2025ApJ...994...20G

Extended regions of very high-energy γ-ray emission associated with middle-aged pulsars have been found by γ-ray observatories. These regions, called TeV halos or pulsar halos, are thought to be created when energetic electrons from a pulsar or pulsar wind nebula transport into the interstellar medium and undergo inverse Compton scattering with the cosmic microwave background radiation and the interstellar radiation field. The same electrons are expected to emit synchrotron emission in the X-ray band in the interstellar magnetic field. HESS J1813-126 is a pulsar halo candidate from which TeV γ-ray emission with extension 0$\mathop{.}\limits^{\unicode{x000b0}}$21 and a hard E^–2 spectrum is observed. We searched for the synchrotron component of this pulsar halo with Swift-X-Ray Telescope (XRT). In particular, we observed two fields within the region covered by HESS J1813-126 for 35 ks each and a region nearby as a background reference for 10 ks. We also analyzed 21 ks of archival Swift-XRT observations of PSR J1813-1246 and archival Swift-XRT observations of the nearby Galactic X-ray background. We find no evidence for excess X-ray emission from the two observations near HESS J1813-126 and place an upper limit differential flux of 2.18 × 10^–3 and 2.02 × 10^–3 keV^–1 cm^–2 s^–1 at 1 keV, assuming a power-law spectrum in Fields 1 and 2. The archival differential flux upper limit is 3.36 × 10^–3 keV^–1 cm^–2 s^–1 at 1 keV. The nondetection implies that the magnetic field inside the halo is not significantly enhanced compared to the average Galactic magnetic field.