Absorption Measure Distribution Research Articles

Supernova Archaeology with X-Ray Binary Winds: The Case of GRO J1655−40

Supernovae are responsible for the elemental enrichment of the galaxy, and some are postulated to leave behind a black hole. In a stellar binary system, the supernova pollutes its companion, and the black hole can accrete back its own debris and emit X-rays. In this sequence of events, which is only poorly understood, winds are ejected and observed through X-ray absorption lines. Measuring abundances of elements in the wind can lead to inferences about the historical explosion and possibly identify the long-gone progenitor of the compact object. Here, we reanalyze the uniquely rich X-ray spectrum of the 2005 outburst of GRO J1655−40. We reconstruct the absorption measure distribution (AMD) of the wind and find that it increases sharply with ionization from H-like O up to H-like Ca and then flattens out. The AMD is then used to measure the relative abundances of 18 different elements. The present abundances are in partial agreement with a previous work with discrepancies mostly for low-Z elements. The overabundance of odd-Z elements hints at a high-metallicity, high-mass (≃25 M ⊙) progenitor. Interestingly, the abundances are different from those measured in the companion atmosphere, indicating that the wind entrains lingering ambient supernova debris. This can be expected since the current total stellar mass of the binary (<10 M ⊙) is much less than the progenitor mass.

Ionization Distributions in Outflows of Active Galaxies: Universal Trends and Prospect of Future XRISM Observations

The physics behind the ionization structure of outflows from black holes is yet to be fully understood. Using archival observations with the Chandra/HETG gratings over the past two decades, we measured an absorption measure distribution for a sample of outflows in nine active galactic nuclei (AGNs), namely the dependence of outflow column density, N H, on the ionization parameter, ξ. The slope of N H versus is found to be between 0.00 and 0.72. We find an anticorrelation between the log of total column density of the outflow and the log of AGN luminosity, and none with the black hole mass and accretion efficiency. A major improvement in the diagnostics of AGN outflows will potentially occur with the launch of the XRISM/Resolve spectrometer. We study the ability of Resolve to reveal the outflow ionization structure by constructing the absorption measure distribution from simulated Resolve spectra, utilizing its superior resolution and effective area. Resolve constrains the column density as well as HETG, but with much shorter observations.

Warm Absorbers in the Radiation-driven Fountain Model of Low-mass Active Galactic Nuclei

Abstract To investigate the origins of the warm absorbers in active galactic nuclei (AGNs), we study the ionization-state structure of the radiation-driven fountain model in a low-mass AGN and calculate the predicted X-ray spectra utilizing the spectral synthesis code Cloudy. The spectra show many absorption and emission line features originating in the outflowing ionized gas. The O viii 0.654 keV lines are produced mainly in the polar region much closer to the supermassive black hole than the optical narrow-line regions. The absorption measure distribution of the ionization parameter (ξ) at a low inclination spreads over 4 orders of magnitude in ξ, indicating the multiphase ionization structure of the outflow, as actually observed in many type 1 AGNs. We compare our simulated spectra with the high energy resolution spectrum of the narrow-line Seyfert 1 galaxy NGC 4051. The model reproduces slowly outflowing (a few hundred kilometers per second) warm absorbers. However, the faster components with a few thousand kilometers per second observed in NGC 4051 are not reproduced. The simulation also underproduces the intensity and width of the O viii 0.654 keV line. These results suggest that the ionized gas launched from subparsec or smaller regions inside the torus, which is not included in the current fountain model, must be an important ingredient of the warm absorbers with a few thousand kilometers per second. The model also consistently explains the Chandra/HETG spectrum of the Seyfert 2 galaxy Circinus.

