arXiv subjects

D. R. Ballantyne

Publications and source records attributed to D. R. Ballantyne.

At least 19 recordsLinked to original sources

arXiv2026

Do Accretion Disks Get Bent Out of Shape? Investigating the Origin of Rapid Fluctuations in the Tails of Long Thermonuclear X-ray Bursts

Thermonuclear X-ray bursts from the surfaces of neutron stars affect the surrounding accretion flow, revealing details of the underlying physical processes influencing accretion physics. In this context, we perform a spectral analysis of 5 long X-ray bursts that exhibit temporary rapid flux variations during the tails of their light curves. In all cases, spectra extracted from before, during and after the time of fluctuations show evidence for relativistic, ionized reflection with the blackbody from the neutron star often hidden from view. Both the properties of the reflecting region and the observed fraction of the blackbody vary as the fluctuations start and stop. These results are compared to a sample of 5 similar bursts without fluctuations in their light curves, and we find that spectra from 4 of the bursts in the control sample are best described by a simple absorbed blackbody. The bursts with fluctuations are longer and more energetic than those without fluctuations, in agreement with the prediction that radiatively-driven warps are impacting the accretion disks of the bursts with fluctuations. This result would also imply that these accretion disks must have large viscosity parameters. The lack of reflection in bursts from the control sample may result from lower accretion rates, as these disks would have smaller surface densities and would expand and become Compton-thin due to heating from the burst. High-throughput, time-resolved spectral analysis of X-ray bursts that undergo warping would give insight into the strength of accretion disk viscosity.

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arXiv2025

The Compton-thick AGN Population and the $N_{\rm H}$ Distribution of Low-mass AGN in our Cosmic Backyard

We present a census of the Compton-thick (CT) active galactic nucleus (AGN) population and the column density ($N_{\rm{H}}$) distribution of AGN in our cosmic backyard using a mid-infrared selected AGN sample within 15 Mpc. The column densities are measured from broadband X-ray spectral analysis, mainly using data from $\textit{Chandra}$ and $\textit{NuSTAR}$. Our sample probes AGN with intrinsic 2-10 keV luminosities of $L_{\rm 2-10, int} = 10^{37}$-$10^{43}$ erg s$^{-1}$, reaching a parameter space inaccessible to more distant samples. We directly measure a 32$^{+30}_{-18}\%$ CT AGN fraction and obtain an $N_{\rm{H}}$ distribution that agrees with that inferred by the $\textit{Swift}$-BAT survey. Restricting the sample to the largely unexplored domain of low-luminosity AGN with $L_{\rm 2-10, int}$ $\leq$ $10^{42}$ erg s$^{-1}$, we found a CT fraction of 19$^{+30}_{-14}\%$, consistent with those observed at higher luminosities. Comparing the host-galaxy properties between the two samples, we find consistent star formation rates, though the majority of our galaxy have lower stellar masses (by $\approx 0.3$ dex). In contrast, the two samples have very different black hole mass ($M_{\rm BH}$) distributions, with our sample having $\approx$1.5 dex lower mean mass ($M_{\rm BH}$ $\sim$ 10$^{6}$ $M_\odot$). Additionally, our sample contains a significantly higher number of LINERs and H$_{\rm{II}}$-type nuclei. The Eddington ratio range probed by our sample, however, is the same as $\textit{Swift}$-BAT, although the latter dominates at higher accretion rates, and our sample is more evenly distributed. The majority of our sample with $λ_{\rm Edd} \ge$ 10$^{-3}$ tend to be CT, while those with $λ_{\rm Edd} <$ 10$^{-3}$ are mostly unobscured or mildly obscured.

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arXiv2024

Examining the Relationship Between the Persistent Emission and the Accretion Rate During a Type I X-ray Burst

The accretion flow onto a neutron star will be impacted due to irradiation by a Type I X-ray burst. The burst radiation exerts Poynting-Robertson (PR) drag on the accretion disk, leading to an enhanced mass accretion rate. Observations of X-ray bursts often find evidence that the normalization of the disk-generated persistent emission (commonly denoted by the factor $f_a$) increases during a burst, and changes in $f_a$ have been used to infer the evolution in the mass accretion rate due to PR drag. Here, we examine this proposed relationship between $f_a$ and mass accretion rate enhancement using time-resolved data from simulations of accretion disks impacted by Type I X-ray bursts. We consider bursts from both spinning and non-spinning neutron stars and track both the change in accretion rate due to PR grad and the disk emission spectra during the burst. Regardless of the neutron star spin, we find that $f_a$ strongly correlates with the disk temperature and only weakly follows the mass accretion rate (the Pearson correlation coefficients are $\leq 0.63$ in the latter case). Additionally, heating causes the disk to emit at higher energies, reducing its contribution to a soft excess. We conclude that $f_a$ cannot accurately capture the mass accretion rate enhancement and is rather a tracer of the disk temperature.

