Study of X-ray emission from the S147 nebula by SRG/eROSITA: Supernova-in-the-cavity scenario

The Simeis 147 nebula (S147) is particularly well known for a spectacular net of H α -emitting filaments. It is often considered one of the largest and oldest (∼10 5 yr) cataloged supernova remnants in the Milky Way, although the kinematics of the pulsar PSR J0538+2817 suggests that this supernova remnant might be a factor of three younger. The former case is considered in a companion paper, while here we pursue the latter. Both studies are based on the data of SRG/eROSITA All-Sky Survey observations. Here, we confront the inferred properties of the X-ray emitting gas data with the scenario of a supernova explosion in a low-density cavity, such as a wind-blown-bubble. This scenario assumes that a ∼20 M ⊙ progenitor star has had a low velocity with respect to the ambient interstellar medium, and so stayed close to the center of a dense shell created during its main-sequence evolution till the moment of the core-collapse explosion. The ejecta first propagate through the low-density cavity until they collide with the dense shell, and only then does the reverse shock go deeper into the ejecta and power the observed X-ray emission of the nebula. The part of the remnant inside the dense shell remains non-radiative till this point, plausibly in a state with T e < T i and nonequilibrium ionization. On the contrary, the forward shock becomes radiative immediately after entering the dense shell, and, being subject to instabilities, gives the nebula its characteristic “foamy” appearance in H α and radio emission.