Multiphase AGN Winds from X-Ray-irradiated Disk Atmospheres

Abstract The mechanism of thermal driving for launching mass outflows is interconnected with classical thermal instability (TI). In a recent paper, we demonstrated that as a result of this interconnectedness, radial wind solutions of X-ray-heated flows are prone to becoming clumpy. In this paper, we first show that the Bernoulli function determines whether or not the entropy mode can grow due to TI in dynamical flows. Based on this finding, we identify a critical “unbound” radius beyond which TI should accompany thermal driving. Our numerical disk wind simulations support this result and reveal that clumpiness is a consequence of buoyancy disrupting the stratified structure of steady-state solutions. Namely, instead of a smooth transition layer separating the highly ionized disk wind from the cold phase atmosphere below, hot bubbles formed from TI rise up and fragment the atmosphere. These bubbles first appear within large-scale vortices that form below the transition layer, and they result in the episodic production of distinctive cold phase structures referred to as irradiated atmospheric fragments (IAFs). Upon interacting with the wind, IAFs advect outward and develop extended crests. The subsequent disintegration of the IAFs takes place within a turbulent wake that reaches high elevations above the disk. We show that this dynamics has the following observational implications: dips in the absorption measure distribution are no longer expected within TI zones, and there can be a less sudden desaturation of X-ray absorption lines such as O viii Lyα as well as multiple absorption troughs in Fe xxv Kα.

Swift Spectroscopy of the Accretion Disk Wind in the Black Hole GRO J1655–40

Abstract Chandra obtained two High Energy Transmission Grating spectra of the stellar-mass black hole GRO J1655−40 during its 2005 outburst, revealing a rich and complex disk wind. Soon after its launch, the Neil Gehrels Swift Observatory began monitoring the same outburst. Some X-ray Telescope (XRT) observations were obtained in a mode that makes it impossible to remove strong Mn calibration lines, so the Fe Kα line region in the spectra was previously neglected. However, these lines enable a precise calibration of the energy scale, facilitating studies of the absorption-dominated disk wind and its velocity shifts. Here we present fits to 15 Swift/XRT spectra, revealing variability and evolution in the outflow. The data strongly point to a magnetically driven disk wind: both the higher-velocity (e.g., ) and lower-velocity (e.g., ) wind components are typically much faster than is possible for thermally driven outflows ( ), and photoionization modeling yields absorption radii that are two orders of magnitude below the Compton radius that defines the typical inner extent of thermal winds. Moreover, correlations between key wind parameters yield an average absorption measure distribution that is consistent with magnetohydrodynamic wind models. We discuss our results in terms of recent observational and theoretical studies of black hole accretion disks and outflows, as well as future prospects.

The ionic composition of the local absorber towards 3C 273

ABSTRACT Hot ionized gas is observed in the local vicinity of our galaxy through spectral absorption features. The most common hypothesis is that this gas forms a halo surrounding our Milky Way, in collisional ionization equilibrium. In this paper, we investigate the elemental abundance of this hot and ionized local gas. We use a 2.4 Ms stacked X-ray spectrum of the bright blazar 3C 273 and probe the local absorption features. Using ion-by-ion fitting of the X-ray absorption lines, we derive the column density of each ionization species. Based on the column densities, we reconstruct the absorption measure distribution, namely the hydrogenic column density as a function of temperature. We report the elemental abundances of C, N, Ne, and Fe relative to solar O. Previous measurements of local X-ray emission lines in conjunction with the present column densities indicate a scale height of 1–80 kpc and hydrogen number density of 10−4–10−3 cm−3 for the hot ionized gas. Additionally, we detect He-like O lines from the quasar broad line region with velocities of 6400 ± 1500 km s−1.

What Shapes the Absorption Measure Distribution in AGN Outflows?

