Abstract
Variations in the flux from active galactic nuclei (AGN) allow the testing of models of their emission mechanisms. Various suggestions for the energy generation in AGNs require the existence of a compact object, with X rays generated deep in that object's potential well and, therefore, X-ray luminosity related directly to the object responsible for their enormous luminosities. During a 5-h X-ray observation of NGC4151 we observed a significant drop in intensity lasting ∼2,000s. The event is similar to a short flux decline previously observed1 in NGC6814. Simultaneous intensity monitoring of a quasar with coordinates close to those of NGC4151 revealed no variability in its flux, allowing instrumental causes of the event to be discounted. The short timescale of the intensity dip imposes limits of 2×107 M⊙≤M≤3×108 M⊙ on the mass of the central black hole. We suggest here that the sudden drop in intensity may have been caused by a change in the ionization state of gas clouds surrounding the black hole or, possibly, by the passage of a star across the line-of-sight in the dense nuclear region of the galaxy.
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