Energy Distribution of Individual Quasars from Far‐Ultraviolet to X‐Rays. I. Intrinsic Ultraviolet Hardness and Dust Opacities

We report on a remarkable change in the spectral energy distribution (SED) of Mrk 841, providing new insights on how the soft X-ray excess emission in active galactic nuclei (AGNs) is produced. By Swift monitoring of a sample of Seyfert-1 galaxies, we found an X-ray spectral hardening event in Mrk 841. We thereby triggered our XMM-Newton, NuSTAR, and Hubble Space Telescope observations in 2022 to study this event. Our previous investigations of such events in other AGNs had shown that they are caused by obscuring winds. However, the event in Mrk 841 has different spectral characteristics and origin. We find it is the soft X-ray excess component that has become dimmer. This is, importantly, accompanied by a similar decline in the optical/UV continuum, suggesting a connection to the soft X-ray excess. In contrast, there is relatively little change in the X-ray power law and the reflection components. Our SED modeling suggests that the soft X-ray excess is the high-energy extension of the optical/UV disk emission, produced by warm Comptonization. We find the temperature of the disk dropped in 2022, explaining the observed SED dimming. We then examined the Swift data, taken over 15 yr, to further decipher the UV and X-ray variabilities of Mrk 841. A significant relation between the variabilities of the X-ray spectral hardness and that of the UV continuum is found, again suggesting that the soft excess and the disk emission are interlinked. This is readily explicable if the soft excess is produced by warm Comptonization.

The soft X-ray excess – the excess of X-rays below 2 keV with respect to the extrapolation of the hard X-ray spectral continuum model – is a very common feature among type 1 active galactic nuclei (AGNs); yet the nature of the soft X-ray excess is still poorly understood and hotly debated. To shed some light on this issue, we have measured in a model-independent way the soft excess strength in a flux-limited sample of broad- and narrow-line Seyfert 1 galaxies (BLS1s and NLS1s) that are matched in X-ray luminosity but different in terms of the black hole mass and the accretion rate values, with NLS1s being characterized by smaller MBH and larger $\dot{m}$ values. Our analysis, in agreement with previous studies carried out with different AGN samples, indicates that: (1) a soft excess is ubiquitously detected in both BLS1s and NLS1s; (2) the strength of the soft excess is significantly larger in the NLS1 sample, compared to the BLS1 sample; and (3) combining the two samples, the strength of the soft excess appears to positively correlate with the photon index as well as with the accretion rate, whereas there is no correlation with the black hole mass. Importantly, our work also reveals the lack of an anticorrelation between the soft excess strength and the luminosity of the primary X-ray component, predicted by the absorption and reflection scenarios. Our findings suggest that the soft excess is consistent with being produced by a warm Comptonization component. Larger, more complete samples of NLS1s and BLS1s are needed to confirm these conclusions.

Context. Active galactic nuclei (AGN) stand as extreme X-ray emitters where disk-corona interplay shapes their spectral energy distribution. The soft X-ray excess, a unique feature of AGN in the 0.5 − 2.0 keV, encodes critical information on the “warm corona” structure bridging the disk and hot corona. However, the systematic evolution of this feature with fundamental accretion parameters in large AGN samples – particularly those studied through the spectral stacking technique – remains observationally unconstrained. Aims. The eROSITA All-Sky Survey (eRASS:5) provides an unprecedented sample to statistically map AGN spectral properties. We present a multiwavelength investigation of how the average AGN X-ray spectra evolve with accretion parameters (αox, LUV, λEdd, MBH), and we explore the disk-corona connection by further combining stacked UV data. Methods. We have developed Xstack, a novel X-ray spectral stacking code that consistently stacks rest-frame pulse invariant (PI) spectra and associated responses using optimized response weighting to preserve spectral shapes. With Xstack, we stacked 17 929 AGNs (“spec-z” sample, total exposure ∼23 Ms) with similar X-ray loudness, αox, and UV luminosity, LUV, and 4159 AGNs (“BH-mass” sample, ∼3 Ms) with similar Eddington ratios, λEdd, and black hole masses, MBH. We analyzed the resulting stacked X-ray spectra with a phenomenological model for both samples. We further fit the stacked optical-UV X-ray SED with the physical AGNSED model on a 3 × 3 MBH – λEdd grid. Results. We observed that the soft excess strength rises strongly with increasing αox and λEdd binning (by a factor of five), while the hard X-ray spectral shape remains largely unchanged, consistent with the interpretation that soft excess is primarily driven by the warm corona rather than reflection. The trends are weaker with LUV binning and reversed for MBH binning. The analysis of the optical-UV X-ray SEDs with AGNSED revealed that the warm corona radius (in units of Rg) generally increases with λEdd and decreases with MBH, or equivalently the disk-to-warm-corona transition consistently occurs near ∼1 × 104 K. The hot corona contracts with λEdd, and the radius remains independent of MBH, aligning with disk evaporation predictions. Conclusions. The soft excess is likely warm-corona dominated, with the disk-to-warm-corona transition potentially linked to hydrogen ionization instability at ∼1 × 104 K, which is consistent with previous work utilizing eFEDS-HSC stacked data. Our work highlights the power of spectral stacking for revealing the AGN disk-corona connection.

