L.C. Ho, A. Ptak, Y. Terashima, H. Kunieda, P.J. Serlemitsos, T. Yaqoob, and A.P. Koratkar
Low-luminosity active galactic nuclei (AGNs), of which NGC 4639 is a good example, constitute an important but poorly understood constituent of the nearby galaxy population. We obtained observations of NGC 4639 with ASCA in order to investigate its mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. have previously shown that the nucleus is a pointlike source in the ROSAT soft X-ray band. We detected in the 2-10 keV band a compact central source with a luminosity of 8.3x1040 ergs s-1 (for an adopted distance of 25.1 Mpc). Comparison of the ASCA data with archival data taken with the Einstein and ROSAT satellites shows that the nucleus varies on timescales of months to years. The variability could be intrinsic, or it could be caused by variable absorption. More rapid variability, on a timescale of ~104 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is well described by a model consisting of a lightly absorbed (NH=7.3x1020 cm-2) power law with a photon index of ¡=1.68+/-0.12. We find no evidence for significant emission from a thermal plasma; if present, it can account for no more than ~25% of the flux in the 0.5-2.0 keV band. The limited photon statistics of our data do not allow us to place significant limits on the presence of iron K emission. Despite its low luminosity, the X-ray properties of the nucleus of NGC 4639 appear quite normal compared with those of more luminous AGNs. The strength of its broad H¿ line follows the correlation between broad H¿ luminosity and hard X-ray luminosity that was previously known for luminous objects. Images taken with the Hubble Space Telescope detected the nucleus in the ultraviolet at a strength relative to the X-ray band that appears to be quite typical of that found in other AGNs. NGC 5033, another low-luminosity Seyfert 1 galaxy with optical characteristics that closely resemble those of NGC 4639, has also been studied recently with ASCA, and we highlight some of the similarities between these two objects.