Tag Archives: gamma-ray

Positrons from normal, not dark, matter

Gamma-rays from electron-positron annihilations in center of Milky Way
Gamma-rays from electron-positron annihilations in center of Milky Way

The ESA space observatory Integral had observed gamma-rays from the center of the galaxy, which indicated the presence of positrons distributed in a way that couldn't quite be explained with known phenomenon. Hence some physicists speculated that the positrons may have been the result of dark matter annihilation. Not only did the distribution of positrons within our galaxy turn out to be lopsided, arguing against dark matter annihilation as the source, but it has now been explained how supernovae could be responsible for the distribution of positrons.

Some had thought that supernovae could not be the source of most of the positrons because it was assumed that they would all annihilate very close to their origin, which would not match the observed distribution of positrons. But it turns out that the positrons from supernovae, which are the result of the decay of heavy elements from the stellar explosion, travel nearly at the speed of light and can travel for thousands of light-years before slowing down and annihilating with an electron. By considering how electrons move in galactic magnetic fields, they were able to model how positrons would travel before being annihilated, and the results seem to be consistent with the Integral observations.

This deals a blow to the hypothesis that dark matter annihilation may be responsible for the positron distribution. I wonder if the same implication can be inferred for the PAMELA observations?

Pulsing with gamma-rays

CTA 1 supernova remnant
CTA 1 supernova remnant

Pulsars are rapidly spinning neutron stars which emit beams of light, which can include radio waves, visible light, and even X-rays, from the magnetic poles powered by the neutron star's powerful rotation. We see pulsars as they seemingly "blink" on and off with extreme regularity as the beam rotates and points to our direction once every rotation. In fact, the regularity had astronomers thinking they may have discovered signals from an extraterritorial intelligence when pulsars were first discovered. Most pulsars discovered before now "blinked" in radio waves.

Via the Bad Astronomer, we hear news that the Fermi Gamma-ray Space Telescope has discovered a new kind of pulsar, one which only "blinks" in gamma-rays. One speculation is that it's not really a different kind of pulsar, but rather that the gamma-ray beam is more broader than the radio wave beam so that we only see it "blinking" in gamma-rays. This strikes me as odd since I would have thought it would be the other way around. Whatever the case may be, it's mind-blowing to consider the amounts of energy the pulsar must be emitting.

First light from "GLAST"

First light all sky map from GLAST

"GLAST", a space-based gamma-ray observatory launched this June, has collected enough data for us to see what the entire sky looks like in gamma-rays. The impressive thing is that it was able to get a map of the entire sky with less than a hundred hours of data collection, which is staggeringly short when compared with how it took years for the Compton Gamma Ray Observatory to collect enough data to construct a similar map.

You can see what the sky looks like in gamma-rays in the video that follows, which when unrolled and flattened becomes the image above.

You might have noticed that I've put "GLAST" between quotation marks. This is because the gamma-ray observatory has also been renamed to the Fermi Gamma-ray Space Telescope. It's named after Enrico Fermi, a physicist whose accomplishments include overseeing the construction of the first nuclear reactor, which was almost literally a pile of bricks.

GLAST is operating smoothly

NASA's space-based gamma-ray observatory GLAST is working smoothly, and they have made available images of the detection of individual gamma rays. After a few weeks, when GLAST would have collected tons of these events and had time to process them, they will be releasing images of the sky as seen with gamma-rays. Considering that GLAST can detect gamma-rays with high energies that have never been systematically observed before, we might get to see things that we've never even suspected of before.

Single event display from GLAST

In other news, NASA plans to rename GLAST in a few weeks at the same time it releases images from the observatory.