Exploring the nature of an ultraluminous X-ray source in NGC 628

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Table 2.

Best-fit X-ray spectral parameters of X-1 in all epochs calculated with TBABS*TBABS*PO and TBABS*TBABS*DISKBB models.

Obs.	Model	N_H^(a)	Γ/kT_in (keV)	N^(b)	$χ_{ν}^{2}$ $\chi_{\nu}^{2}$ (d.o.f.)	L_X^(c)	α_ox^(d)

		(10²² cm⁻²)		(10⁻⁴)		(10³⁹ erg cm⁻²)	c1	c2
C1	PO	0.00	$2 . 02_{- 0.17}^{+ 0.18}$ $2.02_{-0.17}^{+0.18}$	$0 . 09_{- 0.01}^{+ 0.01}$ $0.09_{-0.01}^{+0.01}$	1.96 (13)	$0 . 63_{- 0.06}^{+ 0.06}$ $0.63_{-0.06}^{+0.06}$	$- 0 . 60_{- 0.02}^{+ 0.02}$ $-0.60_{-0.02}^{+0.02}$	$- 0 . 55_{- 0.02}^{+ 0.02}$ $-0.55_{-0.02}^{+0.02}$
	DISKBB	0.00	0.48	370	3.95 (13)	0.36	...	...
C2	PO	0.00	$1 . 76_{- 0.11}^{+ 0.12}$ $1.76_{-0.11}^{+0.12}$	$0 . 24_{- 0.01}^{+ 0.01}$ $0.24_{-0.01}^{+0.01}$	1.78 (38)	$1 . 90_{- 0.10}^{+ 0.10}$ $1.90_{-0.10}^{+0.10}$	$- 0 . 41_{- 0.01}^{+ 0.01}$ $-0.41_{-0.01}^{+0.01}$	$- 0 . 36_{- 0.01}^{+ 0.01}$ $-0.36_{-0.01}^{+0.01}$
	DISKBB	0.00	0.60	378	3.55 (38)	1.06	...	...
XM1	PO	0.00	$1 . 94_{- 0.14}^{+ 0.15}$ $1.94_{-0.14}^{+0.15}$	$0 . 21_{- 0.01}^{+ 0.01}$ $0.21_{-0.01}^{+0.01}$	1.22 (93)	$1 . 54_{- 0.14}^{+ 0.14}$ $1.54_{-0.14}^{+0.14}$	$- 0 . 44_{- 0.02}^{+ 0.02}$ $-0.44_{-0.02}^{+0.02}$	$- 0 . 39_{- 0.01}^{+ 0.01}$ $-0.39_{-0.01}^{+0.01}$
	DISKBB	0.00	$0 . 60_{- 0.01}^{+ 0.01}$ $0.60_{-0.01}^{+0.01}$	$75_{- 5}^{+ 5}$ $75_{-5}^{+5}$	1.73 (93)	$0 . 86_{- 0.06}^{+ 0.06}$ $0.86_{-0.06}^{+0.06}$	...	...
XM2	PO	$0 . 03_{- 0.02}^{+ 0.02}$ $0.03_{-0.02}^{+0.02}$	$2 . 06_{- 0.18}^{+ 0.22}$ $2.06_{-0.18}^{+0.22}$	$0 . 15_{- 0.02}^{+ 0.02}$ $0.15_{-0.02}^{+0.02}$	1.25 (21)	$0 . 99_{- 0.12}^{+ 0.12}$ $0.99_{-0.12}^{+0.12}$	$- 0 . 53_{- 0.03}^{+ 0.03}$ $-0.53_{-0.03}^{+0.03}$	$- 0 . 47_{- 0.02}^{+ 0.02}$ $-0.47_{-0.02}^{+0.02}$
	DISKBB	0.00	0.75	72	2.00 (21)	0.59	...	...
C5	PO	$0 . 29_{- 0.06}^{+ 0.07}$ $0.29_{-0.06}^{+0.07}$	$1 . 85_{- 0.11}^{+ 0.13}$ $1.85_{-0.11}^{+0.13}$	$0 . 73_{- 0.07}^{+ 0.07}$ $0.73_{-0.07}^{+0.07}$	0.96 (12)	$6 . 37_{- 0.06}^{+ 0.06}$ $6.37_{-0.06}^{+0.06}$	$- 0 . 21_{- 0.01}^{+ 0.01}$ $-0.21_{-0.01}^{+0.01}$	$- 0 . 16_{- 0.01}^{+ 0.01}$ $-0.16_{-0.01}^{+0.01}$
	DISKBB	$0 . 08_{- 0.06}^{+ 0.07}$ $0.08_{-0.06}^{+0.07}$	$1 . 36_{- 0.04}^{+ 0.04}$ $1.36_{-0.04}^{+0.04}$	$38_{- 4}^{+ 4}$ $38_{-4}^{+4}$	0.75 (12)	$3 . 96_{- 0.04}^{+ 0.04}$ $3.96_{-0.04}^{+0.04}$	...	...
