Secondary Radiation from the Pamela/atic Excess and Relevance for Fermi

The excess of electrons/positrons observed by the Pamela and ATIC experiments gives rise to a noticeable amount of synchrotron and Inverse Compton Scattering (ICS) radiation when the ee interact with the Galactic Magnetic Field, and the InterStellar Radiation Field (ISRF). In particular, the ICS signal produced within the WIMP annihilation interpretation of the Pamela/ATIC excess shows already some tension with the EGRET data. On the other hand, 1 yr of Fermi data taking will be enough to rule out or confirm this scenario with a high confidence level. The ICS radiation produces a peculiar and clean “ICS Haze” feature, as well, which can be used to discriminate between the astrophysical and Dark Matter scenarios. This ICS signature is very prominent even several degrees away from the galactic center, and it is thus a very robust prediction with respect to the choice of the DM profile and the uncertainties in the ISRF. PACS: 95.35.+d, 95.85.Bh, 95.85.Pw, 98.70.Vc Subject headings: dark matter — gamma rays: observations — cosmic rays — radio continuum: ISM — ISM: general — Galaxy: general The Pamela and ATIC results have recently raised a great interest in the scientific community due to the possibility that the observed ee excesses could be a signature of the, so-far elusive, particle associated to Dark Matter. The raise in the positron fraction above 10 GeV until ∼100 GeV seen by Pamela (Adriani et al. 2008a) and the excess of the sum of e and e between ∼100 GeV and ∼700 GeV seen by ATIC (Chang et al. 2008) can be hardly explained in a standard Cosmic Ray production scenario and, instead, seem to point to a new source of e and e. Hints of this anomaly were reported also by different experiments like HEAT (Barwick et al. 1997), AMS-01 (Aguilar et al. 2007; Alcaraz et al. 2000) and PPBBETS (Torii et al. 2008). In addition, HESS has recently presented a measurement of the electron spectrum in the range 0.6 < E < 5 TeV (Aharonian et al. 2008). This anomaly can have a standard astrophysical interpretation (Atoian et al. 1995, Zhang and Cheng 2001, Profumo 2008,Yuksel et al. 2008, Hooper et al. 2009) or an exotic one involving decaying (Liu et al. 2008, Hisano et al. 2008b, Yin et al. 2008, Chen et al. 2008, Ibarra and Tran, Hamaguchi et al. 2008) or the annihilation of DM particles (Hisano et al. 2008a, Mardon et al. 2009, Zurek 2008, Cholis et al. 2008a, Bergstrom et al. 2008a, Arkani-Hamed et al. 2009, Meade et al. 2009, Ishiwata et al. 2008a, Hu et al. 2009, Nomura and Thaler 2008, Hall and Hooper 2008, Barger et al. 2009, deBoer 2009, Cholis et al. 2008b, Fox and Poppitz 2008). The latter description, in particular, seems to favor a DM particle in the TeV range and with a thermally averaged annihilation cross section 〈σAv〉 ∼ 10 −23 cms. However, this scenario faces several difficulties. A first problem is that, differently from the positron ratio, no excess is observed by Pamela in the antiproton over proton ratio (Adriani et al. 2008b). This means that DM decay/annihilation into hadronic channels is mainly forbidden or at least strongly suppressed (Cirelli et al. 2008; Donato et al. 2008), and hence one has to resort to models in which only the leptonic channels are allowed. The second problem is that the annihilation rate required to explain the anomaly is about three orders of magnitude above the natural expectation of 〈σAv〉 ∼ 3 × 10 −26 cms for a DM thermal relic which accounts for the cosmological DM abundance. This requires either the introduction of large annihilation boost factors from the presence of galactic substructure, or some enhancing annihilation mechanism like the Sommerfeld process (Lattanzi and Silk 2008; Ibe et al. 2008). The fact that hadronic channels have to be suppressed to explain the Pamela/ATIC anomaly implies that only few (energetic) photons are produced either if the annihilation takes place through the μμ or ττ channels or in the case of the ee channel through the presence of Final State Radiation. With the limited contribution of gamma rays accompanying the annihilation process, the constraints from gamma observations become thus quite weak. Anyway, even though only ee were produced in the DM annihilation process, these leptons, once in the galactic environment, would interact with the Galactic Magnetic Field (GMF) and the Interstellar Radiation Field (ISRF). Thus they would lose energy producing synchrotron radiation in the radio band and Inverse Compton Scattering (ICS) Radiation in the gamma band. This secondary radiation thus represents a complementary observable to constrain the DM signal (Bergstrom et al. 2008b, Ishiwata et al. 2008b, Cholis et al. 2008c, Nardi et al. 2008, Zhang et al. 2008, Bertone et al. 2008, Borriello et al. 2008). In the following we will focus on the synchrotron and ICS signals which are expected in the galactic halo. With