A core sample of 59 unobscured type 1 active galactic nuclei (AGNs) with simultaneous XMM-Newton X-ray and UV observations is compiled from the archives to probe the nature of soft X-ray excess (SE). In the first paper of this series, our focus centers on scrutinizing the spectral profile of the soft excess. Of the sources, ≈71% (42/59) exhibit power-law-like (po-like) soft excess, while ≈29% (17/59) exhibit blackbody-like (bb-like) soft excess. We show that a cutoff power law could uniformly characterize both types of soft excesses, with a median E cut of 1.40 keV for po-like and 0.14 keV for bb-like. For the first time, we report a robust and quantitative correlation between the SE profile and SE strength (the ratio of SE luminosity to that of the primary power-law continuum in 0.5–2.0 keV), indicating that stronger soft excess is more likely to be po-like, or effectively has a higher E cut. This correlation cannot be explained by ionized disk reflection alone, which produces mostly bb-like soft excess (E cut ~ 0.1 keV) as revealed by relxilllp simulation. Remarkably, we show with simulations that a toy hybrid scenario, where both ionized disk reflection (relxilllp, with all reflection parameters fixed at default values except for ionization of the disk) and warm corona (compTT, with temperature fixed at 1 keV) contribute to the observed soft excess, can successfully reproduce the observed correlation. This highlights the ubiquitous hybrid nature of the SE in AGNs, and underscores the importance of considering both components while fitting the spectra of soft excess.

We report the results from a recent 133 ks XMM-Newton observation of a highly super-Eddington narrow-line Type-1 QSO RX J0439.6-5311. This source has one of the steepest AGN hard X-ray slopes, in addition to a prominent and smooth soft X-ray excess. Strong variations are found throughout the 0.3 to 10 keV energy range on all time-scales covered by the observation, with the soft excess mainly showing low frequency variations below 0.1 mHz while the hard X-rays show stronger variability at higher frequencies. We perform a full set of spectral-timing analysis on the X-ray data, including a simultaneous modelling of the time-average spectra, frequency-dependent RMS and covariance spectra, lag-frequency and lag-energy spectra. Especially, we find a significant time-lag signal in the low frequency band, which indicates that the soft X-rays lead the hard by $\sim$4 ks, with a broad continuum-like profile in the lag spectrum. Our analysis strongly supports the model where the soft X-ray excess is dominated by a separate low temperature, optically thick Comptonisation component rather than relativistic reflection or a jet. This soft X-ray emitting region is several tens or hundreds of $R_{g}$ away from the hot corona emitting hard X-rays, and is probably associated with a geometrically thick (`puffed-up') inner disc region.

We show that the soft X-ray excess in the Coma cluster can be explained by a cosmic background of relativistic axion-like particles (ALPs) converting into photons in the cluster magnetic field. We provide a detailed self-contained review of the cluster soft X-ray excess, the proposed astrophysical explanations and the problems they face, and explain how a 0.1- 1 keV axion background naturally arises at reheating in many string theory models of the early universe. We study the morphology of the soft excess by numerically propagating axions through stochastic, multi-scale magnetic field models that are consistent with observations of Faraday rotation measures from Coma. By comparing to ROSAT observations of the 0.2- 0.4 keV soft excess, we find that the overall excess luminosity is easily reproduced for gaγγ ∼ 2 × 10-13 Ge -1. The resulting morphology is highly sensitive to the magnetic field power spectrum. For Gaussian magnetic field models, the observed soft excess morphology prefers magnetic field spectra with most power in coherence lengths on \U0001d4aa(3 kpc) scales over those with most power on \U0001d4aa(12 kpc) scales. Within this scenario, we bound the mean energy of the axion background to 50 eV≲ ⟨ Ea ⟩ ≲ 250 eV, the axion mass to ma ≲ 10-12 eV, and derive a lower bound on the axion-photon coupling gaγγ ≳ √(0.5/Δ Neff) 1.4 × 10-13 Ge -1.

