Chandra X-ray spectroscopy of a clear dip in GX 13+1

Abstract

Context. The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of ~400 km s-1 and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. Aims: We give the first spectral characterization of the periodic dip of GX 13+1 found in this archival Chandra observation performed in 2010. Methods: We used Chandra/HETGS data (1.0-10 keV band) and contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broad-band X-ray spectrum. We adopted different spectral models to describe the continuum emission and used the XSTAR-derived warm absorber component to constrain the highly ionized absorption features. Results: The 1.0-25 keV continuum emission is consistent with a model of soft accretion-disk emission and an optically thick, harder Comptonized component. The dip event, lasting ~450 s, is spectrally resolved with an increase in the column density of the neutral absorber, while we do not find significant variations in the column density and ionization parameter of the warm absorber with respect to the out-of-dip spectrum. Conclusions: We argue that the very low dipping duty-cycle with respect to other sources of the same class can be ascribed to its long orbital period and the mostly neutral bulge that is relatively small compared with the dimensions of the outer disk radius.