MUSE observations of V1425 Aql reveal an arc-shaped nova shell

Abstract: Nova shells are the remnants of a nova eruption in a cataclysmic variable system. By studying their geometry we can better understand the physical mechanisms that shape them during the nova eruption. A nova shell that challenges our current understanding of these processes is the shell observed around V1425 Aql. It has at least two different components: an inner and symmetric shell, and an outer and asymmetric shell, with the latter expanding faster than the former. The physical reason behind the asymmetric ejecta is not clear. We aim to characterize the properties and differences between these two components to understand the origin behind the unusual shape. We acquired MUSE data to study the spatial position and kinematics of the expanding gas across the shell. Our analysis includes channel maps, position-velocity diagrams, and the reconstruction of the 3D geometry of the nova shell. Several emission lines are detected within the MUSE wavelength coverage, including but not limited to Balmer, Oxygen, Nitrogen, and Helium lines. There are significant differences in the spectra of the inner and outer shells, with the latter being observed only in forbidden transitions, while the inner shows a mix of forbidden and allowed ones. Our analysis reveals that the outer shell has a geometry consistent with an arc-shaped structure that partially encircles the more spherical inner shell. Within the inner shell, clumpy structures start to be noticeable in the lines of H$\alpha$+[Nii]. We have constrained the geometry of the outer shell to an arc-shaped structure, although the physical reason behind its origin is still eluding us. Further monitoring of the evolution of both shells in this object might help to clarify the mechanism behind this unusual configuration.