X-ray spectropolarimetric characterisation of the Z source GX 340+0 in the normal branch

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Volume 702 (October 2025)

A&A, 702 (2025) A101

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Table 3.

Best-fit parameters of the broadband joint spectral analysis for GX 340+0 in the NB obtained by NICER, NuSTAR, and IXPE.

Model	Parameter (units)	Model A	Model B
`TBabs`	N_H (10²² cm⁻²)	$9 . 76_{- 0.07}^{+ 0.04}$ $9.76_{-0.07}^{+0.04}$	$9 . 06_{- 0.04}^{+ 0.07}$ $9.06_{-0.04}^{+0.07}$

`diskbb`	kT_in (keV)	$1 . 303_{- 0.019}^{+ 0.002}$ $1.303_{-0.019}^{+0.002}$	1.390 ± 0.014
	norm ([R_in/D₁₀]² cos θ)	$245 . 8_{- 0.3}^{+ 6.5}$ $245.8_{-0.3}^{+6.5}$	$181_{- 6}^{+ 7}$ $181_{-6}^{+7}$

`comptt`	T₀ (keV)	$1 . 706_{- 0.032}^{+ 0.004}$ $1.706_{-0.032}^{+0.004}$	1.72 ± 0.03
	kT (keV)	$3 . 199_{- 0.042}^{+ 0.004}$ $3.199_{-0.042}^{+0.004}$	3.18 ± 0.07
	τ	$3 . 564_{- 0.010}^{+ 0.013}$ $3.564_{-0.010}^{+0.013}$	$3 . 1_{- 0.2}^{+ 0.3}$ $3.1_{-0.2}^{+0.3}$
	norm	$0 . 3069_{- 0.0144}^{+ 0.0005}$ $0.3069_{-0.0144}^{+0.0005}$	$0 . 267_{- 0.013}^{+ 0.012}$ $0.267_{-0.013}^{+0.012}$

`Gaussian`	E (keV)	$6 . 62_{- 0.04}^{+ 0.05}$ $6.62_{-0.04}^{+0.05}$	–
	σ (keV)	$1 . 00_{- 0.05}^{+ 0.03}$ $1.00_{-0.05}^{+0.03}$	–
	norm (photon cm⁻² s⁻¹)	0.013 ± 0.001	–
	Equivalent width (eV)	$11 . 95_{- 0.03}^{+ 0.01}$ $11.95_{-0.03}^{+0.01}$	–

`relxillNS`	Emissivity	–	$2 . 30_{- 0.13}^{+ 0.20}$ $2.30_{-0.13}^{+0.20}$
	R_in (ISCO)	–	1.2 ± 0.2
	R_out (GM/c²)	–	[1000]
	Inclination (deg)	–	$33_{- 3}^{+ 2}$ $33_{-3}^{+2}$
	log ξ	–	$2 . 77_{- 0.08}^{+ 0.10}$ $2.77_{-0.08}^{+0.10}$
	A_Fe	–	$1 . 5_{- 0.2}^{+ 0.4}$ $1.5_{-0.2}^{+0.4}$
	log N	–	[19]
	norm (10⁻³)	–	$1 . 20_{- 0.12}^{+ 0.08}$ $1.20_{-0.12}^{+0.08}$

	χ²/d.o.f.	1793/1690 = 1.1	1898/1687 = 1.1

Cross normalisation factors
	C_NICER	[1.0]	[1.0]
	C_{NuSTAR − A}	$1 . 295_{- 0.001}^{+ 0.009}$ $1.295_{-0.001}^{+0.009}$	$1 . 301_{- 0.007}^{+ 0.011}$ $1.301_{-0.007}^{+0.011}$
	C_{NuSTAR − B}	$1 . 264_{- 0.001}^{+ 0.010}$ $1.264_{-0.001}^{+0.010}$	$1 . 270_{- 0.007}^{+ 0.010}$ $1.270_{-0.007}^{+0.010}$
	C_{IXPE − DU1}	0.878 ± 0.006	0.861 ± 0.006
	C_{IXPE − DU2}	0.890 ± 0.006	0.870 ± 0.006
	C_{IXPE − DU3}	0.878 ± 0.006	0.860 ± 0.006

Photon flux ratios in 2–8 keV
	F_diskbb/F_tot	0.63	0.66
	F_comptt/F_tot	0.35	0.31
	F_{gauss, relxillNS}/F_tot	0.02	0.03

Notes. The estimated unabsorbed flux in 2–8 keV is ∼1.2 × 10⁻⁸ erg s⁻¹ cm⁻², corresponding to a luminosity of ∼1.7 × 10³⁸ erg s⁻¹ for a distance to the source of 11 kpc (Fender & Hendry 2000). The errors are reported at 90% CL.

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