National Aeronautics and Space Administration

NGC 3516 is an Active Galaxy--- the brightest, most strongly variable Seyfert I observable by RXTE without gaps due to Earth occultation. A key to unlocking the mysteries of the central engines of AGN is their X-ray time variability. In some models of the central engine, (commonly believed to be an accretion disk fed black hole) X-ray variability could be the only way to probe the smallest spatial scales. Measuring a characteristic variability time scale in AGN would place limits on the source size, and black hole mass. The key is to study variability over both as short and as long a time scale as possible.

RXTE, with its large continuous viewing zone, extended viewing "season," and high throughput, extends both the shortest and longest time scales observed in NGC 3516, characterizing variability on time scales from minutes to weeks.

Fig. 1: XTE 2--10 keV light curve of NGC 3516. The source was sampled once every 12.8 hr from March through July 1997 (top panel) and quasi-continuously (except for SAA passage and interruptions for other time-critical observations) for 4 days in the middle (bottom panel, binned in 700 sec intervals). The inner error bars are the statistical errors, while the outer ones also include a 0.3 ct/s systematic uncertainty added in quadrature due to the background. The peak-to-peak variation was a factor of ~4, and real variability is visible down to the shortest time scales sampled (1.5 ksec).

Fig. 2: PDS of NGC 3516, derived from the data in Fig. 1 by combining the PDS measured from the quasi-continuous 4 day monitoring (dots on the right) with that from the overlapping monitoring every 12.8 hr (triangles on the left). A fit to the short-term data shows a slope of a = -1.80 (P(f) proportional to f^a, where P(f) is the fluctuation power at temporal freqency f) while the long-term data has a = -1.05 . This break of delta s ~ 0.75 is a real flattening to longer time scales, so the combined PDS was fitted with the function P(f) proportional to { 1 / {{ 1 + f/f_t }^a} to derive a turnover frequency of f_t ~ 3 * 10^{-7} Hz (about 1 month; arrow in the figure) and a = -1.70 . Although the turnover is real, its exact value is not well defined because it lies so close to the longest time scale sampled.

Figures and captions courtesy of Rick Edelson.

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