Compton Reflection Model Research Articles

The Remarkable X-Ray Spectra and Variability of the Ultraluminous Weak-line Quasar SDSS J1521+5202

We present a focused X-ray and multiwavelength study of the ultraluminous weak-line quasar (WLQ) SDSS J1521+5202, one of the few X-ray weak WLQs that is amenable to basic X-ray spectral and variability investigations. J1521+5202 shows striking X-ray variability during 2006–2023, by up to a factor of ≈32 in 0.5–2 keV flux, and our new 2023 Chandra observation caught it in its brightest X-ray flux state to date. Concurrent infrared/optical observations show only mild variability. The 2023 Chandra spectrum can be acceptably described by a power law with intrinsic X-ray absorption, and it reveals a nominal intrinsic level of X-ray emission relative to its optical/ultraviolet emission. In contrast, an earlier Chandra spectrum from 2013 shows apparent spectral complexity that is not well fit by a variety of models, including ionized absorption or standard Compton-reflection models. Overall, the observations are consistent with the thick-disk plus outflow model previously advanced for WLQs, where a nominal level of underlying X-ray emission plus variable absorption leads to the remarkable observed X-ray variability. In the case of J1521+5202, it appears likely that the outflow, and not the thick disk itself, lies along our line of sight and causes the X-ray absorption.

NuSTARSPECTROSCOPY OF MULTI-COMPONENT X-RAY REFLECTION FROM NGC 1068

We report on observations of NGC1068 with NuSTAR, which provide the best constraints to date on its $>10$~keV spectral shape. We find no strong variability over the past two decades, consistent with its Compton-thick AGN classification. The combined NuSTAR, Chandra, XMM-Newton, and Swift-BAT spectral dataset offers new insights into the complex reflected emission. The critical combination of the high signal-to-noise NuSTAR data and a spatial decomposition with Chandra allow us to break several model degeneracies and greatly aid physical interpretation. When modeled as a monolithic (i.e., a single N_H) reflector, none of the common Compton-reflection models are able to match the neutral fluorescence lines and broad spectral shape of the Compton reflection. A multi-component reflector with three distinct column densities (e.g., N_H~1.5e23, 5e24, and 1e25 cm^{-2}) provides a more reasonable fit to the spectral lines and Compton hump, with near-solar Fe abundances. In this model, the higher N_H components provide the bulk of the Compton hump flux while the lower N_H component produces much of the line emission, effectively decoupling two key features of Compton reflection. We note that ~30% of the neutral Fe Kalpha line flux arises from >2" (~140 pc), implying that a significant fraction of the <10 keV reflected component arises from regions well outside of a parsec-scale torus. These results likely have ramifications for the interpretation of poorer signal-to-noise observations and/or more distant objects [Abridged].

The second INTEGRAL AGN catalogue

The INTEGRAL mission provides a large data set for studying the hard X-ray properties of AGN and allows testing of the unified scheme for AGN. We present analysis of INTEGRAL IBIS/ISGRI, JEM-X, and OMC data for 199 AGN supposedly detected by INTEGRAL above 20 keV. The data analysed here allow a significant spectral extraction on 148 objects and an optical variability study of 57 AGN. The slopes of the hard X-ray spectra of Seyfert 1 and Seyfert~2 galaxies are found to be consistent within the uncertainties, whereas higher cut-off energies and lower luminosities are measured for the more absorbed / type 2 AGN. The intermediate Seyfert 1.5 objects exhibit hard X-ray spectra consistent with those of Seyfert 1. When applying a Compton reflection model, the underlying continua appear the same in Seyfert 1 and 2 with photon index 2, and the reflection strength is about R = 1, when assuming different inclination angles. A significant correlation is found between the hard X-ray and optical luminosity and the mass of the central black hole in the sense that the more luminous objects appear to be more massive. There is also a general trend toward the absorbed sources and type 2 AGN having lower Eddington ratios. The black holemass appears to form a fundamental plane together with the optical and X-ray luminosity of the form Lv being proportional to Lx^0.6 M^0.2, similar to that found between radio luminosity Lr, Lx, and M. The unified model for Seyfert galaxies seems to hold, showing in hard X-rays that the central engine is the same in Seyfert 1 and 2, but seen under different inclination angles and absorption. (Abridged)

A longer XMM-Newton look at I Zwicky 1 - distinct modes of X-ray spectral variability

