Showing posts tagged gamma ray astronomy.

Science Briefs

Ghostly Gamma-ray Beams Blast from Milky Way's Center →

As galaxies go, our Milky Way is pretty quiet. Active galaxies have cores that glow brightly, powered by supermassive black holes, and often spit twin jets in opposite directions. The Milky Way’s center shows little activity, but it wasn’t always so peaceful. New evidence of gamma-ray jets suggests that the Milky Way’s central black hole was much more active in the past.

"These faint jets are an after-image of what existed a million years ago," said Meng Su, at the Harvard-Smithsonian Center for Astrophysics (CfA). "They strengthen the case for an active galactic nucleus in the Milky Way’s relatively recent past."

The two jets, were revealed by NASA’s Fermi space telescope. They extend from the galactic center to a distance of 27,000 light-years above and below the galactic plane. They are the first such gamma-ray jets ever found, and the only ones close enough to resolve with Fermi.

The jets may be related to gamma-ray bubbles that Fermi detected in 2010. Those also stretch 27,000 light-years from the center of the Milky Way. However, where the bubbles are perpendicular to the galactic plane, the gamma-ray jets are tilted at an angle of 15 degrees. This may reflect a tilt of the accretion disk surrounding the supermassive black hole.

"The central accretion disk can warp as it spirals in toward the black hole, under the influence of the black hole’s spin," explained co-author Douglas Finkbeiner of the CfA. "The magnetic field embedded in the disk therefore accelerates the jet material along the spin axis of the black hole, which may not be aligned with the Milky Way."


Ghostly jets seen streaming from Milky Way’s core
Ghosts of Milky Way’s Powerful Past Revealed
Towering Gamma-Ray Jets Point to Past Outbursts from Milky Way’s Black Hole

— 2 years ago
#active galaxies  #gamma ray astronomy 
Gamma rays hint at dark matter →

Using a new statistical technique to analyse publicly available data from NASA’s Fermi Space Telescope, an astrophysicist in Germany says he may have spotted a tell-tale sign of exotic particles annihilating within the Milky Way. If proved to be real, this “gamma-ray line” would, he claims, be a “smoking-gun signature” of dark matter.

A gamma-ray line would leave little room for alternative explanations. The dark-matter particles believed to exist in a halo surrounding our galaxy are slow moving because they have been slowed down as the universe has expanded. As a result, the total energy of the photons produced by the collision and annihilation of two such particles is, essentially, twice the rest mass of a dark-matter particle. Conservation of momentum requires that the energy of each photon equals the mass of one dark-matter particle – and would appear as a very narrow peak, or line, in gamma-ray spectra. This is unlike the radiation emitted by all standard astrophysical phenomena, which have much broader energy distributions.

In the latest work, Christoph Weniger of the Max Planck Institute for Physics in Munich looked for such lines in about 3.5 years’ worth of gamma-ray observations carried out by the Fermi satellite’s Large Area Telescope (LAT). In regions close to the centre of the galaxy, Weniger found that the gamma rays collected by Fermi showed evidence for a line, at about 130 GeV, with a statistical significance of 4.6σ.


Closing in on Dark Matter: Another “Tentative” Step

— 2 years ago
#dark matter  #gamma ray astronomy 
Fermi Uses Gamma Rays to Unearth Clues About "Empty" Space →

The SLAC-built Large Area Telescope (LAT), the main instrument of the Fermi Gamma-ray Space Telescope, has been studying the gamma-ray sky for almost four years. During that time, the LAT has identified hundreds of gamma-ray sources, including pulsars and active galactic nuclei. But, as reported in a paper soon to appear in The Astrophysical Journal, most of the gamma rays detected by the LAT cannot be attributed to individual point sources.

The team discovered that the pulsars, active galactic nuclei, and the all the rest of the gamma-ray sources pinpointed by the LAT account for only about 10 percent of the gamma-ray photons that have been detected. Extragalactic diffuse emission, a glow that pervades the universe and originates in distant, indistinct sources, comprises approximately 15 percent of the total. Most of the gamma rays detected by the LAT are “diffuse” emissions originating from our own Milky Way galaxy.

The main source of these pervasive, diffuse gamma-ray emissions are high-energy charged particles – cosmic rays – that interact with the gas and radiation fields between the stars – the interstellar medium, which is far from empty space. Scientists are busy gathering crucial information about the gas, dust and high-energy charged particles that fill it.

Cosmic rays can come from pulsars, supernova explosions and other powerful astrophysical sources. There may be an additional contribution from annihilating dark-matter particles or the black hole at the center of our galaxy.

— 2 years ago
#gamma ray astronomy 
Powerhouse in the Crab Nebula →

The pulsar at the center of the Crab Nebula is a bundle of energy. This was confirmed by the two MAGIC Telescopes on the Canary island of La Palma. They observed the pulsar in very high energy gamma radiation from 25 up to 400 gigaelectronvolts (GeV), a region that was previously difficult to access with high energy instruments, and discovered that it actually emits pulses with the maximum measurable energy of up to 400 GeV – at least 50 to 100 times higher than theorists thought possible. These latest observations are difficult for astrophysicists to explain. “There must be processes behind this that are as yet unknown”, says Razmik Mirzoyan, project head at the Max Planck Institute for Physics.

Data measured by MAGIC over the past two years show that the pulsed emissions far exceed all expectations, reaching 400 GeV in extremely short pulses of about a millisecond duration. This finding casts doubt on existing theories, since it was thought that all pulsars had significantly lower energy limits. The recent measurements by MAGIC, together with those of the orbiting Fermi satellite at much lower energies, provide an uninterrupted spectrum of the pulses from 0.1 GeV to 400 GeV. These clear observational results create major difficulties for most of the existing pulsar theories that predict significantly lower limits for highest energy emission.

— 2 years ago
#neutron stars  #pulsars  #gamma ray astronomy