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 2.

Best-fit parameters obtained applying Model 3 (Rfxconv) and Model 4 (Relxillns) to the data.

Model	Parameter	`Model 3`^(#)		`Model 4 ^(#)`
		Oct.	15 Apr.	Oct.	15 Apr.
`Tbfeo`	nH (10²²)	$0 . 179_{- 0.009}^{+ 0.011}$ $0.179^{+0.011}_{-0.009}$	$0 . 173_{- 0.0079}^{+ 0.011}$ $0.173^{+0.011}_{-0.0079}$	$0 . 181_{- 0.005}^{+ 0.006}$ $0.181^{+0.006}_{-0.005}$	$0 . 180_{- 0.007}^{+ 0.009}$ $0.180^{+0.009}_{-0.007}$
	O	$1 . 35_{- 0.08}^{+ 0.06}$ $1.35^{+0.06}_{-0.08}$	$1 . 333_{- 0.098}^{+ 0.13}$ $1.333^{+0.13}_{-0.098}$	$1 . 36_{- 0.06}^{+ 0.07}$ $1.36^{+0.07}_{-0.06}$	$1 . 340_{- 0.01}^{+ 0.10}$ $1.340^{+0.10}_{-0.01}$
	Fe	0.5 ± 0.4	$0 . 81_{- 0.5}^{+ 0.45}$ $0.81^{+0.45}_{-0.5}$	0.5 ± 0.3	0.7 ± 0.5
`Expabs`	E (keV)	kT_s^( † )	kT_s^( † )	kT_s^( † )	kT_s^( † )
`Powerlaw`	Index	2.7 ± 0.2	$2 . 77_{- 0.28}^{+ 0.18}$ $2.77^{+0.18}_{-0.28}$	2.606 ^( * )	$2 . 8_{- 0.3}^{+ 0.2}$ $2.8^{+0.2}_{-0.3}$
	Norm	$0 . 4_{- 0.2}^{+ 0.3}$ $0.4^{+0.3}_{-0.2}$	$0 . 46_{- 0.4}^{+ 0.22}$ $0.46^{+0.22}_{-0.4}$	0.25 ± 0.04	$0 . 7_{- 0.5}^{+ 0.6}$ $0.7^{+0.6}_{-0.5}$
`diskbb`	T_in (keV)	$0 . 60_{- 0.05}^{+ 0.15}$ $0.60^{+0.15}_{-0.05}$	$0 . 684_{- 0.057}^{+ 0.083}$ $0.684^{+0.083}_{-0.057}$	0.61 ± 0.06	$0 . 726_{- 0.08}^{+ 0.11}$ $0.726^{+0.11}_{-0.08}$
	Norm	$910_{- 590}^{+ 600}$ $910^{+600}_{-590}$	$660_{- 210}^{+ 260}$ $660^{+260}_{-210}$	$960_{- 230}^{+ 330}$ $960^{+330}_{-230}$	$570_{- 240}^{+ 320}$ $570^{+320}_{-240}$
`Comptb`	kT_s (keV)	$0 . 76_{- 0.05}^{+ 0.14}$ $0.76^{+0.14}_{-0.05}$	$0 . 978_{- 0.069}^{+ 0.096}$ $0.978^{+0.096}_{-0.069}$	$0 . 79_{- 0.08}^{+ 0.10}$ $0.79^{+0.10}_{-0.08}$	$1 . 1_{- 0.1}^{+ 0.2}$ $1.1^{+0.2}_{-0.1}$
	Γ	3 ^( * )	3 ^( * )	3 ^( * )	3 ^( * )
	α	$0 . 85_{- 0.02}^{+ 0.03}$ $0.85^{+0.03}_{-0.02}$	$0 . 877_{- 0.038}^{+ 0.044}$ $0.877^{+0.044}_{-0.038}$	$0 . 93_{- 0.03}^{+ 0.04}$ $0.93^{+0.04}_{-0.03}$	$1 . 01_{- 0.06}^{+ 0.12}$ $1.01^{+0.12}_{-0.06}$
	Δ	0 ^( * )	0 ^( * )	0 ^( * )	0 ^( * )
	kT_e (keV)	2.88 ± 0.02	$2 . 834_{- 0.033}^{+ 0.029}$ $2.834^{+0.029}_{-0.033}$	2.91 ± 0.04	$2 . 86_{- 0.04}^{+ 0.06}$ $2.86^{+0.06}_{-0.04}$
	log (A)	8 ^( * )	8 ^( * )	8 ^( * )	8 ^( * )
	Norm	0.051 ± 0.001	0.065 ± 0.002	0.052 ± 0.001	0.067 ± 0.002
`Reflection`	Index	$- 2 . 6_{- 1.2}^{+ 0.7}$ $-2.6^{+0.7}_{-1.2}$	$- 1 . 7_{- 0.5}^{+ 0.8}$ $-1.7^{+0.8}_{-0.5}$	$- 2 . 8_{- 0.7}^{+ 1.5}$ $-2.8^{+1.5}_{-0.7}$	2.3 ^( * )
	R_in (Rg)	$22_{- 7}^{+ 25}$ $22^{+25}_{-7}$	10 ± 2 ^( ⊙ )	< 35	$10_{- 2}^{+ 3}$ $10^{+3}_{-2}$ ^( ⊙ )
	R_out	1.0 ^( * )	1.0 ^( * )	1.0 ^( * )	1.0 ^( * )
	Incl (deg)	$31_{- 5}^{+ 9}$ $31^{+9}_{-5}$	$15_{- 13}^{+ 16}$ $15^{+16}_{-13}$	$34_{- 5}^{+ 7}$ $34^{+7}_{-5}$	$35_{- 4}^{+ 6}$ $35^{+6}_{-4}$
	Rel refl	$- 0 . 157_{- 0.032}^{+ 0.053}$ $-0.157^{+0.053}_{-0.032}$	$- 0 . 085_{- 0.072}^{+ 0.037}$ $-0.085^{+0.037}_{-0.072}$	−1 ^( * )	−1 ^( * )
	Fe_abund	$0 . 28_{- 0.1}^{+ 0.2}$ $0.28^{+0.2}_{-0.1}$	$0 . 5_{- 0.2}^{+ 0.4}$ $0.5^{+0.4}_{-0.2}$	< 0.77 ^( ⋆ )	< 1.26 ^( ⋆ )
	log N	–	–	< 17.6	< 16.5
	log ξ	$2 . 69_{- 0.19}^{+ 0.05}$ $2.69^{+0.05}_{-0.19}$	$2 . 8_{- 0.2}^{+ 0.1}$ $2.8^{+0.1}_{-0.2}$	$3 . 1_{- 0.3}^{+ 0.2}$ $3.1^{+0.2}_{-0.3}$	2.8 ± 0.2
	Norm (10⁻⁴)	–	–	$9_{- 2}^{+ 3}$ $9^{+3}_{-2}$	$13_{- 3}^{+ 4}$ $13^{+4}_{-3}$

	χ²/d.o.f.	762/810	317/414	763/810	326/414

Notes. Uncertainties are calculated at the 90% confidence level. ^( † )Linked to the seed-photon temperature. ^( * )Kept frozen during the fit. ^( ⊙ )Linked to the emitting radius of the diskbb component. ^( ⋆ )Upper limit at 90% confidence level obtained fixing the seed photons and blackbody temperature. ^(#)Model 3: const*TBfeo*(expabs*powlaw+rdblur*rfxconv*Comptb+Comptb+diskbb) Model 4: const*TBfeo*(expabs*powlaw+Relxillns+Comptb+diskbb).

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