Identification of three new ultraluminous X-ray sources in NGC 4631 and NGC 1097: Evidence for stellar-mass black holes

Open Access

Table 1.

X-ray spectral parameters of NGC 4631 X-6 in Chandra and XMM–Newton observations.

Data label	Net Rate	N_H	Γ/T_in	Γ^/T $_{in}^{}$ $Mathematical equation: $ _{in}^{*} $$	Cstat/dof	F_x	L_x	$F_{x}^{}$ $Mathematical equation: $ F_{x}^{} $$	$L_{x}^{}$ $Mathematical equation: $ L_{x}^{} $$
	(10⁻³ counts/s)	(10²² cm⁻²)	(/keV)	(/keV)		(10⁻¹³ erg cm⁻² s⁻¹)	(10³⁹ erg s⁻¹)	(10⁻¹³ erg cm⁻² s⁻¹)	(10³⁹ erg s⁻¹)
C1	< 0.08
tbabstbabspower-law

C2	17.64 ± 0.78	$1 . 76_{- 0.14}^{+ 0.16}$ $Mathematical equation: $ 1.76^{+0.16}_{-0.14} $$	$2 . 69_{- 0.07}^{+ 0.08}$ $Mathematical equation: $ 2.69^{+0.08}_{-0.07} $$	$2 . 07_{- 0.07}^{+ 0.07}$ $Mathematical equation: $ 2.07^{+0.07}_{-0.07} $$	21.56/28^b	$3 . 07_{- 0.29}^{+ 0.30}$ $Mathematical equation: $ 3.07^{+0.30}_{-0.29} $$	$7 . 78_{- 0.58}^{+ 0.58}$ $Mathematical equation: $ 7.78^{+0.58}_{-0.58} $$	$3 . 26_{- 0.29}^{+ 0.29}$ $Mathematical equation: $ 3.26^{+0.29}_{-0.29} $$	$4 . 12_{- 0.31}^{+ 0.31}$ $Mathematical equation: $ 4.12^{+0.31}_{-0.31} $$
C3	7.93 ± 0.59	$0 . 92_{- 0.19}^{+ 0.21}$ $Mathematical equation: $ 0.92^{+0.21}_{-0.19} $$	$2 . 32_{- 0.12}^{+ 0.13}$ $Mathematical equation: $ 2.32^{+0.13}_{-0.12} $$	$2 . 31_{- 0.12}^{+ 0.13}$ $Mathematical equation: $ 2.31^{+0.13}_{-0.12} $$	27.48/29	$1 . 48_{- 0.23}^{+ 0.29}$ $Mathematical equation: $ 1.48^{+0.29}_{-0.23} $$	$2 . 16_{- 0.25}^{+ 0.27}$ $Mathematical equation: $ 2.16^{+0.27}_{-0.25} $$	$1 . 50_{- 0.21}^{+ 0.24}$ $Mathematical equation: $ 1.50^{+0.24}_{-0.21} $$	$2 . 12_{- 0.25}^{+ 0.27}$ $Mathematical equation: $ 2.12^{+0.27}_{-0.25} $$
C4	9.75 ± 0.73	$2 . 06_{- 0.21}^{+ 0.23}$ $Mathematical equation: $ 2.06^{+0.23}_{-0.21} $$	$3 . 38_{- 0.13}^{+ 0.14}$ $Mathematical equation: $ 3.38^{+0.14}_{-0.13} $$	$2 . 33_{- 0.13}^{+ 0.14}$ $Mathematical equation: $ 2.33^{+0.14}_{-0.13} $$	21.99/30	$1 . 65_{- 0.25}^{+ 0.31}$ $Mathematical equation: $ 1.65^{+0.31}_{-0.25} $$	$8 . 89_{- 1.04}^{+ 1.13}$ $Mathematical equation: $ 8.89^{+1.13}_{-1.04} $$	$1 . 88_{- 0.27}^{+ 0.32}$ $Mathematical equation: $ 1.88^{+0.32}_{-0.27} $$	$4 . 75_{- 0.56}^{+ 0.61}$ $Mathematical equation: $ 4.75^{+0.61}_{-0.56} $$
C_combine1	9.06 ± 0.47	$1 . 41_{- 0.14}^{+ 0.15}$ $Mathematical equation: $ 1.41^{+0.15}_{-0.14} $$	$2 . 78_{- 0.10}^{+ 0.09}$ $Mathematical equation: $ 2.78^{+0.09}_{-0.10} $$	$2 . 37_{- 0.16}^{+ 0.17}$ $Mathematical equation: $ 2.37^{+0.17}_{-0.16} $$	58.47/60	$1 . 55_{- 0.17}^{+ 0.19}$ $Mathematical equation: $ 1.55^{+0.19}_{-0.17} $$	$3 . 76_{- 0.32}^{+ 0.33}$ $Mathematical equation: $ 3.76^{+0.33}_{-0.32} $$	$1 . 64_{- 0.16}^{+ 0.18}$ $Mathematical equation: $ 1.64^{+0.18}_{-0.16} $$	$2 . 07_{- 0.19}^{+ 0.19}$ $Mathematical equation: $ 2.07^{+0.19}_{-0.19} $$
