Tuesday, May 1, 2012

"Paper of the day": IS IT A BUBBLE?

Recently I read a notice and after the relative paper titled “A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope”. The Fermi Large Area Telescope (also known with the name GLAST, Gamma-Ray Large Area Telescope) is an international mission designed with the aim to explore the sky at very high energies. It is a space telescope capable of detecting photons in the range from 30 MeV to 300 GeV. The Fermi space observatory is a NASA mission with a broad international collaboration (Italy, Japan, France, Sweden). After the activation in orbit, the mission has been dedicated to Enrico Fermi and nowadays it is known as the Fermi Gamma-Ray Telescope. The autor of the paper, independently from the Fermi official collaboration, claimed to observe a gamma-ray signature in the cosmic-ray coming from dark matter annihilation in the Universe (indeed the pair-annihilation of Galactic dark matter produce a monochromatic or internal bremsstrahlung gamma rays that rise from the continuous background). 

The data were collected during the last 4 years by the Fermi satellite using a new sophisticated data technique to select optimized target regions of the sky depending on the density profile of the Galactic dark matter halo. In regions near to the Galactic center, the analysis reveals a bump over the gamma ray continuum near 130 GeV, with a significance of the observed excess of 4.6σ ,that after taking into account the so-called "lookelsewhere effect" drops to 3.3σ. If this bump is interpreted in terms of dark matter particles annihilation into a photon pair, the observations imply a dark matter of mass of mχ = 129.8 ± 2.4 GeV.

But yesterday (30 April 2012) a new paper appeared on arXiv and seems to contradict the conclusions of the previous work. The authors interpeted the bump in a different way: since the 130 GeV excess appears making a specific selection of the target region that vastly overlap with the region corresponding to the so-called “Fermi bubbles”, they finally think that the reason of this phenomenon are releted to the bubbles themselves. To understand this claming, a short digression is needed. The “Fermi bubbles” are two massive bubbles of plasma, each extending for tens of thousands of light-years, with the property of emitting high-energy radiation above and below the plane of the galaxy (actually this structure was found in data thanks to the Fermi Telescope again). The gamma rays origine is not completely clear, but one possible explanation could be that populations of high-energy electrons, produced when the matter falls into the galaxy's central black hole, swept up in a hot plasma emitting conseguently gamma rays (see also this for another interpretation).

"Fermi bubbles" representation in our Milky Way.

So the authors argue that the tentative evidence for a line feature is likely due to hard photons in the Fermi bubbles regions, where the gamma-ray spectrum contains a spectral break at the energy range of interest. Although the origin of this broken power-law is unclear, it is probably related to standard astrophysical processes and not to dark matter annihilation. A broken power-law provides a fit as good as the one for the line “excess”, as shown in figure below.

The gamma-ray spectrum in two regions and the best-fit power-law (red dashed), power-law plus monochromatic line at Eγ = 130 GeV and a broken power-law (blue) similar to the spectrum observed in the Fermi bubbles regions.

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