Inverse Compton constraints on the Dark Matter e+e- excesses

TL;DR

The paper addresses whether Dark Matter (DM) annihilations proposed to explain the cosmic ray $e^{\pm}$ excesses can be reconciled with high-energy gamma-ray data. It develops a semi-analytic inverse Compton scattering (ICS) framework to predict the diffuse gamma-ray flux from DM-produced $e^{\pm}$ interacting with the interstellar radiation field, considering multiple sky regions, DM density profiles, and annihilation channels over a broad mass range $M_{\rm DM}=100~\text{GeV}$ to $20~\text{TeV}$; constraints are derived by requiring the ICS flux to not exceed observational data at the $3\sigma$ level. The results show that for $M_{\rm DM} \gtrsim 1~{\rm TeV}$ and leptonic channels, especially with benchmark Einasto or NFW profiles, large portions of the PAMELA-favored parameter space are excluded, with isothermal profiles offering milder constraints and hadronic channels remaining comparatively less constrained. The study highlights the power of ICS gamma-ray observations as a complementary, relatively DM-distribution–robust probe that challenges DM interpretations of the $e^{\pm}$ excess and anticipates stronger constraints with future FERMI data, while corroborating and connecting with limits from prompt gamma rays and synchrotron emission.

Abstract

Recent results from experiments like PAMELA have pointed to excesses of e+e- in cosmic rays. If interpreted in terms of Dark Matter annihilations, they imply the existence of an abundant population of e+e- in the galactic halo at large. We consider the high energy gamma ray fluxes produced by Inverse Compton scattering of interstellar photons on such e+e-, and compare them with the available data from EGRET and some preliminary data from FERMI. We consider different observation regions of the sky and a range of DM masses, annihilation channels and DM profiles. We find that large portions of the parameter space are excluded, in particular for DM masses larger than 1 TeV, for leptonic annihilation channels and for benchmark Einasto or NFW profiles.

Figures (4)

• Figure 1: Left: The Dark Matter galactic distribution profiles considered in the text. Right: Modelization of the total Inter-Stellar Radiation Field (ISRF): the dashed lines reproduce the computations of ISRFISRF2. From top to bottom: Galactic Center ($R=0$ kpc, $z=0$ kpc -- black), $R=4$ kpc, $z=0$ kpc (blue), 5 kpc outside of the galactic plane (magenta). The solid lines represent our modelizations in terms of (renormalized) blackbody spectra for the StarLight and InfraRed photons.
• Figure 2: Some examples of ICS signals from selected DM models, in the three different regions of the sky that we consider, superimposed to the relevant datasets. Left: in the region '5$\times$30', EGRET datapoints and the signal from a $3$ TeV DM candidate annihilating into $\tau^+\tau^-$ with $\sigma_{\rm ann}v = 1.7 \cdot 10^{-22}\, {\rm cm}^3/{\rm sec}$, choosing the Einasto DM profile of eq. (\ref{['eq:Einasto']}) (in analogy with 700+, where however the Einasto profile differs slightly). Center: in the region '10$\times$60', EGRET datapoints and a signal from a $1.5$ TeV DM candidate annihilating into $\mu^+\mu^-$ with $\sigma_{\rm ann}v = 5 \cdot 10^{-23}\, {\rm cm}^3/{\rm sec}$, choosing the isothermal DM profile of eq. (\ref{['eq:isoT']}) (in analogy with Cuoco, where however the isothermal profile differs slightly). Right: in the region '10$-$20' strips, EGRET datapoints and preliminary FERMI datapoints, together with a signal from a $10$ TeV DM candidate annihilating into $W^+W^-$ with $\sigma_{\rm ann}v = 5 \cdot 10^{-22}\, {\rm cm}^3/{\rm sec}$, choosing the NFW DM profile of eq. (\ref{['eq:NFW']}).
• Figure 3: The regions favored by PAMELA (green bands), containing in particular the areas favored by PAMELA+ATIC (yellow areas), are compared with the bounds from ICS secondary radiation. 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 Einasto, an NFW and an isothermal profile. In each panel, the bounds from EGRET data in the '5$\times$30' region are plotted with a short dashed red line, those from EGRET data in the '10$\times$60' region with a solid red line and those from EGRET data in the '10$-$20' strips with a dashed red line. The preliminary FERMI bounds in the '10$-$20' strips are plotted with a dashed blue line. The ICS bounds are computed adopting the simplifying assumption of neglecting the diffusion of the source $e^\pm$, see text for the full discussion.
• Figure 4: As in the previous fig. \ref{['fig:exclusion']}, but for $W^+W^-$, $b \bar{b}$ and $t \bar{t}$ annihilation channels. Since a DM particle fitting the PAMELA data has to be multi-TeV, the green bands start at larger masses. There is no possibility to fit the ATIC data in these channels.