X-ray, optical, and radio follow-up of five thermally emitting isolated neutron star candidates

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Volume 705 (January 2026)

A&A, 705 (2026) A148

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Table 3.

X-ray spectral analysis results.

J0723
Instruments	Model	N_H	N_H, gal^(a)	Dist.^(b)	kT	R^(c)	Γ^(d)	ϵ^(e)	σ	EW^(f)	χ_ν²(ν)	Absorbed flux^(g)
		[10²⁰ cm⁻²]	[10²⁰ cm⁻²]	[kpc]	[eV]	[km]		[eV]	[eV]	[eV]		[10⁻¹³ erg s⁻¹ cm⁻²]

EPIC-pn	`BB`	$4 . 9_{- 0.6}^{+ 0.7}$ $4.9^{+0.7}_{-0.6}$	64.7	< 1.4	$71 . 1_{- 1.6}^{+ 1.6}$ $71.1^{+1.6}_{-1.6}$	$4 . 5_{- 0.5}^{+ 0.6}$ $4.5^{+0.6}_{-0.5}$					1.02(16)	$1 . 292_{- 0.023}^{+ 0.023}$ $1.292^{+0.023}_{-0.023}$
EPIC-pn	`PL`	$16 . 3_{- 1.0}^{+ 1.1}$ $16.3^{+1.1}_{-1.0}$	64.7	< 2.1			$7 . 75_{- 0.19}^{+ 0.2}$ $7.75^{+0.2}_{-0.19}$				4.20(16)	$1 . 220_{- 0.022}^{+ 0.022}$ $1.220^{+0.022}_{-0.022}$
All	`BB`	$4 . 6_{- 0.5}^{+ 0.6}$ $4.6^{+0.6}_{-0.5}$	64.7	< 1.4	$71 . 6_{- 1.4}^{+ 1.4}$ $71.6^{+1.4}_{-1.4}$	$4 . 3_{- 0.4}^{+ 0.5}$ $4.3^{+0.5}_{-0.4}$					1.04(41)	$1 . 320_{- 0.020}^{+ 0.020}$ $1.320^{+0.020}_{-0.020}$

J0819

Instruments	Model	N_H	N_H, gal^(a)	Dist.^(b)	kT	R^(c)	Γ^(d)	ϵ^(e)	σ	EW^(f)	χ_ν²(ν)	Absorbed flux^(g)
		[10²⁰ cm⁻²]	[10²⁰ cm⁻²]	[kpc]	[eV]	[km]		[eV]	[eV]	[eV]		[10⁻¹³ erg s⁻¹ cm⁻²]

EPIC-pn	`BB`	$0 . 6_{- 0.4}^{+ 0.4}$ $0.6^{+0.4}_{-0.4}$	6.7	< 0.3	$47 . 6_{- 1.3}^{+ 1.3}$ $47.6^{+1.3}_{-1.3}$	$15 . 8_{- 1.9}^{+ 2.9}$ $15.8^{+2.9}_{-1.9}$					0.92(20)	$3 . 97_{- 0.06}^{+ 0.06}$ $3.97^{+0.06}_{-0.06}$
EPIC-pn	`PL`	$7 . 2_{- 0.6}^{+ 0.6}$ $7.2^{+0.6}_{-0.6}$	6.7				$9 . 21_{- 0.25}^{+ 0.27}$ $9.21^{+0.27}_{-0.25}$				1.10(20)	$3 . 58_{- 0.05}^{+ 0.05}$ $3.58^{+0.05}_{-0.05}$
All	`BB`	$0 . 54_{- 0.29}^{+ 0.4}$ $0.54^{+0.4}_{-0.29}$	6.7	< 0.3	$48 . 0_{- 1.1}^{+ 1.1}$ $48.0^{+1.1}_{-1.1}$	$15 . 2_{- 1.7}^{+ 2.4}$ $15.2^{+2.4}_{-1.7}$					0.97(45)	$3 . 88_{- 0.05}^{+ 0.05}$ $3.88^{+0.05}_{-0.05}$

J0840

Instruments	Model	N_H	N_H, gal^(a)	Dist.^(b)	kT	R^(c)	Γ^(d)	ϵ^(e)	σ	EW^(f)	χ_ν²(ν)	Absorbed flux^(g)
		[10²⁰ cm⁻²]	[10²⁰ cm⁻²]	[kpc]	[eV]	[km]		[eV]	[eV]	[eV]		[10⁻¹³ erg s⁻¹ cm⁻²]

