Spectral energy-loss bump and $\gamma$-ray pulsar halos

Abstract

The parent electron spectrum of $\gamma$-ray pulsar halos is determined by continuous injection and radiative energy losses, with the nonlinear nature of the latter expected to produce an age-dependent energy-loss bump in the halo spectrum. We demonstrate that the broadband $\gamma$-ray spectrum of the Geminga halo measured by Fermi-LAT and HAWC is consistent with this expectation. Its energy-loss bump appears in the GeV region and is primarily produced by electrons injected at the earliest stage. Therefore, fitting the data allows us to constrain parameters governing early electron injection, such as the pulsar braking index $n$ and initial spin period $P_0$. In addition, we show that another crucial condition for explaining the Geminga halo spectrum is a power-law index of the injection spectrum close to $1$, consistent with the X-ray observations of the Geminga pulsar wind nebula. Currently, no significant energy-loss bump has been detected in the spectrum of the Monogem halo; based on this, we also place preliminary constraints on $n$ and $P_0$ of Monogem. This work demonstrates that pulsar halo spectra can be used not only to constrain electron injection parameters, but also to provide independent estimates of certain intrinsic pulsar properties.