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

K. Iwasawa, A. C. Fabian, O. Almaini, P. Lira, A. Lawrence, K. Hayashida, H. Inoue

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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 100s. Such X-ray variability is in contrast to the behaviour of other low-luminosity active galaxies and resembles that of higher luminosity Seyfert 1galaxies. 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 Γ=1.7+/-0.3 with a Fe K line marginally detected at ~6.4keV. The soft-X-ray emission below 3keV 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 2-10keV luminosity is 4×1039ergs-1 for a source distance of 2.6Mpc, and at 1keV 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 104Msolar, 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 (LBolLEdd) 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 higher-luminosity Seyfert nuclei, with an intermediate-mass (104-105Msolar) black hole, unlike the nearby low-luminosity active galaxies in which underfed massive black holes are suspected to reside.
Original languageEnglish
Pages (from-to)879-888
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 1 Nov 2000



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