Searching for ultra-fast outflows in AGN using variability spectra

Abstract

We present a qualitative search for ultra-fast outflows (UFOs) in excess variance spectra of radio-quiet active galactic nuclei (AGN). We analyse 42 sources from the Tombesi et al. (2010) spectroscopic UFO detection sample, and an additional 22 different sources from the Kara et al. (2016) variability sample. A total of 58 sources have sufficient observational data from XMM-Newton EPIC-pn and variability for an excess variance spectrum to be calculated. We examine these spectra for peaks corresponding to variable blue-shifted H-and He-like ion absorption lines from UFOs. We find good evidence for such outflows in 28% of the AGN sample and weak evidence in a further 31%, meaning that ∼ 30-60% of the AGN sample hosts such UFOs. The mean and median blue-shifted velocity is found to be ∼ 0.14c and 0.12c, respectively. Current variability methods allow for a fast, model-independent determination of UFOs, however, further work needs to be undertaken to better characterize the statistical significance of the peaks in these spectra by more rigorous modelling. Detecting good evidence for variable UFO lines in a large number of sources also lays the groundwork for detailed analysis of the variability timescales of the absorbers. This will allow us to probe their densities and hence distances from the central super-massive black hole.

Characterizing continuum variability in the radio-loud narrow-line Seyfert 1 galaxy IRAS 17020+4544

“…Blue-shifted Fe K absorption features at energies >7.1 keV have been reported in numerous AGNs (e.g. Tombesi et al 2010;Gofford et al 2013;Igo et al 2020), and indeed there is evidence of ultra-fast outflows from other RGS based analyses (Longinotti et al 2015;Sanfrutos et al 2018) having already been detected in this source with velocities of ∼0.05−0.1c. Longinotti et al (2015) reported finding marginal evidence of blue-shifted Fe K absorption that would correspond to an outflow of ∼0.3c.…”

Modelling X-ray RMS spectra II: the ultrafast outflow of PDS 456

“…Firstly, it can function as a method for UFO detection without complex spectral analysis, and without the same systematic effects that plague conventional spectroscopic detection. Igo et al (2020) used this method with a sample of bright, variable AGN, and found evidence for UFOs in 30-60% of sources studied, consistent with the results from conventional spectroscopy. Secondly, by modelling the variance spectra of UFOs we can probe the physical properties of the outflow, measuring properties that cannot be measured from a static count spectrum.…”

“…Selecting variable sources for UFO searches, as in Igo et al (2020), is therefore likely to be a promising way of detecting more UFOs. However, it is worth noting that such surveys may be biased towards a higher prevalence of UFOs because of this.…”

“…These models gave a good description of the data, and allowed the different physical processes contributing to the variance to be quantified. the UFO disappear as the X-ray flux rises (Parker et al 2017b;Pinto et al 2018;Parker et al 2018;Igo et al 2020). This means that the flux in the energy bands affected by absorption rises more, leading to a higher variability amplitude in these bands.…”

“…Variance spectra show the energy dependence of variability, so they can be used to try and understand the spectral components responsible for the variability. For example, a common result is that the soft X-ray band is less variable than the hard band, which implies that the commonly observed soft excess in AGN is less variable than the power-law continuum (e.g., Vaughan & Fabian 2004;Parker et al 2015;Igo et al 2020).…”