C5	8.25 ± 0.64	$1 . 35_{- 0.26}^{+ 0.28}$ $Mathematical equation: $ 1.35^{+0.28}_{-0.26} $$	$1 . 93_{- 0.12}^{+ 0.12}$ $Mathematical equation: $ 1.93^{+0.12}_{-0.12} $$	$1 . 59_{- 0.12}^{+ 0.12}$ $Mathematical equation: $ 1.59^{+0.12}_{-0.12} $$	35.92/29	$1 . 89_{- 0.33}^{+ 0.44}$ $Mathematical equation: $ 1.89^{+0.44}_{-0.33} $$	$2 . 53_{- 0.31}^{+ 0.33}$ $Mathematical equation: $ 2.53^{+0.33}_{-0.31} $$	$2 . 00_{- 0.31}^{+ 0.37}$ $Mathematical equation: $ 2.00^{+0.37}_{-0.31} $$	$2 . 07_{- 0.26}^{+ 0.28}$ $Mathematical equation: $ 2.07^{+0.28}_{-0.26} $$
C6	9.63 ± 0.81	$0 . 60_{- 0.25}^{+ 0.28}$ $Mathematical equation: $ 0.60^{+0.28}_{-0.25} $$	$1 . 83_{- 0.14}^{+ 0.15}$ $Mathematical equation: $ 1.83^{+0.15}_{-0.14} $$	$2 . 06_{- 0.14}^{+ 0.15}$ $Mathematical equation: $ 2.06^{+0.15}_{-0.14} $$	32.27/22	$2 . 29_{- 0.47}^{+ 0.87}$ $Mathematical equation: $ 2.29^{+0.87}_{-0.47} $$	$2 . 29_{- 0.30}^{+ 0.33}$ $Mathematical equation: $ 2.29^{+0.33}_{-0.30} $$	$2 . 18_{- 0.35}^{+ 0.43}$ $Mathematical equation: $ 2.18^{+0.43}_{-0.35} $$	$2 . 68_{- 0.35}^{+ 0.39}$ $Mathematical equation: $ 2.68^{+0.39}_{-0.35} $$
C7	9.69 ± 0.81	$2 . 10_{- 0.29}^{+ 0.32}$ $Mathematical equation: $ 2.10^{+0.32}_{-0.29} $$	$2 . 71_{- 0.13}^{+ 0.14}$ $Mathematical equation: $ 2.71^{+0.14}_{-0.13} $$	$1 . 83_{- 0.13}^{+ 0.14}$ $Mathematical equation: $ 1.83^{+0.14}_{-0.13} $$	20.52/23	$1 . 68_{- 0.30}^{+ 0.32}$ $Mathematical equation: $ 1.68^{+0.32}_{-0.30} $$	$5 . 26_{- 0.69}^{+ 1.06}$ $Mathematical equation: $ 5.26^{+1.06}_{-0.69} $$	$2 . 14_{- 0.34}^{+ 0.41}$ $Mathematical equation: $ 2.14^{+0.41}_{-0.34} $$	$2 . 36_{- 0.31}^{+ 0.34}$ $Mathematical equation: $ 2.36^{+0.34}_{-0.31} $$
C8	4.59 ± 0.39	$1 . 09_{- 0.21}^{+ 0.23}$ $Mathematical equation: $ 1.09^{+0.23}_{-0.21} $$	$2 . 65_{- 0.15}^{+ 0.16}$ $Mathematical equation: $ 2.65^{+0.16}_{-0.15} $$	$2 . 47_{- 0.15}^{+ 0.16}$ $Mathematical equation: $ 2.47^{+0.16}_{-0.15} $$	32.46/23	$0 . 73_{- 0.14}^{+ 0.20}$ $Mathematical equation: $ 0.73^{+0.20}_{-0.14} $$	$1 . 44_{- 0.20}^{+ 0.21}$ $Mathematical equation: $ 1.44^{+0.21}_{-0.20} $$	$0 . 77_{- 0.12}^{+ 0.15}$ $Mathematical equation: $ 0.77^{+0.15}_{-0.12} $$	$1 . 21_{- 0.16}^{+ 0.18}$ $Mathematical equation: $ 1.21^{+0.18}_{-0.16} $$
C_combine2	2.49 ± 0.20	$1 . 67_{- 0.18}^{+ 0.20}$ $Mathematical equation: $ 1.67^{+0.20}_{-0.18} $$	$3 . 65_{- 0.16}^{+ 0.17}$ $Mathematical equation: $ 3.65^{+0.17}_{-0.16} $$	$2 . 80_{- 0.16}^{+ 0.17}$ $Mathematical equation: $ 2.80^{+0.17}_{-0.16} $$	22.76/26	$0 . 39_{- 0.06}^{+ 0.08}$ $Mathematical equation: $ 0.39^{+0.08}_{-0.06} $$	$2 . 47_{- 0.31}^{+ 0.34}$ $Mathematical equation: $ 2.47^{+0.34}_{-0.31} $$	$0 . 44_{- 0.06}^{+ 0.08}$ $Mathematical equation: $ 0.44^{+0.08}_{-0.06} $$	$0 . 89_{- 0.11}^{+ 0.12}$ $Mathematical equation: $ 0.89^{+0.12}_{-0.11} $$

