PHL 1092 as a transient extreme X-ray weak quasar

TL;DR

This paper reports a dramatic X-ray flux drop in the quasar PHL 1092, with stable UV emission, suggesting a transient disruption of its X-ray emitting corona rather than absorption.

Contribution

It provides the first detailed observation of extreme X-ray variability in PHL 1092, indicating a transient corona disruption as the likely cause.

Findings

X-ray flux dropped by a factor of ~200 in 4.5 years

UV flux remained constant during the X-ray decline

X-ray weakness attributed to corona disruption, not absorption

Abstract

We report a dramatic variability event in the X-ray history of the Narrow-Line quasar PHL 1092 (z=0.396). Our latest 2008 XMM-Newton observation reveals a flux drop of ~200 with respect to the previous observation performed about 4.5 years earlier, and a drop of ~135 with respect to its historical flux. Despite the huge X-ray variation, the UV flux remains constant producing a very significant steepening of the optical to X-ray slope alpha_ox from -1.56 to -2.44, making PHL 1092 one of the most extreme X-ray weak quasars. The similarity in the soft X-ray spectral shape between the present and previous observations, together with the persistent UV flux and the lack of any dramatic change in the optical spectrum suggest that an absorption event is not likely to be the origin of the observed variation. If absorption is ruled out, the sudden X-ray weakness of PHL 1092 must be produced by a transient significant weakening or disruption of the X-ray emitting corona.