Synopsis: Do cosmic rays account for all the gamma rays in diffuse galactic radiation?

New data are inconsistent with previous measurements that showed an unexpected excess of diffuse gamma-ray emission in the Galaxy.
Synopsis figure
Illustration: Fermi LAT Collaboration, Phys. Rev. Lett. (2009)

The interactions of cosmic rays with the galactic interstellar gas and radiation field result in the diffuse emission of gamma rays. If these cosmic ray interactions are the only source of such diffuse emission, then models can be constructed which fit both the cosmic ray spectra and measurements of the diffuse gamma radiation. However, several years ago, the Energetic Gamma-Ray Experiment Telescope (EGRET) at the Compton Gamma-Ray Observatory reported [1] measurements of the diffuse galactic gamma-ray emission that were larger than what the models expected. A variety of explanations for this discrepancy have been offered, with particular excitement focused on the possibility that it is the annihilation of dark matter particles that produces the excess gamma rays.

Now, new data on the diffuse galactic gamma-ray emission from the Large Area Telescope (LAT) on the Fermi Gamma-Ray Space Telescope are reported in Physical Review Letters. The LAT instrument represents a significant improvement in sensitivity and resolution compared with EGRET. The Fermi LAT Collaboration reports that both their results and the measured cosmic-ray spectra are consistent with a model of diffuse galactic gamma-ray emission—which means that the EGRET gamma-ray excess is not confirmed. – Stanley Brown

[1] S. D. Hunter et al., Astrophys. J. 481, 205 (1997).


Announcements

More Announcements »

Subject Areas

Particles and FieldsAstrophysicsCosmology

Previous Synopsis

Soft Matter

A scaffold for soft matter

Read More »

Next Synopsis

Superconductivity

Dirac connection

Read More »

Related Articles

Viewpoint: Cosmic Clues from Mini Clumps of Dark Matter
Cosmology

Viewpoint: Cosmic Clues from Mini Clumps of Dark Matter

Searches for ultracompact clumps of cold dark matter have come up empty, but these nondetections place new limits on the early expansion history of the Universe. Read More »

Synopsis: Anisotropy Limits for the Universe
Cosmology

Synopsis: Anisotropy Limits for the Universe

A new study of the cosmic microwave background places the strictest limits to date on a rotating Universe and other forms of cosmic anisotropy. Read More »

Synopsis: Spotting Dark Matter with Supermaterials
Particles and Fields

Synopsis: Spotting Dark Matter with Supermaterials

Superconducting aluminum or superfluid helium could be used to detect superlight dark matter particles. Read More »

More Articles