A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1

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Volume 707 (March 2026)

A&A, 707 (2026) L20

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Table A.1.

Phase-resolved spectral fitting results for the pulse-off and pulse-on intervals defined from the candidate pulsation. Fits were performed using an absorbed power-law model (Base) and with the addition of a Gaussian absorption line (+ gabs). Unabsorbed fluxes are reported in the 0.3–10 keV band, with uncertainties quoted at the 1σ level.

Parameter	`Pulse-off`		`Pulse-on`

	`Base`	`+ gabs`	`Base`	`+ gabs`
N_H, int (10²² cm⁻²)	0.50 ± 0.02	0.50 ± 0.02	0.46 ± 0.03	0.46 ± 0.03
Γ	1.88 ± 0.04	1.85 ± 0.04	1.71 ± 0.06	1.70 ± 0.06
Norm (×10⁻⁴)	4.8 ± 0.2	4.8 ± 0.2	4.3 ± 0.3	4.3 ± 0.3
Flux (10⁻¹² cgs)	2.93 ± 0.08	2.95 ± 0.07	3.0 ± 0.1	3.0 ± 0.1

Absorption Line
E_line (keV)	–	3.29 ± 0.02	–	$3 . 26_{- 0.05}^{+ 0.08}$ $Mathematical equation: $ 3.26^{+0.08}_{-0.05} $$
σ (keV)	–	$0 . 04_{- 0.01}^{+ 0.02}$ $Mathematical equation: $ 0.04^{+0.02}_{-0.01} $$	–	$0 . 04_{- 0.02}^{+ 0.03}$ $Mathematical equation: $ 0.04^{+0.03}_{-0.02} $$
S_line (keV)	–	$0 . 22_{- 0.07}^{+ 0.02}$ $Mathematical equation: $ 0.22^{+0.02}_{-0.07} $$	–	$0 . 07_{- 0.05}^{+ 0.08}$ $Mathematical equation: $ 0.07^{+0.08}_{-0.05} $$

χ²/d.o.f.	114.43/84	89.01/81	47.77/62	45.02/59

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