The short-term spectral variability of the narrow-line Seyfert 1 galaxy I Zwicky 1 (I Zw 1) as observed in an 85ks XMM-Newton observation is discussed in detail. I Zw 1 shows distinct modes of variability prior to and after a flux dip in the broad-band light curve. Before the dip the variability can be described as arising from changes in shape and normalization of the spectral components. Only changes in normalization are manifested after the dip. The change in the mode of behaviour occurs on dynamically short time-scales in I Zw 1. The data suggest that the accretion-disc corona in I Zw 1 could have two components that are co-existing. The first, a uniform, physically diffuse plasma responsible for the `typical' long-term (e.g. years) behaviour; and a second compact, centrally located component causing the rapid flux and spectral changes. This compact component could be the base of a short or aborted jet as sometimes proposed for radio-quiet active galaxies. Modelling of the average and time-resolved rms spectra demonstrate that a blurred Compton-reflection model can describe the spectral variability if we allow for pivoting of the continuum component prior to the dip.

GRO J1655−40: Early Stages of the 2005 Outburst

The black hole X-ray binary transient GRO J1655-40 underwent an outburst beginning in early 2005. We present the results of our multiwavelength observational campaign to study the early outburst spectral and temporal evolution, which combines data from X-ray (RXTE and INTEGRAL), radio (VLA), and optical (ROTSE and SMARTS) instruments. During the reported period, the source left quiescence and went through four major accreting black hole states: low-hard, hard intermediate, soft intermediate, and high-soft. We investigated dipping behavior in the RXTE band and compare our results to the 1996-1997 case, when the source was predominantly in the high-soft state, finding significant differences. We consider the evolution of the low-frequency quasi-periodic oscillations and find that the frequency strongly correlates with the spectral characteristics, before shutting off prior to the transition to the high-soft state. We model the broadband high-energy spectrum in the context of empirical models, as well as more physically motivated thermal and bulk motion Comptonization and Compton reflection models. RXTE and INTEGRAL data together support a statistically significant high-energy cutoff in the energy spectrum at ≈100-200 keV during the low-hard state. The RXTE data alone also show it very significantly during the transition, but not in the high-soft state spectra. We consider radio, optical, and X-ray connections in the context of possible synchrotron and synchrotron self-Compton origins of X-ray emission in low-hard and intermediate states. In this outburst of GRO J1655-40, the radio flux does not rise strongly with the X-ray flux.

A puzzling event during the X-ray emission of the binary system GX 1+4

We report on a long X-ray observation of the the slow-rotating binary pulsar GX 1+4. BeppoSAX observed, in the 0.1–200 keV energy range, an event in which the source flux dropped for almost a day, and then recovered. During this event only the high-energy emission was found to be pulsed and the pulsations were shifted in phase of ∼0.2. The spectrum during the event was well fitted by a Compton-reflection model. A broad iron line at ∼6.55 keV was present outside of the event, where instead two narrow emission lines at ∼6.47 and ∼7.05 keV were detected. The pulse profile was highly variable as a function of both energy and time. We interpret this low-flux event as an occultation of the direct X-ray emission, due to the increase of a torus-like accretion disk; we then discuss similarities between this source and the recently discovered highly absorbed INTEGRAL sources.

Downscattering Due to Wind Outflows in Compact X‐Ray Sources: Theory and Interpretation

A number of recent lines of evidence point towards the presence of hot, out-flowing plasma from the central regions of compact Galactic and extragalactic X-ray sources. Additionally, it has long been noted that many of these sources exhibit an ``excess'' continuum component, above 10 keV, usually attributed to Compton Reflection from a static medium. Motivated by these facts, as well as by recent observational constraints on the Compton reflection models - specifically apparently discrepant variability timescales for line and continuum components in some cases -- we consider possible effects of out-flowing plasma on the high-energy continuum spectra of accretion powered compact objects. We present a general formulation for photon downscattering diffusion which includes recoil and Comptonization effects due to divergence of the flow. We then develop an analytical theory for the spectral formation in such systems that allows us to derive formulae for the emergent spectrum. Finally we perform the analytical model fitting on several Galactic X-ray binaries. Objects which have been modeled with high-covering-fraction Compton reflectors, such as GS1353-64 are included in our analysis. In addition, Cyg X-3, is which is widely believed to be characterized by dense circumstellar winds with temperature of order 10^6 K, provides an interesting test case. Data from INTEGRAL and RXTE covering the 3-300 keV range are used in our analysis. We further consider the possibility that the widely noted distortion of the power-law continuum above 10 keV may in some cases be explained by these spectral softening effects.

