A Monte Carlo spectropolarimetric model for the high-soft state of neutron star low-mass X-ray binaries

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Table 2.

Best-fit spectral parameters of the model applied to the IXPE (I, Q, U) and NuSTAR spectra of Cyg X-2 and GX 9+9.

Parameter	Cyg X-2	GX 9+9
N_H (×10²² cm⁻²)	0.91 $_{- 0.11}^{+ 0.14}$ $^{+0.14}_{-0.11}$	1.34 $_{- 0.16}^{+ 0.12}$ $^{+0.12}_{-0.16}$
E_edge (keV)	9.06 $_{- 0.09}^{+ 0.09}$ $^{+0.09}_{-0.09}$	9.59 $_{- 0.36}^{+ 0.30}$ $^{+0.30}_{-0.36}$
τ_edge (×10⁻²)	4.4 $_{- 0.7}^{+ 0.6}$ $^{+0.6}_{-0.7}$	1.3 $_{- 0.5}^{+ 0.4}$ $^{+0.4}_{-0.5}$
ψ (deg)	49 $_{- 7}^{+ 7}$ $^{+7}_{-7}$	−23 $_{- 10}^{+ 10}$ $^{+10}_{-10}$

`bldisk`

θ_bl (deg)	90 ( > 44)	23 $_{- 3}^{+ 2}$ $^{+2}_{-3}$
kT_bb (keV)	1.42 $_{- 0.02}^{+ 0.14}$ $^{+0.14}_{-0.02}$	1.79 $_{- 0.01}^{+ 0.03}$ $^{+0.03}_{-0.01}$
kT_e (keV)	3.66 $_{- 0.07}^{+ 0.03}$ $^{+0.03}_{-0.07}$	2.14 $_{- 0.02}^{+ 0.04}$ $^{+0.04}_{-0.02}$
τ_bl	4.3 $_{- 0.4}^{+ 0.3}$ $^{+0.3}_{-0.4}$	5.3 $_{- 0.3}^{+ 1.2}$ $^{+1.2}_{-0.3}$
ṁ	0.15 $_{- 0.03}^{+ 0.05}$ $^{+0.05}_{-0.03}$	0.10 ( < 0.11)
τ_disk	[5]	5.0 ( > 4.7)
$h / \tilde{r}$ $h/\tilde{r}$	0.10 ( > 0.09)	0.10 ( > 0.08)
i_obs (deg)	48 $_{- 8}^{+ 2}$ $^{+2}_{-8}$	63 $_{- 3}^{+ 2}$ $^{+2}_{-3}$
N_bldisk	2.8 $_{- 0.8}^{+ 0.2}$ $^{+0.2}_{-0.8}$	2.5 $_{- 0.3}^{+ 0.1}$ $^{+0.1}_{-0.3}$

`gaussian`

E_ga (keV)	6.52 $_{- 0.05}^{+ 0.05}$ $^{+0.05}_{-0.05}$	6.43 $_{- 0.11}^{+ 0.12}$ $^{+0.12}_{-0.11}$
σ_ga (keV)	0.58 $_{- 0.07}^{+ 0.08}$ $^{+0.08}_{-0.07}$	0.50 ( > 0.43)
N_ga ( × 10⁻³)	5.9 $_{- 1}^{+ 1}$ $^{+1}_{-1}$	0.9 $_{- 0.2}^{+ 0.2}$ $^{+0.2}_{-0.2}$
EW (eV)	76 $_{- 6}^{+ 97}$ $^{+97}_{-6}$	20 $_{- 19}^{+ 33}$ $^{+33}_{-19}$
F_x (erg cm⁻² s⁻¹)	1.59 × 10⁻⁸	8.95 × 10⁻⁹
χ²/d.o.f.	1555/1439	1387/1480

`constant`

C_FPMB/FPMA	1.002 ± 0.002	1.001 ± 0.001
C_DU1/FPMA	0.94 ± 0.02	0.99 ± 0.02
C_DU2/FPMA	0.91 ± 0.02	0.96 ± 0.02
C_DU3/FPMA	0.82 ± 0.01	0.88 ± 0.01

`gain`

α_DU1	0.941 ± 0.002	0.981 ± 0.003
β_DU1 (eV)	−47 ± 14	−103 ± 17
α_DU2	0.946 ± 0.002	0.979 ± 0.003
β_DU2 (eV)	−52 ± 17	−92 ± 18
α_DU3	0.946 ± 0.002	0.987 ± 0.003
β_DU3 (eV)	−9 ± 20	−82 ± 18

Notes. Parameters between square bracket are frozen in the fit. The errors are at the 90 per cent confidence level for a single parameter (Δχ² = 2.71). Model flux is computed in the energy range 0.1–50 keV.

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