All-Moscow Seminar of Astrophysicists
(ASA)
The seminar is supported by the grant of Federal Programm "Integration"
annotation of talk
Stefan John Collier (MSSL, University College of London)
Measuring the Hubble Constant from Reverberating Accretion Discs in AGNs
We show how wavelength-dependent time delays between continuum flux variations of active galactic nuclei (AGNs) can be
used to test the standard black hole--accretion disc paradigm, by measuring the temperature structure T(R) of the gaseous material
surrounding the purported black hole. Reprocessing of high-energy radiation in a steady-state blackbody accretion disc with T ~
R-3/4 incurs a wavelength-dependent light travel time delay tau ~ lambda4/3. The International AGN Watch
multiwavelength monitoring campaign on NGC7469 showed optical continuum variations lagging behind those in the UV by
about 1d at 4800A and about 2d at 7500A. These UV/optical continuum lags imply a radial temperature structure
T ~ R-3/4, consistent with the classical accretion disc model, and hence strongly support the existence of a disc in this system. We
assume that the observed time delays are indeed caused by a classical accretion disc structure, and derive a redshift-independent
luminosity distance to NGC 7469. The luminosity distance allows us to estimate a Hubble constant of
H_0 (cos i / 0.7)1/2 = 42 +/- 9 km/s/Mpc. The interpretation of the observed
time delays and spectral energy distribution in the context of an
accretion disc structure requires further validation. At the same time, efforts to minimize the systematic uncertainties in our
method to derive a more accurate measurement of H_0, e.g. by obtaining an independent accurate determination of the disc
inclination i or statistical average of a moderate sample of active galaxies, are required. However, this remains a promising new
method of determining redshift-independent distances to AGNs.
(see. MNRAS 302, L24-L28, 1999)