X-ray Excess Research Articles

Inferring Compton-thick AGN candidates at z > 2 with Chandra using the >8 keV rest-frame spectral curvature

To fully understand cosmic black hole growth, we need to constrain the population of heavily obscured active galactic nuclei (AGNs) at the peak of cosmic black hole growth (z ∼1–3). Sources with obscuring column densities higher than 1024 atoms cm−2, called Compton-thick (CT) AGNs, can be identified by excess X-ray emission at ∼20–30 keV, called the ‘Compton hump’. We apply the recently developed Spectral Curvature (SC) method to high-redshift AGNs (2 < z < 5) detected with Chandra. This method parametrizes the characteristic ‘Compton hump’ feature cosmologically redshifted into the X-ray band at observed energies <10 keV. We find good agreement in CT AGNs found using the SC method, and bright sources fit using their full spectrum with X-ray spectroscopy. In the Chandra Deep Field-South, we measure a CT fraction of 17|$^{+19}_{-11}\hbox{\,per\,cent}$| (3/17) for sources with observed luminosity >5 × 1043erg s−1. In the Cosmological Evolution Survey (COSMOS), we find an observed CT fraction of |$15^{+4}_{-3}\hbox{\,per\,cent}$| (40/272) or 32 ± 11 per cent when corrected for the survey sensitivity. When comparing to low redshift AGNs with similar X-ray luminosities, our results imply that the CT AGN fraction is consistent with having no redshift evolution. Finally, we provide SC equations that can be used to find high-redshift CT AGNs (z > 1) for current (XMM–Newton) and future (eROSITA and ATHENA) X-ray missions.

On the Origin of the Hard X-Ray Excess of High-Synchrotron-Peaked BL Lac Object Mrk 421

Abstract For the first time, Kataoka &amp; Stawarz reported a clear detection of a hard X-ray excess, above , in the high-synchrotron-peaked BL Lac object Mrk 421. We find that this feature may not be produced by the low-energy part of the same electron population that produced the Fermi/LAT γ-ray. Because of that, it is required that the power-law electron energy go down to , which predicts a very strong radio emission (radio flux larger than the observed), even considering the synchrotron self-absorption effect. We investigate the possibility of this excess being produced from the spine/layer jet structure, which has been clearly detected in Mrk 421. We find that (1) similar to one-zone modeling, the spine emissions provide good modeling of the broadband spectral energy distribution, except for the hard X-ray excess; and (2) the hard X-ray excess can be well represented by the synchrotron photons (from the layer) being inverse Compton scattered by the spine electrons.

The origin of the UV/optical lags in NGC 5548

The new multi-wavelength monitoring campaign on NGC 5548 shows clearly that the variability of the UV/optical lightcurves lags by progressively longer times at longer wavelengths, as expected from reprocessing of an optically thick disk, but that the timescales are longer than expected for a standard Shakura-Sunyaev accretion disc. We build a full spectral-timing reprocessing model to simulate the UV/optical lightcurves of NGC 5548. We show that disc reprocessing of the observed hard X-ray lightcurve produces optical lightcurves with too much fast variability as well as too short a lag time. Supressing the fast variability requires an intervening structure preventing the hard X-rays from illuminating the disc. We propose this is the disc itself, perhaps due to atomic processes in the UV lifting the photosphere, increasing the scale-height, making it less dense and less able to thermalise, so that it radiates low temperature Comptonised emission as required to produce the soft X-ray excess. The outer edge of the puffed-up Comptonised disc region emits FUV flux, and can illuminate the outer thin blackbody disc but while this gives reprocessed variable emission which is much closer to the observed UV and optical lightcurves, the light travel lags are still too short to match the data. We reverse engineer a solution to match the observations and find that the luminosity and temperature of the lagged emission is not consistent with material at the light travel lag distance responding to the irradiating flux (either FUV or X-ray) from the AGN. We conclude that the UV/optical lags of NGC 5548 are not the light travel time from X-ray reprocessing, nor the light travel time from FUV reprocessing, but instead could be the timescale for the outer blackbody disc vertical structure to respond to the changing FUV illumination.

