ChandraX-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.

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

“…This result is consistent with that found for other accreting NSs and also for XMM-Newton and Chandra spectra of GX 13+1 (e.g. Díaz Trigo et al 2012;D'Aì et al 2014 and reference therein). The soft component provides an inner temperature consistent with ∼ 0.6 keV for the two corrections.…”

“…It also results more opaque, and probably cloudy, close to the plane of the disc. In addition, the absorption lines associated to the warm absorber are produced by highly ionised species and indicate bulk outflow velocities of ∼ 400 km s −1 (Ueda et al 2004;Madej et al 2013;D'Aì et al 2014). …”

Dust absorption and scattering in the silicon K-edge

“…In the case of the observation with obsid 11814, a second XABS model is introduced in order to fit the non-outflowing ionized gas along the line of sight. The column density of GX 13+1 is in agreement with values found by Pintore et al (2014) andD'Aì et al (2014).…”

Red-skewed Kα iron lines in GX 13+1

“…They showed that the RDPHA calibration provides more consis-tent centroid line energies and more physical fit parameters (such as the emission Gaussian line centroid energy and source inclination parameters obtained through the diskline model best fit). Using this correction, they found the energy of the iron mission line at 6.6 keV, and stated that it was consistent with the energies found by Díaz Trigo et al (2012) andD'Aì et al (2014). In the RDPHA correction, the energy scale is calibrated by fitting the peaks in derivative PHA spectra corresponding to the Si (∼1.7 keV) and Au (∼ 2.3keV) edges of the instrumental responses, where the gradient of the effective area is largest (XMM-SOC-CAL-TN-0306).…”