EPIC-pn	`BB`	$2 . 0_{- 0.4}^{+ 0.5}$ $2.0^{+0.5}_{-0.4}$	5.71	< 1.3	$67 . 2_{- 1.4}^{+ 1.4}$ $67.2^{+1.4}_{-1.4}$	$5 . 4_{- 0.5}^{+ 0.6}$ $5.4^{+0.6}_{-0.5}$					1.32(22)	$2 . 27_{- 0.04}^{+ 0.04}$ $2.27^{+0.04}_{-0.04}$
EPIC-pn	`PL`	$10 . 5_{- 0.6}^{+ 0.7}$ $10.5^{+0.7}_{-0.6}$	5.71				$7 . 44_{- 0.17}^{+ 0.18}$ $7.44^{+0.18}_{-0.17}$				3.96(22)	$2 . 1_{- 0.04}^{+ 0.04}$ $2.1^{+0.04}_{-0.04}$
EPIC-pn	`BBGABS`	$2 . 4_{- 1.2}^{+ 0.6}$ $2.4^{+0.6}_{-1.2}$	5.71	< 1.3^★	$63 . 5_{- 2.3}^{+ 2.2}$ $63.5^{+2.2}_{-2.3}$	$7 . 1_{- 1.0}^{+ 1.6}$ $7.1^{+1.6}_{-1.0}$		$359_{- 50}^{+ 17}$ $359^{+17}_{-50}$	$29_{- 28}^{+ 50}$ $29^{+50}_{-28}$	$30_{- 22}^{+ 60}$ $30^{+60}_{-22}$	0.89(19)	$2 . 32_{- 0.04}^{+ 0.04}$ $2.32^{+0.04}_{-0.04}$
All	`BBGABS`	< 1.1	5.71	< 1.2	$67 . 0_{- 1.8}^{+ 1.6}$ $67.0^{+1.6}_{-1.8}$	$5 . 2_{- 0.5}^{+ 0.9}$ $5.2^{+0.9}_{-0.5}$		$282_{- 22}^{+ 29}$ $282^{+29}_{-22}$	$67_{- 22}^{+ 14}$ $67^{+14}_{-22}$	$90_{- 50}^{+ 40}$ $90^{+40}_{-50}$	1.15(49)	$2 . 50_{- 0.04}^{+ 0.04}$ $2.50^{+0.04}_{-0.04}$

J1347

Instruments	Model	N_H	N_H, gal^(a)	Dist.^(b)	kT	R^(c)	Γ^(d)	ϵ^(e)	σ	EW^(f)	χ_ν²(ν)	Absorbed flux^(g)
		[10²⁰ cm⁻²]	[10²⁰ cm⁻²]	[kpc]	[eV]	[km]		[eV]	[eV]	[eV]		[10⁻¹³ erg s⁻¹ cm⁻²]

EPIC-pn	`BB`	$2 . 6_{- 0.8}^{+ 0.9}$ $2.6^{+0.9}_{-0.8}$	4.32	< 1.3^★	$80 . 4_{- 2.3}^{+ 2.3}$ $80.4^{+2.3}_{-2.3}$	$2 . 51_{- 0.28}^{+ 0.4}$ $2.51^{+0.4}_{-0.28}$					1.21(18)	$1 . 192_{- 0.026}^{+ 0.026}$ $1.192^{+0.026}_{-0.026}$
EPIC-pn	`PL`	$14 . 6_{- 1.4}^{+ 1.5}$ $14.6^{+1.5}_{-1.4}$	4.32				$7 . 05_{- 0.23}^{+ 0.25}$ $7.05^{+0.25}_{-0.23}$				3.83(18)	$1 . 051_{- 0.024}^{+ 0.024}$ $1.051^{+0.024}_{-0.024}$
EPIC-pn	`BBGABS`	$2 . 2_{- 0.8}^{+ 0.9}$ $2.2^{+0.9}_{-0.8}$	4.32	< 1.7	$78 . 0_{- 2.5}^{+ 2.5}$ $78.0^{+2.5}_{-2.5}$	$2 . 8_{- 0.4}^{+ 0.5}$ $2.8^{+0.5}_{-0.4}$		$373_{- 15}^{+ 15}$ $373^{+15}_{-15}$	$3 . 1_{- 1.0}^{+ 40}$ $3.1^{+40}_{-1.0}$	$27_{- 14}^{+ 400}$ $27^{+400}_{-14}$	0.89(15)	$1 . 263_{- 0.028}^{+ 0.028}$ $1.263^{+0.028}_{-0.028}$
All	`BBGABS`	$1 . 45_{- 0.6}^{+ 0.28}$ $1.45^{+0.28}_{-0.6}$	4.32	< 1.1	$79 . 6_{- 1.9}^{+ 1.9}$ $79.6^{+1.9}_{-1.9}$	$2 . 53_{- 0.24}^{+ 0.3}$ $2.53^{+0.3}_{-0.24}$		$363_{- 12}^{+ 12}$ $363^{+12}_{-12}$	$2 . 1_{- 0.7}^{+ 40}$ $2.1^{+40}_{-0.7}$	$23_{- 17}^{+ 300}$ $23^{+300}_{-17}$	1.08(50)	$1 . 357_{- 0.025}^{+ 0.025}$ $1.357^{+0.025}_{-0.025}$