ASCA Observation of Unusually X-Ray-Hard Radio-Quiet QSO Kaz102

Abstract We observed the radio-quiet QSO Kaz102 $(z = 0.136)$ with ASCA as a part of our program for a complete spectral characterization of hard X-ray selected AGNs. We found that Kaz102 shows unusual spectral properties. A simple power-law with absorption in our Galaxy gave a satisfactory description of the spectrum. However, it showed a very hard photon index of $\Gamma \sim 1.0$ with no sign of deep absorption or a prominent spectral feature. We further explored the Compton reflection with the Fe $\mathrm{K}\alpha$ line and warm absorber models for hardening the spectra. Both gave statistically satisfactory fits. However, the Compton reflection model requires a very low metal abundance (0.03–0.07 in solar units). The warm absorber model with no direct component is preferred, and gave a very high ionization parameter of $\xi \sim 10^{2.3}$. If this is the case, the values of $\xi$, warm absorber column density, and variability over $\sim 10\;\mathrm{yr}$ may suggest that the warm absorber resides in the broad-line region and crosses the line of sight to the central X-ray source.

X‐Ray Spectra of Intermediate‐Luminosity, Radio‐loud Quasars

We present new hard X-ray spectra of three radio-loud active galactic nuclei (AGNs) of moderately high X-ray luminosity (LX ≈ 1045 ergs s-1; PKS 2349-01, 3C 323.1, and 4C 74.26) obtained with ASCA and BeppoSAX. The X-ray continua are described in all three cases with a power-law model with photon indices Γ ≈ 1.85, modified at low energies by absorption in excess of the galactic value, which appears to be due to neutral gas. At higher energies, an Fe Kα emission line is detected in PKS 2349-01 and 4C 74.26 and is tentatively detected in 3C 323.1. The equivalent widths of the lines are consistent, albeit within large uncertainties, with the values for radio-quiet AGNs of comparable X-ray luminosity. The Fe Kα line is unresolved in 4C 74.26. In the case of PKS 2349-01, however, the inferred properties of the line depend on the model adopted for the continuum: if a simple power-law model is used, the line is resolved at more than 99% confidence with a full width at half-maximum corresponding to approximately 50,000 km s-1 and a rest-frame equivalent width of 230 ± 120 eV; however, if a Compton reflection model is used, the line is found to be a factor of 2 weaker for an assumed full width at half-maximum of 50,000 km s-1. In 4C 74.26, a strong Compton reflection component is detected. Its strength suggests that the scattering medium subtends a solid angle of 2π to the illuminating source. Overall, the spectral indices of these radio-loud quasars are remarkably similar to those of their radio-quiet counterparts. On the other hand, if the absorber is indeed neutral, as our results suggest, this would be consistent with the typical properties of radio-loud AGNs.

XMM-Newton observation of the Seyfert 1 galaxy Fairall 9

We report an X-ray observation of the Fairall 9 Seyfert 1 galaxy performed in July 2000 with the EPIC cameras and Reflection Grating Spectrometers (RGS) on board the XMM-Newton observatory. Above 2 keV, the high signal to noise ratio of the EPIC spectra shows an emission line at keV with a measured equivalent width of 120 eV. An absorption edge is also detected at in the source rest frame that has not been reported before. Spectral fitting of the measured spectra with reflection models indicates that these imprinted features are likely the result of partial reprocessing of a primary X-ray continuum by optically thick material. The positions in energy of the emission line and absorption edge are consistent with Compton reflection models from material containing iron in states of ionization lower than FeXVII. The Fe K emission line is narrow suggesting that the reflection process arises from material relatively far from a putative central black hole. The best fit models show that the reflected spectrum mask a steeper intrinsic power law slope even at energies lower than 5 keV. The extrapolation of this primary continuum to energies lower than 2 keV reveals the presence of a soft excess component contributing to of the overall flux in the 0.3-1.0 keV energy range. No evidence is found for a warm absorber in this low energy part of the spectrum. The analysis results are discussed within the frame of recent accretion disc models in which X-rays from the primary continuum source heat up the upper layers of the disc thus creating very hot, optically thin plasmas on top of cooler denser material.

