Cosmic Rays from Microquasars: a Narrow Component to the CR Spectrum?

Abstract

We propose that relativistic Galactic jets like those observed in GRS 1915+105 and GRO J1655-40 may produce small but measurable contributions to the cosmic ray (CR) spectrum. If these jets contain cold protons and heavy ions (as in SS433), it is plausible that this component will consist of a narrow spectral feature, with a mean particle energy corresponding roughly to the bulk kinetic particle energy in the beam, Gamma_jet m c^2, which, based on current estimates of Gamma_jet, falls into the range of 3-10 GeV. The presence of several sources with different Gamma_jet will lead to superposition of several such peaks. Additionally, diffusive particle acceleration should produce a powerlaw whose low energy cutoff at or above Gamma_jet^2 m c^2 would be visible as another spectral feature. The large metallicities measured in several microquasar companions suggest that this CR component could have an anomalous composition compared to the bulk Galactic CRs. We provide estimates of the effects of adiabatic losses, which pose the greatest challenge to models of narrow band CR production in microquasar jets. While the total energy contained in the microquasar CR component is highly uncertain, the CR spectrum in the vicinity of microquasars would be severely affected. AMS02 will be able to probe the low energy CR spectrum for such components and composition anomalies. The spectrally peculiar gamma-ray emission produced by interaction of the ISM with CRs surrounding microquasars might be detectable by GLAST. If the presence of a microquasar CR proton component can be ruled out observationally, this argument could be turned around in favor of electron-positron jets. We show that existing OSSE/GRO and future INTEGRAL data on the Galactic 511 keV line flux put interesting constraints on the particle content of microquasar jets.