X-ray spectral variability in PG 1535+547: the changing look of a “soft X-ray weak” AGN

Abstract

Context. PG 1535+547 is a bright Narrow Line Seyfert 1 galaxy, whose high-energy emission shows strong variability both in shape and flux. On the basis of ROSAT observations, it is classified as "soft X-ray weak quasi-stellar object (QSO)", a class of objects whose X-ray-to-optical flux ratio is smaller than in typical QSOs. Their X-ray spectra are often characterized by highly-ionized, complex absorbers and/or reflection from the inner accretion disk, and the relative importance of the two is currently debated. Whatever the correct interpretation may be, the presence of such features implies that we are looking at matter located in the innermost regions of these AGN. Aims. We want to clarify the nature of the X-ray emission of PG 1535+547, and constrain the physical properties of its innermost regions, where this emission originates. Methods. We present new XMM-Newton observations of PG 1535+547 (90 ks exposure time, January 2006), from which we obtained two spectra European Photon Imaging Camera (EPIC) separated by about one week. We compare these spectra with those from a previous (November 2002) XMM-Newton observation. Results. These latest observations support the complex and variable nature of the X-ray emission of PG 1535+547. The broadband observed flux increases by a factor ∼2.3 in three years, and then decreases by a factor ∼1.3 in about one week. In the new spectra, strong absorption features at E < 3 keV and a complex spectral shape in the iron-line energy range are evident, coupled with a drop in the emission at higher energies. We describe all the different states in a consistent way, assuming either a warm absorber plus a relativistically-blurred ionized reflection, or a two-phase warm absorber partially covering the source with the addition of a scattered component. Conclusions. The observed variability can be ascribed mostly to warm absorbing gas in the innermost regions of PG 1535+547, which appears to vary in its physical properties on timescales of both years and days. In the blurred reflection scenario, all the analyzed states require a high fraction of reflection from the disk, calling for some mechanisms able to increase the reflection component with respect to the intrinsic continuum. Finally, the strong variability observed in the X-ray band opposed to a more constant emission at optical frequencies, changes the value of the X-ray-to-optical spectral index, implying that PG 1535+547 can not actually be classified as a soft X-ray weak active galactic nucleus (AGN).

Anatomy of the AGN in NGC 5548

“…Despite being recorded here for the first time in this source, this kind of "obscuration event" may not be unique to NGC 5548. Albeit with less quality data, other fast and variable obscuration events have been recorded in the past in a number of intermediate-type or type 1 sources, such as NGC 3227 (Lamer et al 2003), NGC 1365 (Risaliti et al 2005), NGC 4388 (Elvis et al 2004), NGC 4151 (Puccetti et al 2007), PG1535+547 (Ballo et al 2008), NGC 7582 ), H0557-385 (Longinotti et al 2009;Coffey et al 2014), Mrk 766 (Risaliti et al 2011), SwiftJ2127.4+5654 (Sanfrutos et al 2013), Mrk 335 (Longinotti et al 2013), NGC 5506 (Markowitz et al 2014), NGC 985 (Ebrero et al 2016a,b), Fairall 51 (Svoboda et al 2015), and ESO 323-G77 (Sanfrutos et al 2016), not to mention the systematic surveys by Malizia et al (1997), Markowitz et al (2014), andTorricelli-Ciamponi et al (2015). Based on various arguments, such as the ones above, and including the spectral properties and variability timescales of the absorbers, most authors have associated the origin of the absorbing clouds as either broad line region (BLR) clouds, a clumpy torus, or the inner boundary of a dusty torus.…”

The peculiar optical-UV X-ray spectra of the X-ray weak quasar PG 0043+039

“…Further measurements of extreme X-ray weak quasars (Saez et al 2012) are incorporated by cyan circles. Additional observations are given for the quasars PG 2112+059 (blue open triangle; Schartel et al 2010Schartel et al , 2007, PG 1535+547 (green star; Ballo et al 2008), and PG 1700+518 (yellow open circle; Ballo et al 2011). Among all galaxies PG 0043+039 shows the most extreme α ox gradient (α ox = −2.55) based on the data taken in 2005.…”

An X-ray variable absorber within the broad line region in Fairall 51

“…The variability time scale of months to years is typical of a clumpy torus at the distance ∼1 pc. However, the absorption variability was later found on a time scale of days in several sources: NGC 4388 (Elvis et al 2004), NGC 1365 (Risaliti et al 2005), NGC 4151 (Puccetti et al 2007), NGC 7582 (Bianchi et al 2009b), PG 1535+547 (Ballo et al 2008), Mrk 766 (Risaliti et al 2011), NGC 5506 , NGC 3227 (Lamer et al 2003;Beuchert et al, in prep. ), or Swift J2127.4+5654 (Sanfrutos et al 2013).…”