Diffuse gamma ray constraints on annihilating or decaying Dark Matter after Fermi

TL;DR

This work evaluates Fermi-LAT first-year diffuse gamma-ray data to constrain dark matter (DM) annihilation $\langle \sigma v \rangle$ or decay $\tau_{\rm dec}$ by predicting the full gamma-ray signal (prompt plus Inverse Compton, IC) for multiple sky regions and DM halo profiles, including a cosmological decay component. It derives exclusion curves in the $m_\chi$–$\langle \sigma v \rangle$ and $m_\chi$–$\tau_{\rm dec}$ planes and tests DM interpretations of the PAMELA/FERMI/HESS lepton anomalies, finding that for NFW/Einasto halos leptonic DM explanations are strongly constrained or excluded, whereas a subset of muon-dominated, cored-halo scenarios remain marginally viable. Decaying DM is largely disfavored by the isotropic gamma-ray background, with the strongest constraints for the $\tau^+\tau^-$ channel, while the muon channel faces tighter limits but some space remains depending on the halo profile. Overall, the diffuse gamma-ray constraints from Fermi provide powerful, nearly background-free tests of DM in the Galactic halo, underscoring the need for careful background modeling to interpret potential signals and guiding future multi-messenger DM searches.

Abstract

We consider the diffuse gamma ray data from FERMI first year observations and compare them to the gamma ray fluxes predicted by Dark Matter annihilation or decay (both from prompt emission and from Inverse Compton Scattering), for different observation regions of the sky and a range of Dark Matter masses, annihilation/decay channels and Dark Matter galactic profiles. We find that the data exclude large regions of the Dark Matter parameter space not constrained otherwise and discuss possible directions for future improvements. Also, we further constrain Dark Matter interpretations of the e+e- PAMELA/FERMI spectral anomalies, both for the annihilating and the decaying Dark Matter case: under very conservative assumptions, only models producing dominantly mu+mu- and assuming a cored Dark Matter galactic profile can fit the lepton data with masses around 2 TeV.

Figures (5)

• Figure 1: Gamma ray fluxes for a few sample DM candidates, compared to the Fermi datapoints in the different observation regions that we consider. See text for details.
• Figure 2: The regions on the parameter space $m_\chi$--$\langle \sigma v \rangle$ that are excluded by the diffuse galactic gamma ray measurements by the Fermi satellite. The first column of panels refers to DM annihilations into $e^+e^-$, the second into $\mu^+\mu^-$ and the third into $\tau^+\tau^-$; the three rows assume respectively an NFW, an Einasto and a cored Isothermal profile. Each panel shows the exclusion contour due to Fermi observations of the '$3^\circ \times 3^\circ$' region (blue short dashed line), '$5^\circ \times 30^\circ$' region (orange dashed line), the '$10^\circ - 20^\circ$ strip' (red long dashed line) and the 'Galactic Poles' $|b| > 60^\circ$ region (black long dashed line). We also report the regions that allow to fit the PAMELA positron data (green and yellow bands, 95 % and 99.999 % C.L. regions) and the PAMELA positron + Fermi and HESS data (red and orange blobs, 95% and 99.999% C.L. regions).
• Figure 3: Like figure \ref{['fig:exclusionann']}, but for DM annihilations into $b\bar{b}$, $W^+W^-$, and $t\bar{t}$.
• Figure 4: Similarly to figure \ref{['fig:exclusionann']} but for decaying Dark Matter. The vertical axis reports here the half-life $\tau_{\rm dec}$ in seconds. The exclusion contours are due to Fermi observations of the '$10^\circ - 20^\circ$ strip' (red dashed line), the $|b| > 60^\circ$ 'Galactic Poles' region (black long dashed line) and the isotropic flux (magenta dotted line). We also report the regions that allow to fit the PAMELA positron data (green and yellow bands, 95% and 99.999% C.L. regions) and the PAMELA positron + Fermi and HESS data (red and orange blobs, 95% and 99.999% C.L. regions) in terms of decaying Dark Matter. We here report only the case of an Einasto galactic DM profile: the cases of an Isothermal or a NFW profile are essentially identical (see text for details).
• Figure 5: For one selected observational region (the '$10^\circ-20^\circ$ strips') and profile, and for two exemplar annihilating DM candidates, we show the current bounds derived by our conservative procedure of requiring that the DM signal must not exceed the observational data points by more than 3$\sigma$ (red dashed line) and the prospective regions that can be explored by Fermi if an astrophysical gamma ray background is assumed and the exclusion criterion is relaxed to 2$\sigma$ (blue short dashed line).