SimultaneousEUVE/ASCA/RXTEObservations of NGC 5548

We present simultaneous observations by EUVE, ASCA, and RXTE of the type?1 Seyfert galaxy NGC?5548. These data indicate that variations in the EUV emission (at ~0.2 keV) appear to lead similar modulations in higher energy (1 keV) X-rays by ~10-30 ks. This is contrary to popular models which attribute the correlated variability of the EUV, UV and optical emission in type?1 Seyferts to reprocessing of higher energy radiation. This behavior instead suggests that the variability of the optical through EUV emission is an important driver for the variability of the harder X-rays which are likely produced by thermal Comptonization. We also compute the power density spectra at the various energy bands probed by these observations. Over 10-300 ks timescales, the emission in EUV shows about a factor of two greater rms variability than that of the 2-20 keV RXTE-PCA band?18. ? 1.4% versus 7.4 ? 0.6%. On longer timescales, we construct a PDS from 1-12 keV RXTE-ASM data which shows evidence for a break at about 6 ? 10-8 Hz. Furthermore, we find that the combined RXTE-ASM/PCA power spectrum is remarkably similar in shape to PDSs found for the low/hard states of Galactic black hole candidates such as Cygnus X-1. The implied scaling factor of ~106 is comparable to the expected mass ratio for these two objects. In addition, we investigate the spectral characteristics of the fluorescent iron K? line and Compton reflection emission. In contrast to prior measurements of these spectral features, we find that the iron K? line has a relatively small equivalent width (WK? ~ 100 eV) and that the reflection component is consistent with a covering factor which is significantly less than unity (?/2? ~ 0.4-0.5). Notably, although the 2-10?keV X-ray flux varies by ~?25% and the derived reflection fraction appears to be constant throughout our observations, the flux in the iron K? line is also constant. This behavior is difficult to reconcile in the context of standard Compton reflection models.

X‐Ray Emission from the Prototypical LINER Galaxy NGC 1052

We examine the 0.1 to 10.0 keV X-ray spectrum of the bright nuclear LINER galaxy NGC 1052, one of two elliptical galaxies known to contain a luminous H2O maser. The observed 2.0-10.0 keV spectrum is unusually flat (photon index Gamma~0.2) and best described as intrinsically power-law shaped nuclear flux that is either (1) attenuated by a complex absorber with ~70% of the nuclear flux absorbed by a column density of NH ~ 3x10^23 cm^{-2} and ~30% absorbed by a column density of NH ~ 3-5x10^{22} cm^{-2}, or (2) reprocessed, with the nuclear source blocked and the X-rays Compton reflected in our direction by high column density (>=10^{24} cm^{-2}) gas. The moderate equivalent width of the Fe K-alpha line favors the dual absorption model as the most likely scenario. The 0.1-2.0 keV spectrum does not resemble the few times 10^6 to 10^7 K thermal emission typically found in other elliptical galaxies, but instead is best described as nuclear X-rays leaking through a patchy absorber or scattered in our direction by low-density, ionized gas with the thermal contribution limited to about 15% for solar abundances. The absorption-corrected 2-10 keV luminosity of the nuclear source is L_X ~ 8x10^{41} ergs s^{-1} or L_X ~ 2x10^{43} ergs s^{-1} for the dual-absorption and Compton-reflection models, respectively. The absorbing and H2O masing gases appear to be spatially separate in this galaxy.

X-Ray/Soft Gamma-Ray Observation of Centaurus A and Its Implication on the Emission Mechanism

We have studied Cen A (NGC 5128) in the X-ray band (3–20 keV) and soft γ-ray band (40–600 keV) with the Large Area Counter (LAC) of the Ginga satellite (1989 March and 1990 February) and with a balloon-borne low background detector (Welcome-1, 1991 November), respectively. The observed continuous spectra show a power-law shape (Γ∼ 1.8) with relatively heavy absorption (NH ∼ 1.5 × 1023 H cm−2) at the low-energy end and a possible break at ∼ 180 keV. We analyzed the total spectra as the sum of the direct power-law flux from the central engine, the Compton-scattered flux from a cold cloud near to the engine, and the iron fluorescence-line flux. By assuming that the geometry around the central engine remained unchanged during the two-year period, we studied two possible cold cloud geometries by comparing Monte-Carlo simulations with our observations: the first is the Compton-reflection model in which the cloud forms a slab covering 2π sr behind the central engine, the second is a case where the central source is totally surrounded by a cold cloud. We found that the latter geometry reproduces our data well; the observed power-law spectrum is then identified as direct flux from the central engine which undergoes a break at ∼ 180 keV.