C6	PO	$0 . 07_{- 0.02}^{+ 0.02}$ $0.07_{-0.02}^{+0.02}$	$1 . 59_{- 0.05}^{+ 0.06}$ $1.59_{-0.05}^{+0.06}$	$0 . 61_{- 0.02}^{+ 0.02}$ $0.61_{-0.02}^{+0.02}$	1.14 (55)	$5 . 23_{- 0.03}^{+ 0.03}$ $5.23_{-0.03}^{+0.03}$	$- 0 . 25_{- 0.01}^{+ 0.01}$ $-0.25_{-0.01}^{+0.01}$	$- 0 . 20_{- 0.01}^{+ 0.01}$ $-0.20_{-0.01}^{+0.01}$
	DISKBB	0.00	$1 . 43_{- 0.02}^{+ 0.02}$ $1.43_{-0.02}^{+0.02}$	$28_{- 1}^{+ 1}$ $28_{-1}^{+1}$	1.27 (55)	$3 . 68_{- 0.02}^{+ 0.02}$ $3.68_{-0.02}^{+0.02}$	...	...
C7	PO	$0 . 23_{- 0.05}^{+ 0.05}$ $0.23_{-0.05}^{+0.05}$	$1 . 92_{- 0.09}^{+ 0.10}$ $1.92_{-0.09}^{+0.10}$	$0 . 97_{- 0.07}^{+ 0.07}$ $0.97_{-0.07}^{+0.07}$	1.07 (21)	$6 . 26_{- 0.04}^{+ 0.04}$ $6.26_{-0.04}^{+0.04}$	$- 0 . 21_{- 0.01}^{+ 0.01}$ $-0.21_{-0.01}^{+0.01}$	$- 0 . 16_{- 0.01}^{+ 0.01}$ $-0.16_{-0.01}^{+0.01}$
	DISKBB	$0 . 01_{- 0.01}^{+ 0.01}$ $0.01_{-0.01}^{+0.01}$	$1 . 34_{- 0.03}^{+ 0.03}$ $1.34_{-0.03}^{+0.03}$	$57_{- 4}^{+ 4}$ $57_{-4}^{+4}$	0.99 (21)	$3 . 86_{- 0.03}^{+ 0.03}$ $3.86_{- 0.03}^{+0.03}$	...	...
C8	PO	$0 . 25_{- 0.04}^{+ 0.05}$ $0.25_{-0.04}^{+0.05}$	$1 . 99_{- 0.09}^{+ 0.10}$ $1.99_{-0.09}^{+0.10}$	$0 . 95_{- 0.07}^{+ 0.07}$ $0.95_{-0.07}^{+0.07}$	1.18 (22)	$6 . 49_{- 0.03}^{+ 0.03}$ $6.49_{-0.03}^{+0.03}$	$- 0 . 21_{- 0.01}^{+ 0.01}$ $-0.21_{-0.01}^{+0.01}$	$- 0 . 16_{- 0.01}^{+ 0.01}$ $-0.16_{-0.01}^{+0.01}$
	DISKBB	$0 . 01_{- 0.01}^{+ 0.01}$ $0.01_{-0.01}^{+0.01}$	$1 . 35_{- 0.03}^{+ 0.03}$ $1.35_{-0.03}^{+0.03}$	$44_{- 3}^{+ 3}$ $44_{-3}^{+3}$	1.38 (22)	$3 . 89_{- 0.03}^{+ 0.03}$ $3.89_{-0.03}^{+0.03}$	...	...
C9	PO	$0 . 03_{- 0.03}^{+ 0.07}$ $0.03_{-0.03}^{+0.07}$	$1 . 52_{- 0.11}^{+ 0.13}$ $1.52_{-0.11}^{+0.13}$	$0 . 56_{- 0.06}^{+ 0.06}$ $0.56_{-0.06}^{+0.06}$	1.55 (11)	$5 . 05_{- 0.05}^{+ 0.05}$ $5.05_{-0.05}^{+0.05}$	$- 0 . 26_{- 0.01}^{+ 0.01}$ $-0.26_{-0.01}^{+0.01}$	$- 0 . 21_{- 0.01}^{+ 0.01}$ $-0.21_{-0.01}^{+0.01}$
	DISKBB	0.00	$1 . 43_{- 0.04}^{+ 0.04}$ $1.43_{-0.04}^{+0.04}$	$32_{- 3}^{+ 3}$ $32_{-3}^{+3}$	1.84 (11)	$3 . 58_{- 0.05}^{+ 0.04}$ $3.58_{-0.05}^{+0.04}$	...	...
