Broad-Band X-ray study of the Hyper-Eddington AGN IRAS 04416+1215

The XMM-Newton EPIC-pn light curves showed the presence of rapid variability with a rapid increase followed by a decrease of the count rate by a factor two in less than two hours during the observation. This feature is present in the light curves extrapolated in different energy bands, but it does not affect the ratio between the flux in the 2.5-10 keV and the 0.2-2.5 keV band (see left panels of Fig. 1). The NuSTAR light-curves show no evidence of strong flux and spectral variability (see right panels of Fig. 1). To check whether flux variability affects the spectral analysis we extracted, together with the total time-averaged EPIC-pn spectrum, two spectra applying two different cuts in count-rate (c/s ≥ 0.6, and c/s < 0.6) and we looked for any spectral differences in these spectra by fitting all of them in the 0.3–10 keV energy band using our best-fitting model. For both the spectra with the two different cuts in count-rate the values of the parameters are consistent within the errors with the values obtained by fitting the total time averaged EPIC-pn spectrum. 

We looked at the variability spectrum of IRAS 04416+1215 using the fractional root mean square variability amplitude (F_var, Vaughan et al., 2003) for the XMM-Newton observation. We computed F_var using the background-subtracted EPIC-pn light curves with a temporal bin of 1000 seconds and with an increasing energy binning. We fitted the variability spectrum with a simple linear relation: y = ax + b obtaining the following slope and intercept values: a = −2.09 ± 0.23 & b = 25.19 ± 0.37.

The X-ray variability spectrum of IRAS 04416+1216 shows a different trend respect to the median of the Seyfert 1 sample from the BAT AGN Spectroscopic Survey (BASS, grey in the plot) with almost the same black hole mass as IRAS 04416+1215, with a higher variability in the soft band, suggesting that the most variable components are the ones found at low energy, such as the soft excess and/or ionized absorption (see Fig. 2).

We carried out a detailed spectral analysis of the broad-band data obtained from the simultaneous observation of the source by XMM-Newton and NuSTAR using a large variety of spectral models. The best-fitting model is composed of a soft-excess, three ionized outflows, neutral absorption and a reflection component, which was modelled with xillver in xspec. We found that the reprocessed radiation is dominating (R_refl > 8.01) over the primary continuum, which shows a slope of Γ = 1.77, as in the case of other sources with high accretion rate. Indeed, when the accretion rate is close or above the Eddington limit, the accretion disc can be radiation-pressure dominated and not perfectly flat, but clumpy or it can have the shape of deep rings or sheets. In this case the reflection component can be be stronger and the reflection fraction can be >1.

We searched for possible degeneracies between the fitting parameters, performing Monte Carlo Markov Chain (MCMC) using the xspec-emcee code by Jeremy Sanders. This is an implementation of the emcee code (Foreman-Mackey et al. 2013), to analyze X-ray spectra in xspec. We used 50 walkers with 10,000 iterations each, burning the first 1,000. The walkers started at the best fit values found in xspec, following a Gaussian distribution in each parameter, with the standard deviation set to the delta value of that parameter. Thanks to the NuSTAR unprecedented hard-band sensitivity, we obtained a very good constraint of the high energy cutoff, which is ~ 45 keV. 

From the analysis of the location (see Tab.2) and the energetic (see Tab. 3) of the X-ray winds, two main scenarios arise:

• The three winds are co-spatial absorbers, the disc emitted luminosity does not have enough power to accelerate the winds because, due to the extremely high accretion rate, much of the luminosity is advected inside the disc and the winds receive a relatively low acceleration from radiation pressure alone, suggesting that the outflows observed in IRAS 04416+1215 could be accelerated by magnetohydrodynamic processes;
• Multi-phase and multi-scale X-ray winds consistent with being a warm absorber (WA), an ultrafast outflow (UFO) and an entrained-ultrafast outflow (E-UFO) producing an important feedback on the environment surrounding the AGN. 

The coronal plasma electron temperature is expected to be related to the cut-off energy by E_cut = 2 − 3 kT_e (Petrucci et al. 2000, 2001). Following this relation applied to the value of the cutoff energy from the best fitting model (i,e, 45 keV) we derived the coronal temperature, which is kT_e ∼ 15 − 22 keV. Moreover, to directly measure the coronal temperature parameter, assuming that the primary emission is due to the Comptonization of thermal disc photons in a hot corona, we used the xillverCp versions of the tables in which the reflection spectrum is calculated by using a more physically motivated primary continuum, implemented with the analytical Comptonization model nthcomp (Zdziarski et al. 1996; Życki et al. 1999), instead of a simple cutoff power law. In this model the seed photon temperature is fixed at 50 eV. We found a photon index Γ = 1.88±0.04 and a coronal temperature kT_e= 2.95 ± 0.25 keV.

IRAS 04416+1215 has a very well constrained value of the coronal temperature, one of the lowest coronal temperature measured so far by NuSTAR. Depending on the modelling we found a coronal temperature between ∼ 3 keV and ∼ 22 keV. The location of IRAS 04416+1215 in the compactness-temperature diagram (see Fig. 6) suggests that, unlike most of the AGN, its X-ray emission is not created in a pair-dominated thermal plasma. However, considering a scenario in which the corona is made of an hybrid plasma, composed of both thermal and non-thermal electrons, the non-thermal electrons that exceed the temperature of ∼ 1MeV can emit photons which give rise to a pair run-away situation. In this case, the primary continuum emission would seem to originate in a thermal coronal plasma with low-temperature which is not pair-dominated. A possible explanation for this very low value of the coronal temperature of IRAS 04416+1215 can be found considering that, in Super- Eddington accretions flows, the comptonizing corona could originate from radiation-pressure driven optically thick (τ>3) outflows which act like a corona above the disc at relatively low temperatures