XMM-Newton first X-ray detection of the low-ionization broad absorption line quasar PG 1700+518

Abstract

We report the first high-energy detection of PG 1700+518, a well-known low-ionization broad absorption line quasar (quasi-stellar object, QSO). Due to previous X-ray non-detection, it was classified as soft X-ray weak QSO. We observed PG 1700+518 with XMM-Newton for about 60 ks divided in three exposures. The spectrum below 2 keV is very steep, Γ∼ 2.4-3.8, while at higher energies the extremely flat emission (photon index Γ∼ 0.15, when modelled with a power law) suggests the presence of strong absorption (N_{H, pl}∼ 2 × 10²³ cm^-2, Γ fixed to 1.8) or a reflection-dominated continuum. The broad-band flux is consistent with previous non-detection. Simultaneous European Photon Imaging Camera (EPIC) and Optical Monitor (OM) data confirm its X-ray weakness (observedα_ox∼-2.2). The level of obscuration derived from the X-ray spectra of PG 1700+518 cannot explain its soft X-ray nuclear weakness unless a column density of N_H≳ 2 × 10²⁴ cm^-2 is present.