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arXiv2024

X-ray view of dissipative warm corona in active galactic nuclei

In the X-ray spectra of AGNs, a noticeable excess of soft X-rays is typically detected beyond the extrapolation of the power-law trend observed between 2-10 keV. In the scenario of warm Comptonization, observations propose a warm corona temperature ranging from 0.1-1 keV and an optical depth of approximately 10-20. Furthermore, according to radiative constraints derived from spectral analyses employing Comptonization models, it is suggested that the majority of the accretion power is released within the warm corona, while the disk beneath it is largely non-dissipative, emitting mainly the reprocessed radiation from the corona. We test the dissipative warm corona model using the radiative transfer code-TITAN/NOAR on a sample of 82 XMM-Newton observations of AGNs. Through spectral modeling of the X-ray data, we aim to estimate the total amount of internal heating inside the warm corona situated on top of the accretion disk. By modeling the 0.3-10 keV EPIC-pn spectra, we estimate the internal heating and optical depth of the warm corona and check their correlations with global parameters blackhole parameters. From model normalization, we compute the radial extent of warm corona on top of cold accretion disk. Our model infers the presence of dissipative warm corona, with optical depths distributed in the range 6-30 and total internal heating in the range 1-29 x 1e-23 erg/s-cm3. The extent of warm corona is spread across a large range from 7-408 gravitational radii, and we find that warm corona is more extended for larger accretion rates. Soft excess emission is ubiquitous in AGNs across wide mass range and accretion rate. We confirm that warm corona responsible for producing the soft-excess is highly dissipative in nature with larger optical depths being associated with lower internal heating and vice versa. The cold standard accretion disk regulates the extent of warm corona.

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arXiv2024

The origin of the soft excess in the luminous quasar HE 1029-1401

The enigmatic and intriguing phenomenon of the "soft excess" observed in the X-ray spectra of luminous quasars continues to be a subject of considerable interest and debate in the field of high-energy astrophysics. This study focuses on the quasar HE 1029-1401 ($z=0.086$, $\log(L_{\rm{bol}}/[\rm{erg\,s^{-1}}])= 46.0 \pm 0.2$), with a particular emphasis on investigating the properties of the hot corona and the physical origin of the soft excess. In this study, we present the results of a joint \textit{XMM-Newton}/\textit{NuSTAR} monitoring campaign of this quasar conducted in May 2022. The source exhibits a cold and narrow Fe $\rm{K}α$ emission line at 6.4 keV, in addition to the detection of a broad component. Our findings suggest that the soft excess observed in HE 1029-1401 can be adequately explained by Comptonized emission originating from a warm corona. Specifically, fitting the spectra with two \nthcomp\, component we found that the warm corona is characterized by a photon index ($Γ^{w}$) of $2.75\pm0.05$ and by an electron temperature ($kT_{e}^{w}$) of $0.39^{+0.06}_{-0.04}$ keV, while the optical depth ($τ^{w}$) is found to be $23\pm3$. We also test more physical models for the warm corona, corresponding to two scenarios: pure Comptonization and Comptonization plus reflection. Both models provide a good fit to the data, and are in agreement with a radially extended warm corona having a size of a few tens of gravitational radii.

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arXiv2024

Unveiling Energy Pathways in AGN Accretion Flows with the Warm Corona Model for the Soft Excess

