The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies

LINER nuclei, which are present in many nearby galactic bulges, may be themanifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whether thecompact UV nuclear sources present in many LINERs are clusters of massivestars, rather than being directly related to the accretion process. We haveused HST to monitor the UV variability of a sample of 17 galaxies with LINERnuclei and compact nuclear UV sources. Fifteen of the 17 galaxies were observedmore than once, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlated variations areseen in two independent bands (F250W and F330W). Comparison to previous UVmeasurements indicates, for many objects, long-term variations by factors of afew over decade timescales. Variability is detected in LINERs with and withoutdetected compact radio cores, in LINERs that have broad H-alpha wings detectedin their optical spectra (``LINER 1's''), and in those that do not (``LINER2s''). This variability demonstrates the existence of a non-stellar componentin the UV continuum of all types, and sets a lower limit to the luminosity ofthis component. We note a trend in the UV color (F250W/F330W) with spectraltype - LINER 1s tend to be bluer than LINER 2s. This trend may indicate a linkbetween the shape of the nonstellar continuum and the presence or thevisibility of a broad-line region. In one target, the post-starburst galaxy NGC4736, we detect variability in a previously noted UV source that is offset by2.5" (60 pc in projection) from the nucleus. This may be the nearest example ofa binary active nucleus, and of the process leading to black hole merging.Comment: accepted to Ap