Energy dependent morphology of the pulsar wind nebula HESS J1825-137 with Fermi-LAT

Aims: Taking advantage of more than 11 years of Fermi-LAT data, we perform a new and deep analysis of the pulsar wind nebula (PWN) HESS J1825-137. Combining this analysis with recent H.E.S.S. results we investigate and constrain the particle transport mechanisms at work inside the source as well as the system evolution. Methods: The PWN is studied using 11.6 years of Fermi-LAT data between 1 GeV and 1 TeV. In particular, we present the results of the spectral analysis and the first energy-resolved morphological study of the PWN HESS J1825-137 at GeV energies, which provide new insights into the γ-ray characteristics of the nebula. Results: An optimised analysis of the source returns an extended emission region larger than 2°, corresponding to an intrinsic size of about 150 pc, making HESS J1825-137 the most extended γ-ray PWN currently known. The nebula presents a strong energy dependent morphology within the GeV range, moving from a radius of ∼1.4° below 10 GeV to a radius of ∼0.8° above 100 GeV, with a shift in the centroid location. Conclusions: Thanks to the large extension and peculiar energy-dependent morphology, it is possible to constrain the particle transport mechanisms inside the PWN HESS J1825-137. Using the variation of the source extension and position, as well as the constraints on the particle transport mechanisms, we present a scheme for the possible evolution of the system. Finally, we provide an estimate of the electron energy density and we discuss its nature in the PWN and TeV halo-like scenario.