The soft excess in active galactic nuclei (AGNs) may arise through a combination of relativistic reflection and the effects of a warm corona at the surface of the accretion disc. Detailed examination of the soft excess can therefore constrain models of the transport and dissipation of accretion energy. Here, we analyze 34 XMM-Newton observations from 14 Type I AGNs with the reXcor spectral model which self-consistently combines emission from a warm corona with relativistic reflection assuming a lamppost corona. The model divides accretion energy between the disc, the warm corona, and the lamppost. The XMM-Newton observations span a factor of 188 in Eddington ratio ($λ_{\mathrm{obs}}$) and 350 in black hole mass, and we find that a warm corona is a significant contributor to the soft excess for 13 of the 14 AGNs with a mean warm corona heating fraction of $0.51$. The reXcor fits reveal that the fraction of accretion energy dissipated in the lamppost is anti-correlated with $λ_{\mathrm{obs}}$. In contrast, the relationship between $λ_{\mathrm{obs}}$ and both the optical depth and heating fraction of the warm corona appears to transition from an anti-correlation to a correlation at $λ_{\mathrm{obs,t}} \approx 0.15$. Therefore, at least one other physical process in addition to the accretion rate is needed to explain the evolution of the warm corona. Overall, we find that a warm corona appears to be a crucial depository of accretion energy in AGNs across a broad range of $λ_{\mathrm{obs}}$ and black hole mass.

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arXiv2023

The High Energy X-ray Probe (HEX-P): The Future of Hard X-ray Dual AGN Science

A fundamental goal of modern-day astrophysics is to understand the connection between supermassive black hole (SMBH) growth and galaxy evolution. Merging galaxies offer one of the most dramatic channels for galaxy evolution known, capable of driving inflows of gas into galactic nuclei, potentially fueling both star formation and central SMBH activity. Dual active galactic nuclei (dual AGNs) in late-stage mergers with nuclear pair separations $<10$ kpc are thus ideal candidates to study SMBH growth along the merger sequence since they coincide with the most transformative period for galaxies. However, dual AGNs can be extremely difficult to confirm and study. Hard X-ray ($>10$ keV) studies offer a relatively contamination-free tool for probing the dense obscuring environments predicted to surround the majority of dual AGN in late-stage mergers. To date, only a handful of the brightest and closest systems have been studied at these energies due to the demanding instrumental requirements involved. We demonstrate the unique capabilities of HEX-P to spatially resolve the soft and - for the first time - hard X-ray counterparts of closely-separated ($\sim2''-5''$) dual AGNs in the local Universe. By incorporating state-of-the-art physical torus models, we reproduce realistic broadband X-ray spectra expected for deeply embedded accreting SMBHs. Hard X-ray spatially resolved observations of dual AGNs - accessible only to HEX-P - will hence transform our understanding of dual AGN in the nearby Universe.

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arXiv2023

The High Energy X-ray Probe (HEX-P): Constraining Supermassive Black Hole Growth with Population Spin Measurements

Constraining the primary growth channel of supermassive black holes (SMBH) remains one the most actively debated questions in the context of cosmological structure formation. Owing to the expected connection between SMBH spin parameter evolution and the accretion and merger history of individual black holes, population spin measurements offer a rare observational window into the SMBH cosmic growth. As of today, the most common method for estimating SMBH spin relies on modeling the relativistically broaden atomic profiles in the reflection spectrum observed in X-rays. In this paper, we study the observational requirements needed to confidently distinguish between the primary SMBH growth channels, based on their distinct spin-mass distributions predicted by the Horizon-AGN cosmological simulation. In doing so, we characterize outstanding limitations associated with the existing measurements and discuss the landscape of future observational campaigns, which can be planned and executed with future X-ray observatories. We focus our attention on the High-Energy X-ray Probe (HEX-P), a concept probe-class mission aimed to serve the high-energy community in the 2030s.

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arXiv2023

Impact of neutron star spin on Poynting-Robertson drag during a Type I X-ray burst

External irradiation of a neutron star (NS) accretion disc induces Poynting-Robertson (PR) drag, removing angular momentum and increasing the mass accretion rate. Recent simulations show PR drag significantly enhancing the mass accretion rate during Type I X-ray bursts, which could explain X-ray spectral features such as an increase in the persistent emission and a soft excess. However, prograde spin of the NS is expected to weaken PR drag, challenging its importance during bursts. Here, we study the effect of spin on PR drag during X-ray bursts. We run four simulations, with two assuming a non-spinning NS and two using a spin parameter of $a_*=0.2$, corresponding to a rotation frequency of 500 Hz. For each scenario, we simulate the disc evolution subject to an X-ray burst and compare it to the evolution found with no burst. PR drag drains the inner disc region during a burst, moving the inner disc radius outward by $\approx1.6$ km in the $a_*=0$ and by $\approx2.2$ km in the $a_*=0.2$ simulation. The burst enhances the mass accretion rate across the innermost stable circular orbit $\approx7.9$ times when the NS is not spinning and $\approx11.2$ times when it is spinning. The explanation for this seemingly contradictory result is that the disc is closer to the NS when $a_*=0.2$, and the resulting stronger irradiating flux offsets the weakening effect of spin on the PR drag. Hence, PR drag remains a viable explanation for the increased persistent emission and soft excess observed during X-ray bursts in spinning NS systems.