The Absorption Measure Distribution (AMD) in the X-ray outflows of Seyfert active galactic nuclei (AGN) describes the distribution of absorbing column density as a function of ionization parameter. Up to now, AMD has been measured only for seven objects with high-resolution X-ray data that contain absorption lines from ionized heavy elements. Even though the number of measured AMDs is not large, they display a universal broad shape containing a prominent dip, for which the absorbing column drops by around two orders of magnitude. In this paper, we tested a range of photoionization models against the overall shape of the AMD as observed in Seyferts. In particular, we demonstrate that the shape of the AMD depends both on the spectral energy distribution (SED) of radiation which enters the outflow, and the density of the warm absorber (WA). The model that best reproduces the observed shape of the AMD is one wherein the gas density of the WA is of the order of $10^{12}$ cm$^{-3}$, irradiated by an SED whose optical/UV luminosity is 100 times higher than the X-ray luminosity. When the cloud density is higher than $\sim 10^{11}$ cm$^{-3}$, free-free heating dominates the entire absorber, and only one instability zone occurs, which is in agreement with observations.

Magnetized Disk Winds in NGC 3783

Abstract We analyze a 900 ks stacked Chandra/HETG spectrum of NGC 3783 in the context of magnetically driven accretion-disk wind models in an effort to provide tight constraints on the global conditions of the underlying absorbers. Motivated by the earlier measurements of its absorption measure distribution (AMD) indicating X-ray-absorbing ionic columns that decrease slowly with decreasing ionization parameter, we employ 2D magnetohydrodynamic (MHD) disk wind models to describe the global outflow. We compute its photoionization structure along with the wind kinematic properties, allowing us to further calculate in a self-consistent fashion the shapes of the major X-ray absorption lines. With the wind radial density profile determined by the AMD, the profiles of the ensemble of the observed absorption features are determined by the two global parameters of the MHD wind; i.e., disk inclination and wind density normalization n o . Considering the most significant absorption features in the ∼1.8–20 Å range, we show that the MHD wind is best described by cm−3 and . We argue that winds launched by X-ray heating or radiation pressure, or even MHD winds but with steeper radial density profiles, are strongly disfavored by data. Considering the properties of Fe K-band absorption features (i.e., Fe xxv and Fe xxvi), while typically prominent in the active galactic nucleus X-ray spectra, they appear to be weak in NGC 3783. For the specific parameters of our model obtained by fitting the AMD and the rest of the absorption features, these features are found to be weak, in agreement with observations.

Fast ionized X‐ray absorbers in AGNs

AbstractWe investigate the physics of the X‐ray ionized absorbers often identified as warm absorbers (WAs) and ultra‐fast outflows (UFOs) in Seyfert AGNs from spectroscopic studies in the context of magnetically‐driven accretion‐disk wind scenario. Launched and accelerated by the action of a global magnetic field anchored to an underlying accretion disk around a black hole, outflowing plasma is irradiated and ionized by an AGN radiation field characterized by its spectral energy density (SED). By numerically solving the Grad‐Shafranov equation in the magnetohydrodynamic (MHD) framework, the physical property of the magnetized disk‐wind is determined by a wind parameter set, which is then incorporated into radiative transfer calculations with xstar photoionization code under heating‐cooling equilibrium state to compute the absorber's properties such as column density NH, line‐of‐sight (LoS) velocity v, ionization parameter ς, among others. Assuming that the wind density scales as n ∝ r–1, we calculate theoretical absorption measure distribution (AMD) for various ions seen in AGNs as well as line spectra especially for the Fe Kα absorption feature by focusing on a bright quasar PG 1211+143 as a case study and show the model's plausibility. In this note we demonstrate that the proposed MHD‐driven disk‐wind scenario is not only consistent with the observed X‐ray data, but also help better constrain the underlying nature of the AGN environment in a close proximity to a central engine. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The thermal instability of the warm absorber in NGC 3783

