AGN Spectra Research Articles

Surveying the Onset and Evolution of Supermassive Black Holes at High-z with AXIS

The nature and origin of supermassive black holes (SMBHs) remain an open matter of debate within the scientific community. While various theoretical scenarios have been proposed, each with specific observational signatures, the lack of sufficiently sensitive X-ray observations hinders the progress of observational tests. In this white paper, we present how AXIS will contribute to solving this issue. With an angular resolution of 1.5″ on-axis and minimal off-axis degradation, we designed a deep survey capable of reaching flux limits in the [0.5–2] keV range of approximately 2 × 10−18 erg s−1 cm−2 over an area of 0.13 deg2 in approximately 7 million seconds (7 Ms). Furthermore, we planned an intermediate depth survey covering approximately 2 deg2 and reaching flux limits of about 2 × 10−17 erg s−1 cm−2 in order to detect a significant number of SMBHs with X-ray luminosities (LX) of approximately 1042 erg s−1 up to z∼10. These observations will enable AXIS to detect SMBHs with masses smaller than 105 M⊙, assuming Eddington-limited accretion and a typical bolometric correction for Type II AGN. AXIS will provide valuable information on the seeding and population synthesis models of SMBHs, allowing for more accurate constraints on their initial mass function (IMF) and accretion history from z∼0–10. To accomplish this, AXIS will leverage the unique synergy of survey telescopes such as the JWST, Roman, Euclid, Vera Rubin Telescope, and the new generation of 30 m class telescopes. These instruments will provide optical identification and redshift measurements, while AXIS will discover the smoking gun of nuclear activity, particularly in the case of highly obscured AGN or peculiar UV spectra as predicted and recently observed by the JWST in the early Universe.

Absorption in the Spectrum of a Background AGN by C ii* by the Supernova Remnant G354-33

A number of very large, very faint supernova remnants (SNRs) have been discovered in the past few years. Those in the Galactic halo occur in low density gas, so they might be more easily observed in absorption than in emission. The SNR G354-33 is a shell of faint UV and optical filaments about 10° in diameter. Here we point out that a spectrum of the QSO J2017-4516 used study the galaxy cluster J2016-4517 shows an absorption line from the excited fine structure level of C ii at −50 km s−1. Absorption features of Si ii and Si iii are also seen at this velocity. The observed column density compares well with shock wave models. Doppler shifted C ii* absorption features provide an excellent means to find and characterize SNR shocks because they arise from compressed gas.

ReveaLLAGN 0: First Look at JWST MIRI Data of Sombrero and NGC 1052

We present the first results from the Revealing Low-Luminosity Active Galactic Nuclei (ReveaLLAGN) survey, a JWST survey of seven nearby LLAGNs. We focus on two observations with the Mid-Infrared Instrument (MIRI)’s Medium-Resolution Spectrometer of the nuclei of NGC 1052 and Sombrero (NGC 4594/M104). We also compare these data to public JWST data of higher-luminosity AGNs, NGC 7319 and NGC 7469. JWST clearly separates the AGN spectrum from the galaxy light even in Sombrero, the faintest target in our survey; the AGN components have very red spectra. We find that the emission-line widths in both NGC 1052 and Sombrero increase with increasing ionization potential, with FWHM > 1000 km s−1 for lines with ionization potential ≳ 50 eV. These lines are also significantly blueshifted in both LLAGNs. The high-ionization-potential lines in NGC 7319 show neither broad widths nor significant blueshifts. Many of the lower-ionization-potential emission lines in Sombrero show significant blue wings extending >1000 km s−1. These features and the emission-line maps in both galaxies are consistent with outflows along the jet direction. Sombrero has the lowest-luminosity high-ionization-potential lines ([Ne v] and [O iv]) ever measured in the mid-infrared, but the relative strengths of these lines are consistent with higher-luminosity AGNs. On the other hand, the [Ne v] emission is much weaker relative to the [Ne iii] and [Ne ii] lines of higher-luminosity AGNs. These initial results show the great promise that JWST holds for identifying and studying the physical nature of LLAGNs.

