The persistent nature of the Be X-ray binary pulsar 4U 0728-25

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Table 2.

Best-fit parameters of the time-averaged EPIC spectrum of 4U 0728-25, in the case of the four best-fitting continuum models.


Model		TBPCF × PL	CPL	PL + BB	CPL + APEC
Parameter	Unit	Value	Value	Value	Value
TBABS N_H	×10²¹ cm⁻²	7.5 $_{- 0.2}^{+ 0.3}$ ${}^{+0.3}_{-0.2}$	5.2 $_{- 0.2}^{+ 0.1}$ ${}^{+0.1}_{-0.2}$	6.4 $_{- 0.2}^{+ 0.3}$ ${}^{+0.3}_{-0.2}$	5.2 ± 0.2
TBPCF N_H	×10²² cm⁻²	6.4 $_{- 0.5}^{+ 0.6}$ ${}^{+0.6}_{-0.5}$	–	–	–
TBPCF Covering Fraction	–	0.51 ± 0.03	–	–	–
Γ	–	1.79 $_{- 0.04}^{+ 0.05}$ ${}^{+0.05}_{-0.04}$	0.73 $_{- 0.06}^{+ 0.05}$ ${}^{+0.05}_{-0.06}$	1.52 ± 0.06	0.66 ± 0.06
E_cut	keV	–	6.5 $_{- 0.5}^{+ 0.7}$ ${}^{+0.7}_{-0.5}$	–	6.0 $_{- 0.4}^{+ 0.6}$ ${}^{+0.6}_{-0.4}$
Flux_PLorCPL (0.2–12 keV) ^(a)	×10⁻¹¹ erg cm⁻² s⁻¹	3.3 ± 0.2	1.91 ± 0.03	1.57 $_{- 0.07}^{+ 0.06}$ ${}^{+0.06}_{-0.07}$	1.88 ± 0.03
kT_{BB or APEC}	keV	–	–	1.48 $_{- 0.07}^{+ 0.07}$ ${}^{+0.07}_{-0.07}$	1.4 $_{- 0.3}^{+ 0.4}$ ${}^{+0.4}_{-0.3}$
R_BB ^(b)	m	–	–	240 $_{- 20}^{+ 30}$ ${}^{+30}_{-20}$	–
N_APEC	cm⁻⁵	–	–	–	1.4 $_{- 0.8}^{+ 1.3} \times 10^{- 4}$ ${}^{+1.3}_{-0.8} \times 10^{-4}$
Flux_{BB or APEC} (0.2–12 keV) ^(a)	×10⁻¹² erg cm⁻² s⁻¹	–	–	5.1 ± 0.7	0.2 $_{- 0.1}^{+ 0.2}$ ${}^{+0.2}_{-0.1}$

Flux_{BB or APEC}/Flux_TOT (0.2–12 keV)	–	–	–	24.2%	1.1%
Flux_{BB or APEC}/Flux_TOT (0.01–100 keV)	–	–	–	9.5%	1.1%
Unabsorbed flux (0.2–12 keV)	×10⁻¹¹ erg cm⁻² s⁻¹	3.3 ± 0.2	1.91 ± 0.03	2.07 $_{- 0.02}^{+ 0.03}$ ${}^{+0.03}_{-0.02}$	1.90 $_{- 0.02}^{+ 0.03}$ ${}^{+0.03}_{-0.02}$
Luminosity (0.2–12 keV) ^(b)	×10³⁵ erg s⁻¹	2.2 ± 0.1	1.25 ± 0.02	1.35 $_{- 0.01}^{+ 0.02}$ ${}^{+0.02}_{-0.01}$	1.24 $_{- 0.01}^{+ 0.02}$ ${}^{+0.02}_{-0.01}$
$χ_{ν}^{2}$ $\chi ^{2}_{\nu }$ /d.o.f.	–	1.063/2506	1.063/2507	1.050/2506	1.058/2505

Notes.

^(a)

Corrected for absorption;

^(b)

Assuming a source distance d = 7.6 kpc.

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