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

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Table 4.

Best-fit parameters of GX 13+1 spectrum from IXPE data divided into five equal time bins.

Component	Parameter (units)	Bin 1	Bin 2	Bin 3	Bin 4	Bin 5
`tbabs`	N_H (10²² cm⁻²)	5.9 (frozen)	5.9 (frozen)	5.9 (frozen)	5.9 (frozen)	5.9 (frozen)
`diskbb`	kT_in (keV)	1.49 $_{- 0.05}^{+ 0.03}$ $^{+0.03}_{-0.05}$	1.44 $_{- 0.03}^{+ 0.04}$ $^{+0.04}_{-0.03}$	1.45±0.04	1.57 $_{- 0.07}^{+ 0.06}$ $_{-0.07}^{+0.06}$	1.55±0.06
	`norm`	131 $_{- 8}^{+ 14}$ $^{+14}_{-8}$	142 $_{- 12}^{+ 13}$ $^{+13}_{-12}$	130 $_{- 11}^{+ 10}$ $_{-11}^{+10}$	79 $_{- 8}^{+ 10}$ $_{-8}^{+10}$	102 $_{- 7}^{+ 10}$ $_{-7}^{+10}$
`bbodyrad`	kT (keV)	2.0 (frozen)	2.0 (frozen)	2.0 (frozen)	2.0 (frozen)	2.0 (frozen)
	`norm`	14±4	14 $_{- 5}^{+ 2}$ $^{+2}_{-5}$	14±3	10±4	8 $_{- 5}^{+ 4}$ $^{+4}_{-5}$
	R_bb (km)	2.6±0.4	2.6 $_{- 0.5}^{+ 0.2}$ $^{+0.2}_{-0.5}$	2.6 ± 0.3	2.2 ± 0.4	$2 . 0_{- 0.6}^{+ 0.5}$ $2.0^{+0.5}_{-0.6}$
`constant`	DU1	1 (frozen)	1 (frozen)	1 (frozen)	1 (frozen)	1 (frozen)
	DU2	0.951 ± 0.004	0.956 ± 0.004	0.948 ± 0.004	0.951 ± 0.004	0.950 ± 0.006
	DU3	0.908 ± 0.003	0.910 ± 0.004	0.904 ± 0.004	0.912 ± 0.004	0.911 ± 0.005

	χ²/d.o.f.	392/420	415/416	389/411	420/411	403/409

Flux_{2 − 8 keV} (10⁻⁹ erg s⁻¹ cm⁻²)		6.42	5.98	5.77	4.96	5.65
Flux_diskbb/Flux_{2 − 8 keV}		0.82	0.83	0.81	0.81	0.91
Flux_bbodyrad/Flux_{2 − 8 keV}		0.18	0.17	0.19	0.19	0.09

Notes. The errors are at the 68% CL. A gain slope of ≈0.96 and gain offsets ≈2–80 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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