XMM-Newton observation of the deep minimum state of PG 2112+059

A spectrum dominated by reflection from the accretion disk?

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 Astrophysics Mission Division, ESA, SCT-SA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 Mexico DF, México
⁴ LAEFF-INTA, Apdo 50727, 28080 Madrid, Spain
⁵ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00040 Monteporzio Catone, Italy

Received: 8 May 2007
Accepted: 6 August 2007

Abstract

Context.Highly ionised absorbers and the frequent occurrence of relativistically broad iron fluorescence lines characterize the 0.2-10 keV spectra of (soft) X-ray weak quasars.

Aims.We constrain the physical conditions of the absorber and the broad iron line of the X-ray weak quasar PG 2112+059 in greater detail than in previous studies.

Methods.We analyse a 75 ks XMM-Newton observation of PG 2112+059 performed in November 2005 and compare it with a 15 ks XMM-Newton observation taken in May 2003.

Results. PG 2112+059 was found in a deep minimum state as its 0.2-12 keV flux decreased by a factor of 10 in comparison to the May 2003 observation. During the deep minimum state the spectra show strong emission in excess of the continuum in the 3-6 keV region. The excess emission corresponds to an whereas its shape resembles that of heavily absorbed objects. 
The spectra of both observations of PG 2112+059 can be explained statistically by a combination of two absorbers where one shows a high column density, , and the other high ionisation parameters. As the ionisation parameter of the high flux state, , is lower than the value found for the deep minimum state, , either the absorbers are physically different or the absorbing material is moving with respect to the X-ray source.
The spectra can also be explained by a continuum plus X-ray ionised reflection on the accretion disk, seen behind a warm absorber. The ionisation parameter of the high state () is higher than the ionisation parameter of the deep minimum state (), as expected for a stationary absorber. The values found for the ionisation parameters are in the range typical for AGNs. The spectra observed during the deep minimum state are reflection dominated and show no continuum emission. These can be understood in the context of light bending near the supermassive black hole as predicted by Minutti and Fabian.

Conclusions. Light bending offers an alternative explanation for X-ray weak quasars and might challenge the suggestion that absorption is the primary cause of their X-ray weakness. If on a class level the weakness of X-ray weak quasars is caused by light bending then they offer unique possibilities to observe accretion disks near the supermassive black hole and even to test general relativity in the strong field.