GeV Observations of the Extended Pulsar Wind Nebulae Constrain the Pulsar Interpretations of the Cosmic-Ray Positron Excess

Abstract

It has long been suggested that nearby pulsars within ∼1 kpc are the leading candidate of the 10-500 GeV cosmicray positron excess measured by PAMELA and other experiments. The recent measurement of the surface brightness profile of TeV nebulae surrounding Geminga and PSRB0656+14 by the High-Altitude Water Cherenkov Observatory (HAWC) suggests inefficient diffusion of particles from the sources, giving rise to a debate on the pulsar interpretation of the cosmic-ray positron excess. Here we argue that GeV observations provide more direct constraints on the positron density in the TeV nebulae in the energy range of 10-500 GeV and hence on the origin of the observed positron excess. Motivated by this, we search for GeV emission from the TeV nebulae with the Fermi Large Area Telescope (LAT). No spatially extended GeV emission is detected from these two TeV nebulae in the framework of two-zone diffusion spatial templates, suggesting a relatively low density of GeV electrons/positrons in the TeV nebulae. A joint modeling of the data from HAWC and Fermi-LAT disfavors Geminga and PSRB0656+14 as the dominant sources of the positron excess at ∼50-500 GeV for the usual Kolmogorov-type diffusion, while for an energy-independent diffusion, a dominant part of the positron excess contributed by them cannot be ruled out by the current data.

Cosmic ray models

“…No GeV photon halo around Geminga and PSR B0656+14 has been found in the search performed in Ref. [240], and the derived upper limits were used to constrain their contribution to the observed positron flux as < ∼ 15%. The similar analysis [241] detected a weak GeV halo around Geminga and set an upper limit of 20% to its contribution to the observed positron flux.…”

Self-consistent interpretations of the multi-wavelength gamma-ray spectrum of LHAASO J0621$+$3755

“…The surface brightness profile (SBP) of the Geminga halo measured by HAWC constrains the diffusion of particles away from the pulsar to be much slower than that in the typical ISM [4]. This anomalously slow diffusion arouses extensive discussions on how particles propagate in pulsar halos [5][6][7][8][9] and whether nearby pulsars can contribute significant positron flux at Earth [10][11][12][13][14][15][16].…”

Magnetohydrodynamic turbulence and propagation of cosmic rays: theory confronted with observations

“…The detection of such a diffuse TeV emission has been interpreted as the presence of a slow diffusion zone around the pulsar [77], which has spurred immense interest in the community. Different proposals have been put forward [78][79][80], which shows the urgent need of understanding of basic plasma processes in the astroparticle physics. It restates that the picture of simple uniform diffusion dictated by the same hydrodynamic Kolmogorov turbulence is far from reality.…”