A search for changing-look AGN in the Grossan catalog

Abstract

We observed with XMM-Newton 4 objects selected from the Grossan (1992, Ph.D. Thesis) catalog, with the aim to search for new "changinglook" AGN. The sample includes all the sources which showed in subsequent observations a flux much lower than the one measured with HEAO A-1: NGC 7674, NGC 4968, IRAS 13218+0552 and NGC 1667. None of the sources was caught in a high flux state during the XMMNewton observations, whose analysis reveal they are all likely Compton-thick objects. We suggest that, for all the sources, potential problems with the HEAO A-1 source identification and flux measurement prevent us from being certain that the HEAO A-1 data represent a putative "high" state for these objects. Nonetheless, based on the high flux state and Compton-thin spectrum of its GINGA observation, NGC 7674 represents probably the sixth known case of a "changing-look" Seyfert 2 galaxy. From the X-ray variability pattern, we can estimate a likely lower limit of a few parsec to the distance of the inner walls of the torus in this object. Remarkably, IRAS 13218+0552 was not detected by XMM-Newton, despite being currently classified as a Seyfert 1 with a large [OIII] flux. However, the original classification was likely to be affected by an extreme velocity outflow component in the emission lines. The object likely harbors an highly obscured AGN and should be re-classified as a type 2 source.

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674

“…Fig. 6 shows the long-term X-ray light curve of NGC 7674 over a period of about 37 yr. As first noted by Bianchi et al (2005), the source showed a decline by a factor of ∼3 in the 2-10 keV band between the first detection by HEAO in the late 1970s and the Ginga measurement in 1989, followed by a further order of magnitude flux decrease in 1996 when BeppoSAX identified the source as a CTAGN (Malaguti et al 1998). Thereafter, the source flux has remained constant with no significant spectral or flux variation for the past ≈ 20 yr.…”

“…However, as discussed by Bianchi et al (2005), the background for the observation of NGC 7674 is based on a scanning observation obtained close in time to the target (Awaki et al 1991). These background scans result in noise estimates that are more reliable than model estimates typically adopted for non-imaging detectors.…”

“…The switched-off AGN scenario was examined by Bianchi et al (2005), motivated by the fact that, unlike other changing-look AGN in which N H variations occur relatively frequently, NGC 7674 did not show any flux or spectral variations in post-BeppoSAX observations. In this scenario, the observed reflection component dominating at hard X-rays is expected to be delayed with respect to the incident PL AGN that has now faded.…”

“…As detailed in a historical X-ray analysis by Bianchi et al (2005), the source was likely detected by the HEAO mission in the late 1970s at a 2-10 keV flux level almost 30 times brighter (Grossan 1992) and subsequently with an intermediate flux by Ginga (Awaki et al 1991). Bianchi et al (2005) discuss several potential caveats to these detections, including HEAO contamination by nearby sources, and systematic uncertainties related to the background level measured by Ginga, and conclude that the combined weight of evidence favours the historical source detections being real, although the Ginga flux measurement is considered to be more reliable of the two, due to a more robust background determination. The Ginga spectrum is a lightly absorbed power law (PL, with N H < 2 × 10 22 cm −2 ) with an upper limit of 80 eV to the equivalent width (EW) of any neutral Fe Kα emission line.…”

“…Bianchi et al (2005) interpret the source as potentially having switched-off, with the spectrum observed by BeppoSAX and XMM-Newton being the reflection component that is delayed with respect to the direct, illuminating PL. Another source that has been discussed from these two opposing perspectives recently is NGC 7582 (Rivers et al 2015).…”