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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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Take a tour of the Crab Nebula

THE CRAB NEBULA IS ONE OF THE BRIGHTEST sources of high-energy radiation in the sky. Little wonder—it’s the expanding remains of an exploded star, a supernova seen in 1054.

Scientists have used virtually every telescope at their disposal, including NASA’s Chandra X-ray Observatory, to study the Crab.

The supernova left behind a magnetised neutron star—a pulsar. It’s about the size of Washington DC, but it spins 30 times per second. Each rotation sweeps a lighthouse-like beam past us, creating a pulse of electromagnetic energy detectable across the spectrum.

The pulsar in the Crab Nebula is among the brightest sources of high-energy gamma rays. Recently, NASA’s Fermi Gamma Ray Observatory and Italy’s AGILE Satellite detected strong gamma-ray flares from the Crab, including a series of “superflares” in April 2011.

To help pinpoint the location of these flares, astronomers enlisted Chandra space telescope.

With its keen X-ray eyes, Chandra saw lots of activity, but none of it seems correlated with the superflare. This hints that whatever is causing the flares is happening with about a third of a light-year from the pulsar. And rapid changes in the rise and fall of gamma rays imply that the emission region is very small, comparable in size to our Solar System.

The Chandra observations will likely help scientists to home in on an explanation of the gamma-ray flares one day.

Even after a thousand years, the heart of this shattered star still offers scientists glimpses of staggering energies and cutting edge science.

Adapted from information issued by Harvard-Smithsonian Centre for Astrophysics. Still 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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Black hole eats a star

Artist's impression of a star being eaten by a black hole

The aftermath of a black hole's banquet of a star, was jets of energy blasted from the black hole, fortuitously pointed in our direction and detected by the Swift satellite. (Artist's impression)

A BRIGHT FLASH OF GAMMA RAYS observed on March 28 by the Swift satellite may have been the death rattle of a star falling into a massive black hole and being ripped apart.

When Swift detected the flash, astronomers initially thought it was a gamma-ray burst from a collapsing star.

However, research led by astronomers at the University of Warwick has confirmed that the flash—one of the biggest and brightest bangs yet recorded by astronomers—came from a massive black hole at the centre of a distant galaxy.

The black hole appears to have ripped apart a star that wandered too close, creating a powerful beam of energy that crossed the 3.8 billion light years to Earth.

Gamma-ray flare in a distant galaxy

A gamma-ray flare seen in a distant galaxy is thought to have been the death throes of a star being eaten by a black hole.

Careful analysis of the data and subsequent observations by the Hubble Space Telescope and the Chandra X-ray Observatory confirmed Bloom’s initial insight.

“Despite the power of this the cataclysmic event we still only happen to see this event because our Solar System happened to be looking right down the barrel of this jet of energy,” said Dr Andrew Levan, lead researcher from the University of Warwick.

What made this gamma-ray flare, called Sw 1644+57, stand out from a typical burst were its long duration and the fact that it appeared to come from the centre of a galaxy nearly 4 billion light-years away.

Since most, if not all, galaxies are thought to contain a massive black hole at the centre, a long-duration burst could conceivably come from the relatively slow disruption of an infalling star, the astronomers said.

“This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than two and a half months later,” said Joshua Bloom, an associate professor of astronomy at the University of California Berkeley. “That’s because as the black hole rips the star apart, the mass swirls around like water going down a drain, and this swirling process releases a lot of energy.”

Adapted from information issued by the University of California, Berkeley, and University of Warwick. Images courtesy NASA / Swift / Stefan Immler / ESA / A. Fruchter, STScI / Mark A. Garlick.

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