Abstract
The very high energy Galactic γ-ray sky is partially opaque in the (0.1–10) PeV energy range. In the light of the recently detected high energy neutrino flux by IceCube, a comparable very high energy γ-ray flux is expected in any scenario with a sizable Galactic contribution to the neutrino flux. Here we elaborate on the peculiar energy and anisotropy features imposed upon these very high energy γ-rays by the absorption on the cosmic microwave background photons and Galactic interstellar light. As a notable application of our considerations, we study the prospects of probing the PeV-scale decaying DM scenario, proposed as a possible source of IceCube neutrinos, by extensive air shower (EAS) cosmic ray experiments. In particular, we show that anisotropy measurements at EAS experiments are already sensitive to τDM∼ \U0001d4aa(1027) s and future measurements, using better gamma/hadron separation, can improve the limit significantly.
Highlights
The associated gamma-ray flux should still be detectable at VHE, albeit with a significantly suppressed spectrum
The important role of extensive air shower (EAS) probes of this scenario has been discussed in the past, here we revisit the calculation of the expected γ-ray flux, with a triple goal: i) To estimate more precisely the spectral and angular shape of a decaying dark matter (DDM) signal, with state of the art treatment for the primary γ-ray absorption and the inverse Compton component. ii) To point out that due to the generically anisotropic nature of the VHE γ-ray component, even detectors with little or without gamma-hadron rejection capability should be able to put constraints on these contributions based merely on the expected anisotropy. iii) To motivate experimental collaborations to constrain some angular-energy templates, to optimize their constraining power for specific models
We discussed some effects that this phenomenon has onto expected signals in extensive air shower (EAS) detectors
Summary
The γ-ray flux in the approximate range 10−2 ÷ 102 PeV will suffer attenuation in the Galaxy due to the pair production γγ → e−e+ process onto photon baths: at the lower energies, starlight (SL) and infrared (IR) photons constitute important targets (mostly for directions towards the inner Galaxy), while at ∼ PeV energies and above the homogeneous cosmic microwave background (CMB) is dominant. In the following we calculate the optical depth τγγ for both CMB and SL+IR, for different incoming directions and energies. For the technically simpler case of pair production on CMB photons, the optical depth for photons of energy Eγ coming from a source at distance L can be calculated as (here and in the following, we use natural units with c = kB = 1). The optical depth due to pair production on the SL+IR photon bath can be calculated to eq (2.1), with the extra complication that the integral along the line of sight is non-trivial, since the photon bath number density nSL+IR depends on position x, and the optical depth depends on the Galactic coordinates (b, l). The optical depth due to SL+IR photon bath for two different distances and various directions are shown in figure 3.
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