On possible interpretations of the high energy electron-positron spectrum measured by the Fermi Large Area Telescope

TL;DR

The paper investigates interpretations of the Fermi-LAT cosmic-ray electron-positron spectrum between $20~\mathrm{GeV}$ and $1~\mathrm{TeV}$, testing a smooth large-scale Galactic CRE component against additional sources from nearby pulsars and dark matter annihilation. Using GALPROP and analytic treatments, it shows that single-component models struggle to reconcile PAMELA’s positron fraction and H.E.S.S. data, while a two-component picture with local pulsars (notably Monogem and Geminga) can consistently fit both Fermi and PAMELA; dark matter scenarios can also fit under leptonic final states but face strong constraints from H.E.S.S., neutrino, and BBN limits. A key takeaway is that CRE anisotropy offers a potential discriminator between pulsar and dark matter origins, and future data from AMS-02 and gamma-ray observations will be crucial to distinguishing these possibilities. Overall, pulsars emerge as the favored interpretation given current data, though a DM contribution remains a viable, albeit constrained, alternative depending on the model class and multi-messenger bounds.

Abstract

The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV. The spectrum shows no prominent spectral features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron-positron primary sources, e.g. nearby pulsars or particle Dark Matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100 GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.

Figures (12)

• Figure 1: In this figure we compare Fermi-LAT CRE data (Abdo et al. 2009 Fermi_CRE_1), as well as several other experimental data sets (HEAT: Du Vernois et al. 2001 DuVernois:2001bb; AMS-01: Aguilar et al. 2002 ams1; ATIC: Chang et al. 2008 atic; PPB-BETS:Tori et al. 2008 Torii:2008xu; H.E.S.S. 2008: Aharonian et al. 2008, hess; H.E.S.S. 2009 Aharonian et al. 2009, hess:09) with the electron plus positron spectrum modeled with GALPROP under the conditions discussed in Sec.\ref{['sec:galprop3']}. The gray band represents systematic errors on the CRE spectrum measured by Fermi-LAT. The black continuos line corresponds to the conventional model used in (Strong et al. 2004 Strong:04) to fit pre-Fermi data model (model 0 in Tab. 1). The red dashed (model 1 in Tab. \ref{['table1']}) and blue dot-dashed lines (model 2 in Tab. \ref{['table1']}) are obtained with modified injection indexes in order to fit Fermi-LAT CRE data. Both models account for solar modulation using the force field approximation assuming a potential $\Phi = 0.55~{\rm GV}$.
• Figure 2: Results of an analytical calculation for stochastic sources, including Gould's Belt (see Pohl et al. 2003 GouldBelt). The propagation parameters are those of model 1 in Tab. \ref{['table1']}, and all spectra are normalized to the fiducial flux at 100 GeV. The solid line gives the average spectrum that one would obtain, if the sources were continuously distributed. The shaded are indicates the 1-$\sigma$ fluctuation range of the electron flux at each energy. The dashed line indicates one randomly chosen, actual electron spectrum. Fermi-LAT and H.E.S.S. data points are represented in red and black respectively.
• Figure 3: In this figure we compare the positron fraction corresponding to the same models used to draw Fig. \ref{['fig:elepos_242reac']} with several experimental data sets (HEAT: Barwick et al. 1997 Barwick:1997ig; CAPRICE: Boezio et al. 2000 Boezio:00; AMS-01: Aguilar et al. 2002 Aguilar et al. 2002 ams1; PAMELA: Adriani et al. 2009, 2009b PAMELAPAMELA_Nature). The line styles are coherent with those in that figure. Note that our results use a solar modulation potential $\Phi = 0.55~{\rm GV}$ which is appropriate for the AMS-01 and HEAT data taking periods (Barwick et al. 1997 Barwick:1997ig). It is not appropriate for the PAMELA data taking period, and impacts agreement among the experiments and our model with the PAMELA data below 10 GeV.
• Figure 4: In this figure we represent the electron-plus-positron spectrum (blue continuos line) computed in a case in which only observed pulsars from the ATNF catalogue (Manchester 2005 Manchester:05) with distance $d < 1~{\rm kpc}$ plus the large-scale Galactic component (GCRE) give a significant contribution. The dominant contribution of Monogem and Geminga pulsars, analytically computed for a representative choice of the relevant parameters (see text) is shown as colored dot-dashed lines, while the GCRE, computed with GALPROP is shown as a black-dotted line. The gray band represents systematic errors on the CRE Fermi-LATdata. Solar modulation is accounted as done in Fig.\ref{['fig:elepos_242reac']}.
• Figure 5: The positron fraction for the same scenario as in Fig.\ref{['fig:elepos_Monogem_242reac']}. Line styles are coherent with those in that figure. Solar modulation is accounted as done in Fig.\ref{['fig:pos_ratio_242reac']}.