X-ray polarization of Z-type neutron star low-mass X-ray binaries - II. Spectropolarimetric analysis

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Volume 711 (July 2026)

A&A, 711 (2026) A208

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Table A.1.

Best-fitting model parameters for Cyg X-2.

	Parameter	HB^★	NB	FB
`TBabs`	N_H (10²² cm⁻²)	[0.12]	0.16 $_{- 0.05}^{+ 0.05}$ $Mathematical equation: $ ^{+0.05}_{-0.05} $$
`edge`	E_c (keV)	/	[1.839]	[1.839]
	D (10⁻²)	/	3.9 $_{- 0.2}^{+ 0.2}$ $Mathematical equation: $ ^{+0.2}_{-0.2} $$	5.4 $_{- 0.5}^{+ 0.6}$ $Mathematical equation: $ ^{+0.6}_{-0.5} $$
`diskbb`	kT_in (keV)	0.65 $_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ ^{+0.02}_{-0.02} $$	1.12 $_{- 0.03}^{+ 0.03}$ $Mathematical equation: $ ^{+0.03}_{-0.03} $$	0.81 $_{- 0.04}^{+ 0.04}$ $Mathematical equation: $ ^{+0.04}_{-0.04} $$
	$R_{d} \sqrt{cos i}$ $Mathematical equation: $ R_{\mathrm{d}} \sqrt{\cos i} $$ (km)	23.3 $_{- 0.8}^{+ 0.8}$ $Mathematical equation: $ ^{+0.8}_{-0.8} $$	10.2 $_{- 0.5}^{+ 0.5}$ $Mathematical equation: $ ^{+0.5}_{-0.5} $$	15.7 $_{- 1.2}^{+ 1.2}$ $Mathematical equation: $ ^{+1.2}_{-1.2} $$
`thcomp`	kT_e (keV)	3.4 $_{- 0.1}^{+ 0.1}$ $Mathematical equation: $ ^{+0.1}_{-0.1} $$	3.1 $_{- 0.1}^{+ 0.1}$ $Mathematical equation: $ ^{+0.1}_{-0.1} $$	2.9 $_{- 0.1}^{+ 0.1}$ $Mathematical equation: $ ^{+0.1}_{-0.1} $$
	τ	8.5 $_{- 0.4}^{+ 0.5}$ $Mathematical equation: $ ^{+0.5}_{-0.4} $$	9.0 $_{- 0.7}^{+ 0.6}$ $Mathematical equation: $ ^{+0.6}_{-0.7} $$	8.8 $_{- 0.8}^{+ 0.8}$ $Mathematical equation: $ ^{+0.8}_{-0.8} $$
	f	[1]	[1]	[1]
`bbodyrad`	kT (keV)	1.10 $_{- 0.01}^{+ 0.01}$ $Mathematical equation: $ ^{+0.01}_{-0.01} $$	1.28 $_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ ^{+0.02}_{-0.02} $$	1.13 $_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ ^{+0.02}_{-0.02} $$
	R_bb (km)	11.6 $_{- 0.3}^{+ 0.3}$ $Mathematical equation: $ ^{+0.3}_{-0.3} $$	8.2 $_{- 0.4}^{+ 0.5}$ $Mathematical equation: $ ^{+0.5}_{-0.4} $$	11.5 $_{- 0.6}^{+ 0.6}$ $Mathematical equation: $ ^{+0.6}_{-0.6} $$
`relxillNS`	q_em	1.6 $_{- 0.2}^{+ 0.2}$ $Mathematical equation: $ ^{+0.2}_{-0.2} $$	1.7 $_{- 0.2}^{+ 0.2}$ $Mathematical equation: $ ^{+0.2}_{-0.2} $$
	a		[0.1]
	i (deg)	[62]	62 $_{- 4}^{+ 4}$ $Mathematical equation: $ ^{+4}_{-4} $$
	R_in (R_ISCO)	< 5.3	< 1.6	[1]
	kT_bb (keV)	= kT	= kT	= kT
	log ξ	3.0 $_{- 0.1}^{+ 0.1}$ $Mathematical equation: $ ^{+0.1}_{-0.1} $$	2.6 $_{- 0.1}^{+ 0.1}$ $Mathematical equation: $ ^{+0.1}_{-0.1} $$	2.8 $_{- 0.1}^{+ 0.1}$ $Mathematical equation: $ ^{+0.1}_{-0.1} $$
	A_Fe	[1.4]	[1.4]
	log n_e (cm⁻³)	[18]	[18]
	N_r (10⁻³)	2.8 $_{- 0.9}^{+ 0.8}$ $Mathematical equation: $ ^{+0.8}_{-0.9} $$	5.5 $_{- 0.2}^{+ 0.2}$ $Mathematical equation: $ ^{+0.2}_{-0.2} $$	8.5 $_{- 0.1}^{+ 0.2}$ $Mathematical equation: $ ^{+0.2}_{-0.1} $$

