—are blocked by the Earth’s atmosphere. We can study them only by using telescopes in space.

Space and ground working together

The CSIRO-NASA collaboration shows we get the best results by combining land and space-based detectors.

First, the Fermi space telescope is finding unidentified gamma-ray sources, which the Parkes telescope can investigate for radio wave pulses.

“That’s how we were able to find those 25 millisecond pulsars, an incredible haul,” Dr Johnston said.

Simon Johnston in the control room at Parkes

Dr Simon Johnston (foreground) in the control room of CSIRO's Parkes radio telescope.

Second, Parkes is doing very precise timing of 168 radio pulsars that Fermi might be able to study.

“We work out exactly when the pulsar’s radio beam sweeps over us. That tells us how fast the pulsar is rotating,” Dr Johnston said.

“That knowledge helps us make use of the gamma-ray photons that Fermi detects. If Parkes can get the timing precisely right through the radio wave pulses, we can build up a picture of the gamma-ray pulses by collecting a few photons every time the pulsar beam sweeps past.”

Intriguing results

The collaboration has thrown up some intriguing results. Of the 60 objects Fermi has found that emit gamma-ray pulses, about twenty lack detectable radio pulses.

“The most likely explanation is that these pulsars do have radio beams, but they are just not sweeping across the Earth, so we can’t detect them,” Dr Johnston said.

“In other words, we think the beam of gamma rays is a big fat beam, which is easier to detect, and the radio beam is more tightly directed, less spread out.

“This suggests certain things about where on the pulsar the two beams come from, and how they are made. It’s only when we work together that we can crack these long-standing mysteries.”

CSIRO's Parkes radio telescope

CSIRO's Parkes radio telescope

International collaboration

Innovation Minister Senator Kim Carr said the research exemplified the sorts of international collaboration that the Australian Government was fostering across the board.

“We have a proud history of cooperation and involvement with NASA on a number of fronts, from assisting with communicating with the Apollo missions to the moon, to deep space exploration, and understanding how our universe works,” Senator Carr said.

“It’s all about exploring new frontiers and building Australian capacity as a research intensive and innovative nation.

“While this might seem remote from everyday life, experience has shown that space exploration in all its forms has unforeseen spin-offs that provide wide-reaching benefits through new technologies and new approaches to a range of challenges.”

Adapted from information issued by CSIRO. Images courtesy CSIRO and NASA.

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Crab’s candle starts to flicker

  • Crab Nebula is 6,500 light-years from Earth
  • It is the remains of an exploded star (a supernova)
  • Now shown to unexpectedly vary its energy output

DATA FROM SEVERAL NASA satellites has astonished astronomers by revealing unexpected changes in X-ray emission from the Crab Nebula, once thought to be the steadiest high-energy source in the sky.

“For 40 years, most astronomers regarded the Crab as a standard candle,” said Colleen Wilson-Hodge, an astrophysicist at NASA’s Marshall Space Flight Centre, who presented the findings recently at the American Astronomical Society meeting in Seattle.

“Now, for the first time, we’re clearly seeing how much our candle flickers.”

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054. Located 6,500 light-years away, it is one of the most studied objects in the sky.

At the heart of the expanding gas cloud lies what’s left of the original star’s core, a superdense neutron star that spins 30 times a second. All of the Crab’s high-energy emissions are thought to be the result of physical processes that tap into this rapid spin.

For decades, astronomers have regarded the Crab’s X-ray emissions as so stable that they’ve used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in “millicrabs,” a unit derived from the nebula’s output.

Crab Nebula

This view of the Crab Nebula comes from the Hubble Space Telescope and spans 12 light-years. The supernova remnant, located 6,500 light-years away, is among the best-studied objects in the sky. Image courtesy NASA / ESA / ASU / J. Hester.

“The Crab Nebula is a cornerstone of high-energy astrophysics,” said team member Mike Cherry at Louisiana State University (LSU), “and this study shows us that our foundation is slightly askew.”

Satellite tag teams

The story unfolded when Cherry and Gary Case, also at LSU, first noticed the Crab’s dimming in observations by the Gamma-ray Burst Monitor (GBM) aboard NASA’s Fermi Gamma-ray Space Telescope.

The team then analysed GBM observations of the object from August 2008 to July 2010 and found an unexpected but steady decline of several percent at four different “hard” X-ray energies.

With the Crab’s apparent constancy well established, the scientists needed to prove that the fadeout was real and was not an instrumental problem associated with the GBM.

“If only one satellite instrument had reported this, no one would have believed it,” Wilson-Hodge said.