We model the observed X-ray spectral continuum shape, ionic column densities, and absorption measure distribution (AMD) of the warm absorber in the Seyfert galaxy NGC 3783. We assume a photo-ionized medium with a uniform total (gas+radiation) pressure. The irradiation causes the wind to be radiation pressure compressed (RPC). We compare the observational characteristics derived from the 900 ksec Chandra observation to radiative transfer computations in pressure equilibrium using the radiative transfer code TITAN. We explore different values of the ionization parameter xi of the incident flux and adjust the hydrogen-equivalent column density, N_H0 of the warm absorber to match the observed soft X-ray continuum. We derive theoretical column densities for a broad range of ionic species of iron and neon and an AMD that we compare to the observations. We find an extension of the degeneracy between xi and N_H0 for the constant pressure models previously discussed for NGC 3783. Including the ionic column densities of iron and neon in the comparison between observations and data we conclude that a range of ionization parameters between 4000 and 8000 ergs cm/s is preferred. For the first time, we present theoretical AMD for a constant pressure wind in NGC 3783 that correctly reproduces the observed level and is in approximate agreement with the observational appearance of an instability region. Using a variety of observational indicators, we confirm that the X-ray outflow of NGC 3783 can be described as an RPC medium in pressure equilibrium. The observed AMD agrees with a uniformly hot or a uniformly cold thermal state. The measured ionic column densities suggest that the wind tends to the uniformly cold thermal state. The occurrence of thermal instability in the warm absorber model may depend on the computational method and the spatial scale on which the radiative transfer is solved.

ABSORPTION MEASURE DISTRIBUTION IN Mrk 509

In this paper we model the observed absorption measure distribution (AMD) in Mrk 509, which spans three orders of magnitude in ionization level with a single-zone absorber in pressure equilibrium. AMD is usually constructed from observations of narrow absorption lines in radio-quiet active galaxies with warm absorbers. We study the properties of the warm absorber in Mrk~509 using recently published broad-band spectral energy distribution observed with different instruments. This spectrum is an input in radiative transfer computations with full photoionisation treatment using {\sc titan} code. We show that the simplest way to fully reproduce the shape of AMD is to assume that the warm absorber is a single zone under constant total pressure. With this assumption we found theoretical AMD which matches the observed one determined on the basis of 600 ks RGS XMM-Newton spectrum of Mrk~509. The softness of the source spectrum and the important role of the free-free emission breaks the usual degeneracy in the ionization state calculations, and the explicit dependence of the depths of AMD dips on density open a new path to density diagnostic for the warm absorber. In Mrk 509 the implied density is of the order of $10^8$ cm$^{-3}$.

Radiation pressure confinement – III. The origin of the broad ionization distribution in AGN outflows

The winds of ionized gas driven by Active Galactic Nuclei (AGN) can be studied through absorption lines in their X-ray spectra. A recurring feature of these outflows is their broad ionization distribution, including essentially all ionization levels (e.g., Fe^0+ to Fe^25+). This characteristic feature can be quantified with the absorption measure distribution (AMD), defined as the distribution of column density with ionization parameter |dN / dlog xi|. Observed AMDs extend over 0.1 < xi < 10^4 (cgs), and are remarkably similar in different objects. Power-law fits (|dN /dlog xi| ~ N_1 xi^a) yield N_1 = 3x10^{21} cm^-2 +- 0.4 dex and a = 0 -- 0.4. What is the source of this broad ionization distribution, and what sets the small range of observed $N_1$ and $a$? A common interpretation is a multiphase outflow, with a wide range of gas densities in a uniform gas pressure medium. However, the incident radiation pressure leads to a gas pressure gradient in the photoionized gas, and therefore to a broad range of ionization states within a single slab. We show that this compression of the gas by the radiation pressure leads to an AMD with |dN / dlog xi| = 8x10^21 xi^0.03 cm^-2, remarkably similar to that observed. The calculated values of $N_1$ and $a$ depend weakly on the gas metallicity, the ionizing spectral slope, the distance from the nucleus, the ambient density, and the total absorber column. Thus, radiation pressure compression (RPC) of the photoionized gas provides a natural explanation for the observed AMD. RPC predicts that the gas pressure increases with decreasing ionization, which can be used to test the validity of RPC in ionized AGN outflows.

