HS 1700+6416: the first high redshift unlensed narrow absorption-line QSO showing variable high-velocity outflows

We present a detailed analysis of the X-ray emission of HS 1700+6416, a high-redshift (z = 2.7348) luminous quasar classified as a narrow absorption line (NAL) quasar on the basis of its SDSS spectrum. The source has been observed nine times by Chandra and once by XMM-Newton from 2000 to 2007. Long-term variability is clearly detected between the observations in the 2-10 keV flux, where it varies by a factor of three (~3-9 × 10-14 erg s-1 cm-2), and in the amount of neutral absorption (NH < 10^22 cm-2 in 2000 and 2002 and NH = 4.4 ± 1.2 × 10^22 cm-2 in 2007). Most interestingly, one broad absorption feature is clearly detected at 10.3 ± 0.7 keV (rest frame) in the 2000 Chandra observation, while two similar features at 8.9 ± 0.4 and at 12.5 ± 0.7 keV are visible when the eight contiguous Chandra observations of 2007 are stacked together. In the XMM-Newton observation of 2002, which is strongly affected by background flares, there is a hint of a similar feature at 8.0 ± 0.3 keV. We interpreted these features as absorption lines from a high-velocity highly ionized (i.e. Fe XXV, FeXXVI) outflowing gas. In this scenario, the outflow velocities inferred are in the range v = 0.12-0.59c. To reproduce the observed features, the gas must have a high column density (NH > 3 × 10^23 cm-2), high ionization parameter (log ξ > 3.3 erg cm s-1) and a wide range of velocities (ΔV ~ 104 km s-1). This absorption line quasar is the fourth high-z quasar that displays X-ray signatures of variable high-velocity outflows, and among these, it is the only one that is not lensed. A rough estimate of the minimum kinetic energy carried by the wind of up to 18% Lbol, based on a biconical geometry of the wind, implies that the amount of energy injected into the outflow environment is large enough to produce effective mechanical feedback.