Chandra spectral analysis. Top: data with best-fit single power-law model. Each colored line represents a different observation (identified by its ObsID, see Table 1). We note that the ObsID 4187 spectrum (blue curve) was performed with ACIS-I, while the other three were conducted with ACIS-S. Bottom: residues (data-model) in terms of the contribution to the χ² for each energy bin in the spectrum.