Extreme ultra-soft X-ray variability in an eROSITA observation of the narrow-line Seyfert 1 galaxy 1H 0707−495

Abstract

The ultra-soft narrow-line Seyfert 1 galaxy 1H 0707−495 is a well-known and highly variable active galactic nucleus (AGN), with a complex, steep X-ray spectrum, and has been studied extensively with XMM-Newton. 1H 0707−495 was observed with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) aboard the Spectrum-Roentgen-Gamma (SRG) mission on October 11, 2019, for about 60 000 s as one of the first calibration and pointed verification phase (CalPV) observations. The eROSITA light curves show significant variability in the form of a flux decrease by a factor of 58 with a 1 σ error confidence interval between 31 and 235. This variability is primarily in the soft band, and is much less extreme in the hard band. No strong ultraviolet variability has been detected in simultaneous XMM-Newton Optical Monitor observations. The UV emission is LUV ≈ 1044 erg s−1, close to the Eddington limit. 1H 0707−495 entered the lowest hard flux state seen in 20 yr of XMM-Newton observations. In the eROSITA All-Sky Survey (eRASS) observations taken in April 2020, the X-ray light curve is still more variable in the ultra-soft band, but with increased soft and hard band count rates more similar to previously observed flux states. A model including relativistic reflection and a variable partial covering absorber is able to fit the spectra and provides a possible explanation for the extreme light-curve behaviour. The absorber is probably ionised and therefore more transparent to soft X-rays. This leaks soft X-rays in varying amounts, leading to large-amplitude soft-X-ray variability.

The nature of the extreme X-ray variability in the NLS1 1H 0707-495

“…The 2019 variance spectrum is qualitatively different from those in other epochs. We attribute this to the obscuration variability discovered by Boller et al (2021), which seems only to contribute significantly in this observation. However, as Boller et al point out, there are other low state count spectra of 1H 0707-495 that appear very similar to the 2019 spectrum, notably the 2011 spectrum.…”

“…The 2019 spectrum observed by eROSITA (see Fig. 3 of Boller et al 2021) is drastically different from the other spectra, as it is much more variable at low energies. The simultaneous XMM-Newton spectrum shows the same basic shape.…”

“…This is likely due to obscuration, which enhances the variance at low energies, modified by the photoionised lines, which cause a sharp drop in variance at 1 keV. Boller et al (2021) were able to fit flux-resolved eROSITA and XMM-Newton spectra from the joint 2019 observation with a model consisting of partial covering absorption and relativistic reflection, and attribute the differences between the three spectra primarily to absorption. We note, however, that the ionization parameter of the reflection in their model is also free to vary between the three spectra (along with the normalization and the covering fraction of the absorber), and the effect of this is crucial to understanding how the model is able to fit the spectra.…”

1H 0707‐495: An AGN is dimming its X‐ray light

“…1H 0707-495 was observed with eROSITA cameras 1,2,5,6,7 and simultaneously with XMM-Newton EPIC-pn and MOS2 (c.f. table 1 of Boller et al (2021)). During the eROSITA observations, 1H 0707-495 showed a dramatic flux drop in about 1 day (Figure 5 upper panel).…”

“…Dauser et al (2012); Done & Jin (2016)). In the paper by Boller et al (2021), strong relativistic reflection in combination with partial covering has been suggested to account both for relativistic effects in the strong gravitational regime close to the central black hole and the presence of a clumpy outflowing wind that mimics the effect of partial covering. The distance of the absorber has been estimated with about 20–1000 gravitational radii from the central black hole, well inside the broad‐line region.…”