Resolving the X-ray obscuration in a low flux observation of the quasar PDS 456

Abstract

Simultaneous XMM-Newton, NuSTAR and HST observations, performed in March 2017, of the nearby (z=0.184 ) luminous quasar PDS 456 are presented. PDS 456 had a low X-ray flux compared to past observations, where the first of the two new XMM-Newton observations occurred during a pronounced dip in the X-ray lightcurve. The broad-band X-ray spectrum is highly absorbed, attenuated by a soft X-ray absorber of column density N H =6×10 ²² cm ⁻² . An increase in obscuration occurs during the dip, which may be due to an X-ray eclipse. In addition, the persistent, fast Fe K outflow is present, with velocity components of −0.25c and −0.4c . The soft absorber is less ionized (logξ=3 ) compared to the iron K outflow (logξ=5 ) and is outflowing with a velocity of approximately −0.2c . A soft X-ray excess is present below 1 keV against the highly absorbed continuum and can be attributed to the re-emission from a wide angle wind. The complex X-ray absorption present in PDS 456 suggests that the wind is inhomogeneous, whereby the soft X-ray absorber originates from denser clumps or filaments which may form further out along the outflow. In contrast to the X-ray observations, the simultaneous UV spectrum of PDS 456 is largely unabsorbed, where only a very weak broad absorption trough is present bluewards of Lyα , compared to a past observation in 2000 when the trough was significantly stronger. The relative weakness of the UV absorption may be due to the soft X-ray absorber being too highly ionized and almost transparent in the UV band.