tbabstbabsdiskbb

C2		$0 . 84_{- 0.14}^{+ 0.16}$ $Mathematical equation: $ 0.84^{+0.16}_{-0.14} $$	$1 . 21_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ 1.21^{+0.02}_{-0.02} $$	$1 . 39_{- 0.03}^{+ 0.03}$ $Mathematical equation: $ 1.39^{+0.03}_{-0.03} $$	17.62/28^b	$2 . 94_{- 0.28}^{+ 0.29}$ $Mathematical equation: $ 2.94^{+0.29}_{-0.28} $$	$3 . 07_{- 0.23}^{+ 0.23}$ $Mathematical equation: $ 3.07^{+0.23}_{-0.23} $$	$3 . 05_{- 0.27}^{+ 0.28}$ $Mathematical equation: $ 3.05^{+0.28}_{-0.27} $$	$2 . 70_{- 0.20}^{+ 0.20}$ $Mathematical equation: $ 2.70^{+0.20}_{-0.20} $$
C3		$0 . 25_{- 0.19}^{+ 0.21}$ $Mathematical equation: $ 0.25^{+0.21}_{-0.19} $$	$1 . 39_{- 0.03}^{+ 0.03}$ $Mathematical equation: $ 1.39^{+0.03}_{-0.03} $$	$1 . 24_{- 0.04}^{+ 0.04}$ $Mathematical equation: $ 1.24^{+0.04}_{-0.04} $$	27.22/29	$1 . 47_{- 0.23}^{+ 0.33}$ $Mathematical equation: $ 1.47^{+0.33}_{-0.23} $$	$1 . 14_{- 0.14}^{+ 0.14}$ $Mathematical equation: $ 1.14^{+0.14}_{-0.14} $$	$1 . 40_{- 0.19}^{+ 0.23}$ $Mathematical equation: $ 1.40^{+0.23}_{-0.19} $$	$1 . 25_{- 0.15}^{+ 0.16}$ $Mathematical equation: $ 1.25^{+0.16}_{-0.15} $$
C4		$1 . 01_{- 0.21}^{+ 0.22}$ $Mathematical equation: $ 1.01^{+0.22}_{-0.21} $$	$0 . 89_{- 0.03}^{+ 0.03}$ $Mathematical equation: $ 0.89^{+0.03}_{-0.03} $$	$1 . 09_{- 0.03}^{+ 0.03}$ $Mathematical equation: $ 1.09^{+0.03}_{-0.03} $$	23.44/30	$1 . 55_{- 0.22}^{+ 0.27}$ $Mathematical equation: $ 1.55^{+0.27}_{-0.22} $$	$1 . 93_{- 0.23}^{+ 0.25}$ $Mathematical equation: $ 1.93^{+0.25}_{-0.23} $$	$1 . 65_{- 0.23}^{+ 0.27}$ $Mathematical equation: $ 1.65^{+0.27}_{-0.23} $$	$1 . 52_{- 0.18}^{+ 0.19}$ $Mathematical equation: $ 1.52^{+0.19}_{-0.18} $$
C_combine1		$0 . 57_{- 0.14}^{+ 0.14}$ $Mathematical equation: $ 0.57^{+0.14}_{-0.14} $$	$1 . 13_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ 1.13^{+0.02}_{-0.02} $$	$1 . 17_{- 0.03}^{+ 0.03}$ $Mathematical equation: $ 1.17^{+0.03}_{-0.03} $$	59/60	$1 . 49_{- 0.16}^{+ 0.18}$ $Mathematical equation: $ 1.49^{+0.18}_{-0.16} $$	$1 . 41_{- 0.12}^{+ 0.13}$ $Mathematical equation: $ 1.41^{+0.13}_{-0.12} $$	$1 . 51_{- 0.15}^{+ 0.16}$ $Mathematical equation: $ 1.51^{+0.16}_{-0.15} $$	$1 . 37_{- 0.11}^{+ 0.12}$ $Mathematical equation: $ 1.37^{+0.12}_{-0.11} $$
