Appendix A: Interstellar absorption towards XB 1745-25

A value of $N_{\rm H}$ was reported for XB 1745-25 in Terzan 5 on the basis of ROSAT/PSPC observations (Verbunt et al. 1995): $2 \times 10 ^{22}$ cm^-2. The BeppoSAX/WFCs observed XB 1745-25 through the various stages of its 2000 outburst for a net exposure of 761 ksec. The source was quite bright (about 0.5 Crab), allowing a good measure of $N_{\rm H}$, since the absorption is already visible at $\simeq$4 keV. We accumulated a dead-time corrected spectrum using data from WFC unit 2 obtained between UT 2000 August 8 20:44:38 to August 9 20:53:48, giving an effective exposure time of 31.4 ksec.

We fitted the WFC spectrum with similar models as used by Sidoli et al. (2001), as well as a black body plus a power-law. The fit results are displayed in Table A.1. The disk black body plus Comptonization model provided the best fits, similar to the results by Sidoli et al. (2001). We therefore use the value of $N_{\rm H}$ from this model for our analysis, see Table 2, which is somewhat higher than that found by Verbunt et al. (1995). The mean persistent flux was $\simeq$1. $16 \times 10 ^{-8}$ erg cm^-2 s (2-20 keV).

 

 

Table A.1: X-ray spectral fits results to the persistent emission of XB 1745-25 during outburst observed with the BeppoSAX/WFCs. Three models were used; the first two (disk black body [ DISKBB in XSPEC] plus a Comptonization component [ COMPTT, see Titarchuk 1994] and black body [ BBODYRAD] plus a Comptonization component) were similar to those studied by Sidoli et al. (2001). The third (black body plus a power law) is used for comparison. The DISKBB model has as free parameters: $kT_{\rm in}$, the temperature at the inner disk radius, and $r_{\rm in}/d_{10}(\cos{i})^{0.5}$, where $r_{\rm in}$ is the inner disk radius, d₁₀ the distance to the source in units of 10 kpc, and i the inclination angle of the disk. The COMPTT model has as free parameters: $kT_{\rm e}$, the temperature of the Comptonizing electrons, $\tau _{\rm p}$, the plasma optical depth with respect to electron scattering, kT₀, the input temperature of the soft photon (Wien) distribution, and a normalization constant. The BBODYRAD model has as free parameters: $kT_{\rm bb}$, the black-body temperature, and $r_{\rm bb}$, the black-body radius for a source at 10 kpc. The POWERLAW model has as free parameters: $\Gamma$, the power-law photon index, and the power-law normalization in photons keV^-1 cm^-2 s^-1 at 1 keV.

model	$N_{\rm H}$	$kT_{\rm in}$	$r_{\rm in}/d_{10}(\cos{i})^{0.5}$	kT0	$kT_{\rm e}$	$\tau _{\rm p}$	norm	$\chi ^2_{\rm red}$/d.o.f.
	(1022 cm-2)	(keV)	(km)	(keV)	(keV)
DISKBB+COMPTT	3.8 +0.9-0.7	0.60 +0.07-0.04	72 +57-21	1.28 +0.06-0.09	3.7 +1.0-0.5	3.6 +0.8-0.9	0.57 +0.12-0.14	1.15/21
model	$N_{\rm H}$	$kT_{\rm bb}$	$r_{\rm bb}$	kT0	$kT_{\rm e}$	$\tau _{\rm p}$	norm	$\chi ^2_{\rm red}$/d.o.f.
	(1022 cm-2)	(keV)	(km)	(keV)	(keV)
BBODYRAD+COMPTT	2.3 +0.6-0.8	1.69 +0.07-0.08	8.3 +0.8-0.5	0.35 +0.10-0.31	4.3 +2.3-0.8	4.2 +1.3-1.4	0.7 +0.7-0.2	1.38/21
model	$N_{\rm H}$	$kT_{\rm bb}$	$r_{\rm bb}$	$\Gamma$	norm			$\chi ^2_{\rm red}$/d.o.f.
	(1022 cm-2)	(keV)	(km)
BBODYRAD+POWERLAW	3.4 $\pm$ 0.3	1.79 $\pm$ 0.02	7.9 $\pm$ 0.2	2.63 +0.06-0.07	6.1 +0.9-0.8			1.40/23