J1445

Instruments	Model	N_H	N_H, gal^(a)	Dist.^(b)	kT	R^(c)	Γ^(d)	ϵ^(e)	σ	EW^(f)	χ_ν²(ν)	Absorbed flux^(g)
		[10²⁰ cm⁻²]	[10²⁰ cm⁻²]	[kpc]	[eV]	[km]		[eV]	[eV]	[eV]		[10⁻¹³ erg s⁻¹ cm⁻²]

EPIC-pn	`BB`	$3 . 2_{- 1.0}^{+ 1.1}$ $3.2^{+1.1}_{-1.0}$	5.77	< 1.1^★	$210_{- 5}^{+ 5}$ $210^{+5}_{-5}$	$0 . 281_{- 0.018}^{+ 0.022}$ $0.281^{+0.022}_{-0.018}$					0.99(36)	$1 . 332_{- 0.027}^{+ 0.027}$ $1.332^{+0.027}_{-0.027}$
EPIC-pn	`PL`	$42 . 0_{- 2.8}^{+ 2.9}$ $42.0^{+2.9}_{-2.8}$	5.77				$4 . 88_{- 0.16}^{+ 0.17}$ $4.88^{+0.17}_{-0.16}$				1.75(36)	$1 . 322_{- 0.027}^{+ 0.027}$ $1.322^{+0.027}_{-0.027}$
All	`BB`	$2 . 7_{- 0.9}^{+ 0.9}$ $2.7^{+0.9}_{-0.9}$	5.77	< 1.2	$206_{- 4}^{+ 4}$ $206^{+4}_{-4}$	$0 . 288_{- 0.016}^{+ 0.018}$ $0.288^{+0.018}_{-0.016}$					1.26(94)	$1 . 327_{- 0.022}^{+ 0.022}$ $1.327^{+0.022}_{-0.022}$

Notes. 1σ confidence intervals are provided for the best-fit parameters.

^(a)

Galactic column density in the direction of the sources as inferred from HI4PI Collaboration (2016).

^(b)

Upper distance limit based on the lower confidence level limit of the N_{H, E(B − V)} value in the direction of the source equal to the 99% confidence region upper limit of the best-fit N_H value. Distance values were inferred from the online tool described in Doroshenko (2024). Values marked with a star (★) indicate cases where the upper limit on N_H is always above the lower confidence limit on N_{H, E(B − V)} regardless of distance, indicating a possible extragalactic nature. Here, we give the distance according to the absolute N_{H, E(B − V)} value.

^(c)

A distance of 1 kpc assumed for the BB emission radius at infinity.

^(d)

Photon index as given by the XSPEC powerlaw model component.

^(e)

Central line energy of the Gaussian absorption component.

^(f)

The equivalent width estimated from $\int \frac{f_{c} - f_{o}}{f_{c}} d E$ $\int \frac{f_c-f_o}{f_c} dE$ , with f_c being the continuum and f_o the observed flux. The errors provide the maximum and minimum EW values obtained from all possible combinations of the upper and lower 1σ confidence interval limits of the model parameters.

^(g)

Absorbed model flux in the 0.2 − 12 keV range.

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