Testing rate-dependent corrections on timing mode EPIC-pn spectra of the accreting neutron star GX 13+1

Abstract

When the EPIC-pn instrument on board XMM-Newton is operated in Timing mode, high count rates (>100 counts s-1) of bright sources may affect the calibration of the energy scale, resulting in a modification of the real spectral shape. The corrections related to this effect are then strongly important in the study of the spectral properties. Tests of these calibrations are more suitable in sources which spectra are characterized by a large number of discrete features. Therefore, in this work, we carried out a spectral analysis of the accreting neutron star GX 13+1, which is a dipping source with several narrow absorption lines and a broad emission line in its spectrum. We tested two different correction approaches on an XMM-Newton EPIC-pn observation taken in Timing mode: the standard rate-dependent charge transfer inefficiency (RDCTI or epfast) and the new, rate-dependent pulse height amplitude (RDPHA) corrections. We found that, in general, the two corrections marginally affect the properties of the overall broad-band continuum, while hints of differences in the broad emission line spectral shape are seen. On the other hand, they are dramatically important for the centroid energy of the absorption lines. In particular, the RDPHA corrections provide a better estimate of the spectral properties of these features than the RDCTI corrections. Indeed the discrete features observed in the data, applying the former method, are physically more consistent with those already found in other Chandra and XMM-Newton observations of GX 13+1.