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arXiv2022

Radiation Driven Warping of Accretion Discs Due to X-ray Bursts

The outpouring of radiation during an X-ray burst can affect the properties of accretion discs around neutron stars: the corona can cool and collapse, the inner regions can be bled away due to enhanced accretion, and the additional heating will lead to changes in the disc height. In this paper, we investigate whether radiation from bursts can cause the disc to distort through a warping instability. Working in the limit of isotropic viscosity and linear growth, we find that bursts are more likely to drive disc warps when they have larger luminosities and longer durations. Therefore, warps will be most probable during intermediate duration bursts (IMDBs) and superbursts with evidence for photospheric radius expansion. Further, the development of warps depends on the disc viscosity with larger values of $α$ increasing the likelihood of warp growth. We perform time-dependent evolution calculations of the development of warps during Type I bursts and IMDBs. Depending on the initial warp prior to the burst, we find the burst produces warps at $r$ <~ 50 $r_g$ that rapidly grow and decay on second-long timescales, or ones that grow more slowly and cover a large fraction of the disc. The pulsations of warp at small radii appear to have the properties needed to explain the achromatic fluctuations that have been observed during the tails of some IMDBs. The large scale, slowly growing warps could account for the large reflection strengths and absorbing column densities inferred late in the 4U 1820-30 and 4U 1636-53 superbursts.

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arXiv2022

reXcor: A Model of the X-ray Spectrum of Active Galactic Nuclei that Combines Ionized Reflection and a Warm Corona

The X-ray spectra of active galactic nuclei (AGNs) often exhibit an excess of emission above the primary power-law at energies <~ $2$ keV. Two models for the origin of this `soft excess' are ionized relativistic reflection from the inner accretion disc and Comptonization of thermal emission in a warm corona. Here, we introduce reXcor, a new AGN X-ray ($0.3$-$100$ keV) spectral fitting model that self-consistently combines the effects of both ionized relativistic reflection and the emission from a warm corona. In this model, the accretion energy liberated in the inner disc is distributed between a warm corona, a lamppost X-ray source, and the accretion disc. The emission and ionized reflection spectrum from the inner $400$ $r_g$ of the disc is computed, incorporating the effects of relativistic light-bending and blurring. The resulting spectra predict a variety of soft excess shapes and sizes that depend on the fraction of energy dissipated in the warm corona and lamppost. We illustrate the use of reXcor by fitting to the joint XMM-Newton and NuSTAR observations of the Seyfert 1 galaxies HE 1143-1820 and NGC 4593, and find that both objects require a warm corona contribution to the soft excess. Eight reXcor table models, covering different values of accretion rate, lamppost height and black hole spin, are publicly available through the XSPEC website. Systematic use of reXcor will provide insight into the distribution of energy in AGN accretion flows.

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arXiv2021

Evolution of accretion disc reflection spectra due to a Type I X-ray burst

Irradiation of the accretion disc causes reflection signatures in the observed X-ray spectrum, encoding important information about the disc structure and density. A Type I X-ray burst will strongly irradiate the accretion disc and alter its properties. Previous numerical simulations predicted the evolution of the accretion disc due to an X-ray burst. Here, we process time-averaged simulation data of six time intervals to track changes in the reflection spectrum from the burst onset to just past its peak. We divide the reflecting region of the disc within $r\lesssim50$ km into 6-7 radial zones for every time interval and compute the reflection spectra for each zone. We integrate these reflection spectra to obtain a total reflection spectrum per time interval. The burst ionizes and heats the disc, which gradually weakens all emission lines. Compton scattering and bremsstrahlung rates increase in the disc during the burst rise, and the soft excess at $<$3 keV rises from $\approx4$% to $\approx38$% of the total emission at the burst peak. A soft excess is expected to be ubiquitous in the reflection spectra of X-ray bursts. Structural disc changes such as inflation because of heating or drainage of the inner disc due to Poynting-Robertson drag affect the strength of the soft excess. Further studies on the dependence of the reflection spectrum characteristics to changes in the accretion disc during an X-ray burst may lead to probes of the disc geometry.

