Multiwavelength Observations of Short-Timescale Variability in NGC 4151. III. X-Ray and Gamma-Ray Observations

A series of ROSAT, ASCA and Compton Gamma Ray Observatory (CGRO) observations of the Seyfert galaxy NGC 4151 were carried out during the period 1993 November 30-December 13 as part of an intensive campaign to study the multiwavelength spectral characteristics of its short-timescale 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 observing period. However, in an adjacent band (1.0-2.0 keV) significant variability was evident, the most obvious feature being a marked increase (a factor of 1.45) in the count rate over a timescale of ~2 days commencing roughly 3 days into the monitoring period. In contrast, only a low amplitude of variability (<10%) was measured in the four ASCA observations in the 2-10 keV band (but note that the first ASCA observation was performed somewhat after the 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 ~300 keV spectrum of NGC 4151 is dominated by a hard power- law continuum which is cutoff at both high (~90 keV) and low (~4 keV) energy. A high-energy cutoff is characteristic of a continuum generated by the process of thermal Comptonization, whereas that at low energy arises from absorption in line-of-sight gas. In NGC 4151 this gas may be partially photoionized 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 changes 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 brightens, consistent with earlier `observations. As noted in earlier studies, there is a soft excess below 1 keV which probably arises from more than one scattered and/or thermal component. The 1-2 keV soft X-ray and the ultraviolet continuum light curves (e.g., near 1440 A) show reasonably good correspondence, although the relative amplitude of the variations is much higher in the X-ray data. The observed ultraviolet to X-ray correlation has a slope similar to that established in 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 continuum by the disk. The residual ultraviolet flux may then arise from the reprocessing and/or the viscous heating of the disk.