A Cosmic Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?

TL;DR

This paper investigates how two nearby middle-aged pulsars, Geminga and B0656+14, could significantly contribute to local cosmic ray positrons and induce anisotropy in their flux, depending on pulsar and diffusion parameters.

Contribution

It presents a model linking pulsar properties and cosmic ray diffusion to explain local positron excess and anisotropy, providing constraints on pulsar birth periods and diffusion coefficients.

Findings

Geminga and B0656+14 can significantly contribute to local positron flux.

The model constrains pulsar birth periods based on observed spectra.

Predicted anisotropy levels depend on pulsar and diffusion parameters.

Abstract

Geminga and B0656+14 are the closest pulsars with characteristic ages in the ran ge of 100 kyr to 1 Myr. They both have spindown powers of the order 3e34 erg/s at present. The winds of these pulsars had most probably powered pulsar wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of the pulsars. Assuming that leptonic particles accelerated by the pulsars were confined in th e PWNe and were released into the interstellar medium (ISM) on breakup of the PW Ne, we show that, depending on the pulsar parameters, both pulsars make a non-ne gligible contribution to the local cosmic ray (CR) positron spectrum, and they m ay be the main contributors above several GeV. The relatively small angular dist ance between Geminga and B0656+14 thus implies an anisotropy in the local CR po sitron flux at these energies. We calculate the contribution of these pulsars to the locally observed CR electr on and positron spectra depending on the pulsar birth period and the magnitude o f the local CR diffusion coefficient. We further give an estimate of the expecte d anisotropy in the local CR positron flux. Our calculations show that within the framework of our model, the local CR posit ron spectrum imposes constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar period at birth, and also the local interstellar diffusion co efficient for CR leptons.