Super-Eddington QSO RX J0439.6−5311 – I. Origin of the soft X-ray excess and structure of the inner accretion flow

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.

Identification of the Hard X-Ray Source Dominating the E &gt; 25 keV Emission of the Nearby Galaxy M31

Abstract We report the identification of a bright hard X-ray source dominating the M31 bulge above 25 keV from a simultaneous NuSTAR–Swift observation. We find that this source is the counterpart to Swift J0042.6+4112, which was previously detected in the Swift BAT All-sky Hard X-ray Survey. This Swift BAT source had been suggested to be the combined emission from a number of point sources; our new observations have identified a single X-ray source from 0.5 to 50 keV as the counterpart for the first time. In the 0.5–10 keV band, the source had been classified as an X-ray Binary candidate in various Chandra and XMM-Newton studies; however, since it was not clearly associated with Swift J0042.6+4112, the previous E &lt; 10 keV observations did not generate much attention. This source has a spectrum with a soft X-ray excess (kT ∼ 0.2 keV) plus a hard spectrum with a power law of and a cutoff around 15–20 keV, typical of the spectral characteristics of accreting pulsars. Unfortunately, any potential pulsation was undetected in the NuSTAR data, possibly due to insufficient photon statistics. The existing deep HST images exclude high-mass (&gt;3 ) donors at the location of this source. The best interpretation for the nature of this source is an X-ray pulsar with an intermediate-mass (&lt;3 ) companion or a symbiotic X-ray binary. We discuss other possibilities in more detail.

Applications of nonlocal means algorithm in low-dose X-ray CT image processing and reconstruction: A review.

Low-dose X-ray computed tomography (LDCT) imaging is highly recommended for use in the clinic because of growing concerns over excessive radiation exposure. However, the CT images reconstructed by the conventional filtered back-projection (FBP) method from low-dose acquisitions may be severely degraded with noise and streak artifacts due to excessive X-ray quantum noise, or with view-aliasing artifacts due to insufficient angular sampling. In 2005, the nonlocal means (NLM) algorithm was introduced as a non-iterative edge-preserving filter to denoise natural images corrupted by additive Gaussian noise, and showed superior performance. It has since been adapted and applied to many other image types and various inverse problems. This paper specifically reviews the applications of the NLM algorithm in LDCT image processing and reconstruction, and explicitly demonstrates its improving effects on the reconstructed CT image quality from low-dose acquisitions. The effectiveness of these applications on LDCT and their relative performance are described in detail.

Chandra Survey of Nearby Galaxies: The Catalog

Abstract We searched the public archive of the Chandra X-ray Observatory as of 2016 March and assembled a sample of 719 galaxies within 50 Mpc with available Advanced CCD Imaging Spectrometer observations. By cross-correlation with the optical or near-infrared nuclei of these galaxies, 314 of them are identified to have an X-ray active galactic nucleus (AGN). The majority of them are low-luminosity AGNs and are unlikely X-ray binaries based upon their spatial distribution and luminosity functions. The AGN fraction is around 60% for elliptical galaxies and early-type spirals, but drops to roughly 20% for Sc and later types, consistent with previous findings in the optical. However, the X-ray survey is more powerful in finding weak AGNs, especially from regions with active star formation that may mask the optical AGN signature. For example, 31% of the H ii nuclei are found to harbor an X-ray AGN. For most objects, a single power-law model subject to interstellar absorption is adequate to fit the spectrum, and the typical photon index is found to be around 1.8. For galaxies with a non-detection, their stacked Chandra image shows an X-ray excess with a luminosity of a few times 1037 erg s−1 on average around the nuclear region, possibly composed of faint X-ray binaries. This paper reports on the technique and results of the survey; in-depth analysis and discussion of the results will be reported in forthcoming papers.

