Possible origin of the slow-diffusion region around Geminga

Abstract

Geminga pulsar is surrounded by a multi-TeV γ-ray halo radiated by the high energy electrons and positrons accelerated by the central pulsar wind nebula (PWN). The angular profile of the γ-ray emission reported by HAWC indicates an anomalously slow diffusion for the cosmic-ray electrons and positrons in the halo region around Geminga. In the paper we study the possible mechanism for the origin of the slow diffusion. At first, we consider the self-generated Alfvén waves due to the streaming instability of the electrons and positrons released by Geminga. However, even considering a very optimistic scenario for the wave growth, we find this mechanism DOES NOT work to account for the extremely slow diffusion at the present day if taking the proper motion of Geminga pulsar into account. The reason is straightforward as the PWN is too weak to generate enough high energy electrons and positrons to stimulate strong turbulence at the late time. We then propose an assumption that the strong turbulence is generated by the shock wave of the parent supernova remnant (SNR) of Geminga. Geminga may still be inside the SNR, and we find that the SNR can provide enough energy to generate the slow-diffusion circumstance. The TeV halos around PSR B0656+14, Vela X, and PSR J1826-1334 may also be explained under this assumption.

Possible origin of the Geminga slow-diffusion halo

“…Residing inside the parent SNR may not be the unique picture for the preexisting-type interpretation. It is also possible that Geminga is now running into the stellar-wind bubble that creates the Gemini Hα Ring [16]. The stellar wind from the several OB type stars can provide adequate and continuous energy for the turbulence generation.…”

The Crab Pulsar and Nebula as Seen in Gamma-Rays

“…On the other hand, the extension is too small to be produced by particles that propagate in the standard Galactic diffusion coefficient, since the expected size would be a factor of ∼100 larger in that case. A possible explanation has been searched for in a modification of the diffusion properties around that source, possibly conveyed by self-generated turbulence associated with electrons and positrons leaking the nebula [146], or due to the injection of MHD turbulence by the parent SNR [147]. At present, understanding the formation of TeV halos is one of the big challenges in high-energy astrophysics (see e.g., Lopez-Coto et al in preparation), both for their possible implications for galactic cosmic ray transport and for their implications for future gamma-ray observations.…”

Status of Ground-based and Galactic Gamma-ray Astronomy

“…This may be explained by some CR self-confinement effect, leading to suppression of the diffusion coefficient due to increased magnetic turbulence in the immediate vicinity of the accelerator, which several studies have shown for pulsar environments [83], and [82] demonstrated that a similar effect is seen in SNRs. Alternative explanations have also been sought, such as turbulence of the surrounding medium generated by the shock wave of the parent SNR [84]. [85] presented a study investigating whether a ballistic (of the gyro-centre) component to the particle transport description may alleviate the requirement of a slow diffusion coefficient, finding that a more typical value of (1GeV) ∼ 10 28 cm −2 s −1 can be kept, although a highly efficient conversion of the pulsar spin-down energy into leptons is required in this case (∼ 60% compared to 0.3 − 3% for Geminga).…”