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Active galactic nuclei as scaled-up Galactic black holes


A long-standing question is whether active galactic nuclei (AGN) vary like Galactic black hole systems when appropriately scaled up by mass1,2,3. If so, we can then determine how AGN should behave on cosmological timescales by studying the brighter and much faster varying Galactic systems. As X-ray emission is produced very close to the black holes, it provides one of the best diagnostics of their behaviour. A characteristic timescale—which potentially could tell us about the mass of the black hole—is found in the X-ray variations from both AGN and Galactic black holes1,2,3,4,5,6, but whether it is physically meaningful to compare the two has been questioned7. Here we report that, after correcting for variations in the accretion rate, the timescales can be physically linked, revealing that the accretion process is exactly the same for small and large black holes. Strong support for this linkage comes, perhaps surprisingly, from the permitted optical emission lines in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1 galaxies) correlate strongly with the characteristic X-ray timescale, exactly as expected from the AGN black hole masses and accretion rates. So AGN really are just scaled-up Galactic black holes.

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Figure 1: Confidence contours for the mass and bolometric indices A and B.
Figure 2: Edge-on projection of our sample and the T B M BH L bol plane.
Figure 3: Correlation of optical emission linewidth with PSD break timescale.


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This work was supported by the UK Particle Physics and Astronomy Research Council (PPARC). We thank P. Smith for discussions about statistics, P. Arevalo for determining the break timescale in GRS 1915+105 and D. Summons for providing some RXTE lightcurves of Cyg X-1.

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Correspondence to I. M. McHardy.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary information

This file contains the study of the scaling relationship between Active Galactic Nuclei (AGN) and Galactic Black Hole X-ray binary systems (GBHs) by means of comparing the way in which their X-ray emission varies. The study also shows how the widths of the permitted optical emission lines, a fundamental tracer of larger scale AGN properties often used to classify AGN, are intimately related to the way in which the timing properties from large to small black holes are scaled. (DOC 86 kb)

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McHardy, I., Koerding, E., Knigge, C. et al. Active galactic nuclei as scaled-up Galactic black holes. Nature 444, 730–732 (2006).

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