Dark Matter Signals in the gamma-ray sky

Aldo Morselli, L. Bravina, Y. Foka, S. Kabana
2014 EPJ Web of Conferences  
The energy range between 10 and 50 Me V is an experimentally very difficul range and remained uncovered since the time of COMPTEL. Here we propose a possible mission to cover this energy range. One of the major scientific objectives of ')'-ray instruments is the indirect search for dark matter (DM), by means of the production of secondary ')'-rays after the annihilation (or decay) of the DM particle candidates. The search strategy, which was assessed with a detailed study 1 , comprises the
more » ... of targets with an expected relatively large ')'-ray signal (such as the Galactic Center 2 • 3 • 4,which was previously studied with EGRET data 5), or with a very low foreseen conventional ')'-ray emission 6 • 7, the search for annihilation lines S ,9 and also the search of possible anisotropies generated by the DM halo substructures 11. The indirect DM searches with 1' rays are complemented with those performed with the detection of cosmic-ray electrons by the LAT 1 2 • 13 • 10. All these searches and limits were possible thanks to the extraordinary performances of the Fermi-Lat 1 6 • 17 that allowed the Fermi collaboration to compile the Second Fermi-LAT catalog (2FGL) 15 that is the deepest catalog ever produced in the energy band between 100 MeV and 100 GeV. Compared to the First Fermi-LAT (lFGL) 14 , it features several significant improvements: it is based on data from 24 (vs. 11) months of observation and makes use of the new Pass 7 event selection. The energy flux map is shown in figure 1. In the catalog 127 sources of the 1873 sources are firmly identified, based either on periodic variability (e.g. pulsars) or on spatial morphology or on correlated variability. In addition to that 1170 are reliably associated with sources known at other wavelengths, while 576 (i.e. 313 of the total number of entries in the catalog) are still unassociated. The association is particulary difficult in the Galactic plane region ( sec figure 2 ) where many sources are condensed in small regions of the sky. The task of source association as well as the search for dark matter coud have a great benefit from an instrument with better angular resolution and effective area in the energy range centered near 10 MeV. This energy range is notoriously difficult to study, and since COMPTEL on board of CGRO there has been no space instrument devoted to the 5-50 MeV energy range. The main problem is the multiple scattering of the electron and positron pair that ruin the reconstruction of the direction of the incoming gamma. In figure 3 it is shown the effect of Multiple Scattering on the angular distribution of electrons for different thickness of material traversed (in Radiation Lengths (Xo)) and from this figure it is clear that a tracker sistem without passive converter is the only way to minimize the multiple scattering effects. The importance of the 5-50 MeV energy range the search for dark matter can be seen in figure 4 where the differential ')'-ray
doi:10.1051/epjconf/20147100094 fatcat:px3odl5j7jhafhi5jtu2cj4e5y