Table 2: Spectral fitsa to the observations of RX J0720.4-3125.
Orbit kT (eV) $L_{\rm bb}^b$ $\Gamma^c$ Flux d
        (10-23 Å) (23-38 Å) (10-38 Å)
0078 81.3(3) 2.54(3) 1.44(5) 1.86(4) 4.72(7) 6.58(10)
0175 81.4(5) 2.42(3) 1.45(5) 1.77(5) 4.49(7) 6.26(11)
0533 84.6(4) 2.22(3) 1.74(5) 1.89(4) 4.20(7) 6.09(11)
0534 84.0(4) 2.28(3) 1.67(5) 1.88(4) 4.31(7) 6.19(11)
0622 86.3(3) 2.07(2) 1.85(6) 1.93(4) 4.08(7) 6.01(06)
0711 98.2(4) 1.96(2) 2.70(8) 2.81(4) 4.01(6) 6.82(09)
a
Fit range 10-38 Å. For the interstellar absorption component we use cross sections from Verner et al. (1996), and abundances from Wilms et al. (2000). Numbers in parentheses are 1-$\sigma$ confidence limits in the last digit(s). For the fits with a blackbody, the best-fit value for $N_{\rm H}$, constrained to be the same in all observations, is $(4.3 \pm 0.1) \times 10^{20} ~{\rm cm}^{-2}$. For the fits with a blackbody times a power law, the best-fit values for $N_{\rm H}$, kT, and the bolometric luminosity of the blackbody, constrained to be the same in all observations are, respectively, $(1.28 \pm 0.1) \times 10^{20} ~{\rm cm}^{-2}$, $70.3 \pm 0.1$ eV, and $(3.04 \pm 0.02) \times 10^{32} d^2_{\rm 300}$ erg s-1, with $d_{\rm 300}$ the distance to RX J0720.4-3125 in units of 300 pc. For the fits with a blackbody model, $\chi^2 = 874.1$ for 662 degrees of freedom. For the fits with a blackbody times a power-law model $\chi^2 = 776.8$ for 668 degrees of freedom.
b
Bolometric luminosity of the blackbody in units of 10 $^{32} d^2_{\rm 300}$ erg s-1, where $d_{\rm 300}$ is the distance to the source in units of 300 pc.
c
Index of the power law in the model that consists of a blackbody times a power law (see text).
d
Observed flux, obtained from the fits with a blackbody times a power-law; only statistical errors are indicated. The systematic errors in the flux are of order 5-10%.


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