Broad Iron Emission Line Research Articles

The 2022 Outburst of IGR J17091–3624: Connecting the Exotic GRS 1915+105 to Standard Black Hole X-Ray Binaries

While the standard X-ray variability of black hole X-ray binaries (BHXBs) is stochastic and noisy, there are two known BHXBs that exhibit exotic “heartbeat”-like variability in their lightcurves: GRS 1915+105 and IGR J17091–3624. In 2022, IGR J17091–3624 went into outburst for the first time in the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the exotic variability and the corresponding spectral features with unprecedented detail. We find that as in typical BHXBs, the outburst began in the hard state, then continued in the intermediate state, but then transitioned to an exotic soft state, where we identify two types of heartbeat-like variability (Class V and a new Class X). The flux energy spectra show a broad iron emission line due to relativistic reflection when there is no exotic variability, and absorption features from highly ionized iron when the source exhibits exotic variability. Whether absorption lines from highly ionized iron are detected in IGR J17091–3624 is not determined by the spectral state alone, but rather is determined by the presence of exotic variability; in a soft spectral state, absorption lines are only detected along with exotic variability. Our finding indicates that IGR J17091–3624 can be seen as a bridge between the most peculiar BHXB GRS 1915+105 and “normal” BHXBs, because it alternates between the conventional and exotic behaviors of BHXBs. We discuss the physical nature of the absorbing material and exotic variability in light of this new legacy data set.

Moving Corona and the Line Profile of the Relativistic Broad Iron Emission Line

Iron fluorescence emission lines from X-ray binaries and active galactic nuclei are important diagnostic tools for studying the physical processes near the event horizon of both the stellar-mass black holes in X-ray binaries and the supermassive black holes in active galactic nuclei. In this work, we investigate the line profile of the relativistic broad iron lines from the cool accretion disk of a black hole due to the asymmetric illumination of a moving corona, which moves away from the disk with a relativistic velocity. Both the off-axis location and the radial velocity of the moving corona are considered. Our results clearly show that the illumination and the line profile are dependent on the position and velocity of the corona, since the disk region below the corona receives more flux, which is the most important factor affecting the line profiles. As expected, if the corona is close to the receding part of the rotating disk, the red peak is enhanced, while the blue peak is weakened in the broad line profile, and the central energy of the emission line is low. Conversely, if the corona is close to the approaching part of the disk, the blue peak is strong and the central energy of the emission line is high, even higher than the intrinsic energy of the emission line. Due to the beaming effect of the moving corona, the corona with high velocity illuminates the outer region of the disk, which leads to the red peak disappearing and there being only one blue peak in the profile of the emission line.

Studying the Reflection Spectra of the New Black Hole X-Ray Binary Candidate MAXI J1631−479 Observed by NuSTAR: A Variable Broad Iron Line Profile

Abstract We present results from the Nuclear Spectroscopic Telescope Array observations of the new black hole X-ray binary candidate MAXI J1631–479 at two epochs during its 2018–2019 outburst, which caught the source in a disk dominant state and a power-law dominant state. Strong relativistic disk reflection features are clearly detected, displaying significant variations in the shape and strength of the broad iron emission line between the two states. Spectral modeling of the reflection spectra reveals that the inner radius of the optically thick accretion disk evolves from <1.9 to 12 ± 1 r g (statistical errors at 90% confidence level) from the disk dominant to the power-law dominant state. Assuming in the former case that the inner disk radius is consistent with being at the innermost stable circular orbit, we estimate a black hole spin of a* > 0.94. Given that the bolometric luminosity is similar in the two states, our results indicate that the disk truncation observed in MAXI J1631–479 in the power-law dominant state is unlikely to be driven by a global variation in the accretion rate. We propose that it may instead arise from local instabilities in the inner edge of the accretion disk at high accretion rates. In addition, we find an absorption feature in the spectra centered at 7.33 ± 0.03 keV during the disk dominant state, which is evidence for the rare case that an extremely fast disk wind ( ) is observed in a low-inclination black hole binary, with the viewing angle of 29° ± 1° as determined by the reflection modeling.