Dealing with broad-band X-ray spectra of faint AGN: a case study

ABSTRACT In this work, we analyse three average-luminosity hard X-ray selected AGN: ESO 506-G27, IGR J19039+3344, and NGC 7465. They have simultaneous Swift/XRT and NuSTAR data never published before and have been poorly studied at X-ray energies. These sources make for interesting targets both from a methodological and scientific point of view. Scientifically, they are of interest since they are possibly heavily absorbed objects, belong to a peculiar class and are variable both in flux and in spectral shape. Methodologically, because it is an interesting exercise to understand how existing spectral models can be applied to faint sources and how the use of NuSTAR data alone and then simultaneous and/or average data impacts on the spectral parameters determination. In this work, we demonstrate that simultaneous data are not sufficient if their statistical quality is poor. Moreover, we show that also the use of time-averaged data when dealing with faint AGN does not always provide confident results as for brighter AGN. Regardless of the poor data quality employed in our analysis, we are able to provide insights into the spectral characteristics of each source. We analyse in detail for the first time the iron line complex of ESO 506-G27, finding not only the presence of the iron K α and K β lines, but also of the iron K edge around 7 keV in the NuSTAR data. We also highlight changes in the absorption properties of IGR J19039+3344 and confirm NGC 7465 to be an unabsorbed type 1 LINER.

Machine Learning Applied to X-Ray Spectra: Separating Stars in Orion Nebula Cluster from Active Galactic Nuclei in CDFS

Modern X-ray telescopes have detected hundreds of thousands of X-ray sources in the universe. However, current methods to classify these sources using the X-ray data themselves suffer problems—detailed X-ray spectroscopy of individual sources is too time consuming, while hardness ratios often lack accuracy, and can be difficult to use effectively. These methods fail to use the power of X-ray CCD detectors to identify X-ray emission lines and distinguish line-dominated spectra (from chromospherically active stars, supernova remnants, etc.) from continuum-dominated ones (e.g., compact objects or active galactic nuclei, AGN). In this paper, we probe the use of artificial neural networks (ANN) in differentiating Chandra spectra of young stars in the Chandra Orion Ultradeep Project (COUP) survey from AGN in the Chandra Deep Field South (CDFS) survey. We use these surveys to generate 100,000 artificial spectra of stars and AGN, and train our ANN models to separate the two kinds of spectra. We find that our methods reach an accuracy of ∼92% in classifying simulated spectra of moderate-brightness objects in typical exposures, but their performance decreases on the observed COUP and CDFS spectra (∼91%), due in large part to the relatively high background of these long-exposure data sets. We also investigate the performance of our methods with changing properties of the spectra such as the net source counts, the relative contribution of background, the absorption column of the sources, etc. We conclude that these methods have substantial promise for application to large X-ray surveys.

A hard look at the X-ray spectral variability of NGC 7582

ABSTRACT NGC 7582 (z = 0.005264; D = 22.5 Mpc) is a highly variable, changing-look AGN. In this work, we explore the X-ray properties of this source using XMM–Newton and NuSTAR archival observations in the 3 – 40 keV range, from 2001 to 2016. NGC 7582 exhibits a long-term variability between observations but also a short-term variability in two observations that has not been studied before. To study the variability, we perform a time-resolved spectral analysis using a phenomenological model and a physically motivated model (uxclumpy). The spectral fitting is achieved using a nested sampling Monte Carlo method. uxclumpy enables testing various geometries of the absorber that may fit AGN spectra. We find that the best model is composed of a fully covering clumpy absorber. From this geometry, we estimate the velocity, size, and distance of the clumps. The column density of the absorber in the line of sight varies from Compton-thin to Compton-thick between observations. Variability over the time-scale of a few tens of kiloseconds is also observed within two observations. The obscuring clouds are consistent with being located at a distance not larger than 0.6 pc, moving with a transverse velocity exceeding ∼700 km s−1. We could put only a lower limit on the size of the obscuring cloud being larger than 1013 cm. Given the sparsity of the observations, and the limited exposure time per observation available, we cannot determine the exact structure of the obscuring clouds. The results are broadly consistent with comet-like obscuring clouds or spherical clouds with a non-uniform density profile.