view Abstract Citations (37) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Simultaneous ROSAT, GINGA, VLA, IUE, and Optical Observations of the Bright Quasar H1821+643 Kolman, Michiel ; Halpern, Jules P. ; Shrader, Chris R. ; Filippenko, Alexei V. ; Fink, Henner H. ; Schaeidt, Stephan G. Abstract We report on the first simultaneous optical, UV, and X-ray spectroscopy of the low-redshift (z = 0.297), high-luminosity QSO H 1821 + 643. This multiwavelength campaign was complemented by optical imaging and radio mapping with the VLA. Although there was no significant variability during the 40 day monitoring period in either the UV or X-ray bands, a number of new results were obtained. There is significant variability in the optical, UV, and X-ray bands on time scales of years, both in flux level and spectral shape. In the co-added ultraviolet spectrum we detected a Lyman-α absorption line at z = 0.225. In the radio map the QSO is extended in the north-south direction at 20 cm, and can be resolved into two separate components at 6 cm. In the ROSAT data we separated (both spatially and spectroscopically) the emission originating in H1821 + 643 from the nearby hot white dwarf K1 - 16, and thus we isolated the intrinsic soft X-ray excess in the QSO. The IUE and ROSAT data were combined with new optical spectroscopy and hard X-ray data from Ginga, to reveal a strong optical/UV bump with a flat slope (α~ -0.96, f_v_ is proportional to Valpha^) in the UV, a steep soft X-ray excess (α~ -4) between ~0.15 keV and ~0.5 keV, and a typical hard X-ray slope of α~ -0.8 extending up to 10 keV. While the optical/UV bump and the soft X-ray excess can each be fit satisfactorily with standard accretion disk models, a joint fit to the optical/UV/X-ray bump cannot be achieved. This suggests that either the optical/UV or the soft X-ray emission do not arise in an accretion disk, or that the standard bare disk model should be modified (possibly by inclusion of electron scattering) to account for both the strong, flat UV emission and the steep, soft X-ray excess. Publication: The Astrophysical Journal Pub Date: January 1993 DOI: 10.1086/172153 Bibcode: 1993ApJ...402..514K Keywords: Accretion Disks; Iue; Quasars; Rosat Mission; Spaceborne Astronomy; Very Large Array (Vla); Astronomical Models; Astronomical Spectroscopy; Emission Spectra; Red Shift; Spectral Energy Distribution; Ultraviolet Spectroscopy; X Ray Spectroscopy; Astrophysics; RADIO CONTINUUM: GALAXIES; ULTRAVIOLET: GALAXIES; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: H1821; 643; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (8) NED (2) MAST (1) INES (1)

The smooth soft X-ray excess seen in many type 1 active galactic nuclei can be well described by models of absorption in partially ionized material with a large velocity dispersion, often physically interpreted as a radiatively driven accretion disc wind. However, the state-of-the-art xscort code, which calculates the photoionized radiative transfer through a differentially outflowing absorber, shows that terminal velocities of the order of ∼0.9c are required in order to reproduce the soft X-ray excess. Such a high outflow velocity rules out ultraviolet line driving, continuum radiation driving and thermal driving as mechanisms for producing the wind. Entrainment of material by the magnetically driven jet is the only plausible origin of such a high-velocity flow, but numerical simulations of jets and associated outflows do not currently show sufficient material at high enough velocities to reproduce the soft X-ray excess. If the soft excess is produced by absorption then it seems more likely that the material is clumpy and/or only partially covers the source rather than forming a continuous outflow.

