Follow-Up Hubble Space Telescope/Space Telescope Imaging Spectroscopy of Three Candidate Tidal Disruption Events

Abstract

Large-amplitude, high-luminosity, soft X-ray flares were detected by the ROSAT All-Sky Survey in several galaxies with no evidence for Seyfert activity in their ground-based optical spectra. These flares had the properties predicted for a tidal disruption event by a central supermassive black hole: soft X-ray spectrum, timescale of months, and large X-ray luminosity (10⁴²-10⁴⁴ ergs s^-1). In order to evaluate the alternative hypothesis that these flares could have been some form of extreme active galactic nucleus variability, we obtained follow-up optical spectroscopy of three of the flaring galaxies a decade later with the Space Telescope Imaging Spectrograph and a narrow slit to search for or place stringent limits on the presence of any persistent Seyfert-like emission in their nuclei. Two of the galaxies, RX J1624.9+7554 and RX J1242.6-1119, show neither evidence for emission lines nor a nonstellar continuum in their Hubble Space Telescope nuclear spectra, consistent with their ground-based classification as inactive galaxies. They remain the most convincing as hosts of tidal disruption events. NGC 5905, previously known as a starburst H II galaxy due to its strong emission lines, has in its inner 0.1" a nucleus with narrow emission line ratios requiring a Seyfert 2 classification. This weak Seyfert 2 nucleus in NGC 5905, which was masked by the many surrounding H II regions in ground-based spectra, requires a low level of prior nonstellar photoionization, thus raising some uncertainty about the nature of its X-ray flare, which may or may not have been a tidal disruption event. The absence of both broad emission lines and nuclear X-ray absorption in NGC 5905 also characterizes it as a true Seyfert 2 galaxy, yet one that has varied by more than a factor of 100 in X-rays.

On the basis of observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with proposal 9177.