Morphology of Broad Emission Lines from Binary Supermassive Blackholes

Accretion onto a blackhole can indue extreme radiation stimulating emission lines in nearby gases. In the deep gravitational well, these clouds move at speeds up to 10,000 km/s, and so broadens the emission lines via the relativistic Doppler Effect. These broad emission lines on spectra are the most prominent feature of AGNs. The region around central black hole, where such broad emission lines are produced, is known as the broad-line region (BLR). We developed a self-consistent model to describe both the geometry of the BLR and the dynamics of the gas clouds in the BLR. The motion of clouds is described by a series of Keplerian orbits. The spectral shift of emission line can be derived by the radial velocity at each point in Keplerian orbits. Assuming that the BLR is stable, that is, the structure and distribution of gas in BLR doesn’t change over a considerable period of time, the accumulated emission line profile can be constructed by simple addition of all Keplerian orbits in the BLR. By comparing the calculated profiles to spectra observed in SDSS (Sloan Digital Sky Survey), we found that our model can satisfactorily match most observed profiles. We attempted to review and summarize potential binary AGNs. We applied the model to the study of binary AGNs and their profiles of emission lines. We considered the velocity offset due to mutual Keplerian motion of black holes in binary and added their respective profiles to produce the final, resulting emission line profile from binary AGNs. Three kinds of emission line profiles are featured in our simulations: twin-peak structures where two strong broad-lines are present; sub-peak structures where a smaller but still visible emission line accompanies the dominant line; single broad emission peak but with significant velocity offset from the systematic redshift that is derived from the narrow emission-lines on the same spectrum. We then analyzed 1,348 AGN spectra with high S/N ratio in SDSS database, and selected 26 as candidate binary supermassive blackhole systems. These candidates are valuable for following-up observation, their binary nature could be confirmed by detecting the profile variation due to orbital motions.

Efficient analysis routines for single and double peaked Type 2 AGN spectra

ABSTRACT Driven by the imminent need to rapidly process and classify millions of AGN spectra drawn from next generation astronomical facilities, we present a spectral fitting routine for Type 2 AGN spectra optimized for high volume processing, using the quasar spectral fitting library (qsfit). We analyse an optically selected sample of 813 luminous Type 2 AGN spectra at z < 0.83 from the Sloan Digital Sky Survey (SDSS) to qualify its performance. We report a median narrow line H α/H β Balmer decrement of 4.5 ± 0.8, alluding to the presence of dust in the narrow line region (NLR). We publish a specialized qsfit fitting routine for high signal-to-noise ratio spectra and general fitting routine for double peaked Type 2 AGN spectra applied on a subsample of 45 spectra from our parent sample. We report a median red and blue peak velocity separation of 390 ± 60 kms−1. No trend is found for red or blue peaks to exhibit systematically different luminosity or ionization properties. Emission line diagnostics show that the double peaks in all sources are illuminated by an AGN-powered ionizing continuum. Finally, we examine the morphology of host galaxies of our double peaked sample. We find double peaked Type 2 AGN reside in merging systems at a comparable frequency to single peaked AGN. This suggests that the double peaked AGN phenomenon is likely to have a bi-conical outflow origin in the majority of cases. We publicly release the code used for spectral analysis and produced catalogues used in this work.

Constraining energy-dependent emissivity profiles of AGN inflows

Abstract The emissivity of the accretion flow is a key parameter affecting the shape of both the energy and variability power spectrum of AGN. We explore the energy-dependence of the power spectrum for five AGN, across the XMM-Newton bandpass, and across the 0.01-1 mHz frequency range, finding a ubiquitous flattening of the power spectrum towards higher energies. We develop a framework to explore this behaviour and thereby extract the energy dependence of the emissivity assuming a simple disc-like geometry for the inflow. We find that the emissivity ranges from R−2 at energies around the soft excess and increases to R−4 or steeper above 4-6 keV. We describe the changing emissivity index with a linear function in energy, finding the best-fitting slopes to vary between AGN. We attempt to correlate the slope of the linear function against key AGN parameters but, as yet, the sample size is too small to confirm hints of a correlation with Eddington ratio.

The Flux Ratio of the [OIII]λλ5007,4959 Doublet with Redshift

As a test of the time symmetry of forbidden-line emission processes, we measured the [OIII]λλ4959 and 5007 emission lines of more than 12,000 galaxies from the Sloan Digital Sky Survey DR8 to examine the [OIII]λλ4959,5007 flux ratio as a function of redshift z. Using two different approaches, we fitted each line with a Gaussian curve and rejected any spectrum not conforming to requirements of line symmetry, S/N levels, and continuum fit. We found the variance in the ratio of flux to be between 4.7 and −3.3% for 0<z<0.433 which is consistent with no change. After correcting for systematic effects of noise, we found the mean value of the [OIII]λλ4959,5007 flux ratio to be 2.98 ± 0.01 which is consistent with theory and previous studies using AGN spectra. We also used these data to estimate an upper limit on the time dependence of the fine structure constant α of Δα/α(0)<1.4×10−5 for galaxies within the same redshift range. This corresponds to |α−1dα/dt|<2×10−15 yr−1, which is also in line with previous estimates from SDSS QSO data.