X-RAY ABSORPTION ANALYSIS OF NGC 3516: APPEARANCE OF FAST COMPONENTS WITH INCREASED SOURCE FLUX

By analyzing the X-ray spectra of NGC 3516 from 2001 and 2006 obtained with the HETGS spectrometer on board the Chandra observatory, we find that the kinematic structure of the outflow can be well represented by four outflow components intrinsic to NGC 3516. The outflow velocities of the different components are 350 +-100 km s-1, 1500 +-150 km s-1, 2600 +-200 km s-1 and 4000 +-400 km s-1 for components 1, 2, 3 and 4, respectively. A local component at z = 0 could be confused with intrinsic component 3. Components 1 and 2 have a broad range of ionization manifested by absorption from 23 different charge states of Fe. Component 3 and 4 are more highly ionized and show absorption from only 9 different charge states of Fe, however we were able to reconstruct the absorption measure distribution (AMD) for all four. The total column density of each component is NH = (1.8+- 0.5) X10^22 cm-2, NH = (2.5+- 0.3) X10^22 cm-2, NH = (6.9+- 4.3) X10^22 cm-2 and NH = (5.4+- 1.2) X10^22 cm-2, respectively. The fast components 3 and 4 appear only in the high state of 2006 and not in 2001, while the slower components persist during both epochs. On the other hand, there is no significant absorption variability within days during 2001 or during 2006. We find that covering factor plays a minor role for the line absorption.

Multiwavelength campaign on Mrk 509

We present the results of our 600 ks RGS observation as part of the multiwavelength campaign on Mrk 509. The very high quality of the spectrum allows us to investigate the ionized outflow with an unprecedented accuracy due to the long exposure and the use of the RGS multipointing mode. We detect multiple absorption lines from the interstellar medium and from the ionized absorber in Mrk 509. A number of emission components are also detected, including broad emission lines consistent with an origin in the broad line region, the narrow OVII forbidden emission line and also (narrow) radiative recombination continua. The ionized absorber consists of two velocity components (v = -13 \pm 11 km/s and v = -319 \pm 14 km/s), which both are consistent with earlier results, including UV data. There is another tentative component outflowing at high velocity, -770 \pm 109 km/s, which is only seen in a few highly ionized absorption lines. The outflow shows discrete ionization components, spanning four orders of magnitude in ionization parameter. Due to the excellent statistics of our spectrum, we demonstrate for the first time that the outflow in Mrk 509 in the important range of log xi between 1-3 cannot be described by a smooth, continuous absorption measure distribution, but instead shows two strong, discrete peaks. At the highest and lowest ionization parameters we cannot differentiate smooth and discrete components.

MAGNETOHYDRODYNAMIC ACCRETION DISK WINDS AS X-RAY ABSORBERS IN ACTIVE GALACTIC NUCLEI

We present the 2D ionization structure of self-similar MHD winds off accretion disks around irradiated by a central X-ray source. Based on earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r)~1/r. We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. Particular attention is paid to the absorption measure distribution (AMD), namely their Hydrogen-equivalent column per logarithmic ionization parameter \xi interval, d N_H/(d \log \xi), which provides a measure of the winds' radial density profiles. For n(r)~1/r the AMD is found to be independent of \xi, in good agreement with its behavior inferred from the X-ray spectra of several AGNs. For the specific wind structure and X-ray spectrum we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v~100-300 km/sec (at \log \xi~2-3) for Fe XVII and v~1,000-4,000 km/sec (at \log \xi~4-5) for Fe XXV, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate \dot{m} and LOS angle \theta. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing \dot{m} and increases steeply with \theta. As such, we concur with previous authors that these wind configurations, viewed globally, incorporate all the requisite properties of the parsec scale "torii" invoked in AGN unification schemes. We indicate that a combination of the AMD and absorption line profile observations can uniquely determine these model parameters and their bearing on AGN population demographics.

