Fe Kα Research Articles

NuSTAR Hard X-Ray Spectra of Radio Galaxies

Abstract The Nuclear Spectroscopic Telescope Array (NuSTAR) observatory, with its 3–78 keV broadband spectral coverage, enables the detection of the high-energy cutoff in a number of active galaxies, including several individual radio-loud ones. In this work, we present systematic and uniform analyses of 55 Nuclear Spectroscopic Telescope Array (NuSTAR) spectra for a large sample of 28 radio galaxies, 20 of which are FR II galaxies. We perform spectral fitting to measure the high-energy cutoff E cut, photon index Γ, reflection factor R, and Fe Kα line equivalent width (EW). Measurements of E cut are given for 13 sources, and lower limits are given for the rest. We find that those E cut nondetections could primarily be attributed to the obviously smaller net photon counts in their spectra. This indicates that the NuSTAR spectra of the majority of our sample are dominated by the thermal coronal emission, and the E cut distribution of the sample is indistinguishable from that of a radio-quiet one in the literature. The flatter NuSTAR spectra we observed, comparing with radio-quiet sources, are thus unlikely due to jet contamination. The radio galaxies also show weaker X-ray reflection (both in R and Fe Kα line EW) compared with radio-quiet ones. Combining with the radio-quiet sample, we see a correlation between R and EW but with considerably large scatter. Notably, the radio-loud and -quiet sources appear to follow a common Γ–R correlation trend, supporting the outflowing corona model for both populations in which higher bulk outflowing velocity yields weaker reflection and flatter X-ray slope.

Nondetection of the Gamma-Ray Burst X-Ray Emission Line: The Down-Comptonization Effect

Abstract The detection of the gamma-ray burst (GRB) X-ray emission line is important for studying GRB physics and constraining the GRB redshift. Since the line-like feature in the GRB X-ray spectrum was first reported in 1999, several works on line searching have been published over the past two decades. Even though some observations on the X-ray line-like feature were performed, its significance remains controversial to date. In this paper, we utilize the down-Comptonization mechanism and present the time evolution of the Fe Kα line emitted near the GRB central engine. The line intensity decreases with the evolution time, and the time evolution depends on the electron density and the electron temperature. In addition, the initial line with a larger broadening decreases less over time. For instance, when the emission line penetrates material with an electron density above 1012 cm−3 at 1 keV, it generally becomes insignificant enough after 100 s for it not to be detected . The line-like profile deviates from the Gaussian form, and it finally changes to be similar to a blackbody shape at the time of the thermal equilibrium between the line photons and the surrounding material.

Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies

The structure-function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe Kα and Kβ X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

Space Telescope and Optical Reverberation Mapping Project. XI. Disk-wind Characteristics and Contributions to the Very Broad Emission Lines of NGC 5548

Abstract In 2014 the NGC 5548 Space Telescope and Optical Reverberation Mapping campaign discovered a two-month anomaly when variations in the absorption and emission lines decorrelated from continuum variations. During this time the soft X-ray part of the intrinsic spectrum had been strongly absorbed by a line-of-sight (LOS) obscurer, which was interpreted as the upper part of a disk wind. Our first paper showed that changes in the LOS obscurer produces the decorrelation between the absorption lines and the continuum. A second study showed that the base of the wind shields the broad emission-line region (BLR), leading to the emission-line decorrelation. In that study, we proposed the wind is normally transparent with no effect on the spectrum. Changes in the wind properties alter its shielding and affect the spectral energy distribution (SED) striking the BLR, producing the observed decorrelations. In this work we investigate the impact of a translucent wind on the emission lines. We simulate the obscuration using XMM-Newton, NuSTAR, and Hubble Space Telescope observations to determine the physical characteristics of the wind. We find that a translucent wind can contribute a part of the He ii and Fe Kα emission. It has a modest optical depth to electron scattering, which explains the fainter far-side emission in the observed velocity-delay maps. The wind produces the very broad base seen in the UV emission lines and may also be present in the Fe Kα line. Our results highlight the importance of accounting for the effects of such winds in the analysis of the physics of the central engine.

