Open AccessSpectral evolution of magnetic flares and time lags in accreting black hole sources
Author(s) -
Poutanen Juri,
Fabian Andrew C.
Publication year - 1999
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1999.02735.x
Subject(s) - physics , astrophysics , flare , accretion (finance) , black hole (networking) , spectral density , solar flare , magnetic reconnection , light curve , astronomy , magnetic field , computer network , routing protocol , statistics , routing (electronic design automation) , mathematics , quantum mechanics , computer science , link state routing protocol
We present a model for the short time‐scale spectral variability of accreting black holes. It describes the time‐averaged spectra well, and also temporal characteristics such as the power‐density spectrum, time/phase lags, and coherence function of Cygnus X‐1. We assume that X/ γ ‐rays are produced in compact magnetic flares at radii ≲100 GM c 2 from the central black hole. The tendency for magnetic loops to inflate and detach from the underlying accretion disc causes the spectrum of a flare to evolve from soft to hard because of the decrease of the feedback from the cold disc, so causing time delays between hard and soft photons. We identify the observed time lags with the evolution time‐scales of the flares, which are of the order of the Keplerian time‐scale. We model the overall temporal variability using a pulse avalanche model in which each flare has a certain probability of triggering a neighbouring flare, thus occasionally producing long avalanches. The duration of the avalanches determines the Fourier frequencies at which most of the power emerges.