C10	PO	$0 . 21_{- 0.03}^{+ 0.03}$ $0.21_{-0.03}^{+0.03}$	$1 . 85_{- 0.06}^{+ 0.06}$ $1.85_{-0.06}^{+0.06}$	$0 . 73_{- 0.04}^{+ 0.04}$ $0.73_{-0.04}^{+0.04}$	1.03 (52)	$5 . 57_{- 0.02}^{+ 0.02}$ $5.57_{-0.02}^{+0.02}$	$- 0 . 24_{- 0.00}^{+ 0.00}$ $-0.24_{-0.00}^{+0.00}$	$- 0 . 19_{- 0.00}^{+ 0.00}$ $-0.19_{-0.00}^{+ 0.00}$
	DISKBB	0.00	$1 . 49_{- 0.02}^{+ 0.02}$ $1.49_{-0.02}^{+0.02}$	$30_{- 2}^{+ 2}$ $30_{-2}^{+2}$	0.77 (52)	$3 . 71_{- 0.02}^{+ 0.02}$ $3.71_{-0.02}^{+0.02}$	...	...
C11	PO	$0 . 05_{- 0.02}^{+ 0.02}$ $0.05_{-0.02}^{+0.02}$	$1 . 82_{- 0.07}^{+ 0.08}$ $1.82_{-0.07}^{+0.08}$	$0 . 44_{- 0.02}^{+ 0.02}$ $0.44_{-0.02}^{+0.02}$	0.83 (35)	$2 . 99_{- 0.02}^{+ 0.02}$ $2.99_{-0.02}^{+0.02}$	$- 0 . 34_{- 0.01}^{+ 0.01}$ $-0.34_{-0.01}^{+0.01}$	$- 0 . 29_{- 0.01}^{+ 0.01}$ $-0.29_{-0.01}^{+0.01}$
	DISKBB	0.00	1.6	19	2.94 (35)	2.35	...	...
C12	PO	$0 . 03_{- 0.03}^{+ 0.16}$ $0.03_{-0.03}^{+0.16}$	$1 . 71_{- 0.12}^{+ 0.15}$ $1.71_{-0.12}^{+0.15}$	$0 . 60_{- 0.07}^{+ 0.07}$ $0.60_{-0.07}^{+0.07}$	0.44 (6)	$4 . 20_{- 0.05}^{+ 0.05}$ $4.20_{-0.05}^{+0.05}$	$- 0 . 26_{- 0.03}^{+ 0.03}$ $-0.26_{-0.03}^{+0.03}$	$- 0 . 21_{- 0.02}^{+ 0.02}$ $-0.21_{-0.02}^{+0.02}$
	DISKBB	0.00	$1 . 30_{- 0.05}^{+ 0.04}$ $1.30_{-0.05}^{+0.04}$	$30_{- 4}^{+ 4}$ $30_{-4}^{+4}$	1.21 (6)	$2 . 85_{- 0.06}^{+ 0.05}$ $2.85_{-0.06}^{+0.05}$	...	...
XM3	PO	$0 . 12_{- 0.01}^{+ 0.01}$ $0.12_{-0.01}^{+0.01}$	$2 . 14_{- 0.04}^{+ 0.04}$ $2.14_{-0.04}^{+0.04}$	$0 . 06_{- 0.01}^{+ 0.01}$ $0.06_{-0.01}^{+0.01}$	0.92 (486)	$3 . 24_{- 0.01}^{+ 0.01}$ $3.24_{-0.01}^{+0.01}$	$- 0 . 34_{- 0.00}^{+ 0.00}$ $-0.34_{-0.00}^{+0.00}$	$- 0 . 29_{- 0.00}^{+ 0.00}$ $-0.29_{-0.00}^{+0.00}$
	DISKBB	0.00	$1 . 04_{- 0.01}^{+ 0.01}$ $1.04_{-0.01}^{+0.01}$	$79_{- 2}^{+ 2}$ $79_{-2}^{+2}$	1.37 (486)	$2 . 07_{- 0.01}^{+ 0.01}$ $2.07_{-0.01}^{+0.01}$	...	...

Notes. The error values of the individual parameters were estimated at the 90% confidence level.

^(a)

Intrinsic absorption values towards the source.

^(b)

Normalisation parameters of the PO model in units of photons per kiloelectron volt per square centimetre per second at 1 keV and normalisation parameters of the DISKBB model, N_Disk = [(r_in km⁻¹)/(D/10 kpc)]² × cos i.

^(c)

The luminosity values were calculated in the 0.3−10 keV energy range.

^(d)

Optical spectral index parameters in terms of the X-ray–UV correlation.

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