Description
The rare and mysterious class of event sometimes known as luminous fast blue optical transients, typefied by the event AT2018cow, has provided a steady stream of surprises over the past five years: they are extremely radio-luminous, highly X-ray variable, and at least in some cases they produce ultra-fast optical flares for months and leave behind a hot remnant. While the emission from these transients peaks in the UV at all epochs, the only UV spectrum obtained before now has been a low-S/N UVOT grism observation of AT2018cow. I will pretent the first HST spectrum of a LFBOT, using COS and STIS to provide simultaneous coverage over the entire FUV/NUV range, with <1 Angstrom resolution blueward of 2000 Angstroms. Surprisingly, the spectrum is almost a pure single-temperature blackbody with no strong transitions beyond ISM absorption features from the Milky Way and host galaxy. The lack of strong features calls into question most supernova-like models, pointing towards models in which virtually all material from the progenitor system remains bound in a hot and dense plasma surrounding the central engine.
