A long hard look at the minimum state of PG 2112+059 with XMM-Newton

¹ XMM-Newton Science Operations Centre, ESA, Villafranca del Castillo, Apartado 78, 28691 Villanueva de la Cañada, Spain e-mail: Norbert.Schartel@sciops.esa.int
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510-Mexico DF, México
³ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
⁴ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00040 Monteporzio Catone, Italy

Received: 25 April 2009
Accepted: 28 November 2009

Abstract

Aims. Our observational aim is to perform a long X-ray observation of the quasar PG 2112+059 in its low or minimum state. Starting form this very peculiar emission state, we intend to constrain the intrinsic emission mechanism by comparing new and old data, corresponding to different source states.

Methods. XMM-Newton successfully detected the minimum state of PG 2112+059 during a short snapshot observation and performed a long follow-up observation. The high signal-to-noise spectra are modelled assuming different emission scenarios and compared with archival spectra taken by XMM-Newton and Chandra.

Results. The PG 2112+059 X-ray spectra acquired in May 2007 allowed the detection of a weak iron fluorescent line, which is interpreted as being caused by reflection from neutral material at some distance from the primary X-ray emitting source. The X-ray spectra of PG 2112+059 taken at five different epochs during different flux states can be interpreted within two different scenarios. The first consists of two layers of ionised material with column densities of N_H~5 × 10²² cm^-2 and N_H∼3.5 × 10²³ cm^-2, respectively. The first layer is moderately ionised and its ionisation levels follow the flux changes, while the other layer is highly ionised and does not show any correlation with the flux of the source. The spectra can also be interpreted assuming reflection by an ionised accretion disk seen behind a warm absorber. The warm absorber ionisation is consistent with being correlated with the flux of the source, which provides an additional degree of self-consistency with the overall reflection-based model. We explain the spectral variability with light bending according to the models of Miniutti and Fabian and constrain the black hole spin to be a/M > 0.86. Both scenarios also assume that a distant cold reflector is responsible for the Fe K α emission line.

Conclusions. Light bending provides an attractive explanation of the different states of PG 2112+059 and may also describe the physical cause of the observed properties of other X-ray weak quasars. The observations of PG 2112+059 in different states provide valuable constraints, although are unable to break the degeneracy between complex absorption scenarios and reflection from an ionised disk.