Spectral-timing evidence for a Very High State in the Narrow Line Seyfert 1 Ark 564

Abstract

We use a 100 ks long XMM-Newton observation of the Narrow-Line Seyfert 1 galaxy Ark 564 and combine it with the month-long monitoring of the same source produced by ASCA, to calculate the phase lags and coherence between different energy bands, over frequencies ~ 10^{-6} to 10^{-3} Hz. This is the widest frequency range for which these spectral-timing properties have been calculated accurately for any AGN. The 0.7-2 and 2--0 keV ASCA light curves, and the XMM-Newton light curves in corresponding energy bands, are highly coherent (~ 0.9) over most of the frequency range studied. We observe time lags between the energy bands, increasing both with time-scale and with energy separation of the bands. The time lag spectrum shows a broad peak in the 10^{-5} - 5x10^{-4} Hz frequency range, where the time lags follow a power law slope ~ -0.7. Above ~ 5 x 10^{-4} Hz the lags drop below this relation significantly. This change in slope resembles the shape of the lag spectra of black hole X-ray binaries (BHXRB) in the very high or intermediate state. The lags increase linearly with the logarithm of the separation of the energy bands, which poses one more similarity between this AGN and BHXRBs.