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 1.

Best-fitting parameters for the diskbb+powerlaw and ThComp(diskbb) models of the joint pn+MOS2+FPMA/B spectra. S_line is the line strength parameter of the gabs component (in kilo-electronvolts).

Parameter	`Power-law + diskbb`		`ThComp(diskbb)`

	`Base`	`+ gabs`	`Base`	`+ gabs`
N_H, int (10²² cm⁻²)	0.72 ± 0.07	$0 . 79_{- 0.07}^{+ 0.06}$ $Mathematical equation: $ 0.79^{+0.06}_{-0.07} $$	$0 . 68_{- 0.09}^{+ 0.08}$ $Mathematical equation: $ 0.68^{+0.08}_{-0.09} $$	0.76 ± 0.08
Γ/Γ_asy	1.90 ± 0.02	1.93 ± 0.03	1.91 ± 0.02	1.92 ± 0.03
kT_e (keV)	–	–	80 ± 50	$80_{- 50}^{+ 50}$ $Mathematical equation: $ 80^{+50}_{-50} $$
cov_frac	–	–	$0 . 4_{- 0.1}^{+ 0.2}$ $Mathematical equation: $ 0.4^{+0.2}_{-0.1} $$	$0 . 23_{- 0.08}^{+ 0.10}$ $Mathematical equation: $ 0.23^{+0.10}_{-0.08} $$
kT_seed (keV)	0.15 ± 0.01	0.14 ± 0.01	0.15 ± 0.01	0.14 ± 0.01
Flux (10⁻¹² cgs)	3.48 ± 0.05		$4 . 83_{- 0.68}^{+ 0.96}$ $Mathematical equation: $ 4.83^{+0.96}_{-0.68} $$	$5 . 63_{- 0.89}^{+ 1.23}$ $Mathematical equation: $ 5.63^{+1.23}_{-0.89} $$

Absorption line
E_line (keV)	–	3.29 ± 0.02	–	3.29 ± 0.02
σ (keV)	–	$0 . 04_{- 0.02}^{+ 0.03}$ $Mathematical equation: $ 0.04^{+0.03}_{-0.02} $$	–	$0 . 04_{- 0.02}^{+ 0.03}$ $Mathematical equation: $ 0.04^{+0.03}_{-0.02} $$
S_line (keV)	–	$0 . 09_{- 0.02}^{+ 0.05}$ $Mathematical equation: $ 0.09^{+0.05}_{-0.02} $$	–	$0 . 09_{- 0.03}^{+ 0.07}$ $Mathematical equation: $ 0.09^{+0.07}_{-0.03} $$
Line Sig. (σ)	–	3.04 ± 0.09	–	2.53 ± 0.06

χ²/d.o.f.	608.20/582	587.74/579	605.83/581	586.10/578

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