Detection of a γ -ray halo around Geminga with the Fermi -LAT data and implications for the positron flux

Abstract

The HAWC Collaboration has discovered a γ-ray emission extended about 2 degrees around the Geminga and Monogem pulsar wind nebulae (PWNe) at γ-ray energies Eγ > 5 TeV. We analyze, for the first time, almost 10 years of γ-ray data obtained with the Fermi Large Area Telescope at Eγ > 8 GeV in the direction of Geminga and Monogem. Since these two pulsars are close the Galactic plane we run our analysis with 10 different interstellar emission models (IEMs) to study the systematics due to the modeling of this component. We detect a γ-ray halo around Geminga with a significance in the range 7.8 − 11.8σ depending on the IEM considered. This measurement is compatible with e + and e − emitted by the PWN, which inverse-Compton scatter (ICS) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around 1.1 × 10 27 cm 2 /s at 100 GeV. We include in our analysis the proper motion of the Geminga pulsar which is relevant for γ rays produced for ICS in the Fermi-LAT energy range. We find that an efficiency of about 1% for the conversion of the spin-down energy of the pulsar into e + and e − is required to be consistent with γ-ray data from Fermi-LAT and HAWC. The inferred contribution of Geminga to the e + flux is at most 20% at the highest energy AMS-02 data. Our results are compatible with the interpretation that the cumulative emission from Galactic pulsars explains the positron excess.PACS numbers:

Cosmic ray models

“…[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. These limits disfavour young pulsars as explanation for the positron excess.…”

Galactic halo size in the light of recent AMS-02 data

“…However, this was calculated assuming a similar diffusion in the local bubble and in the rest of the disc and halo, which may be questionable. Recent γ-ray observations (Abeysekara et al 2017;Di Mauro et al 2019) -interpreted as pockets of slow diffusion around pulsars (e.g. Profumo et al 2018) -, and the indication that the local ISM properties are affected by several SN explosions a few Myr ago (Fields et al 2019) provide enough motivation to revisit this issue in a future study.…”

“…3.4, which could be very sensitive to the local ISM. However, similarly to radioactive nuclei, inhomogeneous spatial diffusion zones around CR sources (Abeysekara et al 2017;Di Mauro et al 2019), could also affect primary and secondary lepton spectra in different and very non trivial ways. These complications go beyond the scope of this analysis.…”

Cosmic ray transport in the proximity of pulsars and the formation of gamma-ray halos

“…The HAWC data have been mostly analyzed in the assumption that the ± propagate isotropically and diffusively at any time after being released from the source. This lead to the conclusion [1][2][3][4][5][6][7] that, in order to fit the Geminga and Monogem halos surface brightness the energy dependent CR diffusion coefficient, ( ), within few tens pc from pulsars should be at least two orders of magnitude smaller than the typical values employed in standard Galactic CR propagation models [8]. This rather unexpected outcome has since then acquired popularity but so far no satisfactory theoretical explanation has been proposed (see e.g.…”