X-Ray Quasi-periodic Oscillations in the Lense–Thirring Precession Model. II. Variability of the Relativistic Iron Kα Line

Abstract The reprocessing of primary X-ray emission in the accretion disk of black hole X-ray binaries (BHXRBs) produces a reflection spectrum with the characteristic Fe Kα fluorescence line. Strong low-frequency quasi-periodic oscillations (QPOs) are observed from BHXRBs, and the dependence of QPO properties (e.g., phase lag) on the inclination angle suggests that the observed QPO may be associated with a geometrical effect, e.g., the precession of the X-ray source due to frame dragging near the spinning black hole. Here, in the scenario of the Lense–Thirring precession of the X-ray source, we use a Monte Carlo simulation of radiative transfer to study the irradiation/reflection and the resultant spectral properties including the Fe Kα line as a function of precession phase (time). We found that the reflection fraction, i.e., the ratio of incident flux toward the disk and the direct flux toward the observer at infinity, is modulated by the precession phase, which depends on the truncation radius (i.e., the spectral state in the truncated disk model) and the inclination angle. The Fe Kα line profile also changes as the primary X-ray source precesses, with the line luminosity and the flux-weighted centroid energy varying with the precession phase. The periodically modulated 2–10 keV continuum flux could apparently lag the line luminosity in phase, if the truncation radius is small enough for Doppler effects due to disk orbital motion to significantly affect the observed radiation.

Probing cosmic rays with Fe Kα line structures generated by multiple ionization process

Abstract Supernova remnants (SNRs) have been regarded as major acceleration sites of Galactic cosmic rays. Recent X-ray studies revealed neutral Fe Kα line emission from dense gas in the vicinity of some SNRs, which can be best interpreted as K-shell ionization of Fe atoms in the gas by sub-relativistic particles accelerated in the SNRs. In this Letter, we propose a novel method of constraining the composition of particles accelerated in SNRs, which is currently unknown. When energetic heavy ions collide with target atoms, their strong Coulomb field can easily cause simultaneous ejection of multiple inner-shell electrons of the target. This results in shifts in characteristic X-ray line energies, forming distinctive spectral structures. Detection of such structures in the neutral Fe Kα line strongly supports the particle ionization scenario, and furthermore provides direct evidence of heavy ions in the accelerated particles. We construct a model for the Fe Kα line structures by various projectile ions utilizing atomic-collision data.

AGN X-Ray Irradiation of CO Gas in NGC 2110 Revealed by Chandra and ALMA

Abstract We report spatial distributions of the Fe–Kα line at 6.4 keV and the CO(J = 2–1) line at 230.538 GHz in NGC 2110, which are, respectively, revealed by Chandra and Atacama Large Millimeter/submillimeter Array (ALMA) at ≈0.″5. A Chandra 6.2–6.5 keV to 3.0–6.0 keV image suggests that the Fe–Kα emission extends preferentially in a northwest to southeast direction out to ≈3″, or ∼500 pc, on each side. Spatially resolved spectral analyses support this by finding significant Fe–Kα emission lines only in the northwest and southeast regions. Moreover, their equivalent widths are found to be ∼1.5 keV, indicative for the fluorescence by nuclear X-ray irradiation as the physical origin. By contrast, CO(J = 2–1) emission is weak therein. For quantitative discussion, we derive ionization parameters by following an X-ray dominated region (XDR) model. We then find them high enough to interpret the weakness as the result of X-ray dissociation of CO and/or H2. Another possibility also remains that CO molecules follow a superthermal distribution, resulting in brighter emission in higher-J lines. Further follow-up observations are encouraged to draw a conclusion on what predominantly changes the interstellar matter properties and whether the X-ray irradiation eventually affects the surrounding star formation as active galactic nucleus (AGN) feedback.

X-ray spectral and image spatial models of NGC 3081 with Chandra data

AbstractThe physical properties of AGN such as accretion rate, column density, temperature of hot corona and other characteristics can be found from X-ray spectral data. We present the results of spatial and spectral analysis for Sy2 type galaxy NGC 3081 obtained with different mathematical tools of the Chandra Interactive Analysis of Observations software. We found evidence of extended emission in 0.5-3.0 keV as well as derived parameters for model A: photon index , column density , warm component and hot component . We detected the presence of a component of the reflection spectrum, Fe Kα emission line with and .