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arXiv2020

A Spitzer survey of Deep Drilling Fields to be targeted by the Vera C. Rubin Observatory Legacy Survey of Space and Time

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will observe several Deep Drilling Fields (DDFs) to a greater depth and with a more rapid cadence than the main survey. In this paper, we describe the ``DeepDrill'' survey, which used the Spitzer Space Telescope Infrared Array Camera (IRAC) to observe three of the four currently defined DDFs in two bands, centered on 3.6 $μ$m and 4.5 $μ$m. These observations expand the area which was covered by an earlier set of observations in these three fields by the Spitzer Extragalactic Representative Volume Survey (SERVS). The combined DeepDrill and SERVS data cover the footprints of the LSST DDFs in the Extended Chandra Deep Field-South field (ECDFS), the ELAIS-S1 field (ES1), and the XMM Large-Scale Structure Survey field (XMM-LSS). The observations reach an approximate $5σ$ point-source depth of 2 $μ$Jy (corresponding to an AB magnitude of 23.1; sufficient to detect a 10$^{11} M_{\odot}$ galaxy out to $z\approx 5$) in each of the two bands over a total area of $\approx 29\,$deg$^2$. The dual-band catalogues contain a total of 2.35 million sources. In this paper we describe the observations and data products from the survey, and an overview of the properties of galaxies in the survey. We compare the source counts to predictions from the SHARK semi-analytic model of galaxy formation. We also identify a population of sources with extremely red ([3.6]$-$[4.5] $>1.2$) colours which we show mostly consists of highly-obscured active galactic nuclei.

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arXiv2020

NuSTAR Survey of Obscured Swift/BAT-selected Active Galactic Nuclei: II. Median High-energy Cutoff in Seyfert II Hard X-ray Spectra

Broadband X-ray spectroscopy of the X-ray emission produced in the coronae of active galactic nuclei (AGN) can provide important insights into the physical conditions very close to their central supermassive black holes. The temperature of the Comptonizing plasma that forms the corona is manifested through a high-energy cutoff that has been difficult to directly constrain even in the brightest AGN because it requires high-quality data at energies above 10 keV. In this paper we present a large collection of coronal cutoff constraints for obscured AGN based on a sample of 130 AGN selected in the hard X-ray band with Swift/BAT and observed nearly simultaneously with NuSTAR and Swift/XRT. We find that under a reasonable set of assumptions regarding partial constraints the median cutoff is well constrained to 290$\pm$20 keV, where the uncertainty is statistical and given at the 68% confidence level. We investigate the sensitivity of this result to our assumptions and find that consideration of various known systematic uncertainties robustly places the median cutoff between 240 keV and 340 keV. The central 68% of the intrinsic cutoff distribution is found to be between about 140 keV and 500 keV, with estimated uncertainties of 20 keV and 100 keV, respectively. In comparison with the literature, we find no clear evidence that the cutoffs in obscured and unobscured AGN are substantially different. Our analysis highlights the importance of carefully considering partial and potentially degenerate constraints on the coronal high-energy cutoff in AGN.

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arXiv2020

Cooling of Accretion Disc Coronae by Type I X-ray Bursts

Understanding the persistent emission is crucial for studying type I X-ray bursts, which provide insight into neutron star properties. Although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. Recent work suggests that Type I X-ray bursts from accreting neutron stars provide an opportunity to probe the characteristics of coronae. Several studies have observed hard X-ray shortages from the accretion disk during an X-ray burst implying strong coronal cooling by burst photons. Here, we use the plasma emission code EQPAIR to study the impact of X-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. Assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of $\gtrsim 10$ and cause a reduction of emission in the $30$-$50$ keV band to $\lesssim 1\%$ of the pre-burst emission. This hard X-ray drop is intensified when the coronal optical depth and aspect ratio is increased. In contrast, depending on the properties of the burst and corona, the emission in the $8$-$24$ keV band can either increase, by a factor of $\gtrsim20$, or decrease, down to $\lesssim 1\%$ of the pre-burst emission. An increasing accretion rate during the X-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $\gtrsim60\%$. These results indicate that changes of the hard X-ray flux during an X-ray burst probe the geometrical properties of the corona.

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arXiv2020

$NuSTAR$ Observations of Four Nearby X-ray Faint AGN: Low Luminosity or Heavy Obscuration?

