Narrow‐line Seyfert 1 (NLS1) galaxies have low‐mass black holes and mass accretion rates close to (or exceeding) Eddington, so a standard blackbody accretion disc should peak in the extreme ultraviolet. However, the lack of true absorption opacity in the disc means that the emission is better approximated by a colour temperature corrected blackbody, and this colour temperature correction is large enough (∼2.4) that the bare disc emission from a zero spin black hole can extend into the soft X‐ray bandpass. Part of the soft X‐ray excess seen in these objects must be intrinsic emission from the disc unless the vertical structure is very different to that predicted. None the less, this is not the whole story even for the extreme NLS1 as the shape of the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by warm, optically thick material. We associate this with the disc itself, so it must ultimately be powered by mass accretion. We build an energetically self‐consistent model assuming that the emission thermalizes to a (colour temperature corrected) blackbody only at large radii. At smaller radii the gravitational energy is split between powering optically thick Comptonized disc emission (forming the soft X‐ray excess) and an optically thin corona above the disc (forming the tail to higher energies). We show examples of this model fit to the extreme NLS1 RE J1034+396, and to the much lower Eddington fraction broad‐line Seyfert 1 PG 1048+231. We use these to guide our fits and interpretations of three template spectra made from co‐adding multiple sources to track out a sequence of active galactic nucleus (AGN) spectra as a function of L/LEdd. Both the individual objects and template spectra show the surprising result that the Compton upscattered soft X‐ray excess decreases in importance with increasing L/LEdd. The strongest soft excesses are associated with low mass accretion rate AGN rather than being tied to some change in disc structure around Eddington. We argue that this suggests a true break in accretion flow properties between stellar and supermassive black holes. The new model is publicly available within the xspec spectral fitting package.

FAST FRB backend; LQ, GH, XYX, QJZ, SD made key contributions to the overall FAST data processing pipelines; LS, MC, MK provided salient information on FRB 121102 from other observatories, particularly Effelsberg, and contributed to the scientific analysis; SC, JMC, DRL made numerous corrections to the writing and analysis. JMC, in particular, pointed out the errors in the noise floor analysis in the original draft. * Uncertainties in parentheses refer to the last quoted digit. † Reduced χ 2 is obtained by the best fitting method with 20 iterations. ‡ Coefficient of determination, R 2 = 1 − S res /S tot , where S tot is total sum of squares from data, and S res is the minimum fitting residual sum of squares.

We present modelling and interpretation of the continuum and emission lines for a sample of 51 unobscured type 1 active galactic nuclei (AGNs). All of these AGNs have high‐quality spectra from both XMM–Newton and the Sloan Digital Sky Survey. We extend the wavelength coverage where possible by adding simultaneous ultraviolet data from the OM onboard XMM–Newton. Our sample is selected based on low reddening in the optical and low gas columns implied by their X‐ray spectra, except for one case, the broad absorption line quasar PG 1004+130. They also lack clear signatures for the presence of a warm absorber. Therefore, the observed characteristics of this sample are likely to be directly related to the intrinsic properties of the central engine. To determine the intrinsic optical continuum, we subtract the Balmer continuum and all major emission lines (including Fe ii). We also consider possible effects of contamination from the host galaxy. The resulting continuum is then used to derive the properties of the underlying accretion disc. We constrain the black hole masses from spectral fits of the Balmer emission lines and determine the best‐fitting value from the modelling of broad‐band spectral energy distributions (SEDs). In addition to the disc component, many of these SEDs also exhibit a strong soft X‐ray excess, plus a power law extending to higher X‐ray energies. We fit these SEDs by applying a new broad‐band SED model which comprises accretion disc emission, low‐temperature optically‐thick Comptonization and a hard X‐ray tail by introducing the concept of a corona radius. We find that in order to fit the data, the model often requires an additional long‐wavelength optical continuum component, whose origin is discussed in this paper. We also find that the photorecombination edge of the Balmer continuum shifts and broadens beyond the standard limit of 3646 Å, implying an electron number density which is far higher than that in the broad‐line‐region clouds. Our results indicate that the narrow‐line type 1 Seyfert galaxies in this sample tend to have lower black hole masses, higher Eddington ratios, softer 2–10 keV band spectra, lower 2–10 keV luminosities and higher αox, compared with typical broad‐line type 1 Seyfert galaxies, although their bolometric luminosities are similar. We illustrate these differences in properties by forming an average SED for three subsamples, based on the full width at half‐maximum velocity width of the Hβ emission line.

In this third paper in a series of three, we present a detailed study of the broad‐band spectral energy distribution (SED) of active galactic nuclei (AGNs) based on a nearby unobscured type 1 AGN sample. We perform a systematic cross‐correlation study of several key parameters, i.e. Γ2‐10 keV, L2‐10 keV, Lbol/LEdd = λEdd, Lbol/L2‐10 keV = κ2‐10 keV, L bol /L5100Å=κ5100Å, FWHMHβ, MBH, αox, αX and αUV. The well‐defined spectral properties of the sample enable us to improve existing relations and to identify new correlations among these parameters. We confirm a break region around FWHMHβ ≃ 4000 km s−1 in the Γ2‐10 keV versus FWHMHβ correlation and log(MBH) ≃ 8.0 in the Γ2‐10 keV versus MBH correlation, where these correlations appear to change form. Beyond the break point, the intrinsic Γ2‐10 keV index is dispersed around 1.8. Several new correlations are also reported in this paper, e.g. strong correlations in κ5100 versus λEdd, κ5100 versus κ2‐10 keV and κ2‐10 keV versus MBH. The principal component analysis (PCA) is performed on the correlation matrix of the above parameters. This shows that the three physical parameters, i.e. black hole mass, mass accretion rate and Eddington ratio, drive the majority of the correlations. This is consistent with PCA results found from previous optical spectral studies. For each key parameter, we split the AGNs into three subsamples, binned based on increasing value of that parameter. We co‐add the model SEDs for each object in the subsample to see how the SED changes with that parameter. Most parameters, except Lbol, show similar systematic changes in the SED such that the temperature at which the disc peaks is correlated with the ratio of power in the disc versus the Comptonized components and the hard X‐ray spectral index. This underlying change in SED shape shows that AGNs do exhibit intrinsically different spectral states. This is superficially similar to the SED differences in black hole binary (BHB) seen as λEdd increases, but the analogy does not hold in detail. Only objects with the highest λEdd appear to correspond to a BHB spectral state (the disc‐dominated high/soft state). The AGNs with typical mass accretion rates have spectra which do not match well with any state observed in BHB. We speculate that this could be due to the presence of a powerful ultraviolet line‐driven disc wind, which complicates simple mass scaling between stellar and supermassive black holes.

We present the first fully simultaneous fits to the NIR and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A ⋆ , the supermassive black hole at the Milky Way's center. Our study arises from ambitious multi-wavelength monitoring campaigns with XMMNewton, NuSTAR and SINFONI. The average multi-wavelength spectrum is well reproduced by a broken power-law with Γ N IR = 1.7 ± 0.1 and Γ X = 2.27 ± 0.12. The difference in spectral slopes (∆Γ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 10 3 s, when a very steep X-ray spectrum (∆Γ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high energy cut-off in the electron distribution (γ max ) induces an initial cut-off in the optical-UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γ max . We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A ⋆ 's flaring emission.