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stdisc: X-ray polarized reflection from a distant accretion disc

This additive model computes the spectral and polarization (Stokes $I$, $Q$, $U$) properties of a power-law X-ray source of arbitrary incident polarization that is reprocessed by distant, nearly neutral, equatorial regions of a geometrically thin, optically thick accretion disc around a black hole. The local reflection was precomputed with the STOKES code and is interpolated from FITS tables for any primary polarization state. No relativistic effects are included and all components are static.

The model is designed for joint fitting of the three Stokes spectra. With the Stokes selector (par8) set to $-1$, the quantity returned for each spectrum is taken from its Stokes XFLT keyword (“Stokes:0”$\,=I$, “Stokes:1”$\,=Q$, “Stokes:2”$\,=U$), the same convention used by the polconst model and the chistokes statistic. Setting par8 to an integer 0–$10$ instead returns a single quantity ($I$, $Q$, $U$, $V$, polarization degree or angle, or a normalized Stokes ratio), which is convenient for plotting the model against a dummy response. When par8 is $5$–$10$ the normalization (par9) should be frozen at unity.

After evaluation the derived inclination inc_degrees ( $\arccos(\mathtt{cos\_incl})$ in degrees) can be retrieved with the xset command. The reflection tables
(stokes-neutral-iso-*-disc.fits) are installed in the standard model-data directory.

References: Podgorný et al. 2022, MNRAS 510, 4723; Podgorný et al. 2024, MNRAS 530, 2608.

The parameters are :

par1 Size, upper limit $M_i$ of the $\cos i_{\rm inc}$ integration (corona size)
par2 PhoIndex, photon index of the primary power law
par3 cos_incl, cosine of the observer inclination ($1=$pole, $0=$disc)
par4 pol_deg, intrinsic polarization degree of the primary (0–$1$)
par5 chi, intrinsic polarization angle of the primary (degrees, $-90$ to $90$)
par6 pos_ang, position angle of the system axis (degrees, $-90$ to $90$)
par7 zshift, overall Doppler shift
par8 Stokes, output selector ($-1=$per-spectrum via XFLT; 0–$10=I$/$Q$/$U$/$V$/PD/PA/ratios)
par9 norm
next up previous contents
Next: sttorus: X-ray polarized reflection Up: Additive Model Components Previous: stcone: X-ray polarized reflection   Contents