Extreme X-ray Behavior in the Low-Luminosity Active Nucleus of NGC 4395

Abstract

We present the results of a 17 ks Chandra observation of the nearby dwarf spiral galaxy NGC 4395, focusing on the X-ray properties of the moderate-mass black hole that resides in its nucleus. Chandra affords the first high-quality, broadband X-ray detection of this object that is free of contamination from nearby sources in the field. We find that the nuclear X-ray emission is unresolved in the Chandra image and confirm the rapid, large-amplitude X-ray variability reported in previous studies. The spectrum of the nuclear source shows evidence for absorption by an ionized medium. There is also evidence for spectral variability over the course of the Chandra observation, although contrary to prior reports, it appears to be uncorrelated with fluctuations in the hard X-ray count rate. It is possible that the short-term spectral variability results from column density changes in the ionized absorber. By far the most unusual high-energy property of NGC 4395 is the shape of its spectrum above 1 keV. The Chandra data indicate a power-law photon index of Gamma \approx 0.6, which is much flatter than the Gamma \approx 1.8 X-ray spectra typical of active galactic nuclei and the slope of the nuclear X-ray spectrum measured from an earlier ASCA observation of NGC 4395. This extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei. A variety of possibilities for the origin of the flat continuum slope are considered; none provides a fully satisfactory explanation.