The absence of rapid X-ray variability in active galaxies

Abstract

We have searched for variations on time scales ranging from minutes to several hours in the X-ray flux from 54 observations of 38 active galaxies.Our sample is composed mostly of Seyfert I galaxies but also includes radio galaxies, NELG's, BL Lacs and 3C 273. Only NGC 6814 varied or time scales as short as 100 sec. No other source was observed to vary with a time scale of less than 12 hours. We conclude that large-amplitude short-term variations are not a characteristic of the X-ray emission from active galaxies. Upper limits on a I /I ranged from 2% for Cen A, 5% for NGC 4151, to -20% for sources giving 1 ct/sec in our detector. Three objects NGC 3227, NGC 4151 and MCG 5-23-16 show variability consistent with a time scale of -1 day.We consider ways to reconcile the rapid variability seen for NGC 6814 (and NGC 4051) with the general stability observed for the other objects. If the lack of variability is due to the growth of a new, constant source of soft photons (such as thermal re-radiation from dust) then the X-ray plasma must be 2 much larger than the size implied by the variations seen in NGC 6814. Other possibilities are that the "originally lumpy" accretion flow stablizes or that NGC 6814 is different from other galaxies. Subject headings --Galaxies: nuclei -Galaxies: Seyfert -X-ray: sources J i From a dissertation to be submitted to Graduate School, University of Maryland, by Allyn F. Tennant in partial fulfillment of the requirements for the Ph.D. degree in Physics.

${XMM{-}Newton}$ observations of the Seyfert 1 galaxy NGC 3227

“…A high variability of NGC 3227 on time scales of hours−days has been previously reported in the literature. In particular, XMM-Newton observations are reminiscent of the 40% flux increase within 5 hours found in HEAO1 data (Tennant & Mushotzky 1983) and of the differential variability between the EXOSAT LE (0.05-2 keV) and ME (2-10 keV) instruments (Turner & Pounds 1989). …”

“…Observations of NGC 3227 with HEAO1 indicated a strong variability in the X-ray domain. Tennant & Mushotzky (1983) found a 40% flux increase within 5 hours. Variability was also present in the EXOSAT data (Turner & Pounds 1989) and in an ASCA observation (Ptak et al 1994).…”

The Remarkable Eclipsing Asynchronous AM Herculis Binary RX J19402-1025

“…This is because the X-ray flux shows very rapid variations on time scales of a few hundred seconds (Tennant et al 1981;Tennant and Mushotzky 1983), and a strict periodicity of P = 3.4 hr which manifests itself both in the sharp dips and in a smooth variation around the cycle (Mittaz and Branduardi-Raymond 1989;Done et al 1992; 1 Visiting Astronomer, Cerro Tololo Inter-American Observatory, which is operated by the Associated Universities for Research in Astronomy, Inc. (AURA), under contract with the National Science Foundation. 2 A1so at Department of Astronomy and McDonald Observatory, University of Texas at Austin, Austin, TX 78712.…”

Position sensitive multiwire proportional counters for X-ray astronomy

“…-the Seyfert galaxies showing intensity variability over a wide range of time-scales (Tennant and Mushotzky, 1983;Ubertim etal., 1984);…”