X-raying winds in distant quasars: The first high-redshift wind duty cycle

Aims. Theoretical models of wind-driven feedback from active galactic nuclei (AGN) often identify ultra-fast outflows as being the main agent in the generation of galaxy-sized outflows, which are possibly the main actors in establishing so-called AGN-galaxy co-evolution. Ultra-fast outflows are well characterized in local AGN but much less is known in quasars at the cosmic time when star formation and AGN activity peaked (z ≃ 1–3). It is therefore necessary to search for evidence of ultra-fast outflows in high-z sources to test wind-driven AGN feedback models. Methods. Here we present a study of Q2237+030, the Einstein Cross, a quadruply-imaged radio-quiet lensed quasar located at z = 1.695. We performed a systematic and comprehensive temporally and spatially resolved X-ray spectral analysis of all the available Chandra and XMM-Newton data (as of September 2019). Results. We find clear evidence for spectral variability, possibly due to absorption column density (or covering fraction) variability intrinsic to the source. For the first time in this quasar, we detect a fast X-ray wind outflowing at vout ≃ 0.1c that would be powerful enough (Ėkin ≃ 0.1 Lbol) to significantly affect the evolution of the host galaxy. We report also on the possible presence of an even faster component of the wind (vout ∼ 0.5c). For the first time in a high-z quasar, given the large sample and long time interval spanned by the analyzed X-ray data, we are able to roughly estimate the wind duty cycle as ≃0.46 (0.31) at 90% (95%) confidence level. Finally, we also confirm the presence of a Fe Kα emission line with variable energy, which we discuss in the light of microlensing effects as well as considering our findings on the source.

Is Superorbital Modulation in SMC X-1 Caused by Absorption in a Warped Precessing Accretion Disk?

Abstract We present a broadband spectral-timing analysis of SMC X-1 at different intensity states of its superorbital variation using 10 Suzaku and 6 Nuclear Spectroscopic Telescope Array (NuSTAR) observations. The spectrum in all the states can be described by an absorbed power law with a high-energy cutoff and a blackbody component along with an iron emission line. Compared to other supergiant high-mass X-ray binaries, the Fe Kα line equivalent width is low in SMC X-1—from less than 10 eV in the high state to up to ∼270 eV in the low states. The spectral shape is dependent on flux, with the hard X-ray spectrum steepening with increasing flux. We also report a highly variable normalization of the power-law component across these 16 superorbital states. Pulsations in the hard X-rays for both the instruments were detected in all but two observations. The pulse profiles are near sinusoidal, with two peaks and the relative intensity of the second peak decreasing with decreasing luminosity. These findings suggest that the superorbital modulation in SMC X-1 is not caused by absorption in precessing warped accretion disk alone and there are intrinsic changes in X-rays emanating from the neutron star at different superorbital states. We also note a putative cyclotron line at ∼50 keV in the NuSTAR spectra of three bright states, indicating a possible magnetic field of ∼4.2 × 1012 G. Finally, with the new pulse period measurements reported here, the time base for the secular spin-up of SMC X-1 is increased by thirteen years and the complete pulse period history shows a sudden change in the spin-up trend around 1995.

Iron line from neutron star accretion discs in scalar tensor theories

ABSTRACT The Fe Kα fluorescent line at 6.4 keV is a powerful probe of the space–time metric in the vicinity of accreting compact objects. We investigated here how some alternative theories of gravity, namely scalar tensor theories, that invoke the presence of a non-minimally coupled scalar field and predict the existence of strongly scalarized neutron stars (NSs), change the expected line shape with respect to General Relativity. By taking into account both deviations from the general relativistic orbital dynamics of the accreting disc, where the Fe line originates, and the changes in the light propagation around the NS, we computed line shapes for various inclinations of the disc with respect to the observer. We found that both the intensity of the low-energy tails and the position of the high-energy edge of the line change. Moreover, we verified that even if those changes are in general of the order of a few percent, they are potentially observable with the next generation of X-ray satellites.

