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

TL;DR

This paper asks whether two middle-aged nearby pulsars, Geminga and B0656+14, can meaningfully shape the local CR electron and positron spectra and imprint a detectable anisotropy. The authors model leptons accelerated in pulsar wind nebulae prior to breakup, releasing them into the ISM, and propagate them with an energy-dependent diffusion framework using Green's functions to obtain the local interstellar spectra and the anisotropy. They find that these pulsars can yield a non-negligible positron LIS, potentially dominating above several GeV, with anisotropies up to about 3% for B0656+14 and smaller for Geminga, depending on the birth period $P_0$ and the local diffusion coefficient. The LIS and anisotropy considerations place constraints on pulsar birth properties and diffusion, and indicate that a surprisingly large observed anisotropy would point to a younger, closer source; the results have implications for interpreting high-energy CR data from PAMELA and future missions. Overall, mature, nearby pulsars can significantly contribute to the local $e^ obreak{-}$positron population and its directional characteristics.

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.

Figures (2)

• Figure 1: Left panel: Contribution of Geminga to the positron LIS for $k_0=0.1\,$kpc$^2$ Myr$^{-1}$ and $P_0\,=\,40\,$ms, $T=20\,$kyr (long dashed line), $P_0\,=\,40\,$ms, $T=60\,$kyr (dot-dashed line), and $P_0\,=\,60\,$ms, $T=20\,$kyr (dashed) on top of an isotropic background (solid line). The thin lines mark the combined spectra (pulsar contribution plus background), whereas the thick lines give the contribution of the pulsar alone. Also shown are data from 2000ApJ...532..653B (diamonds) and 2001ApJ...559..296D (triangles). Right panel: the expected local anisotropy in case only Geminga contributes to the LIS (thick lines), and in case Geminga contributes on top of an isotropic background positron flux (thin lines) as given by 1998ApJ...498..779B (solid line in left panel). The line styles correspond to the cases as given for the left panel. The thick dashed and long dashed lines coincide.
• Figure 2: Same as Fig. \ref{['fig:geminga']} but for B0656+14.