Nature 325, 694 - 696 (19 February 1987); doi:10.1038/325694a0

Low-frequency divergent X-ray variability in the Seyfert galaxy NGC4051

A. Lawrence^*, M. G. Watson^†, K. A. Pounds^† & M. Elvis^‡

^*School of Mathematical Sciences, Queen Mary College, Mile End Road, London El 4NS, UK
^†Department of Physics, University of Leciester, University Road, Leicester LEI 7RH, UK
^‡Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

The X-ray emission from NGC4051, a nearby low luminosity Type 1 Seyfert galaxy, exhibits variations that are both rapid and apparently quasi-periodic^1,2. Many Seyfert galaxies are variable³ but our knowledge of the form of variability has not advanced substantially since early well-resolved observations⁴ and attempts at statistical description⁵. Here we report an uninterrupted 62-h observation of NGC4051 with the EXOSAT observatory, giving an order of magnitude improvement in the quality of the time series, allowing comparison with X-ray binaries. The observed power spectral density is roughly proportional to 1/f, similar to Cyg X-l, and characteristic of turbulent processes. No preferred timescale or luminosity gradient has been seen so far. If this behaviour holds generally for active galactic nuclei, quantities derived from short samples will be subject to selection effects. Some naive interpretaions of variability are clearly invalid. Similar conclusions have recently been arrived at by McHardy et al. (see accompanying paper⁶).

References

1.	Marshall, F. E., Holt, S. S., Mushotzky, R. F. M. & Becker, R. H. Astrophys. J. 269, L1−L37 (1983).
2.	Lawrence, A., Watson, M. G., Pounds, K. A. & Elvis, M. Mon. Not. R. astr. Soc. 217, 685−699 (1985). | ISI | ChemPort |
3.	Barr, P. & Mushotzky, R. F. Nature 320, 421−423 (1986). | Article | ChemPort |
4.	Mushotzky, R. F., Holt, S. S. & Serlemitsos, P. Astrophys. J. 225, L115−L118 (1978). | Article | ISI |
5.	Lawrence, A. Mon. Not. R. astr. Soc. 192, 83−94 (1980). | ISI |
6.	McHardy, I. & Czerny, B. Nature 325, 696−698 (1987). | Article | ISI |
7.	Nolan, P. L. et al. Astrophys. J. 246, 494−501 (1981). | Article | ISI |
8.	Mandelbrot, B. B. Fractals: Form, Chance and Dimension (Freeman, San Francisco, 1977).
9.	Press, W. H. Comments Astrophys. 7, 103−119 (1978).
10.	Hooge, F. N., Kleinpenning, T. G. M. & Vandamme, L. K. J. Rep. Prog. Phys. 44, 479−532 (1981). | Article |
11.	Dutta, P. & Horn, P. M. Rev. Mod, Phys. 53, 497−516 (1981). | Article | ISI | ChemPort |
12.	Guilbert, P. W. & Fabian, A. C. Nature 296, 226−228 (1982). | Article | ChemPort |
13.	Fabian, A. C. Paper presented at Conf. Physics of Accretion onto Compact Objects, Tenerife, Spain, April 1986 (eds Mason, K. O., Watson, M. G. & White, N. E.) (Springer, Berlin, 1986).
14.	Motch, C., Illovaisky, S. A. & Chevalier, C. Astr. Astrophys. 109, L1−L4 (1982).
15.	Moore, R. L. et al. Astrophys. J. 260, 415−436 (1982). | Article |
16.	Kunkel, W. E. Astr. J. 72, 1341−1348 (1967). | Article |
17.	Terrell, J. & Olsen, K. H. Astrophys. J. 161, 399−413 (1970). | Article | ISI |
18.	Taylor, B. G., Andresen, R. D., Peacock, A. & Zobl, R. Space Sci. Rev. 30, 479−494 (1981). | Article | ISI |
19.	Turner, M. J. L., Smith, A. & Zimmerman, H. U. Space Sci. Rev. 30, 513−524 (1981). | Article | ISI |
20.	de Korte, et al. Space Sci. Rev. 30, 495−512 (1981).