UV/Optical Nuclear Activity in the gE Galaxy NGC 1399

Using HST STIS, we have detected far-ultraviolet nuclear activity in the giant elliptical galaxy NGC 1399, the central and brightest galaxy in the Fornax I cluster. The source reached a maximum observed far-UV luminosity of ~1.2 × 1039 ergs s-1 in 1999 January. It was detectable in earlier HST archival images in 1996 (B band) but not in 1991 (V band) or 1993 (UV). It faded by a factor of ~4 by mid-2000. The source is almost certainly associated with the low-luminosity AGN responsible for the radio emission in NGC 1399. The properties of the outburst are remarkably similar to the UV-bright nuclear transient discovered earlier in NGC 4552 by Renzini and coworkers. The source is much fainter than expected from its Bondi accretion rate (estimated from Chandra high-resolution X-ray images), even in the context of "radiatively inefficient accretion flow" models, and its variability also appears inconsistent with such models. High spatial resolution UV monitoring is a valuable means to study activity in nearby LLAGNs.