A semi-analytical solution of the transfer equation for Compton reflection models

A semi-analytical solution of the transfer equation for Compton reflection models

On Compton reflection in the sources of the cosmic X-ray background

Consideration is given to recent models for the cosmic X-ray background that assume that it originates from unresolved AGN emitting spectra due to enhanced Compton reflection of a power-law photon spectrum incident on cold matter. The parameter space of the Compton reflection model is studied, and the allowed parameter space is found to be severely constrained by physical and cosmological effects. For an incident power-law energy index alpha is greater than about 1, the X-ray peak in the observed spectrum from a population of AGN is necessarily at an energy less than that of the observed peak. Two examples of improved fits to the X-ray background are shown. It is concluded that the Compton reflection models proposed to date do not provide a straightforward explanation of the X-ray background spectrum.

An ionized accretion disk in Cygnus X-1

Results of an analysis of archival Exosat and HEAO 1-A2 observations of Cyg X-1 are presented. It is found, in agreement with Ebisawa (1991), that the X-ray spectrum of Cyg X-1 exhibits a 'high-energy excess' above 8 keV, similar to that seen in Ginga spectra of AGN. It is shown, on the basis of a likelihood ratio test, that the data are significantly better fit by a Compton reflection model than by a partial covering model. Even with the energy resolution of the Exosat GSPC, it is found that the derived iron line energy and shape are very dependent on the continuum model used. The best-fit temperature for a Suniaev-Titarchuk Comptonization model is found to be significantly larger than that found in previous analyses based primarily on data in the 30-140 keV band. This suggests that the 5-50 keV continuum is not well fit by the same spectrum form applicable at high energies and that a multiple temperature thermal Compton model is necessary to fit the broadband data.

Relativistic enhancement of the Compton-reflected component in active galactic nuclei

Compton reflection may be an important process in AGNs, since it provides an improvement over power-law fits to AGN spectra observed by Ginga and can also explain the spectrum of the cosmic X-ray background. The fraction of the total X-ray luminosity which is incident upon a thin steady alpha-disk if the X-rays are produced by inverse-Compton scattering of soft, disk photons off relativistic electrons located above the disk is calculated. This fraction is called the Compton reflection covering factor, f, and it is found that it can range between 0.5 and 0.88. This large range in f, due to a relativistic kinematic effect first calculated in this connection by Ghisellini et al (1991) is sufficient to explain the typical covering (about 0.5) observed in bright AGNs by Ginga as well as the large covering factor (about 0.9) required to explain the cosmic X-ray background in the Compton reflection model.

A Compton reflection model for the cosmic X-ray and gamma-ray backgrounds

view Abstract Citations (17) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Compton Reflection Model for the Cosmic X-Ray and Gamma-Ray Backgrounds Rogers, R. D. ; Field, G. B. Abstract We calculate the gamma-ray Spectrum of an AGN in the Compton reflection model and consider the contribution of such sources to the cosmic gamma-ray background. The spectrum is composed of three parts: an X-ray power law, a broad hump from ~10 to 120 keV due to reprocessing of the power-law X-ray spectrum by cold gas, and gamma rays which are due to the inverse-Compton scattering of X-rays. We compute the contribution of such sources to the cosmic gamma-ray background and find that they provide an excellent fit to the entire background spectrum from 3 keV to 10 MeV. We derive constraints on the physical conditions in these sources based on our results and consider the possibility of observing such sources with GRO. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/186131 Bibcode: 1991ApJ...378L..17R Keywords: Background Radiation; Compton Effect; Cosmic X Rays; Endeavour (Orbiter); Gamma Rays; X Ray Sources; Astronomical Models; Gamma Ray Spectra; Reflected Waves; Spectrum Analysis; Astrophysics; COSMIC BACKGROUND RADIATION; GAMMA RAYS: BURSTS; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (8) NED (8)

Variable soft X-ray excesses in active galactic nuclei from nonthermal electron-positron pair cascades

In the present study of the formation of steep soft X-ray excesses that are superposed on flatter, hard X-ray power-law spectra in nonthermal electron-positron pair cascade sources, the soft excess in pair-cascade AGN models appears as a steep power law superposed on the tail of the UV bump and the flat nonthermal (hard X-ray) power law. The model-parameter space in which an excess in soft X-rays is visible is ascertained, and the time-variability of soft excesses in pair cascade models is examined. It is established that the parameter space in which soft excesses appear encompasses the range of preferred input parameters for a recently development Compton reflection model of UV and X-ray emission from the central engine of an AGN.