Discovery of a strong rotation of the X-ray polarization angle in the galactic burster GX 13+1

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Table 5.

Best-fit parameters of the spectropolarimetric model for GX 13+1.

Component	Parameter (units)	Bin 1, `polconst`	Bin 1, `pollin`	Bin 2–4	Bin 5
`tbabs`	N_H (10²² cm⁻²)	5.9 (frozen)	5.9 (frozen)	5.9 (frozen)	5.9 (frozen)
`diskbb`	kT_in (keV)	$1 . 51_{- 0.04}^{+ 0.05}$ $1.51_{-0.04}^{+0.05}$	$1 . 51_{- 0.04}^{+ 0.05}$ $1.51_{-0.04}^{+0.05}$	$1 . 50_{- 0.03}^{+ 0.02}$ $1.50_{-0.03}^{+0.02}$	$1 . 58_{- 0.06}^{+ 0.05}$ $1.58_{-0.06}^{+0.05}$
	norm	120 ± 11	120 ± 11	$106_{- 5}^{+ 6}$ $106_{-5}^{+6}$	92 ± 10
`bbodyrad`	kT (keV)	2.0 (frozen)	2.0 (frozen)	2.0 (frozen)	2.0 (frozen)
	norm	$13_{- 3}^{+ 6}$ $13_{-3}^{+6}$	$13_{- 3}^{+ 4}$ $13_{-3}^{+4}$	12 ± 2	< 14
	R_bb (km)	$2 . 5_{- 0.3}^{+ 0.6}$ $2.5_{-0.3}^{+0.6}$	$2 . 5_{- 0.3}^{+ 0.4}$ $2.5_{-0.3}^{+0.4}$	2.4 ± 0.2	< 2.6
`polconst`	A (%)	3.0 ± 0.5	–	1.2 ± 0.3	4.5 ± 0.5
	ψ (deg)	−42 ± 5	–	7 ± 7	32 ± 3
`pollin`	A₁ (%)	–	−0.75 ± 1.4	–	–
	A_slope (% keV⁻¹)	–	1.3 ± 0.5	–	–
	ψ₁ (deg)	–	−42 ± 4	–	–
	ψ_slope (deg keV⁻¹)	–	0 (frozen)	–	–
`constant`	DU1	1 (frozen)	1 (frozen)	1 (frozen)	1 (frozen)
	DU2	0.953 ± 0.004	0.953 ± 0.004	0.956 ± 0.002	0.950 ± 0.004
	DU3	0.910 ± 0.004	0.910 ± 0.004	0.910 ± 0.002	0.909 ± 0.004

	χ²/d.o.f.	634/653	626/652	600/653	630/653

Notes. IXPE I, Q, and U spectra are divided into five equal time bins and regrouped based on their spectral and polarimetric behavior. The errors are at the 68% CL. A gain slope of ≈0.97 and gain offsets ≈130–200 eV for the three DUs were needed to obtain a good reduced χ². R_bb is calculated from the normalization of the bbodyrad component, assuming the distance to the source of 7 kpc.

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