The evolution of the low-density H i intergalactic medium from z = 3.6 to 0: data, transmitted flux, and H i column density,,

ABSTRACT We present a new, uniform analysis of the H i transmitted flux (F) and H i column density ($N_{\mathrm{H\,{\small I}}}$) distribution in the low-density IGM as a function of redshift z for 0 < z < 3.6 using 55 HST/COS FUV (Δz = 7.2 at z < 0.5), five HST/STIS + COS NUV (Δz = 1.3 at z ∼ 1) and 24 VLT/UVES, and Keck/HIRES (Δz = 11.6 at 1.7 < z < 3.6) AGN spectra. We performed a consistent, uniform Voigt profile analysis to combine spectra taken with different instruments, to reduce systematics and to remove metal-line contamination. We confirm previously known conclusions on firmer quantitative grounds in particular by improving the measurements at z ∼ 1. Two flux statistics at 0 < F < 1, the mean H i flux and the flux probability distribution function (PDF), show that considerable evolution occurs from z = 3.6 to z = 1.5, after which it slows down to become effectively stable for z < 0.5. However, there are large sightline variations. For the H i column density distribution function (CDDF, f ∝ $N_{\rm H\,{\small I}}^{-\beta }$) at $\log (N_{\mathrm{H\,{\small I}}}/1\, {\mathrm{cm}^{-2}})$ ∈ [13.5, 16.0], β increases as z decreases from β = 1.60 at z ∼ 3.4 to β = 1.82 at z ∼ 0.1. The CDDF shape at lower redshifts can be reproduced by a small amount of clockwise rotation of a higher-z CDDF with a slightly larger CDDF normalization. The absorption line number per z (dn/dz) shows a similar evolutionary break at z ∼ 1.5 as seen in the flux statistics. High-$N_{\mathrm{H\,{\small I}}}$ absorbers evolve more rapidly than low-$N_{\mathrm{H\,{\small I}}}$ absorbers to decrease in number or cross-section with time. The individual dn/dz shows a large scatter at a given z. The scatter increases towards lower z, possibly caused by a stronger clustering at lower z.

Through a mini halo, darkly

In this Letter we explore the effects of the scattering of photons incident on a dark matter halo through their interaction with either electrons or photons produced by dark matter annihilation. Particularly, we examine the effects of this scattering upon the observed spectrum of a distant AGN or of the Cosmic Microwave Background. Our results indicate that ultra-compact mini halos and other dense dark matter clumps can induce an observable Comptonisation of AGN spectra as well as a Sunyaev-Zel'dovich effect (SZE) with an optical depth similar to that attained by thermal electrons in the Coma cluster. The rate of encounters between a distant AGN and these dense mini-halos is also estimated using micro-lensing limits existing on the population of dark compact bodies.

Massive BH binaries as periodically variable AGN

Massive black-hole (MBH) binaries, which are expected to form following the merger of their parent galaxies, produce gravitational waves which will be detectable by Pulsar Timing Arrays at nanohertz frequencies (year periods). While no confirmed, compact MBH binary systems have been seen in electromagnetic observations, a large number of candidates have recently been identified in optical surveys of AGN variability. Using a combination of cosmological, hydrodynamic simulations; comprehensive, semi-analytic binary merger models; and analytic AGN spectra and variability prescriptions; we calculate the expected electromagnetic detection rates of MBH binaries as periodically variable AGN. In particular, we consider two independent variability models: (i) Doppler boosting due to large orbital velocities, and (ii) hydrodynamic variability in which the fueling of MBH accretion disks is periodically modulated by the companion. Our models predict that numerous MBH binaries should be present and distinguishable in the existing data. In particular, our fiducial models produce an expectation value of $0.2$ (Doppler) and $5$ (hydrodynamic) binaries to be identifiable in CRTS, while $20$ and $100$ are expected after five years of LSST observations. The brightness variations in most systems are too small to be distinguishable, but almost $1\%$ of AGN at redshifts $z \lesssim 0.6$ could be in massive binaries. We analyze the predicted binary parameters of observable systems and their selection biases, and include an extensive discussion of our model parameters and uncertainties.

