Soft gamma-ray background and light dark matter annihilation

Abstract

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the (S-or P -wave) annihilation of light Dark Matter particles into e + e − . In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard X-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about 4 keV cm −2 s −1 sr −1 . We also estimate analytically the extragalactic background due to Dark Matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be one order of magnitude lower so that the hypothesis of a light Dark Matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross-sections.PACS numbers: 95.35.+d, 95.85.Nv, 95.85.Pw, 97.60.Bw

Dark matter annihilation: The origin of cosmic gamma-ray background at 1–20 MeV

“…While this effect reduces the intensity of the line contribution, it does not affect the continuum emission (i.e., the internal bremsstrahlung), as the continuum emission is produced before annihilation. Since the major contribution to the γ-ray background at 1-20 MeV comes from the continuum emission, our conclusion in this paper is not affected by the results in [40].…”

“…Note added in proof -A recent article by Rasera et al ( [40]) argues that our predictions for the γ-ray background from the redshifted 511 keV line ( [18]) were too large because annihilation of electrons and positrons cannot take place in halos less massive than ∼ 10 7 M ⊙ , in which baryons cannot collapse. While this effect reduces the intensity of the line contribution, it does not affect the continuum emission (i.e., the internal bremsstrahlung), as the continuum emission is produced before annihilation.…”

U-boson production ine+e−annihilations,ψandFayet1 2007Phys. Rev. DSelf Cite

“…leptons, (27) which acquire their masses through < h In the presence of one or several extra singlets transforming under the extra-U (1) symmetry and acquiring non-vanishing v.e.v. 's [1], expression (26) of the equivalent spin-0 pseudoscalar gets modified, to a = cos ζ ( "standard" A ) + sin ζ ( new singlet ) , (28) in which we define r = cos ζ .…”

“…They are also sensitive to the shape of the dark matter profiles adopted within the bulge, a P -wave cross section requiring a more peaked halo density [26,27].…”

Testing MeV dark matter with neutrino detectors

“…Nevertheless, the required cross section (times the relative velocity) is, by about four orders of magnitude, smaller than the one necessary to explain the observed abundance of DM in the Universe, σ A v ≃ 3 × 10 −26 cm 3 /s. Only velocity-dependent cross sections would be able to accommodate both evidences, but provide a bad fit to the Integral/SPI data for masses below ∼100 MeV [13,14]. In addition, higher order processes would lead to gamma ray production via internal bremsstrahlung [15,16,17] and positron annihilations in flight with the electrons in the interstellar medium [17,18].…”