XMMSL1 J074008.2-853927: a tidal disruption event with thermal and non-thermal components

We study X-ray bright tidal disruption events (TDE), close to the peak of their emission, with the intention of understanding the evolution of their light curves and spectra. Candidate TDE are identified by searching for soft X-ray flares from non-active galaxies in recent XMM-Newton slew data. In April 2014, X-ray emission was detected from the galaxy XMMSL1 J074008.2-853927 (a.k.a. 2MASX 07400785-8539307), a factor 20 times higher than an upper limit from 20 years earlier. Both the X-ray and UV flux subsequently fell, by factors of 70 and 12 respectively. The bolometric luminosity peaked at Lbol~2E44 ergs/s with a spectrum that may be modelled with thermal emission in the UV band, a power-law with slope~2 dominating in the X-ray band above 2 keV and a soft X-ray excess with an effective temperature of ~86 eV. Rapid variability locates the X-ray emission to within <73 Rg of the nuclear black hole. Radio emission of flux density ~1 mJy, peaking at 1.5 GHz was detected 21 months after discovery. Optical spectra indicate that the galaxy, at a distance of 73 Mpc (z=0.0173), underwent a starburst 2 Gyr ago and is now quiescent. We consider a tidal disruption event to be the most likely cause of the flare. If this proves to be correct then this is a very clean example of a disruption exhibiting both thermal and non-thermal radiation.

The contribution of bulk Comptonization to the soft X-ray excess in AGN

Bulk velocities exceed thermal velocities for sufficiently radiation pressure dominated accretion flows. We model the contribution of bulk Comptonization to the soft X-ray excess in AGN. Bulk Comptonization is due to both turbulence and the background shear. We calculate spectra both taking into account and not taking into account bulk velocities using scaled data from radiation magnetohydrodynamic (MHD) shearing box simulations. We characterize our results with temperatures and optical depths to make contact with other warm Comptonization models of the soft excess. We chose our fiducial mass, $M = 2 \times 10^6 M_{\odot}$, and accretion rate, $L/L_{\rm Edd} = 2.5$, to correspond to those fit to the super-Eddington narrow line Seyfert 1 (NLS1) RE1034+396. The temperatures, optical depths, and Compton $y$ parameters we find broadly agree with those fit to RE1034+396. The effect of bulk Comptonization is to shift the Wien tail to higher energy and lower the gas temperature, broadening the spectrum. Observations of the soft excess in NLS1s can constrain the properties of disc turbulence if the bulk Comptonization contribution can be separated out from contributions from other physical effects, such as reflection and absorption.

ALMA and RATIR observations of GRB 131030A

Abstract We report on the first open-use based Atacama Large Millimeter/submm Array (ALMA) 345 GHz observation for the late afterglow phase of GRB 131030A. The ALMA observation constrained a deep limit at 17.1 d for the afterglow and host galaxy. We also identified a faint submillimeter source (ALMA J2300−0522) near the GRB 131030A position. The deep limit at 345 GHz and multifrequency observations obtained using Swift and RATIR yielded forward-shock modeling with a two-dimensional relativistic hydrodynamic jet simulation and described X-ray excess in the afterglow. The excess was inconsistent with the synchrotron self-inverse Compton radiation from the forward shock. The host galaxy of GRB 131030A and optical counterpart of ALMA J2300−0522 were also identified in the Subaru image. Based on the deep ALMA limit for the host galaxy, the 3σ upper limits of IR luminosity and the star formation rate (SFR) are estimated as LIR &amp;lt; 1.11 × 1011 L⊙ and SFR &amp;lt;18.7 (M⊙ yr−1), respectively. Although the separation angle from the burst location (3${^{\prime\prime}_{.}}$5) was rather large, ALMA J2300−0522 may be one component of the GRB 131030A host galaxy, according to previous host galaxy cases.

