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 .
( 1995 ) 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 8.3 \times 10 ^ { 40 } 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 \sim 10 ^ { 4 } 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 ( N _ { H } = 7.3 \times 10 ^ { 20 } cm ^ { -2 } ) power law with a photon index of \Gamma = 1.68 \pm 0.12 .
We find no evidence for significant emission from a thermal plasma ; if present , it can account for no more than \sim 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 to those of more luminous AGNs .
The strength of its broad H \alpha line follows the correlation between broad H \alpha luminosity and hard X-ray luminosity previous 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 which appears to be quite typical of that found in other AGNs .
NGC 5033 , another low-luminosity Seyfert 1 galaxy whose optical characteristics closely resemble those of NGC 4639 , has also been recently studied with ASCA , and we highlight some of the similarities between these two objects .