We investigate a sample of 14 clusters of galaxies observed with XMM-Newton in a search for soft X-ray excess emission. In five of these clusters a significant soft excess is evident. This soft X-ray excess is compared with the thermal emission from both the hot intracluster gas and any cooling (flow) gas that may be present. A warm (kT=0.2 keV), extended (several Mpc), plasma component is particularly clear in the outer parts of the cluster, where the normal cluster X-ray emission is weak. This warm component causes both a thermal soft X-ray excess at low energies (below 0.4-0.5 keV), as well as O VII line emission with a redshift consistent with a cluster origin, and not easily interpreted as Galactic foreground emission. The intensity of this component is commensurate with what has been measured before with the ROSAT PSPC in the 1/4 keV band. We attribute this component to emission from intercluster filaments of the Warm-Hot Intergalactic Medium in the vicinity of these clusters. For the central regions of clusters the detection of lines in the soft X-ray spectrum is more difficult, due to the predominance of the X-ray emitting hot plasma there, hence we cannot discriminate between the thermal and nonthermal origin of the soft excess, leaving several options open. These include thermal emission from warm filaments seen in projection in front of or behind the cluster center, thermal or nonthermal emission in the cluster core itself related to magnetic reconnection, or Inverse Compton emission from the cosmic microwave background on relativistic electrons.

We present XMM–Newton/EPIC spectra for the Laor et al. sample of Palomar Green (PG) quasars. We find that a power law provides a reasonable fit to the 2–5 keV region of the spectra. Excess soft X-ray emission below 2 keV is present for all objects, with the exception of those known to contain a warm absorber. However, a single power law is a poor fit to the 0.3–10.0 keV spectrum and instead we find that a simple model, consisting of a broken power law (plus an iron line), provides a reasonable fit in most cases. The equivalent width of the emission line is constrained in just 12 objects but with low (<2σ) significance in most cases. For the sources whose spectra are well fitted by the broken-power-law model, we find that various optical and X-ray line and continuum parameters are well correlated; in particular, the power-law photon index is well correlated with the FWHM of the Hβ line and the photon indices of the low- and high-energy components of the broken power law are well correlated with each other. These results suggest that the 0.3–10 keV X-ray emission shares a common (presumably non-thermal) origin, as opposed to suggestions that the soft excess is directly produced by thermal disc emission or via an additional spectral component. We present XMM–Newton Optical Monitor (OM) data, which we combine with the X-ray spectra so as to produce broad-band spectral energy distributions (SEDs), free from uncertainties due to long-term variability in non-simultaneous data. Fitting these optical–UV spectra with a Comptonized disc model indicates that the soft X-ray excess is independent of the accretion disc, confirming our interpretation of the tight correlation between the hard and soft X-ray spectra.

The origin of the soft X-ray excess emission observed in many type-1 active galactic nuclei (AGN) has been an unresolved problem in X-ray astronomy for over two decades. We develop the model proposed by Gierliński & Done, which models the soft excess with heavily smeared, ionized, absorption, by including the emission that must be associated with this absorption. We show that, rather than hindering the ionized absorption model, the addition of the emission actually helps this model reproduce the soft excess. The emission fills in some of the absorption trough, while preserving the sharp rise at ∼1 keV, allowing the total model to reproduce the soft excess curvature from a considerably wider range of model parameters. We demonstrate that this model is capable of reproducing even the strongest soft X-ray excesses by fitting it to the XMM–Newton EPIC PN spectrum of PG1211+143, with good results. The addition of the emission reduces the column density required to fit these data by a factor of ∼2 and reduces the smearing velocity from ∼0.28c to ∼0.2c. Gierliński & Done suggested a tentative origin for the absorption in the innermost, accelerating, region of an accretion disc wind, and we highlight the advantages of this interpretation in comparison to accretion disc reflection models of the soft excess. Associating this material with a wind off the accretion disc results in several separate problems however, namely, the radial nature, and the massive implied mass-loss rate, of the wind. We propose an origin in a ‘failed wind’, where the central X-ray source is strong enough to overionize the wind, removing the acceleration through line absorption before the material reaches escape velocity, allowing the material to fall back to the disc at larger radii.

(Abridged) 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 EUV. 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 ($\sim 2.4$) 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. However, the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by cool, 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 thermalises at large radii, but that at smaller radii the gravitational energy is split between powering optically thick Comptonised 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 REJ1034+396, and to the much lower Eddington fraction Broad Line Seyfert 1 PG1048+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 AGN spectra as a function of $L/L_{Edd}$. The new model is publically available within the {\sc xspec} spectral fitting package.

