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

Abstract

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 9 times by Chandra and once by XMM from 2000 to 2007. Long term variability is clearly detected, between the observations, in the 2-10 keV flux varying by a factor of three (~3-9x10^-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+-1.2x10^22 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 8 contiguous Chandra observations of 2007 are stacked together. In the XMM observation of 2002, strongly affected by background flares, there is a hint for 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 high column density (Nh>3x10^23 cm^-2), high ionization parameter (log(xi)>3.3 erg cm s^-1) and a large range of velocities (Delta V~10^4 km s^-1). This Absorption Line QSO is the fourth high-z quasar displaying X-ray signatures of variable, high velocity outflows, and among these, is the only one non-lensed. A rough estimate of the minimum kinetic energy carried by the wind of up to 18% L(bol), based on a biconical geometry of the wind, implies that the amount of energy injected in the outflow environment is large enough to produce effective mechanical feedback.