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Register a new userOn the importance of special relativistic effects in modelling ultra-fast outflows
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Outflows are observed in a variety of astrophysical sources. Remarkably, ultra-fast ($vgeq 0.1c$), outflows in the UV and X-ray bands are often seen in AGNs. Depending on their energy and mass outflow rate, respectively $dot{E}_{out}, dot{M}_{out}$, such outflows may play a key role in regulating the AGN-host galaxy co-evolution process through cosmic time. It is therefore crucial to provide accurate estimates of the wind properties. Here, we concentrate on special relativistic effects concerning the interaction of light with matter moving at relativistic speed relatively to the source of radiation. Our aim is to assess the impact of these effects on the observed properties of the outflows and implement a relativistic correction in the existing spectral modelling routines. We define a simple procedure to incorporate relativistic effects in radiative transfer codes. Following this procedure, we run a series of simulations to explore the impact of these effects on the simulated spectra, for different $v$ and column densities of the outflow. The observed optical depth is usually considered a proxy for the wind $N_H$, independently on its velocity. However, our simulations show that the observed optical depth of an outflow with a given column density $N_H$ decreases rapidly as the velocity of the wind approaches relativistic values. This, in turn, implies that when estimating $N_H$ from the optical depth, it is necessary to include a velocity-dependent correction, already for moderate velocities (e.g. $v geq 0.05c$). This correction linearly propagates to the derived $dot{M}_{out}, dot{E}_{out}$. As an example of these effects, we calculate the relativistically corrected values of $dot{M}_{out}$ and $dot{E}_{out}$ for a sample of $sim 30$ Ultra-Fast Outflows taken from the literature, and find correction factors of $20-120 %$ within the observed range of outflowing velocities.
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The study of winds in active galactic nuclei (AGN) is of utmost importance as they may provide the long sought-after link between the central black hole and the host galaxy, establishing the AGN feedback. Recently, Laha et al. (2014) reported the X-ray analysis of a sample of 26 Seyferts observed with XMM-Newton, which are part of the so-called warm absorbers in X-rays (WAX) sample. They claim the non-detection of Fe K absorbers indicative of ultra-fast outflows (UFOs) in four observations previously analyzed by Tombesi et al. (2010). They mainly impute the Tombesi et al. detections to an improper modeling of the underlying continuum in the E=4-10 keV band. We therefore re-address here the robustness of these detections and we find that the main reason for the claimed non-detections is likely due to their use of single events only spectra, which reduces the total counts by 40%. Performing a re-analysis of the data in the whole E=0.3-10 keV energy band using their models and spectra including also double events, we find that the blue-shifted Fe K absorption lines are indeed detected at >99%. This work demonstrates the robustness of these detections in XMM-Newton even including complex model components such as reflection, relativistic lines and warm absorbers.
Among a number of active galactic nuclei (AGNs) that drive ionized outflows in X-rays, a low-redshift (z = 0.184) quasar, PDS 456, is long known to exhibit one of the exemplary ultra-fast outflows (UFOs). However, the physical process of acceleration mechanism is yet to be definitively constrained. In this work, we model the variations of the Fe K UFO properties in PDS 456 over many epochs in X-ray observations in the context of magnetohydrodynamic (MHD) accretion-disk winds employed in our earlier studies of similar X-ray absorbers. We applied the model to the 2013/2014 XMM-Newton/NuSTAR spectra to determine the UFOs condition; namely, velocity, ionization parameter, column density and equivalent width (EW). Under some provisions on the dependence of X-ray luminosity on the accretion rate applicable to near-Eddington state, our photoionization calculations, coupled to a 2.5-dimensional MHD-driven wind model, can further reproduce the observed correlations of the UFO velocity and the anti-correlation of its EW with X-ray strength of PDS 456. This work demonstrates that UFOs, even without radiative pressure, can be driven as an extreme case purely by magnetic interaction while also producing the observed spectrum and correlations.
Recent X-ray observations show absorbing winds with velocities up to mildly-relativistic values of the order of ~0.1c in a limited sample of 6 broad-line radio galaxies. They are observed as blue-shifted Fe XXV-XXVI K-shell absorption lines, similarly to the ultra-fast outflows (UFOs) reported in Seyferts and quasars. In this work we extend the search for such Fe K absorption lines to a larger sample of 26 radio-loud AGNs observed with XMM-Newton and Suzaku. The sample is drawn from the Swift BAT 58-month catalog and blazars are excluded. X-ray bright FR II radio galaxies constitute the majority of the sources. Combining the results of this analysis with those in the literature we find that UFOs are detected in >27% of the sources. However, correcting for the number of spectra with insufficient signal-to-noise, we can estimate that the incidence of UFOs is this sample of radio-loud AGNs is likely in the range f=(50+/-20)%. A photo-ionization modeling of the absorption lines with XSTAR allows to estimate the distribution of their main parameters. The observed outflow velocities are broadly distributed between v_out<1,000 km s^-1 and v_out~0.4c, with mean and median values of v_out~0.133c and v_out~0.117c, respectively. The material is highly ionized, with an average ionization parameter of logxi~4.5 erg s^-1 cm, and the column densities are larger than N_H > 10^22 cm^-2. Overall, these characteristics are consistent with the presence of complex accretion disk winds in a significant fraction of radio-loud AGNs and demonstrate that the presence of relativistic jets does not preclude the existence of winds, in accordance with several theoretical models.
X-ray evidence for ultra-fast outflows (UFOs) has been recently reported in a number of local AGNs through the detection of blue-shifted Fe XXV/XXVI absorption lines. We present the results of a comprehensive spectral analysis of a large sample of 42 local Seyferts and 5 Broad-Line Radio Galaxies (BLRGs) observed with XMM-Newton and Suzaku. We detect UFOs in >40% of the sources. Their outflow velocities are in the range 0.03-0.3c, with a mean value of ~0.14c. The ionization is high, in the range logxi~3-6 erg s^{-1} cm, and also the associated column densities are large, in the interval ~10^{22}-10^{24} cm^{-2}. Overall, these results point to the presence of highly ionized and massive outflowing material in the innermost regions of AGNs. Their variability and location on sub-pc scales favor a direct association with accretion disk winds/outflows. This also suggests that UFOs may potentially play a significant role in the AGN cosmological feedback besides jets and their study can provide important clues on the connection between accretion disks, winds and jets.
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 $sim$ 30-60% of the AGN sample hosts such UFOs. The mean and median blue-shifted velocity is found to be $sim$ 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.
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