Cross-calibration
𝒞_DU2/DU1		/	1.001 $_{- 0.001}^{+ 0.001}$ $Mathematical equation: $ ^{+0.001}_{-0.001} $$	1.006 $_{- 0.004}^{+ 0.005}$ $Mathematical equation: $ ^{+0.005}_{-0.004} $$
𝒞_DU3/DU1		0.970 $_{- 0.002}^{+ 0.003}$ $Mathematical equation: $ ^{+0.003}_{-0.002} $$	0.976 $_{- 0.001}^{+ 0.002}$ $Mathematical equation: $ ^{+0.002}_{-0.001} $$	1.020 $_{- 0.005}^{+ 0.005}$ $Mathematical equation: $ ^{+0.005}_{-0.005} $$
𝒞_FPMA/DU1		1.371 $_{- 0.004}^{+ 0.004}$ $Mathematical equation: $ ^{+0.004}_{-0.004} $$	1.098 $_{- 0.002}^{+ 0.002}$ $Mathematical equation: $ ^{+0.002}_{-0.002} $$	1.136 $_{- 0.004}^{+ 0.003}$ $Mathematical equation: $ ^{+0.003}_{-0.004} $$
𝒞_FPMB/DU1		1.392 $_{- 0.004}^{+ 0.005}$ $Mathematical equation: $ ^{+0.005}_{-0.004} $$	1.112 $_{- 0.002}^{+ 0.002}$ $Mathematical equation: $ ^{+0.002}_{-0.002} $$	1.148 $_{- 0.004}^{+ 0.004}$ $Mathematical equation: $ ^{+0.004}_{-0.004} $$
𝒞_XTI/DU1		/	1.026 $_{- 0.002}^{+ 0.002}$ $Mathematical equation: $ ^{+0.002}_{-0.002} $$	1.064 $_{- 0.005}^{+ 0.005}$ $Mathematical equation: $ ^{+0.005}_{-0.005} $$

χ²/d.o.f.		549/526	350/309	318/275

Photon flux ratios (2–8 keV)
N_diskbb/N_Tot		24.6%	50.7%	24.1%
N_thcomp*bb/N_Tot		57.6%	33.5%	43.8%
N_relxillNS/N_Tot		17.8%	15.8%	32.1%
Photon flux ratios (2–4 keV)
N_diskbb/N_Tot		35.7%	61.5%	32.9%
N_thcomp*bb/N_Tot		46.3%	24.6%	35.4%
N_relxillNS/N_Tot		18.0%	13.9%	31.7%
Photon flux ratios (4–6 keV)
N_diskbb/N_Tot		6.5%	36.2%	10.3%
N_thcomp*bb/N_Tot		74.5%	45.4%	55.6%
N_relxillNS/N_Tot		19.0%	18.4%	34.1%
Photon flux ratios (6–8 keV)
N_diskbb/N_Tot		0.9%	17.7%	2.7%
N_thcomp*bb/N_Tot		85.3%	61.1%	67.6%
N_relxillNS/N_Tot		13.8%	21.2%	29.7%
Energy Flux (2–8 keV)
F_Tot (10⁻⁹ erg s⁻¹ cm⁻²)		5.38	7.58	7.92

Notes. Uncertainties are at the 90% confidence level for a single parameter. Parameters in common between the two columns were linked during the different branches, while those in square brackets were frozen during the fit. The normalizations of diskbb and bbodyrad are computed by assuming a source distance of 7.2 kpc (Orosz & Kuulkers 1999). ^( ★ )Best-fit taken by Gnarini et al. (2026).

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