C5		$0 . 77_{- 0.25}^{+ 0.28}$ $Mathematical equation: $ 0.77^{+0.28}_{-0.25} $$	$1 . 69_{- 0.06}^{+ 0.06}$ $Mathematical equation: $ 1.69^{+0.06}_{-0.06} $$	$2 . 01_{- 0.08}^{+ 0.08}$ $Mathematical equation: $ 2.01^{+0.08}_{-0.08} $$	36.04/29	$1 . 75_{- 0.31}^{+ 0.46}$ $Mathematical equation: $ 1.75^{+0.46}_{-0.31} $$	$1 . 64_{- 0.20}^{+ 0.22}$ $Mathematical equation: $ 1.64^{+0.22}_{-0.20} $$	$1 . 83_{- 0.30}^{+ 0.40}$ $Mathematical equation: $ 1.83^{+0.40}_{-0.30} $$	$1 . 56_{- 0.18}^{+ 0.20}$ $Mathematical equation: $ 1.56^{+0.20}_{-0.18} $$
C6		$0 . 11_{- 0.11}^{+ 0.28}$ $Mathematical equation: $ 0.11^{+0.28}_{-0.11} $$	$1 . 70_{- 0.07}^{+ 0.07}$ $Mathematical equation: $ 1.70^{+0.07}_{-0.07} $$	$1 . 39_{- 0.05}^{+ 0.05}$ $Mathematical equation: $ 1.39^{+0.05}_{-0.05} $$	31.76/22	$2 . 24_{- 0.52}^{+ 0.57}$ $Mathematical equation: $ 2.24^{+0.57}_{-0.52} $$	$1 . 57_{- 0.21}^{+ 0.23}$ $Mathematical equation: $ 1.57^{+0.23}_{-0.21} $$	$1 . 98_{- 0.32}^{+ 0.42}$ $Mathematical equation: $ 1.98^{+0.42}_{-0.32} $$	$1 . 72_{- 0.23}^{+ 0.25}$ $Mathematical equation: $ 1.72^{+0.25}_{-0.23} $$
C7		$1 . 22_{- 0.29}^{+ 0.31}$ $Mathematical equation: $ 1.22^{+0.31}_{-0.29} $$	$1 . 16_{- 0.04}^{+ 0.04}$ $Mathematical equation: $ 1.16^{+0.04}_{-0.04} $$	$1 . 27_{- 0.05}^{+ 0.05}$ $Mathematical equation: $ 1.27^{+0.05}_{-0.05} $$	17.82/23	$1 . 76_{- 0.23}^{+ 0.35}$ $Mathematical equation: $ 1.76^{+0.35}_{-0.23} $$	$2 . 05_{- 0.27}^{+ 0.29}$ $Mathematical equation: $ 2.05^{+0.29}_{-0.27} $$	$1 . 76_{- 0.28}^{+ 0.35}$ $Mathematical equation: $ 1.76^{+0.35}_{-0.28} $$	$1 . 63_{- 0.22}^{+ 0.24}$ $Mathematical equation: $ 1.63^{+0.24}_{-0.22} $$
C8		$0 . 45_{- 0.21}^{+ 0.23}$ $Mathematical equation: $ 0.45^{+0.23}_{-0.21} $$	$1 . 04_{- 0.04}^{+ 0.04}$ $Mathematical equation: $ 1.04^{+0.04}_{-0.04} $$	$1 . 02_{- 0.04}^{+ 0.04}$ $Mathematical equation: $ 1.02^{+0.04}_{-0.04} $$	30.64/23	$0 . 67_{- 0.12}^{+ 0.19}$ $Mathematical equation: $ 0.67^{+0.19}_{-0.12} $$	$0 . 63_{- 0.08}^{+ 0.09}$ $Mathematical equation: $ 0.63^{+0.09}_{-0.08} $$	$0 . 67_{- 0.10}^{+ 0.13}$ $Mathematical equation: $ 0.67^{+0.13}_{-0.10} $$	$0 . 64_{- 0.08}^{+ 0.09}$ $Mathematical equation: $ 0.64^{+0.09}_{-0.08} $$
C_combine2		$0 . 81_{- 0.18}^{+ 0.20}$ $Mathematical equation: $ 0.81^{+0.20}_{-0.18} $$	$0 . 73_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ 0.73^{+0.02}_{-0.02} $$	$0 . 83_{- 0.02}^{+ 0.02}$ $Mathematical equation: $ 0.83^{+0.02}_{-0.02} $$	20.85/26	$0 . 36_{- 0.06}^{+ 0.05}$ $Mathematical equation: $ 0.36^{+0.05}_{-0.06} $$	$0 . 50_{- 0.06}^{+ 0.07}$ $Mathematical equation: $ 0.50^{+0.07}_{-0.06} $$	$0 . 38_{- 0.05}^{+ 0.06}$ $Mathematical equation: $ 0.38^{+0.06}_{-0.05} $$	$0 . 40_{- 0.05}^{+ 0.05}$ $Mathematical equation: $ 0.40^{+0.05}_{-0.05} $$

