We report the detection of an absorbed central X-ray source and its strong , rapid , variability in NGC4395 , 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 constrast 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 NGC4395 .
The ASCA spectrum shows a power-law continuum of photon-index \Gamma = 1.7 \pm 0.3 with a Fe K line marginally detected at \sim 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 \times 10 ^ { 39 } erg s ^ { -1 } 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 _ { \odot } , as suggested by the optical results and the small bulge of this dwarf galaxy .
Such a light black hole is favoured also in order for the Eddington ratio ( L _ { Bol } / L _ { Edd } ) to be above the range of ADAFs , which would clearly fail to explain the observed X-ray variability .
The nuclear source of NGC4395 is therefore consistent with a scaled-down version of higher luminosity Seyfert nuclei , with an intermediate mass ( 10 ^ { 4 } – 10 ^ { 5 } M _ { \odot } ) black hole , unlike the nearby low luminosity active galaxies in which underfed massive black holes are suspected to reside .