Evidence for Disk Truncation at Low Accretion States of the Black Hole Binary MAXI J1820+070 Observed by NuSTAR and XMM-Newton

Abstract We present results from NuSTAR and XMM-Newton observations of the new black hole X-ray binary MAXI J1820+070 at low accretion rates (below 1% of the Eddington luminosity). We detect a narrow Fe Kα emission line, in contrast to the broad and asymmetric Fe Kα line profiles commonly present in black hole binaries at high accretion rates. The narrow line, with weak relativistic broadening, indicates that the Fe Kα line is produced at a large disk radius. Fitting with disk reflection models assuming standard disk emissivity finds a large disk truncation radius (a few tens to a few hundreds of gravitational radii, depending on the disk inclination). In addition, we detect a quasi-periodic oscillation (QPO) varying in frequency between 11.6 ± 0.2 mHz and 2.8 ± 0.1 mHz. The very low QPO frequencies suggest a large size for the optically thin Comptonization region according to the Lense–Thirring precession model, supporting that the accretion disk recedes from the innermost stable circular orbit and is replaced by advection-dominated accretion flow at low accretion rates. We also discuss the possibility of an alternative accretion geometry that the narrow Fe Kα line is produced by a lamppost corona with a large height illuminating the disk.

Neutron Star Radius-to-mass Ratio from Partial Accretion Disk Occultation as Measured through Fe Kα Line Profiles

Abstract We present a new method to measure the radius-to-mass ratio (R/M) of weakly magnetic, disk-accreting neutron stars by exploiting the occultation of parts of the inner disk by the star itself. This occultation imprints characteristic features on the X-ray line profile that are unique and are expected to be present in low-mass X-ray binary systems seen under inclinations higher than ∼65°. We analyze a Nuclear Spectroscopic Telescope Array observation of a good candidate system, 4U 1636-53, and find that X-ray spectra from current instrumentation are unlikely to single out the occultation features owing to insufficient signal-to-noise. Based on an extensive set of simulations we show that large-area X-ray detectors of the future generation could measure R/M to ∼ % precision over a range of inclinations. Such is the precision in radius determination required to derive tight constraints on the equation of state of ultradense matter and it represents the goal that other methods also aim to achieve in the future.

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.

EUCLIA. II. On the Puzzling Large UV to X-Ray Lags in Seyfert Galaxies

Abstract Recent intense X-ray and UV monitoring campaigns with Swift have detected clear UV lags behind X-ray in several local active galactic nuclei (AGNs). The UV to X-ray lags are often larger (by a factor of up to ∼20) than expected if the UV variation is simply due to the X-ray reprocessing. We previously developed a model in which the UV/optical variations are attributed to disk turbulences, and the effect of large-scale turbulence is considered. Our model, which overcomes many severe challenges to the reprocessing scheme, can explain the observed variations in NGC 5548, particularly the correlations and lags among the UV/optical bands. In this work, assuming the corona heating is associated with turbulences in the inner accretion disk, we extend our study to model the correlations and lags between the X-ray and UV/optical bands. We find that our model, without the need of light echoing, can well reproduce the observed UV to X-ray lags and the optical to UV lags simultaneously in four local Seyfert galaxies, including NGC 4151, NGC 4395, NGC 4593, and NGC 5548. In our scenario, relatively larger UV to X-ray lag is expected for AGN with smaller innermost disk radius and thus more compact corona. Interestingly, for these Seyfert galaxies studied in this work, sources with relatively larger UV to X-ray lags do have broader Fe Kα lines, indicative of relativistic broadening due to more compact corona and smaller innermost disk radius. If confirmed with more X-ray and UV monitoring campaigns, this interesting discovery would provide a new probe to the inner disk/corona.