X-RAY ABSORPTION ANALYSIS OF MCG -6-30-15: DISCERNING THREE KINEMATIC SYSTEMS

By analyzing the X-ray spectrum of MCG-6-30-15 obtained with the HETG spectrometer on board the Chandra Observatory, we identify three kinematically distinct absorption systems; two outflow components intrinsic to MCG-6-30-15, and one local at z = 0. The slow outflow at -100 +/- 50 km s^-1 has a large range of ionization manifested by absorption from 24 different charge states of Fe, which enables a detailed reconstruction of the absorption measure distribution (AMD). This AMD spans five orders of magnitude in ionization parameter: -1.5 < log xi < 3.5 (cgs units), with a total column density of N_H = (5.3 +/- 0.7) x 10^21 cm^-2. The fast outflow at -1900 +/- 150 km s^-1 has a well defined ionization parameter with log xi = 3.82 +/- 0.03 (cgs units) and column density N_H = 8.1 +/- 0.7 x 10^22 cm^-2. Assuming this component is a thin, uniform, spherical shell, it can be estimated to lie within 11 light days of the \agn center. The third component, most clearly detected in the lower oxygen charge states O^+1 - O^+6, has been confused in the past with the fast outflow, but is identified here with local gas z = 0 and a total column density N_H of a few 10^20 cm^-2. Finally, we exploit the excellent spectral resolution of the HETGS and use the present spectrum to determine the rest-frame wavelengths of oxygen inner-shell lines that were previously uncertain.

DENSITY PROFILES IN SEYFERT OUTFLOWS

For the past decade, ionized outflows of a few 100 km/s from nearby Seyfert galaxies have been studied in great detail using high resolution X-ray absorption spectra. A recurring feature of these outflows is their broad ionization distribution including essentially ions (e.g., of Fe) from neutral to fully ionized. The absorption measure distribution (AMD) is defined as the distribution of column density with ionization parameter |d N_H/d (log xi)|. AMDs of Seyfert outflows can span up to five orders of magnitude in xi. We present the AMD of five outflows and show that they are all rather flat, perhaps slightly rising towards high ionization. More quantitatively, a power-law fit for log AMD ~ (log xi)^a yields slopes of 0 < a < 0.4. These slopes tightly constrain the density profiles of the wind, which until now could be addressed only by theory. If the wind is distributed on large scales, the measured slopes imply a generic density radial profile of n ~ r^{-alpha} with 1 < alpha < 1.3. This scaling rules out a mass conserving radial flow of n ~ r^{-2}, or a constant density absorber, but is consistent with a non-spherical MHD outflow model in which n ~ r^{-1} along any given line of sight. On the other hand, if ionization variations are a result of local (delta r) density gradients, e.g. as in the turbulent interstellar medium (ISM), the AMD slopes imply density scaling of n ~ delta r^{-alpha} with 0.7 < alpha < 1.0, which is quite different from the scaling of approximately n ~ delta r^{0.4} found in the Milky Way ISM and typical of incompressible turbulence.

Absorption Measure Distribution of the Outflow in IRAS 13349+2438: Direct Observation of Thermal Instability?

We analyze the Chandra X-ray spectrum obtained with the HETGS grating spectrometer of IRAS 13349+2438, which has one of the richest absorption spectra of a quasar outflow. Absorption from almost all charge states of Fe is detected. This allows for a detailed reconstruction of the absorption measure distribution (AMD), which we define as the continuous distribution of column density as a function of ionization parameter. We find a double peaked AMD for IRAS 13349+2438 with a total (ionized) column density of N_H = (1.2 +- 0.3)* 10^{22} cm^-2} assuming solar iron abundance. For comparison, we perform a similar analysis on the well studied HETGS spectrum of NGC 3783. Both sources feature a deep minimum in column density consistent with no absorption from gas at temperatures of 4.5 < log T < 5 (K). We interpret the minima as observational evidence for thermal instability in this temperature regime.