A PHYSICALLY BASED MODEL OF THE IONIZING RADIATION FROM ACTIVE GALAXIES FOR PHOTOIONIZATION MODELING

ABSTRACT We present a simplified model of active galactic nucleus (AGN) continuum emission designed for photoionization modeling. The new model oxaf reproduces the diversity of spectral shapes that arise in physically based models. We identify and explain degeneracies in the effects of AGN parameters on model spectral shapes, with a focus on the complete degeneracy between the black hole mass and AGN luminosity. Our reparametrized model oxaf removes these degeneracies and accepts three parameters that directly describe the output spectral shape: the energy of the peak of the accretion disk emission , the photon power-law index of the non-thermal emission Γ, and the proportion of the total flux that is emitted in the non-thermal component . The parameter is presented as a function of the black hole mass, AGN luminosity, and “coronal radius” of the optxagnf model upon which oxaf is based. We show that the soft X-ray excess does not significantly affect photoionization modeling predictions of strong emission lines in Seyfert narrow-line regions. Despite its simplicity, oxaf accounts for opacity effects where the accretion disk is ionized because it inherits the “color correction” of optxagnf. We use a grid of mappings photoionization models with oxaf ionizing spectra to demonstrate how predicted emission-line ratios on standard optical diagnostic diagrams are sensitive to each of the three oxaf parameters. The oxaf code is publicly available in the Astrophysics Source Code Library.

A REVERSE SHOCK IN GRB 160509A

ABSTRACT We present the second multi-frequency radio detection of a reverse shock in a γ-ray burst. By combining our extensive radio observations of the Fermi-Large Area Telescope γ-ray burst 160509A at z = 1.17 up to 20 days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at ≲10 days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of , supporting our previous suggestion that a low-density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, N H ≈ 1.5 × 1022 , and a high rest-frame optical extinction, A V ≈ 3.4 mag. We identify a jet break in the X-ray light curve at , and thus derive a jet opening angle of , yielding a beaming-corrected kinetic energy and radiated γ-ray energy of erg and erg (1–104 keV, rest frame), respectively. Consistency arguments connecting the forward shocks and reverse shocks suggest a deceleration time of s ≈ T 90, a Lorentz factor of , and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of . Our study highlights the power of rapid-response radio observations in the study of the properties and dynamics of γ-ray burst ejecta.

RADIO CONSTRAINTS ON LONG-LIVED MAGNETAR REMNANTS IN SHORT GAMMA-RAY BURSTS

ABSTRACT The merger of a neutron star (NS) binary may result in the formation of a rapidly spinning magnetar. The magnetar can potentially survive for seconds or longer as a supramassive NS before collapsing to a black hole if, indeed, it collapses at all. During this process, a fraction of the magnetar’s rotational energy of ∼1053 erg is transferred via magnetic spin-down to the surrounding ejecta. The resulting interaction between the ejecta and the surrounding circumburst medium powers a year-long or greater synchrotron radio transient. We present a search for radio emission with the Very Large Array following nine short-duration gamma-ray bursts (GRBs) at rest-frame times of ≈1.3–7.6 yr after the bursts, focusing on those events that exhibit early-time excess X-ray emission that may signify the presence of magnetars. We place upper limits of ≲18–32 μJy on the 6.0 GHz radio emission, corresponding to spectral luminosities of ≲(0.05–8.3) × 1039 erg s−1. Comparing these limits to the predicted radio emission from a long-lived remnant and incorporating measurements of the circumburst densities from broadband modeling of short GRB afterglows, we rule out a stable magnetar with an energy of 1053 erg for half of the events in our sample. A supramassive remnant that injects a lower rotational energy of 1052 erg is ruled out for a single event, GRB 050724A. This study represents the deepest and most extensive search for long-term radio emission following short GRBs to date, and thus the most stringent limits placed on the physical properties of magnetars associated with short GRBs from radio observations.

On the nature of type 1 AGN: emission properties and correlations

We present a study of emission properties and correlations of 3191 type 1 AGN at z < 0.35, selected from Sloan Digital Sky Survey Data Release 12. We supplement the data with ultraviolet spectra from Hubble Space Telescope and International Ultraviolet Explorer along with X-ray properties from Chandra Source Catalog and XMM-Newton Serendipitous Source Catalog and radio measurements from FIRST. We find that the observed spectral diversity of type 1 AGN can be unified by Eddington ratio (L/LEdd). Objects with higher L/LEdd tend to have narrower broad Hβ component, strong Ee II emission, systematic blueshift of C IV λ1549, soft X-ray excess, and lower ionization parameter compared to those of the lower L/LEdd.

Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM

Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-ray absorption in excess of the Milky Way. The column densities of metals inferred from UV and optical spectra differ from those derived from soft X-ray spectra, at times by an order of magnitude, with the latter being higher. The origin of the soft X-ray absorption excess observed in GRB X-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous investigations on the effect of hot material both internal and external to the GRB host galaxy on our X-ray afterglow observations. Nevertheless, all models proposed so far have either only been able to account for a subset of our observations (i.e. at z > 2), or they have required fairly extreme conditions to be present within the absorbing material. In this paper, we investigate the absorption of the GRB afterglow by a collisionally ionised and turbulent interstellar medium (ISM). We find that a dense (3 per cubic centimeters) collisionally ionised ISM could produce UV/optical and soft X-ray absorbing column densities that differ by a factor of 10, however the UV/optical and soft X-ray absorbing column densities for such sightlines and are 2-3 orders of magnitude lower in comparison to the GRB afterglow spectra. For those GRBs with a larger soft X-ray excess of up to an order of magnitude, the contribution in absorption from a turbulent ISM as considered here would ease the required conditions of additional absorbing components, such as the GRB circumburst medium and intergalactic medium.

Probing acceleration and turbulence at relativistic shocks in blazar jets

Diffusive shock acceleration (DSA) at relativistic shocks is widely thought to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud active galactic nuclei such as blazars. Such acceleration can produce the non-thermal particles that emit the broadband continuum radiation that is detected from extragalactic jets. An important recent development for blazar science is the ability of Fermi-LAT spectroscopy to pin down the shape of the distribution of the underlying non-thermal particle population. This paper highlights how multi-wavelength spectra spanning optical to X-ray to gamma-ray bands can be used to probe diffusive acceleration in relativistic, oblique, magnetohydrodynamic (MHD) shocks in blazar jets. Diagnostics on the MHD turbulence near such shocks are obtained using thermal and non-thermal particle distributions resulting from detailed Monte Carlo simulations of DSA. These probes are afforded by the characteristic property that the synchrotron $\nu F_{\nu}$ peak energy does not appear in the gamma-ray band above 100 MeV. We investigate self-consistently the radiative synchrotron and inverse Compton signatures of the simulated particle distributions. Important constraints on the diffusive mean free paths of electrons, and the level of electromagnetic field turbulence are identified for three different case study blazars, Mrk 501, BL Lacertae and AO 0235+164. The X-ray excess of AO 0235+164 in a flare state can be modelled as the signature of bulk Compton scattering of external radiation fields, thereby tightly constraining the energy-dependence of the diffusion coefficient for electrons. The concomitant interpretations that turbulence levels decline with remoteness from jet shocks, and the probable significant role for non-gyroresonant diffusion, are posited.

Impact of sterile neutrino dark matter on core-collapse supernovae

We summarize the impact of sterile neutrino dark matter on core-collapse supernova explosions. We explore various oscillations between electron neutrinos or mixed [Formula: see text] neutrinos and right-handed sterile neutrinos that may occur within a core-collapse supernova. In particular, we consider sterile neutrino masses and mixing angles that are consistent with sterile neutrino dark matter candidates as indicated by recent X-ray flux measurements. We find that the interpretation of the observed 3.5 keV X-ray excess as due to a decaying 7 keV sterile neutrino that comprises 100% of the dark matter would have almost no observable effect on supernova explosions. However, in the more realistic case in which the decaying sterile neutrino comprises only a small fraction of the total dark matter density due to the presence of other sterile neutrino flavors, WIMPs, etc. a larger mixing angle is allowed. In this case a 7 keV sterile neutrino could have a significant impact on core-collapse supernovae. We also consider mixing between [Formula: see text] neutrinos and sterile neutrinos. We find, however, that this mixing does not significantly alter the explosion and has no observable effect on the neutrino luminosities at early times.