Joint XMM-Newton and NuSTAR observations of the reflection spectrum of III Zw 2

Detecting and modeling the reprocessed hard X-ray emission component in the accretion flow, the so-called reflection spectrum, is a main tool used to estimate black hole spins in a wide range of astrophysical black holes, regardless of their mass or distance. In this work, we study the X-ray spectra of the Seyfert I galaxy III Zw 2 by using multiepoch XMM-Newton, NuSTAR, and Suzaku observations. The X-ray spectra exhibit a soft-excess below 1 keV and a prominent excess at the location of the broad Fe Kα line at 6.4 keV. To account for these spectral features, we fit the spectra with multiple models including an ionized partially covering absorber and an accretion disk reflection model. To fully resolve the reflection component, we analyzed jointly the XMM-Newton and NuSTAR observations taken in 2017 and archival XMM-Newton data from 2000. Assuming the reflection scenario, the resulting model fits support for a rapidly spinning black hole (a ≥ 0.98) in this radio-intermediate source. The X-ray spectra in 2000 and 2017 are remarkably similar; the only difference pertains to the reflection fraction, which is possibly due to a change in the geometry of the accretion flow. However, the Suzaku observation is markedly different, and we suggest this could be an effect of a jet contribution in the X-ray band, which is supported by the elevated radio flux during this observation.

A broadband X-ray view of the NLSy1 1E 0754.6+3928

Context. The soft X-ray band of many active galactic nuclei (AGNs) is affected by obscuration due to partially ionised matter crossing our line of sight. In this context, two past XMM-Newton observations that were six months apart and a simultaneous NuSTAR-Swift exposure of the Narrow Line Seyfert 1 galaxy 1E 0754.6+3928, which was approximately eight years later, revealed an intense and variable warm absorber (WA) and hints of additional absorbers in the Fe Kα band. Aims. We aim to provide the first X-ray characterisation of this AGN by discussing its broadband (0.3–79 keV) spectrum and temporal properties. Methods. We conduct a temporal and spectroscopic analysis on two ∼10 ks (net exposure) XMM-Newton snapshots performed in April and October 2006. We also study the high energy behaviour of 1E 0754.6+3928 and model its broadband spectrum by using simultaneous Swift-NuSTAR data. Both phenomenological and physically motivated models are tested. Results. We find the presence of flux variability of ∼150% and 30% for 0.3–2 and 2–10 keV bands, respectively, and spectral changes at months timescales of ΔΓ ∼ 0.4. A reflection component that is consistent with being constant over years and arising from relatively cold material that is far from the central super massive black hole is detected. The main spectral feature shaping the 1E 0754.6+3928 spectrum is a WA. Such a component is persistent over the years and the variability of its ionisation and column density is observed down on months in the ranges 3 × 1022cm−2 ≲ NH ≲ 7.2 × 1022cm−2 and 1.5 ≲ log (ξ/erg s−1 cm) ≲ 2.1. Despite the short exposures, we find possible evidence of two additional high-ionisation and high-velocity outflow components in absorption. Conclusions. Our analysis suggests the existence of a complex system of absorbers in 1E 0754.6+3928. Longer exposures are required in order to characterise, on more solid grounds, the absorbers in this AGN.

A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52

ABSTRACT It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below ∼10−2 of the Eddington limit (LEdd). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608−52, to study changes in the reflection spectrum. We find that the broad Fe–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼1037 erg s−1 (∼0.05 LEdd), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 1035 erg s−1 (∼0.002 LEdd). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ ≳ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates.

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.

Performance of assembled X-ray optics consisted of a polycapillary X-ray optics and a monocapillary X-ray optics for micro X-ray fluorescence spectrometry

A micro X-ray fluorescence (μ-XRF) spectrometry instrument based on an assembled X-ray optics was constructed in the laboratory. The assembled X-ray optics consisted of a polycapillary slightly focusing X-ray lens and a tapered monocapillary X-ray lens (TMXRL). The output X-ray beam of the polycapillary slightly focusing X-ray lens was focused again by the TMXRL. A TMXRL with output aperture of 12.01 μm was drawn by a nested glass tube. To evaluate the performance of the instrument, a performance comparison between different capillary X-ray optics was conducted. The intensity of the Fe Kα from an iron plate was used to indicate the analysis efficiency of the μ-XRF instrument. The relative gain of the instrument was corrected by comparing the intensity per unit area for a pinhole aperture. Compared with TMXRL single used, the assembled X-ray optics also had a micro focal spot. Furthermore, the relative gain of the assembled X-ray optics and the intensity of the Fe Kα from the iron plate obtained by the assembled X-ray optics were approximately 5 times those of the TMXRL single used. This highly intense micro X-ray beam with several diameters obtained by the assembled X-ray optics will be useful for μ-XRF analysis.