Context. The nature and origin of the soft X-ray excess in radio-quiet AGN is still an open issue. The interpretation in terms of thermal disc emission has been challanged by the discovery of the constancy of the effective temperature despite the wide range of Black Hole masses of the observed sources. Alternative models are reflection from ionized matter and absorption in a relativistically smeared wind. Aims. We analyzed XMM-Newton observations of four luminous radio-quiet AGN with the aim of characterising their main properties and in particular the soft excess. Methods. Different spectral models for the soft excess were tried: thermal disc emission, Comptonization, ionized reflection, relativistically smeared winds. Results. Comptonization of thermal emission and smeared winds provide the best fits, but the other models also provide acceptable fits. All models, however, return parameters very similar from source to source, despite the large differences in luminosities, Black Hole masses and Eddington ratios. Moreover, the smeared wind model requires very large smearing velocities. The UV to X-ray fluxes ratios are very different, but do not correlate with any other parameter. Conclusions. No fully satisfactory explanation for the soft X-ray excess is found. More informative data, such as observations in a broader energy band, are needed to make further progress.

We report the results obtained by a broad-band (0.5-500 keV) data analysis of narrow-line Seyfert 1 galaxy NGC 4748 observed with an XMM-Newton/PN, INTEGRAL/ISGRI and SWIFT/BAT telescopes. This galaxy has a soft X-ray excess that is typical for the class of narrow-line Seyfert 1. The question of the origin of soft excess in such objects is still unclear. We tested and compared two spectral models for the soft X-ray spectra based on the different physical scenarios. The first one is based on the Done & Nayakshin model of two-phase accretion disc in a vertical direction, which includes two reflection zones with different ionization levels. According to this model, we found that a highly ionized reflection has the value of ionization $\xi\sim3000$ $erg~s^{-1}~cm$ and is mostly responsible for the soft excess. This reflection becomes comparable with a low ionized one ($\xi\sim30$ $erg~s^{-1}~cm$) in moderate X-ray range. However, this model requires also an additional component at soft energies with $kT\sim300$ eV. The second model is an energetically self-consistent model and assumes that a soft excess arises from optically thick thermal Comptonization of the disc emission. Combination of the UV (from XMM/Optical monitor) and X-ray data in the latter model allowed us to determine a mass of the central black hole of $6.9\times10^6 M_{\odot}$ and Eddington ratio $log_{L/L_{Edd}}\simeq-0.57$. Also, we were not able to rule out one of competing models using only X-ray spectra of NGC 4748.

Context. The X-ray broadband spectra of the bare active galactic nucleus (AGN) Mrk 110, obtained by simultaneous XMM-Newton and NuSTAR observations performed in November 2019 and April 2020, are characterised by the presence of a prominent and absorption-free smooth soft X-ray excess, moderately broad O VII and Fe Kα emission lines, and a lack of a strong Compton hump. The disc-corona system is almost viewed face-on as inferred from the O VII accretion disc lines. While relativistic reflection as the sole emission is ruled out, a simplified combination of soft and hard Comptonisation (using COMPTT) from a warm and a hot corona, plus mild relativistic disc reflection (occuring at a few 10 s Rg) reproduces the data very well. Aims. We aim to confirm the physical origin of the soft X-ray excess of Mrk 110 and to determine its disc-corona system properties from its energetics using two new sophisticated models: REXCOR and RELAGN, respectively. Methods. We applied these models to the 0.3–79 keV X-ray broadband spectra and to the spectral energy distribution (SED) from UV to hard X-rays, respectively. Results. At both epochs, the inferred high values of the warm-corona heating from the X-ray broadband spectral analysis using REXCOR confirm that the soft X-ray excess of Mrk 110 mainly originates from a warm corona rather than relativistic reflection. The intrinsic best-fit SED determined at both epochs using RELAGN show a high X-ray contribution relative to the UV and are very well reproduced by a warm and hot corona plus mild relativistic reflection. The outer radii of the hot and warm corona are located at a few 10 s and ∼100 Rg, respectively. Moreover, combining the inferred low Eddington ratio (approximatively a few percent) from this work, and previous multi-wavelength spectral and timing studies suggest that Mrk 110 could be classified as a moderate changing-state AGN. Conclusions. Our analysis confirms the existence of a warm corona as a significant contribution to the soft X-ray excess and UV emission in Mrk 110, adding to growing evidence that AGN accretion deviates from standard disc theory. This strengthens the importance of long-term multi-wavelength monitoring on both single targets and large AGN surveys to reveal the real nature of the disc-corona system in AGNs.