Spectral and Polarization Signatures of Relativistic Shocks in Blazars

Relativistic shocks are one of the most plausible sites of the emission of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via the diffusive shock acceleration (DSA) of relativistic particles. This paper summarizes recent results on a self-consistent coupling of diffusive shock acceleration and radiation transfer in blazar jets. We demonstrate that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of hydromagnetic turbulence responsible for pitch-angle scattering by requiring a strongly energy-dependent pitch-angle mean free path. The prominent soft X-ray excess (“Big Blue Bump”) in the SED of the BL Lac object AO 0235+164 can be modelled as the signature of bulk Compton scattering of external radiation fields by the thermal electron population, which places additional constraints on the level of hydromagnetic turbulence. It has further been demonstrated that internal shocks propagating in a jet pervaded by a helical magnetic field naturally produce polarization-angle swings by 180 ∘ , in tandem with multi-wavelength flaring activity, without requiring any helical motion paths or other asymmetric jet structures. The specific application of this model to 3C279 presents the first consistent simultaneous modeling of snap-shot SEDs, multi-wavelength light curves, and time-dependent polarization signatures of a blazar during a polarization-angle (PA) rotation. This model has recently been generalized to a lepto-hadronic model, in which the high-energy emission is dominated by proton synchrotron radiation. It is shown that in this case, the high-energy (X-ray and γ-ray) polarization signatures are expected to be significantly more stable (not showing PA rotations) than the low-energy (electron-synchrotron) signatures.

INVERSE COMPTON X-RAY EMISSION FROM TeV BLAZAR MRK 421 DURING A HISTORICAL LOW-FLUX STATE OBSERVED WITH NuSTAR

ABSTRACT We report on the detection of excess hard X-ray emission from the TeV BL Lac object Mrk 421 during the historical low-flux state of the source in 2013 January. Nuclear Spectroscopic Telescope Array observations were conducted four times between MJD 56294 and MJD 56312 with a total exposure of 80.9 ks. The source flux in the 3–40 keV range was nearly constant, except for MJD 56307 when the average flux level increased by a factor of three. Throughout the exposure, the X-ray spectra of Mrk 421 were well represented by a steep power-law model with a photon index of Γ ≃ 3.1, although a significant excess was noted above 20 keV in the MJD 56302 data when the source was in its faintest state. Moreover, Mrk 421 was detected at more than the 4σ level in the 40–79 keV count maps for both MJD 56307 and MJD 56302 but not during the remaining two observations. The detected excess hard X-ray emission connects smoothly with the extrapolation of the high-energy γ-ray continuum of the blazar constrained by Fermi-LAT during source quiescence. These findings indicate that while the overall X-ray spectrum of Mrk 421 is dominated by the highest-energy tail of the synchrotron continuum, the variable excess hard X-ray emission above 20 keV (on the timescale of a week) is related to the inverse Compton emission component. We discuss the resulting constraints on the variability and spectral properties of the low-energy segment of the electron energy distribution in the source.

SOFT X-RAY EXCESS FROM SHOCKED ACCRETING PLASMA IN ACTIVE GALACTIC NUCLEI

ABSTRACT We propose a novel theoretical model to describe the physical identity of the soft X-ray excess that is ubiquitously detected in many Seyfert galaxies, by considering a steady-state, axisymmetric plasma accretion within the innermost stable circular orbit around a black hole (BH) accretion disk. We extend our earlier theoretical investigations on general relativistic magnetohydrodynamic accretion, which implied that the accreting plasma can develop into a standing shock under suitable physical conditions, causing the downstream flow to be sufficiently hot due to shock compression. We perform numerical calculations to examine, for sets of fiducial plasma parameters, the physical nature of fast magnetohydrodynamic shocks under strong gravity for different BH spins. We show that thermal seed photons from the standard accretion disk can be effectively Compton up-scattered by the energized sub-relativistic electrons in the hot downstream plasma to produce the soft excess feature in X-rays. As a case study, we construct a three-parameter Comptonization model of inclination angle θ obs, disk photon temperature kT in, and downstream electron energy kT e to calculate the predicted spectra in comparison with a 60 ks XMM-Newton/EPIC-pn spectrum of a typical radio-quiet Seyfert 1 active galactic nucleus, Ark 120. Our χ 2-analyses demonstrate that the model is plausible for successfully describing data for both non-spinning and spinning BHs with derived ranges of 61.3 keV ≲ kT e ≲ 144.3 keV, 21.6 eV ≲ kT in ≲ 34.0 eV, and 17.°5 ≲ θ obs ≲ 42.°6, indicating a compact Comptonizing region of three to four gravitational radii that resembles the putative X-ray coronae.