An ultra-fast inflow in the luminous Seyfert PG1211+143

Blueshifted absorption lines in the X-ray spectra of AGN show that ultra-fast outflows with typical velocities $v \sim 0.1c$ are a common feature of these luminous objects. Such powerful AGN winds offer an explanation of the observed M-$\sigma$ relation linking the mass of the supermassive black hole and the velocity dispersion in the galaxy's stellar bulge. An extended XMM-Newton study of the luminous Seyfert galaxy PG1211+143 recently revealed a variable multi-velocity wind. Here we report the detection of a short-lived, ultra-fast inflow during the same observation. Previous reports of inflows used single absorption lines with uncertain identifications, but this new result identifies an array of resonance absorption lines of highly ionised Fe, Ca, Ar, S and Si, sharing a common redshift when compared with a grid of realistic photoionization spectra. The redshifted absorption arises in a column of highly ionized matter close to the black hole, with a line-of-sight velocity, $v \sim 0.3c$, inconsistent with the standard picture of a plane circular accretion disc. This may represent the first direct evidence for chaotic accretion in AGN, where accretion discs are generally misaligned to the black hole spin. For sufficient inclinations, the Lense-Thirring effect can break the discs into discrete rings, which then precess, collide and shock, causing near free-fall of gas towards the black hole. The observed accretion rate for the reported infall is comparable to the hard X-ray luminosity in PG1211+143, suggesting that direct infall may be a significant contributor to inner disc accretion.

The December 2015 re-brightening of V404 Cygni

In December 2015 the black hole binary V404 Cyg underwent a secondary outburst after the main June 2015 event. We monitored this re-brightening with the INTEGRAL and Swift satellites, and in this paper we report the results of the time-resolved spectral analysis of these data. The December outburst shared several characteristics with the June event. The well-sampled INTEGRAL light curve shows up to ten Crab flares, which are separated by relatively weak non-flaring emission phases when compared to the June outburst. The spectra are nicely described by absorbed Comptonization models, with hard photon indices, Γ ≲ 2, and significant detections of a high-energy cut-off only during the bright flares. This is in contrast to the June outburst, where the Comptonization models gave electron temperatures mostly in the 30–50 keV range, while some spectra were soft (Γ ~ 2.5) without signs of any spectral cut-off. Similarly to the June outburst, we see clear signs of a variable local absorber in the soft energy band covered by Swift/XRT and INTEGRAL/JEM-X, which causes rapid spectral variations observed during the flares. During one flare, both Swift and INTEGRAL captured V404 Cyg in a state where the absorber was nearly Compton thick, N H ≈ 1024 cm−2, and the broad-band spectrum was similar to obscured AGN spectra, as seen during the X-ray plateaus in the June outburst. We conclude that the spectral behaviour of V404 Cyg during the December outburst was analogous with the first few days of the June outburst, both having hard X-ray flares that were intermittently influenced by obscuration due to nearly Compton-thick outflows launched from the accretion disc.

Relativistic reflection from accretion disks in the population of Active Galactic Nuclei at z=0.5–4

We report the detection of relativistically broadened iron K alpha emission in the X-ray spectra of AGN detected in the 4Ms CDF-S. Using the Bayesian X-ray analysis (BXA) package, we fit 199 hard band (2-7 keV) selected sources in the redshift range z=0.5--4 with three models: (i) an absorbed power-law, (ii) the first model plus a narrow reflection component, and (iii) the second model with an additional relativistic broadened reflection. The Bayesian evidence for the full sample of sources selects the model with the additional broad component as being 10^5 times more probable to describe the data better than the second model. For the two brightest sources in our sample, CID 190 (z=0.734) and CID 104 (z=0.543), BXA reveals the relativistic signatures in the individual spectra. We estimate the fraction of sources containing a broad component to be 54^{+35}_{-37}% (107/199 sources). Considering that the low signal-to-noise ratio of some spectra prevents the detection of the broad iron K alpha line, we infer an intrinsic fraction with broad emission of around two thirds. The detection of relativistic signatures in the X-ray spectra of these sources suggests that they are powered by a radiatively efficient accretion disk. Preliminary evidence is found that the spin of the black hole is high, with a maximally spinning Kerr BH model (a=1) providing a significantly better fit than a Schwarzschild model (a=0). Our analysis demonstrate the potential of X-ray spectroscopy to measure this key parameter in typical SMBH systems at the peak of BH growth.