We present $NuSTAR$ observations of four active galactic nuclei (AGN) located within 15 Mpc. These AGN, namely ESO 121-G6, NGC 660, NGC 3486 and NGC 5195, have observed X-ray luminosities of $L_{\rm 2-10\ keV, obs} \lesssim$ 10$^{39}$ erg s$^{-1}$, classifying them as low luminosity AGN (LLAGN). We perform broadband X-ray spectral analysis for the AGN by combining our $NuSTAR$ data with $Chandra$ or $XMM-Newton$ observations to directly measure their column densities ($N_{\rm H}$) and infer their intrinsic power. We complement our X-ray data with archival and new high angular resolution mid-infrared (mid-IR) data for all objects, except NGC 5195. Based on our X-ray spectral analysis, we found that both ESO 121-G6 and NGC 660 are heavily obscured ($N_{\rm H}$ > 10$^{23}$ cm$^{-2}$; $L_{\rm 2-10\ keV,\ int} \sim$ 10$^{41}$ erg s$^{-1}$), and NGC 660 may be Compton-thick. We also note that the X-ray flux and spectral slope for ESO 121-G6 have significantly changed over the last decade, indicating significant changes in the obscuration and potentially accretion rate. On the other hand, NGC 3486 and NGC 5195 appear to be unobscured and just mildly obscured, respectively, with $L_{\rm 2-10\ keV,\ int} <$ 10$^{39}$ erg s$^{-1}$; i.e., genuine LLAGN. Both of the heavily obscured AGN have $L_{\rm bol} >$ 10$^{41}$ erg s$^{-1}$ and $λ_{\rm Edd} \gtrsim$ 10$^{-3}$, and are detected in high angular resolution mid-IR imaging, indicating the presence of obscuring dust on nuclear scale. NGC 3486 however, is undetected in high-resolution mid-IR imaging, and the current data do not provide stringent constraints on the presence or absence of obscuring nuclear dust in the AGN.

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arXiv2020

Sustaining a Warm Corona in Active Galactic Nuclei Accretion Discs

Warm coronae, thick ($τ_{\mathrm{T}}\approx 10$-$20$, where $τ_{\mathrm{T}}$ is the Thomson depth) Comptonizing regions with temperatures of $\sim 1$ keV, are proposed to exist at the surfaces of accretion discs in active galactic nuclei (AGNs). By combining with the reflection spectrum, warm coronae may be responsible for producing the smooth soft excess seen in AGN X-ray spectra. This paper studies how a warm corona must adjust in order to sustain the soft excess through large changes in the AGN flux. Spectra from one-dimensional constant density and hydrostatic warm coronae models are calculated assuming the illuminating hard X-ray power-law, gas density, Thomson depth and coronal heating strength vary in response to changes in the accretion rate. We identify models that produce warm coronae with temperatures between $0.3$ and $1.1$ keV, and measure the photon indices and emitted fluxes in the $0.5$-$2$ keV and $2$-$10$ keV bands. Correlations and anti-correlations between these quantities depend on the evolution and structure of the warm corona. Tracing the path that an AGN follows through these correlations will constrain how warm coronae are heated and connected to the accretion disc. Variations in the density structure and coronal heating strength of warm coronae will lead to a variety of soft excess strengths and shapes in AGNs. A larger accretion rate will, on average, lead to a warm corona that produces a stronger soft excess, consistent with observations of local Seyfert galaxies.

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arXiv2019

Examining the Physical Conditions of a Warm Corona in Active Galactic Nuclei Accretion Discs

A warm corona at the surface of an accretion disc has been proposed as a potential location for producing the soft excess commonly observed in the X-ray spectra of active galactic nuclei (AGNs). In order to fit the observed data the gas must be at temperatures of $\sim 1$ keV and have an optical depth of $τ_{\mathrm{T}}\approx 10$--$20$. We present one-dimensional calculations of the physical conditions and emitted spectra of a $τ_{\mathrm{T}}=10$ or $20$ gas layer subject to illumination from an X-ray power-law (from above), a blackbody (from below) and a variable amount of internal heating. The models show that a warm corona with $kT \sim 1$ keV can develop, producing a strong Comptonized soft excess, but only if the internal heating flux is within a relatively narrow range. Similarly, if the gas density of the layer is too large then efficient cooling will stop a warm corona from forming. The radiation from the hard X-ray power-law is crucial in producing a warm corona, indicating that a warm and hot corona may co-exist in AGN accretion discs, and their combined effect leads to the observed soft excess. Intense heating of a warm corona leads to steep X-ray spectra with ionised Fe K$α$ lines, similar to those seen in some narrow-line Seyfert 1 galaxies.

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