XM1	< 2.74
XM2	21.57 ± 0.93	0.71 $_{- 0.22}^{+ 0.17}$ $Mathematical equation: $ ^{+0.17}_{-0.22} $$	1.40 $_{- 0.24}^{+ 0.20}$ $Mathematical equation: $ ^{+0.20}_{-0.24} $$		34.53/33	3.80 $_{- 0.16}^{+ 0.14}$ $Mathematical equation: $ ^{+0.14}_{-0.16} $$	$2 . 30_{- 0.10}^{+ 0.08}$ $Mathematical equation: $ 2.30^{+0.08}_{-0.10} $$	–	–

XM3	< 2.11

Notes. The N_H of one tbabs component was fixed to the line-of-sight value of 0.013 × 10²² cm⁻². The intrinsic N_H (second tbabs component) was left to free to vary; for the entries marked with (*), the intrinsic N_H was fixed at average values of 0.90 × 10²² cm⁻² and 0.49 × 10²² cm⁻² for the power-law and diskbb models, respectively. C_combine1 and C_combine2 correspond to combined observations C3–C4 and C9–C12, respectively. The count rate values for C9–C12 are (1.68 ± 0.38) × 10⁻³, (2.07 ± 0.32) × 10⁻³, (2.42 ± 0.35) × 10⁻³, and (2.57 ± 0.44)×10⁻³, respectively. The superscript b indicates that the corresponding spectral fits were performed using the χ²/d.o.f. statistic. Absorbed fluxes were calculated in the 0.5–8 keV energy range, while unabsorbed luminosities were derived assuming a distance of 7.6 Mpc. Quoted uncertainties correspond to the 90% confidence level for each parameter.

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