X-ray absorption and rapid variability of the dwarf Seyfert nucleus of NGC 4395

Abstract

ABSTRA C T We report the detection of an absorbed central X-ray source and its strong, rapid, variability in NGC 4395, the least luminous Seyfert nucleus known. The X-ray source exhibits a number of flares with factors of 3‐4 flux changes during a half-day ASCA observation. The shortest doubling time observed is about 100 s. Such X-ray variability is in contrast to the behaviour of other low-luminosity active galaxies and resembles that of higher luminosity Seyfert 1 galaxies. It provides further support for an accreting black hole model rather than an extreme stellar process in accounting for the nuclear activity of NGC 4395. The ASCA spectrum shows a power-law continuum of photon index Ga 1:7 ^ 0:3 with a Fe K line marginally detected at ,6.4 keV. The soft-X-ray emission below 3 keV is strongly attenuated by absorption. The energy spectrum in this absorption band shows a dramatic change in response to the variation in continuum luminosity. A variable warm absorber appears to be the most likely explanation to account for the spectral change. The absorption-corrected 2‐ 10 keV luminosity is 4 10 39 erg s 21 for a source distance of 2.6 Mpc, and at 1 keV is one order of magnitude above previous ROSAT estimates, which affects the appearance of the wide-band spectral energy distribution and photoionization calculations. The rapid X-ray variation is consistent with a black hole of a few times 10 4 M(, as suggested by the optical results and the small bulge of this dwarf galaxy. Such a light black hole is also favoured in order for the Eddington ratio (LBol/LEdd) to be above the range of advection-dominated accretion flows, which would clearly fail to explain the observed X-ray variability. The nuclear source of NGC 4395 is therefore consistent with a scaled-down version of higherluminosity Seyfert nuclei, with an intermediate-mass (10 4 ‐10 5 M() black hole, unlike the