A deep X-ray and UV look into the reflaring stage of the accreting millisecond pulsar SAX J1808.4−3658

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Volume 707 (March 2026)

A&A, 707 (2026) A42

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Table 2.

Best-fit parameters of XMM-Newton EPIC-pn 1.0–10 keV spectra of SAX J1808 during the high- and low-flux intervals.

	High-flux values	Low-flux values
TBABS
N_H [10²² cm⁻²]	0.21^(a)	0.21^(a)

BBODYRAD
kT_BB [keV]	$0 . 399_{- 0.004}^{+ 0.003}$ $Mathematical equation: $ 0.399^{+0.003}_{-0.004} $$	$0 . 362_{- 0.013}^{+ 0.011}$ $Mathematical equation: $ 0.362^{+0.011}_{-0.013} $$
R_BB (d_3.5 km)	$3 . 26_{- 0.07}^{+ 0.10}$ $Mathematical equation: $ 3.26^{+0.10}_{-0.07} $$	2.86 ± 0.14
F_{0.5 − 10}^b	$0 . 218_{- 0.018}^{+ 0.014}$ $Mathematical equation: $ 0.218^{+0.014}_{-0.018} $$	$0 . 102_{- 0.013}^{+ 0.010}$ $Mathematical equation: $ 0.102^{+0.010}_{-0.013} $$

GAUSSIAN LINES
Si K-edge E_line [keV]	1.91 ± 0.02	–
σ [keV]	0^(a)	–
Norm [10⁻⁵]	$3 . 3_{- 0.9}^{+ 0.7}$ $Mathematical equation: $ 3.3^{+0.7}_{-0.9} $$	–
Au M-edge E_line [keV]	$2 . 265_{- 0.007}^{+ 0.009}$ $Mathematical equation: $ 2.265^{+0.009}_{-0.007} $$	–
σ [keV]	0^(a)	–
Norm [10⁻⁵]	6.5 ± 0.7	–
Ar XVIII E_line [keV]	3.49 ± 0.05	–
σ [keV]	0.37 ± 0.07	–
Norm [10⁻⁵]	9 ± 2	–

NTHCOMP
Γ	$2 . 04_{- 0.04}^{+ 0.05}$ $Mathematical equation: $ 2.04^{+0.05}_{-0.04} $$	$2 . 06_{- 0.04}^{+ 0.07}$ $Mathematical equation: $ 2.06^{+0.07}_{-0.04} $$
kT_e [keV]	30^(a)	30^(a)
kT_seed [keV]	< 0.11	< 0.15
Redshift	0^(a)	0^(a)
Norm	0.0300 ± 0.001	$0 . 0051_{- 0.0003}^{+ 0.0004}$ $Mathematical equation: $ 0.0051^{+0.0004}_{-0.0003} $$
$F_{0.5 - 10}^{b}$ $Mathematical equation: $ F_{0.5-10}^{b} $$	$1 . 39_{- 0.04}^{+ 0.03}$ $Mathematical equation: $ 1.39^{+0.03}_{-0.04} $$	0.23 ± 0.04

REFLECTION
β	$- 3 . 02_{- 0.20}^{+ 0.10}$ $Mathematical equation: $ -3.02^{+0.10}_{-0.20} $$	$- 3 . 08_{- 1.5}^{+ 0.4}$ $Mathematical equation: $ -3.08^{+0.4}_{-1.5} $$
R_in [R_g]^c	< 6.7	< 14.7
R_out [R_g]^c	1000^(a)	1000^(a)
Incl. [deg]	$58_{- 1}^{+ 2}$ $Mathematical equation: $ 58^{+2}_{-1} $$	$59_{- 6}^{+ 5}$ $Mathematical equation: $ 59^{+5}_{-6} $$
Refl. frac.	−0.62^(a)	−0.62^(a)
Fe abund. [solar]	1^(a)	1^(a)
log ξ	2.32 ± 0.01	2.7 ± 0.2
$F_{0.5 - 10}^{b}$ $Mathematical equation: $ F_{0.5-10}^{b} $$	$0 . 372_{- 0.014}^{+ 0.012}$ $Mathematical equation: $ 0.372^{+0.012}_{-0.014} $$	$0 . 0901_{- 0.007}^{+ 0.005}$ $Mathematical equation: $ 0.0901^{+0.005}_{-0.007} $$

TOTAL
F_{0.5 − 10}^b	$1 . 989_{- 0.04}^{+ 0.006}$ $Mathematical equation: $ 1.989^{+0.006}_{-0.04} $$	$0 . 424_{- 0.010}^{+ 0.014}$ $Mathematical equation: $ 0.424^{+0.014}_{-0.010} $$

χ²/d.o.f.	179.3/142	159.4/134

Notes. The fit models are TBabs*(bbodyrad + gaussian + gaussian + gaussian + nthComp + rdblur*rfxconv*nthComp) for the high-flux state and TBabs*(bbodyrad + nthComp + rdblur*rfxconv*nthComp) for the low-flux state. Uncertainties are given at a 1σ confidence level. ^aParameter kept frozen during the fit.

^bUnabsorbed flux in the 0.5–10 keV band and reported in units of 10⁻¹⁰ erg cm⁻² s⁻¹.

^cR_g = GM/c² is the gravitational radius.

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