Cosmic-Ray Positrons From Millisecond Pulsars

Abstract

Observations by the Fermi Large Area Telescope of γ-ray millisecond pulsar light curves imply copious pair production in their magnetospheres, and not exclusively in those of younger pulsars. Such pair cascades may be a primary source of Galactic electrons and positrons, contributing to the observed enhancement in positron flux above ∼10 GeV. Fermi has also uncovered many new millisecond pulsars, impacting Galactic stellar population models. We investigate the contribution of Galactic millisecond pulsars to the flux of terrestrial cosmic-ray electrons and positrons. Our population synthesis code predicts the source properties of present-day millisecond pulsars. We simulate their pair spectra invoking an offset-dipole magnetic field. We also consider positrons and electrons that have been further accelerated to energies of several TeV by strong intrabinary shocks in black widow and redback systems. Since millisecond pulsars are not surrounded by pulsar wind nebulae or supernova shells, we assume that the pairs freely escape and undergo losses only in the intergalactic medium. We compute the transported pair spectra at Earth, following their diffusion and energy loss through the Galaxy. The predicted particle flux increases for non-zero offsets of the magnetic polar caps. Pair cascades from the magnetospheres of millisecond pulsars are only modest contributors around a few tens of GeV to the lepton fluxes measured by AMS−02, PAMELA, and Fermi, after which this component cuts off. The contribution by black widows and redbacks may, however, reach levels of a few tens of percent at tens of TeV, depending on model parameters.Subject headings: cosmic rays -pulsars: general -stars: neutron 2 Such an additional source of primary positrons may be either of dark matter annihilation origin (e.g., Grasso et al.

Closer view of the IGR J11014-6103 outflows

“…Whereas the counter-jet misalignment with respect to the main jet could also be due to not completely symmetric outflows and polar caps of the pulsar (as seen already in both isolated and accreting pulsars, see e.g. Harding & Muslimov 2011;Bogdanov 2014;Venter et al 2015), we note that a model that includes several simultaneous helices could also qualitatively recover the counter-jet direction. The complexity of the Γ-distance and Γ-flux distributions along the main jet and the surrounding diffuse emission could at least be partially accommodated for in this multiple-helices model.…”

Cosmic ray models

“…Finally, we mention three alternative pulsar scenarios: In the first one, the contribution from the population of all millisecond pulsars was studied [243]. While pair cascades from the magnetospheres of isolated millisecond pulsars cut off around a few tens of GeV and thus cannot contribute to the high-energy rise of R e + /e − , electron-positrons are accelerated up to tens of TeV in the strong intra-binary shocks of black widow and redback binary systems.…”

GeV–TeV Cosmic-Ray Leptons in the Solar System from the Bow Shock Wind Nebula of the Nearest Millisecond Pulsar J0437–4715

“…Malyshev et al (2009) noted that acceleration at the pulsar wind (PW) termination shock is required to produce the multi-GeV positrons consistent with PAMELA data. Büsching et al (2008) and Venter et al (2015) showed that pair cascades from the magnetospheres of millisecond pulsars without wind nebulae could only modestly contribute to the CR lepton fluxes near the Earth at a few tens of GeV, and this component would cut off at higher energies. However, they pointed out that strong intrabinary shocks in redback and black widow type pulsars may allow them to contribute to 10-40 TeV cosmic ray fluxes near the Earth.…”