SN 2023emq: a flash-ionised Ibn supernova with possible CIII emissio

Abstract: SN 2023emq is a fast-evolving transient initially classified as a rare Type Icn supernova (SN), interacting with a H- and He-free circumstellar medium (CSM) around maximum light. Subsequent spectroscopy revealed the unambiguous emergence of narrow He lines, confidently placing SN 2023emq in the more common Type Ibn class. Photometrically SN 2023emq has several uncommon properties regardless of its class, including its extreme initial decay (faster than > 90% of Ibn/Icn SNe) and sharp transition in the decline rate from 0.20 mag/d to 0.07 mag/d at +20 d. The bolometric light curve can be modelled as CSM interaction with 0.32M_Sun of ejecta and 0.12M_Sun of CSM, with 0.006M_Sun of nickel, as expected of fast interacting SNe. Furthermore, broad-band polarimetry at +8.7 days (P = 0.55 +/- 0.30%) is consistent with spherical symmetry. A discovery of a transitional Icn/Ibn SN would be unprecedented and would give valuable insights into the nature of mass loss suffered by the progenitor just before death, but we favour an interpretation that SN 2023emq is a type Ibn SN that exhibited flash-ionised features in the earliest spectrum, as the features are not an exact match with other SNe Icn to date. However, the feature at 5700{\AA}, in the region of C III and N II emission, is significantly stronger in SN 2023emq than in the few other flash-ionised Type Ibn SNe, and if it is related to C III, it possibly implies a continuum of properties between the two classes.