Revisiting the Geminga halo at GeV energies with Fermi-LAT data

Abstract

Nearby pulsars within $\sim1\,{\rm kpc}$ are considered to be possible sources of 10-500 GeV cosmic-ray positron excess measured by PAMELA and AMS-02. A TeV halo around Geminga is detected by HAWC, and the measurements of its surface brightness profile indicate a slow particle diffusion surrounding the source. This result challenges the pulsar interpretation of the positron excess. The observations at GeV energies provide direct information on the electron/positron density in the GeV nebula, which can offer more direct constraints on the origin of the positron excess. Two previous works have performed analyses on the GeV emission of the pulsar halo, but focused on the energy band above 8 GeV. In this work, we use a longer dataset from the Fermi Large Area Telescope (LAT) to re-analyze the GeV halo emission of Geminga, extending the analysis to cover the energy range of 1-1000 GeV. We find that the analysis in this wider energy range results in a low significance of the halo emission. This can be attributed to the Galactic interstellar emission model being unable to perfectly fit the background over this broader energy range, and due to the low measured halo flux at $<$ 10 GeV energies leading to a mismatch between the observation and model expectation. We also derive the spectral energy distribution of the tentative halo emission, which shows a very hard spectrum in the 1-10 GeV range.

Figures (7)

• Figure 1: Difference between the model maps derived from the best-fit IEMs of Config. II and Config. IV (M2$-$M4). The values are all positive, indicating that in Config. II, the IEM background above 10 GeV is slightly elevated to accommodate the newly included $<10$ GeV data, causing lower TS values of halo in Config. II than IV.
• Figure 2: SEDs for the Geminga ICS halo. Our results in the 1-1000 GeV energy range are shown as red points. Upper limits are reported when TS$<4$. The brown points are derived using 8 alternative IEM templates. Also shown are the results from hawc17, dimauro19, xi19 and TorresEscobedo:2021xz.
• Figure 3: The 10-1000 GeV ( left) and 1-10 GeV ( right) TS maps used to examine the existence of a halo-like component. left: The test source is a two-dimensional spatial Gaussian with $\sigma=10^\circ$ (see the main text and Appendix \ref{['sec:ext']} for details on the determination of $\sigma$). The vector demonstrates the direction of the proper motion of the Geminga pulsar. The solid and dashed contours are for 68% and 95% confidence levels, respectively. Right: The test source is a 2D Gaussian with $\sigma=12^\circ$. The blue circle shows the extension of the best-fit Gaussian of the excess B. These two TS maps cover a sky region within a 35-degree radius. They are generated using the Fermi-LAT data from a circular ROI with a 40-degree radius (i.e., following the Config. VI analysis setup). Note that these are not standard point-source TS maps as derived using gttsmap, but instead TS maps that show test results for an additional extended source component centered at each pixel of the map.
• Figure 4: SEDs of the Geminga halo (or say exess A) for different analyes. The Gaussian template is centered on (78$^\circ$, 7$^\circ$) with $\sigma=10^\circ$. The proper template refers to the halo template used to generate the results shown in Fig. \ref{['fig:sed']} and Table \ref{['tab:likers']}.
• Figure 5: Comparisons between observational data and theoretical expectation for the gamma-ray spectrum of Geminga halo. The model curves are derived by changing only the parameters of $D_0$ (upper-left panel), $\eta$ (upper-right panel) and $\gamma_{\rm min}$ (bottom-left panel) on the basis of the benchmark parameters list in Table \ref{['tab:pars']}. In the bottom-right panel, we additionally impose a low-energy cutoff $\gamma_{\rm min}$ of the injected $e^\pm$ spectrum on the benchmark model.