Searches for 3.5 keV absorption features in cluster AGN spectra

We investigate possible evidence for a spectral dip around 3.5 keV in central cluster active galactic nuclei, motivated by previous results for archival Chandra observations of the Perseus cluster and the general interest in novel spectral features around 3.5 keV that may arise from dark matter physics. We use two deep Chandra observations of the Perseus and Virgo clusters that have recently been made public. In both cases, mild improvements in the fit (Δχ2 = 4.2 and Δχ2 = 2.5) are found by including such a dip at 3.5 keV into the spectrum. A comparable result (Δχ2 = 6.5) is found reanalysing archival on-axis Chandra ACIS-S observations of the centre of the Perseus cluster.

ABSORPTION OF SOME SMALL SILVER CLUSTERS: DFT AND CASPT2 CALCULATIONS

Two quantum chemical methods which are the time-dependent density functional theory (TD-DFT) and the complete active space CASPT2/CASSCF have been used in modeling absorption spectra of silver clusters Agn (n = 2, 3, 4, 6, 8). There is an overall good agreement between TD-DFT and CASPT2 results for transition energies. The absorption spectra of the Agn clusters examined can reasonably be simulated using the excitation energies obtained by either TD-DFT or CASPT2 method. The main result emerged from this calculation is that the TD-DFT method is suitable for treatment of excited states of Ag clusters. The choice of specific functionals and basis sets to be used in some cases induces important effects on the calculated spectra. It is also noteworthy to mention that for some clusters, the neutral Ag6 for instance, the effect of noble gas environment is significant, while for some others such as the neutral Ag8, it is not. Therefore, carrying out TD-DFT calculations to reproduce and to assign a given structure to an experimental absorption spectrum of a silver cluster, it is not only to select suitable functionals but also to take enough effects of environments into account.

XMM-Newton Survey of Local O vii Absorption Lines in the Spectra of Galactic X-Ray Sources

Abstract The detection of highly ionized metal absorption lines in the X-ray spectra of the Galactic X-ray binaries (XRBs) implies the distribution of hot gas along the sightline toward the background sources. However, the origin of this hot gas is still unclear: it can arise in the hot interstellar medium (ISM), or is intrinsic to the XRBs. In this paper, we present an XMM-Newton survey of the O vii absorption lines in the spectra of Galactic XRBs. A total of 33 XRBs were selected, with 29 low-mass XRBs and 4 high-mass XRBs. At a more than 3σ threshold, O vii absorption line was detected in 16 targets, among which 4 were newly discovered in this work. The average line equivalent width is centered around ∼20 mÅ. Additionally, we do not find strong correlations between the O vii EWs and the Galactic neutral absorption N H, the Galactic coordinates, or the distance of background targets. Such non-correlation may suggest contamination of the circumstellar material, or a lack of constraints on the line Doppler-b parameter. We also find that regardless of the direction of the XRBs, the O vii absorption lines are always detected when the flux of the background XRBs reaches a certain level, suggesting a uniform distribution of this hot gas. We estimate a ratio of 0.004–0.4 between the hot and neutral phases of the ISM. This is the second paper in the series following Fang et al. (2015), in which we focused on the local O vii absorption lines detected in the background AGN spectra. Detailed modeling of the hot ISM distribution will be investigated in a future paper.

Radiative Magnetic Reconnection Near Accreting Black Holes

Abstract A radiative mechanism is proposed for magnetic flares near luminous accreting black holes. It is based on recent first-principle simulations of magnetic reconnection, which show a hierarchical chain of fast-moving plasmoids. The reconnection occurs in a compact region (comparable to the black hole radius), and the chain experiences fast Compton cooling accompanied by electron–positron pair creation. The distribution of plasmoid speeds is shaped by radiative losses, and the self-regulated chain radiates its energy in hard X-rays. The mechanism is illustrated by Monte-Carlo simulations of the transfer of seed soft photons through the reconnection layer. The emerging radiation spectrum has a cutoff near 100 keV similar to the hard-state spectra of X-ray binaries and AGN. We discuss how the chain cooling differs from previous phenomenological emission models, and suggest that it can explain the hard X-ray activity of accreting black holes from first principles. Particles accelerated at the X-points of the chain produce an additional high-energy component, explaining the “hybrid Comptonization” observed in Cyg X-1.