Unveiling the reflection spectrum in the ultracompact LMXB 4U 1820–30

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Volume 697 (May 2025)

A&A, 697 (2025) A83

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Table 1.

Best value parameters obtained applying Model 2 to the data.

Model	Component	Oct. + 16 Apr.	15 Apr.
`TBfeo`	nH (10²²)	0.168 ± 0.003	0.170 ± 0.004
	O	1.18 ± 0.05	1.20 ± 0.07
	Fe	0.7 ± 0.3	0.7 ± 0.4
`expabs`	E_cut (keV)	kT_s^( † )	kT_s^( † )
`powerlaw`	Index	2.09 ^( * )	2.34 ^( * )
	Norm	0.027 ± 0.006	0.08 ± 0.02
`diskline`	E (keV)	6.57 ± 0.06	$6 . 56_{- 0.07}^{+ 0.05}$ $6.56^{+0.05}_{-0.07}$
	Index	−2.5 ± 0.5	$- 2 . 1_{- 0.5}^{+ 0.3}$ $-2.1^{+0.3}_{-0.5}$
	R_in(R_g)	< 37	< 52
	R_out(R_g)	1000 ^( * )	1000 ^( * )
	Incl (deg)	45 ^( * )	45 ^( * )
	Norm (10⁻³)	$1 . 5_{- 0.2}^{+ 0.3}$ $1.5^{+0.3}_{-0.2}$	$2 . 2_{- 0.4}^{+ 0.5}$ $2.2^{+0.5}_{-0.4}$
`comptb`	kT_s (keV)	$0 . 84_{- 0.03}^{+ 0.05}$ $0.84^{+0.05}_{-0.03}$	1.00 ± 0.04
	α	0.87 ± 0.02	0.91 ± 0.03
	kT_e (keV)	2.93 ± 0.02	2.88 ± 0.03
	Norm	0.055 ± 0.001	0.070 ± 0.001
`diskbb`	T_in (keV)	0.64 ± 0.03	0.67 ± 0.03
	Norm	$910_{- 130}^{+ 160}$ $910^{+160}_{-130}$	$840_{- 120}^{+ 130}$ $840^{+130}_{-120}$

	χ²/d.o.f.	812/813	345/416

Notes. Uncertainties are calculated at the 90% confidence level. ^( † )Linked to the photon seed temperature. ^( * )Kept frozen during the fit.

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