MULTIWAVELENGTH OBSERVATIONS OF SHORT-TIMESCALE VARIABILITY IN NGC-4151 .3. X-RAY AND GAMMMA-RAY OBSERVATIONS

A series of ROSAT, ASCA and Compton Gamma Ray Observatory (CGRO) obser vations of the Seyfert galaxy NGC 4151 were carried out during the per iod 1993 November 30-December 13 as part of an intensive campaign to s tudy the multiwavelength spectral characteristics of its short-timesca le variability. In the softest X-ray bands monitored by ROSAT (0.1-0.4 keV, 0.5-1.0 keV), the source flux remained constant throughout the o bserving period. However, in an adjacent band (1.0-2.0 keV) significan t variability was evident, the most obvious feature being a marked inc rease (a factor of 1.45) in the count rate over a timescale of similar to 2 days commencing roughly 3 days into the monitoring period. In co ntrast, only a low amplitude of variability (less than or similar to 1 0%) was measured in the four ASCA observations in the 2-10 keV band (b ut note that the first ASCA observation was performed somewhat after t he onset of the flux increase seen by ROSAT). The count rates recorded by the Oriented Scintillation Spectrometer Experiment (OSSE) on CGRO are consistent with +/-15% variations in the 50-150 keV gamma-ray band , but there is no direct correspondence between the gamma-ray and soft X-ray light curves. The 0.1 to similar to 300 keV spectrum of NGC 415 1 is dominated by a hard power-law continuum which is cutoff at both h igh (similar to 90 keV) and low (similar to 4 keV) energy. A high-ener gy cutoff is characteristic of a continuum generated by the process of thermal Comptonization, whereas that at low energy arises from absorp tion in line-of-sight gas. In NGC 4151 this gas may be partially photo ionized by the continuum source, but still retains significant opacity below 1 keV. The observed soft X-ray variability may be the result of changes in the level of the underlying soft-hard X-ray continuum or c hanges in the line-of-sight absorption. The data marginally favor the former, in which case the difference between the soft X-ray and gamma- ray light curves implies a steepening of the continuum as the source b rightens, consistent with earlier observations. As noted in earlier st udies, there is a soft excess below 1 keV which probably arises from m ore than one scattered and/or thermal component. The 1-2 keV soft X-ra y and the ultraviolet continuum light curves (e.g., near 1440 Angstrom ) show reasonably good correspondence, although the relative amplitude of the variations is much higher in the X-ray data. The observed ultr aviolet to X-ray correlation has a slope similar to that established i n earlier studies, although a significant residual ultraviolet flux is evident in the recent observations. A possible interpretation is that the X-ray to gamma-ray continuum is produced in a patchy dissipative corona above the surface of an accretion disk and that the correlated ultraviolet flux results from the reprocessing of part of this continu um by the disk. The residual ultraviolet flux may then arise from the reprocessing and/or the viscous heating of the disk.