Observations

Photometric variability of nucleus of the Galaxy NGC 4395

June 21, 2011

Brightness variability is an intrinsic feature of all accreting objects, from interacting binaries with a compact component (cataclysmic variables, dwarf novae, Galactic black holes, etc.) to supermassive accreting systems (quasars, Seyfert nuclei). The cause of the variability is not yet fully understood, but should somehow be related to some instability processes associated with the accretion itself. Therefore studying variability might help to better understand accretion, which appears to be one of the most important processes for energy generation and all-scale structure building in the Universe. Time scales of accretion-driven optical variability are very different and perhaps depend on many unknowns, but as a first approximation they appear to scale with the mass of the compact object. Thus stellar-mass compact accretors may vary significantly within minutes or seconds, while a 10⁹ solar masses (radio-quiet) quasar typically needs weeks or months for such a brightness change. The gap between them is partially filled by the IMBH (intermediate mass black holes, ~10³ solar masses), whose observational appearance is mostly in the X-ray bands, and LLAGN (low-luminosity/mass active galactic nuclei, ~10^5-6 solar masses), which can successfully be studied in optical bands as well. We selected a sample of such low-mass AGN to probe the characteristics (time scales, amplitudes, color dependences, etc.) of their short-term optical variability. All objects are radio-quiet, e.g. no relativistic jet contribution is present to alter the characteristics of the accretion-attributed variability.

[ large version of the image ]

NGC 4395 is a nearby (z = 0.001) Sd-type active galaxy with broad optical emission lines and a point-like hard X-ray source. It is often referred to as the nearest and the least luminous Seyfert 1 type AGN. Its central black hole mass and Eddington ratio, measured trough the reverberation-mapping technique, are found to be M_BH ~4.10⁵ solar masses and L/L_Edd ~0.001 (Peterson et al. 2005). The X-rays continuum of this object shows very rapid variations on seconds-to-minutes time scale (Leighly 1999; Vaughan et al. 2005). On the other hand, rapid (intranight) optical variations have been previously reported by Lira et al. (1999) and Desroches et al. (2006) who, during a low activity state, also found a lag of optical bands behind the UV (~25 min) and X-rays (~45 min), consistent with the idea of a hard radiation reprocessing within the accretion disk into softer wavelengths. Similarly, for this object variations are found in the optical and near IR bands by Minezaki et al. (2006) and Skelton et al. (2005).