Graph showing multi-wavelength observations of the Crab Nebula

Data from four satellites show that the Crab Nebula's energy output has varied. Powerful gamma-ray flares (pink vertical lines) have been detected as well. Graph courtesy NASA Goddard Space Flight Centre.

So the team amassed data from the fleet of sensitive X-ray observatories now in orbit—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 certain energy ranges. They also show that the Crab has brightened and faded by as much as 3.5 percent a year since 1999.

The scientists say that astronomers will need to find new ways to calibrate instruments in flight and to explore the possible effects of the inconstant Crab on past findings.

Showing some flare

Fermi’s other instrument, the Large Area Telescope, has detected unprecedented gamma-ray flares from the Crab, showing that it is also surprisingly variable at much higher energies.

The nebula’s power comes from the central neutron star, which is also a pulsar that emits fast, regular radio and X-ray pulses. This pulsed emission exhibits no changes associated with the decline, so it cannot be the source.

Instead, researchers suspect that the long-term changes probably occur in the nebula’s central light-year, but observations with future telescopes will be needed to know for sure.

Adapted from information issued by NASA MSFC.

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Super-blast amazes scientists

Astronomers using NASA’s Fermi Gamma-ray Space Telescope have detected gamma-rays from a nova for the first time, a finding that stunned observers and theorists alike.

The discovery overturns the notion that novae explosions lack the power to emit such high-energy radiation.

A nova is a sudden, short-lived brightening of an otherwise inconspicuous white dwarf star. The outburst occurs when a white dwarf in a binary system erupts in an enormous thermonuclear explosion.

“In human terms, this was an immensely powerful eruption, equivalent to about 1,000 times the energy emitted by the Sun every year,” said Elizabeth Hays, a Fermi deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“But compared to other cosmic events Fermi sees, it was quite modest. We’re amazed that Fermi detected it so strongly.”

Gamma rays are the most energetic form of light, and Fermi’s Large Area Telescope detected the nova for 15 days. Scientists believe the emission arose as a 1.5 million-kilometre-per-hour shock wave raced from the site of the explosion.

before and after images showing nova

Japanese amateur astronomers discovered Nova Cygni 2010 in an image taken on March 11 (Japan Standard Time). The erupting star (circled) was 10 times brighter than it was in an image taken several days earlier.

Discovered by Japanese amateurs

Japanese amateur astronomers first noticed that the star system, known as V407 Cyg, was 10 times brighter on March 11, 2010, than in an image they had taken three days earlier.

It was quickly followed up by other Japanese amateurs, and then by professional astronomers working with Fermi’s Large Area Telescope (LAT).

V407 Cyg lies 9,000 light-years away. The system is a so-called symbiotic binary containing a compact white dwarf and a red giant star about 500 times the size of the Sun.

“The red giant is so swollen that its outermost atmosphere is just leaking away into space,” said Adam Hill at Joseph Fourier University in Grenoble, France. The phenomenon is similar to the solar wind produced by the sun, but the flow is much stronger.

“Each decade, the red giant sheds enough hydrogen gas to equal the mass of Earth,” Hill added.

The white dwarf intercepts and captures some of this gas, which accumulates on its surface. As the gas piles on over tens to hundreds of years, it eventually becomes hot and dense enough to fuse into helium. This energy-producing process triggers a runaway reaction that explodes the accumulated gas.

The white dwarf itself, however, remains intact.

Fermi gamma ray image showing V407 Cyg nova

The Large Area Telescope aboard NASA's Fermi Gamma-ray Space Telescope saw no sign of a nova in 19 days of data prior to March 10 (left), but the eruption is obvious in data from the following 19 days (right).

A huge shock, in more ways than one

The blast created a hot, dense expanding shell called a shock front, composed of high-speed particles, ionised gas and magnetic fields. The shock wave expanded at 11 million kilometres per hour—or nearly 1 percent the speed of light.

The magnetic fields trapped particles within the shell and whipped them up to tremendous energies. Before they could escape, the particles had reached velocities near the speed of light. Scientists say that the gamma rays likely resulted when these accelerated particles smashed into the red giant’s wind.

“We know that the remnants of much more powerful supernova explosions can trap and accelerate particles like this, but no one suspected that the magnetic fields in novae were strong enough to do it as well,” said NRL’s Soebur Razzaque.

Supernovae remnants can last for 100,000 years and affect regions of space thousands of light-years across.

Adapted from information issued by Francis Reddy, NASA’s Goddard Space Flight Centre. Images courtesy NASA / DOE / Fermi LAT Collaboration / K. Nishiyama and F. Kabashima / H. Maehara, Kyoto Univ. / GSFC / Conceptual Image Lab.

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