Fermi bubble: Giant gamma-ray bubbles in the milky way

Abstract

Data from the Fermi-LAT reveal two gigantic gamma-ray emitting bubble structures (known as the Fermi bubbles), extending ~50° above and below the Galactic center symmetric about the Galactic plane, with a width of ~40° in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN=dE ~ E-2)than the inverse Compton emission from known cosmic ray electrons in the Galactic disk, or the gamma-rays produced by decay of pions from proton-ISM collisions. The bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; the edges of the bubbles also line up with features in the ROSAT soft X-ray maps at 1.5–2 keV. The Fermi bubble is most likely created by some large episode of energy injection in the Galactic center, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last ~ 10 Myr. Study of the origin and evolution of the bubbles also has the potential to improve our understanding of recent energetic events in the inner Galaxy and the high-latitude cosmic ray population.