Colour temperature corrected disc and energetically coupled Comptonisation model for AGN.

This model and description are due to Chris Done.

AGN spectral energy distributions are complex, but can be phenomenologically fit by a disc, optically thick, low temperature thermal Comptonisation (to produce the soft X-ray excess) and an optically thin, high temperature themal Comptonisation (to produce the power law emission which dominates above 2 keV). Here we combine these three components together assuming that they are all ultimately powered by gravitational energy released in accretion. We assume that the gravitational energy released in the disc at each radius is emitted as a (colour temperature corrected) blackbody only down to a given radius, Rcorona. Below this radius, we further assume that the energy can no longer completely thermalise, and is distributed between powering the soft excess component and the high energy tail. This imposes an important energetic self consistency on the model. The key aspect of this model is that the optical luminosity constrains the mass accretion rate through the outer disc, Mdot, provided there is an independent estimate of the black hole mass (from e.g. the Hβ emission line profile). The total luminosity available to power the entire SED is Ltot=eff Mdot c2, where the efficiency is set by black hole spin assuming Novikov-Thorne emissivity.

There are two versions of the model. Optxagnf is the one recommended for most purposes, and has the colour temperature correction calculated for each temperature from the approximations given in Done et al (2011). Optxagn instead allows the user to define their own colour temperature correction, fcol, which is then applied to annuli with effective temperature > Tscatt. In both models the flux is set by the physical parameters of mass, L/LEdd, spin and distance, hence the model normalisations MUST be frozen at unity.

Parameters in Optxagnf:

  • 1: mass, in solar masses
  • 2: dist, luminosity distance in Mpc
  • 3: log(L/LEdd), Eddington ratio
  • 4: astar, dimensionless black hole spin
  • 5: rcor, coronal radius in Rg=GM/c2 marking the transition from (colour temperature corrected) blackbody emission to a Comptonised spectrum. If this parameter is negative then only the blackbody component is used.
  • 6: logrout, log of the outer radius of the disc in units of Rg; if this is -ve the code will use the self gravity radius as calculated from Laor & Netzer (1989)
  • 7: kT_e, electron temperature for the soft Comptonisation component (soft excess) in keV
  • 8: τ, optical depth of the soft Comptonisation component. If this parameter is negative then only the soft Compton component is used.
  • 9: Γ, spectral index of the hard Comptonisation component ('power law') which has temperature fixed to 100 keV. If this parameter is negative then only the hard Compton component is used.
  • 10: fpl, fraction of the power below rcor which is emitted in the hard comptonisation component
  • 11 Redshift

Parameters in Optxagn:

  • 1: mass, in solar masses
  • 2: dist, luminosity distance in Mpc
  • 3: log(L/LEdd), Eddington ratio
  • 4: astar, dimensionless black hole spin
  • 5: rcor, coronal radius in Rg=GM/c2 marking the transition from (colour temperature corrected) blackbody emission to a Comptonised spectrum. If this parameter is negative then only the blackbody component is used.
  • 6: logrout, log of the outer radius of the disc in units of Rg if this is -ve the code will use the self gravity radius as calculated from Laor & Netzer (1989)
  • 7: kT_e, electron temperature for the soft Comptonisation component (soft excess) in keV
  • 8: τ, optical depth of the soft Comptonisation component. If this parameter is negative then only the soft Compton component is used.
  • 9: Γ, spectral index of the hard Comptonisation component ('power law') which has temperature fixed to 100 keV). If this parameter is negative then only the hard Compton component is used.
  • 10: fpl, fraction of the power below rcor which is emitted in the hard comptonisation component
  • 11: fcol, colour temperature correction to apply to the disc blackbody emission for radii below rcor with effective temperature > Tscatt
  • 12: Tscatt, in K
  • 13: Redshift

Download the tar file optxagn.tar. If you are using this without any other models in xspec12 then untar this in a clean directory, fire up xspec12 and type @load. The model should then be set up. Otherwise, build it as normal with other local models.


Keith Arnaud, Lab. for High Energy Astrophysics, NASA/Goddard Space Flight Center

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Last modified: Monday, 01-Aug-2011 10:05:58 EDT