121 research outputs found
Detecting the Stimulated Decay of Axions at Radio Frequencies
Assuming axion-like particles account for the entirety of the dark matter in
the Universe, we study the possibility of detecting their decay into photons at
radio frequencies. We discuss different astrophysical targets, such as dwarf
spheroidal galaxies, the Galactic Center and halo, and galaxy clusters. The
presence of an ambient radiation field leads to a stimulated enhancement of the
decay rate; depending on the environment and the mass of the axion, the effect
of stimulated emission may amplify the photon flux by serval orders of
magnitude. For axion-photon couplings allowed by astrophysical and laboratory
constraints(and possibly favored by stellar cooling), we find the signal to be
within the reach of next-generation radio telescopes such as the Square
Kilometer Array.Comment: Minor changes, references added, matches published versio
Diffuse cosmic rays shining in the Galactic center: A novel interpretation of H.E.S.S. and Fermi-LAT gamma-ray data
We present a novel interpretation of the -ray diffuse emission
measured by Fermi-LAT and H.E.S.S. in the Galactic center (GC) region and the
Galactic ridge (GR). In the first part we perform a data-driven analysis based
on PASS8 Fermi-LAT data: we extend down to few GeV the spectra measured by
H.E.S.S. and infer the primary cosmic-ray (CR) radial distribution between 0.1
and 3 TeV. In the second part we adopt a CR transport model based on a
position-dependent diffusion coefficient. Such behavior reproduces the radial
dependence of the CR spectral index recently inferred from the Fermi-LAT
observations. We find that the bulk of the GR emission can be naturally
explained by the interaction of the diffuse steady-state Galactic CR sea with
the gas present in the Central Molecular Zone. Although our results leave room
for a residual radial-dependent emission associated with a central source, the
relevance of the large-scale background prevents from a solid evidence of a GC
Pevatron.Comment: 5 pages, 3 figures, accepted for publication in Physical Review
Letter
A dark matter interpretation for the ARCADE excess?
The ARCADE 2 Collaboration has recently measured an isotropic radio emission
which is significantly brighter than the expected contributions from known
extra-galactic sources. The simplest explanation of such excess involves a
"new" population of unresolved sources which become the most numerous at very
low (observationally unreached) brightness. We investigate this scenario in
terms of synchrotron radiation induced by WIMP annihilations or decays in
extragalactic halos. Intriguingly, for light-mass WIMPs with thermal
annihilation cross-section, and fairly conservative clustering assumptions, the
level of expected radio emission matches the ARCADE observations.Comment: 5 pages, 3 figures. v2: one benchmark model added, comments and
references expanded, to appear in PR
Hard Cosmic Ray Sea in the Galactic Center: a consistent interpretation of H.E.S.S. and Fermi-LAT -ray data
We present a novel interpretation of the gamma-ray diffuse emission measured
by H.E.S.S. in the Galactic Center (GC) region and the Galactic ridge. Our
starting base is an updated analysis of PASS8 Fermi-LAT data, which allows to
extend down to few GeV the spectra measured by H.E.S.S. and to infer the
primary CR radial distribution above 100 GeV. We compare those results with a
CR transport model assuming a harder scaling of the diffusion coefficient with
rigidity in the inner Galaxy. Such a behavior reproduces the radial dependence
of the CR spectral index recently inferred from Fermi-LAT measurements in the
inner GP. We find that, in this scenario, the bulk of the Galactic ridge
emission can be naturally explained by the interaction of the diffuse,
steady-state Galactic CR sea interacting with the gas present in the Central
molecular zone. The evidence of a GC PeVatron is significantly weaker than that
inferred adopting a conventional (softer) CR sea.Comment: Oral contribution to the International Cosmic Ray Conference (ICRC
2017), 12-20 July 2017, Bexco, Busan, Kore
Gamma-ray lines and One-Loop Continuum from s-channel Dark Matter Annihilations
The era of indirect detection searches for dark matter has begun, with the
sensitivities of gamma-ray detectors now approaching the parameter space
relevant for weakly interacting massive particles. In particular, gamma ray
lines would be smoking gun signatures of dark matter annihilation, although
they are typically suppressed compared to the continuum. In this paper, we pay
particular attention to the 1-loop continuum generated together with the
gamma-ray lines and investigate under which conditions a dark matter model can
naturally lead to a line signal that is relatively enhanced. We study generic
classes of models in which DM is a fermion that annihilates through an
s-channel mediator which is either a vector or scalar and identify the coupling
and mass conditions under which large line signals occur. We focus on the
"forbidden channel mechanism" advocated a few years ago in the "Higgs in space"
scenario for which tree level annihilation is kinematically forbidden today.
Detailed calculations of all 1-loop annihilation channels are provided. We
single out very simple models with a large line over continuum ratio and
present general predictions for a large range of WIMP masses that are relevant
not only for Fermi and Hess II but also for the next generation of telescopes
such as CTA and Gamma-400. Constraints from the relic abundance, direct
detection and collider bounds are also discussed.Comment: 32 pages, 13 figures; v2: minor clarifications, summary paragraph
added; v3: matches published version, minor clarifications, results unchange
Dark Matter's secret liaisons: Phenomenology of a dark U(1) sector with bound states
Dark matter (DM) charged under a dark U(1) force appears in many extensions of the Standard Model, and has been invoked to explain anomalies in cosmic-ray data, as well as a self-interacting DM candidate. In this paper, we perform a comprehensive phenomenological analysis of such a model, assuming that the DM abundance arises from the thermal freeze-out of the dark interactions. We include, for the first time, bound-state effects both in the DM production and in the indirect detection signals, and quantify their importance for Fermi, Ams-02, and CMB experiments. We find that DM in the mass range 1 GeV to 100TeV, annihilating into dark photons of MeV to GeV mass, is in conict with observations. Instead, DM annihilation into heavier dark photons is viable. We point out that the late decays of multi-GeV dark photons can produce significant entropy and thus dilute the DM density. This can lower considerably the dark coupling needed to obtain the DM abundance, and in turn relax the existing constraints
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