2025A&A...697A.131X

Aims. We aim to study the polarization and magnetic field properties of the supernova remnant HB 9 using new 21-cm continuum cube data from the Five-hundred-meter Aperture Spherical Radio Telescope (FAST).
Methods. We computed the Faraday depth at 21 cm and re-analyzed the rotation measures (RMs) of HB 9 using additional Effelsberg 2695-MHz and Urumqi 4800-MHz polarization data. FAST total-intensity images of two subbands were decomposed into components of multiple angular scales to check spectral-index variation via temperature versus temperature plots (TT-plots).
Results. The filamentary emission has a spectral index (S ∼ ν^α) of α = -0.52, which corresponds to freshly accelerated relativistic electrons. The diffuse emission has a steeper spectrum of α = -0.63, which corresponds to confined electrons that are no longer accelerated. The FAST detected 1385-MHz polarized emission that might come from a thin layer in the outer envelope of the shells, with a Faraday depth of 4-28 rad m^–2 from the Faraday rotation synthesis result. The RMs derived from the Effelsberg 2695-MHz and Urumqi 4800-MHz polarization data show about 70 rad m^–2 in the eastern and northern shell, and 124 rad m^–2 in the inner and southern patches. The regular magnetic field is about 5-8 μG over the remnant. The northern shell shows depolarization at 2695 MHz relative to the 4800-MHz polarization data, which indicates an additional random magnetic field of 12 μG on the scale of 0.6 pc. The shock wave might have entered the dense gas environment in the northern-shell region and has driven turbulence to cause depolarization at 2695 MHz.