Continuous rapid X-ray variability and spectral changes in NGC 4051

Abstract

We present the results of an extended 8.5-hr X-ray observation of the very low luminosity Type 1 Seyfert galaxy NGC 4051 with the EXOSAT observatory. Large-amplitude variability is seen both in the Low Energy (LE) imaging system (0.05–2 keV) and the Medium Energy (ME) proportional counters (2–10 keV), with a characteristic variability time-scale of approximately 1 hr. Soft and hard X-ray light curves are highly correlated, with no discernible time lag ( <200 s). A systematic spectral effect is seen in which the source is softer when brighter, by an amount equivalent to a change in spectral index of ~0.3. By combining LE and ME data, we can fit a time-averaged spectrum over the 0.05–10 keV band with a single power law with energy index 0.93 (90 per cent limits 0.79–1.06) and absorbing column $${N}_{H}=4\times{10}^{19}\,\text{cm}^{-2}$$ (2.0–9.3). An acceptable fit can also be made with a sum of two power laws, allowing a ‘canonical’ power law of slope α~0.65 to contribute subsequently to medium-energy X-rays, and a steep soft excess (α~1.6) to dominate in low-energy X-rays. Our data cannot distinguish the single and double power law hypotheses, but assuming a steep soft excess does allow better agreement between our measured absorption column and the H I column in this direction. The observed dominant time-scale, though very rapid, is orders of magnitude longer than might näively be expected in a conventional Eddington-limited black hole model. The spectral changes may reflect real changes in source emission, or alternatively could be explained by postulating a constant flat ($$\alpha \sim 0.65$$) component and a variable steep ($$\alpha \sim 1.6$$) component. However autocorrelation analysis reveals that ME and LE data have characteristically different time-scales, as well as variability amplitudes. Genuine spectral changes with time-scales and amplitudes varying with photon energy in the required fashion could be expected in a thermal Comptonization model. Assuming such a model, we derive properties of the seed photon source, and Comptonizing hot plasma.