The AGASA and SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High Energy Neutrons

Abstract

Recent analysis of data sets from two extensive air shower cosmic-ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays toward the Galactic center region that ''turns on'' around 1018 eV. We demonstrate that the anisotropy could be due to neutrons created in the Galactic center region through charge exchange in proton-proton collisions, where the incident, high-energy protons obey a $E À2 power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746À2851 (ascribed to p-p-induced neutral pion decay at GeV energies), together with a very reasonable spectral index of 2.2, predicts a neutron flux at $10 18 eV fully consistent with the extremely high energy cosmic-ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air Cerenkov telescope at $TeV energies is almost equally compatible with the $10 18 eV cosmic-ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We find a plausible resolution of this discrepancy in an effective two-source model. Finally, we argue that the shock acceleration is probably occurring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic center. In support of this contention we note that (1) the extended shell of this object could provide both of the sources suggested by the gamma-ray data and (2) the unusually strong magnetic field at this remnant, together with a perpendicular shock geometry, allow for acceleration of protons up to the extreme energies required to explain the cosmic-ray anisotropy. If the connection between the anisotropy and Sagittarius A East could be firmly established, it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near 10 19 eV.

TeV neutrinos from supernova remnants embedded in giant molecular clouds

“…Indeed, the total CR energy required to explain both of them is compatible with that typically expected from shock acceleration by the remnant of a single SN. These considerations point to Sgr A East as the most plausible primary source (see [32,24] about the identification of J1745-290 with that SNR).…”

Sgr A East as a possible high energy neutron factory in the Galactic Centre

“…Actually, in Sec.8 we argue that both AGASA and SUGAR anisotropies can hardly be ascribed to a neutron emission from the Galactic centre. In particular we show that the EeV neutron flux estimated in [7] is likely to be incompatible with the energetic of Sgr A East as constrained by Chandra [9] and HESS [12] observations. Here we only use HESS observations to normalise the primary spectrum of nuclei accelerated in Sgr A East assuming that this spectrum remain unchanged up to UHEs.…”

“…Similarly to what done in [27,7] for 3EG J1746-2851, we assume that the high energy γ-rays detected by HESS from the direction of the GC are generated by the hadronic scattering of high energy nuclei (mainly protons) accelerated by Sgr A East onto the ambient gas nuclei (mainly hydrogen). The relevant process is pp → pp + n π π ±,0 (n π is the pion multiplicity) with the neutral pions decaying into photons.…”

“…The relevant process is pp → pp + n π π ±,0 (n π is the pion multiplicity) with the neutral pions decaying into photons. At high energies the differential pp cross section can be approximated by a scaling form [28] and the photon emissivity can be written [29,27,7] …”

“…[14,15,16]). In particular, Crocker et al [7] first proposed Sgr A East as a possible high energy γ-ray and EeV neutron source, although the contribution of nuclei PD to the neutron flux was not determined in their paper. All these works, however, tried to interpret the EeV CR anisotropy claimed by the AGASA [14] and SUGAR [17] experiments.…”