Dark matter and pulsar signals for Fermi LAT, PAMELA, ATIC, HESS and WMAP data

Abstract

We analyze new diffuse gamma-ray data from the Fermi Gamma-ray Space Telescope, which do not confirm an excess in the EGRET data at galactic mid-latitudes, in combination with measurements of electron and positron fuxes from PAMELA, Fermi and HESS within the context of three possible sources: dark matter (DM) annihilation or decay into charged leptons, and a continuum distribution of pulsars. We allow for variations in the backgrounds, consider several DM halo profiles, and account for systematic uncertainties in data where possible. We find that all three scenarios represent the data well. The pulsar description holds for a wide range of injection energy spectra. We compare with ATIC data and the WMAP haze where appropriate, but do not fit these data since the former are discrepant with Fermi data and the latter are subject to large systematic uncertainties. We show that for cusped halo profiles, Fermi could observe a spectacular gamma-ray signal of DM annihilation from the galactic center while seeing no excess at mid-latitudes.

Figures (7)

• Figure 1: $r\rho(r)$ for the Moore, Einasto, NFW, and isothermal profiles with density 0.3 GeV/cm$^3$ at 8.5 kpc. The analytical expressions are provided in the appendix.
• Figure 2: Best-fit $\chi^2$ values for DM annihilation into various charged lepton final states for several values of $E_s$ from a joint analysis of the PAMELA, Fermi $\gamma-$ray, Fermi $e^\pm$ and HESS datasets which have 7, 18, 26 and 8 points, respectively. $(e,\mu,\tau)$ denotes a democratic final state with equal branching fractions into $e^+e^-$, $\mu^+\mu^-$ and $\tau^+\tau^-$. The number of free parameters is 8, and the number of degrees of freedom (dof) is 53, including two energy scale normalizations; see the appendix for details. The $\chi^2$/dof is provided above each column. For $E_s=150$ GeV, all data other than the falling HESS spectrum are well reproduced for the $\mu^+\mu^-$ and $\tau^+\tau^-$ modes. An isothermal halo profile is assumed.
• Figure 3: Illustrative spectra for DM annihilation with $E_s=150$ GeV, 800 GeV, 1 TeV and 2 TeV into $\mu^+\mu^-$, $e^+e^-$, $(e,\mu,\tau)$, and $\tau^+\tau^-$, respectively. The DM-only contribution to $\gamma-$rays for the $\mu^+\mu^-$ mode is tiny (because the boost factor is about 25; see Eq. \ref{['bf']}) and lies outside the range of the figure. The corresponding $\chi^2$ values can be found in Fig. \ref{['fig:baranni']}. The background contribution for each case is displayed in the same line-type as for the signal. From top to bottom in the bottom-right panel, the electron background injection spectral indices are $\gamma_0=2.38$, 2.49, 2.57 and 2.63; see the appendix. The orange, light blue and dark green HESS data points (color coded according to the annihilation channel) are shifted within the energy calibration uncertainty with $\epsilon=1.3$, 1.02 and 0.84, respectively. The HESS data corresponding to annihilation into $(e,\mu,\tau)$ have $\epsilon=0.98$, and are not shown. For $E_s=800$ GeV, 1 TeV and 2 TeV, the Fermi $e^\pm$ data shift downward (shown in black) with an energy calibration scale of 0.9, and for $E_s=150$ GeV (shown in gray), the corresponding value is 1.03. The shifts in the datasets are amplified by the $E_e^3$ factor in the ordinate. The HESS and Fermi error bars have been expanded to approximately include systematic uncertainties (apart from the energy scale uncertainties).
• Figure 4: A comparison of the best-fits for DM annihilation, DM decay and pulsars. In the DM scenarios, the $\mu^+\mu^-$ mode is marginally preferred over the $\tau^+\tau^-$ mode, except for $E_s=2$ TeV.
• Figure 5: Illustrative spectra for DM annihilation into $\mu^+\mu^-$ ($E_s=1$ TeV), DM decay into $\tau^+\tau^-$ ($E_s=2$ TeV), and pulsars with $E_s=1$ TeV. Note that the $e^\pm$ spectra for DM annihilation into $\tau^+\tau^-$ with $E_s=2$ TeV are identical to the DM decay case. The pulsar-only contribution to $\gamma-$rays lies slightly outside the range of the figure. The corresponding $\chi^2$ values can be found in Fig. \ref{['fig:barchart']}. For the DM decay case, the green HESS data points are systematically shifted to lower energies with $\epsilon=0.84$. For pulsars, the light blue HESS data points have $\epsilon=1.0$. The HESS data corresponding to DM annihilation have $\epsilon=0.8$, and are not shown. For the DM cases, the Fermi $e^\pm$ data shift (shown in black) with an energy calibration scale of 0.9, and for pulsars (shown in gray), the corresponding value is 1.0. The electron backgrounds have $\gamma_0 \simeq 2.61$.