Interaction of the accretion flows in corona and disk near the black hole in active galactic nuclei

Accretion flows toward black holes can be of a quite different nature, described as an optically thick cool gas flow in a disk for high accretion rates or as a hot coronal optically thin gas flow for low accretion rates, possibly affected by outflowing gas. The detection of broad iron emission lines in active galactic nuclei (AGN) indicates the coexistence of corona and disk. The appearance and relative strength of such flows essentially depends on their interaction. Liu et al. suggested that condensation of gas from the corona to the disk allows to understand accretion flows of comparable strength of emission. Matter inflow due to gravitational capture of gas is important for the condensation process. We discuss observational features predicted by the model. Data from simultaneous observations of AGN with {\it {Swift's}} X-ray and UV-optical telescopes are compared with the theoretical predictions. The frequent detection of broad iron K$\alpha$ emission lines and the dependence of the emitted spectra on the Eddington ratio, described by the values of the photon index $\Gamma$ and the two-point spectral index $\alpha_{\rm{ox}}$ are in approximate agreement with the predictions of the condensation model; the latter, however, with a large scatter. The model further yields a coronal emission concentrated in a narrow inner region as is also deduced from the analysis of emissivity profiles. The accretion flows in bright AGN could be described by the accretion of stellar wind or interstellar medium and its condensation into a thin disk.

HIGH-RESOLUTION X-RAY SPECTROSCOPY OF THE BURSTING PULSAR GRO J1744-28

The bursting pulsar GRO J1744-28 is a Galactic low-mass X-ray binary that distinguishes itself by displaying type-II X-ray bursts: brief, bright flashes of X-ray emission that likely arise from spasmodic accretion. Combined with its coherent 2.1 Hz X-ray pulsations and relatively high estimated magnetic field, it is a particularly interesting source to study the physics of accretion flows around neutron stars. Here we report on Chandra/HETG observations obtained near the peak of its bright 2014 accretion outburst. Spectral analysis suggests the presence of a broad iron emission line centered at E_l ~ 6.7 keV. Fits with a disk reflection model yield an inclination angle of i ~ 52 degrees and an inner disk radius of R_in ~ 85 GM/c^2, which is much further out than typically found for neutron star low-mass X-ray binaries. Assuming that the disk is truncated at the magnetospheric radius of the neutron star, we estimate a magnetic field strength of B ~ (2-6)E10 G. Furthermore, we identify an absorption feature near ~6.85 keV could correspond to blue-shifted Fe xxv and point to a fast disk wind with an outflow velocity of v_out ~ (7.5-8.2)E3 km/s (~0.025c-0.027c). If the covering fraction and filling factor are large, this wind could be energetically important and perhaps account for the fact that the companion star lost significant mass while the magnetic field of the neutron star remained strong.

XMM-Newtonobservations reveal the disappearance of the wind in 4U 1630−47

We report on XMM-Newton observations of the black hole X-ray binary 4U 1630-47 during its 2012-2013 outburst. The first five observations monitor the source as its luminosity increases across the high-soft state of accretion. In the sixth observation the source has made a transition to an "anomalous" state, characterised by a significant contribution of electron scattering. A thermally/radiatively driven disc wind is present in the first four observations, which becomes more photoionised as the luminosity increases with time. In the fifth observation, the wind is not observed any more as a consequence of strong photoionisation and the low sensitivity of this observation. This overall trend is then consistent with a fully ionised wind causing the electron scattering characteristic of the anomalous state in the sixth observation. A broad iron emission line co-exists with the absorption features from the wind in the first four observations but is not visible in the last two observations. We find that the changes in the state of the wind as measured from modelling the absorption features with a self-consistent warm absorber model are correlated to the changes in the broad iron line. When the latter is modeled with a reflection component we find that the reflection fraction decreases as the illumination increases. We propose that the changes in both the absorption and broad emission lines are caused by the increasing luminosity and temperature of the accretion disc along the soft state. Such changes ultimately enable the transition to a state where the wind is fully ionised and consequently Comptonisation plays a significant role.

Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636–53

Both the broad iron (Fe) line and the frequency of the kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries (LMXBs) can potentially provide independent measures of the inner radius of the accretion disc. We use XMM-Newton and simultaneous Rossi X-ray Timing Explorer observations of the LMXB 4U 1636-53 to test this hypothesis. We study the properties of the Fe-K emission line as a function of the spectral state of the source and the frequency of the kHz QPOs. We find that the inner radius of the accretion disc deduced from the frequency of the upper kHz QPO varies as a function of the position of the source in the colour-colour diagram, in accordance with previous work and with the standard scenario of accretion disc geometry. On the contrary, the inner disc radius deduced from the profile of the Fe line is not correlated with the spectral state of the source. The values of the inner radius inferred from kHz QPOs and Fe lines, in four observations, do not lead to a consistent value of the neutron star mass, regardless of the model used to fit the Fe line. Our results suggest that either the kHz QPO or the standard relativistic Fe line interpretation does not apply for this system. Furthermore, the simultaneous detection of kHz QPOs and broad Fe lines is difficult to reconcile with models in which the broadening of the Fe line is due to the reprocessing of photons in an outflowing wind.

DISC ATMOSPHERES AND WINDS IN X-RAY BINARIES

We review the current status of studies of disc atmospheres and winds in low mass X-ray binaries. We discuss the possible wind launching mechanisms and compare the predictions of the models with the existent observations. We conclude that a combination of thermal and radiative pressure (the latter being relevant at high luminosities) can explain the current observations of atmospheres and winds in both neutron star and black hole binaries. Moreover, these winds and atmospheres could contribute significantly to the broad iron emission line observed in these systems.

BROAD IRON LINES IN NEUTRONS STARS: DYNAMICAL BROADENING OR WIND SCATTERING?

Broad iron emission lines are observed in many accreting systems from black holes in AGN and X-ray binaries to neutron star low-mass X-ray binaries. The origin of the line broadening is often interpreted as due to dynamical broadening and relativistic effects. However, alternative interpretations have been proposed, included broadening due to Compton scattering in a wind or accretion disk atmosphere. Here we explore the observational signatures expected from broadening in a wind, in particular that the iron line width should increase with an increase in the column density of the absorber (due to an increase in the number of scatterings). We study the data from three neutron star low-mass X-ray binaries where both a broad iron emission line and absorption lines are seen simultaneously, and show that there is no significant correlation between line width and column density. This favors an inner disk origin for the line broadening rather than scattering in a wind.

X-ray diagnostics of chemical composition of the accretion disc and donor star in ultra-compact X-ray binaries

We search for the Fe Kα line in spectra of Ultra Compact X-ray Binaries (UCXBs). For this purpose we have analyzed XMM-Newton observations of five confirmed UCXBs. We find that the object 2S 0918-549 – whose optical spectrum bears tentative signatures of a C/O accretion disk – is devoid of any emission features in the 6-7keV range, with an upper limit of less than 10eV for the equivalent width (EW) of the iron line. 4U 1916-05 – whose optical spectrum is consistent with reflection from a He-rich accretion disk – exhibits a bright broad iron emission line. This behavior is in agreement with the theoretical predictions presented in Koliopanos, Gilfanov & Bildsten (2013). Namely, we expect strong suppression of the Fe Kα emission line in spectra originating in moderately bright (LogLx less than � 37.5) UCXBs with C/O or O/Ne/Mg-rich donors. On the other hand the EW of the iron line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of � 100eV. Our analysis also reveals a strong Fe Kα line in the spectrum of 4U 0614+091. This detection points towards a He-rich donor and seems to be at odds with the source’s classification as C/O-rich. Nevertheless, a He-rich donor would explain the bursting activity reported for this system. Lastly, based on our theoretical predictions, we attribute the lack of a strong iron emission line – in the two remaining UCXB sources in our sample (XTE J1807-294, 4U 0513-40) – as an indication of a C/O or O/Ne/Mg white dwarf donor. From the upper limits of the Fe Kα line EW in 4U 0513-40, 2S 0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron ratio, O/Fe> 10 × [O/Fe]⊙.

The X-ray spectrum of the black hole candidate Swift J1753.5−0127

We present a spectral analysis of the black hole candidate and X-ray transient source Swift J1753.5 0127 making use of simultaneous observations of XMM-Newton and Rossi X-ray Timing Explorer (RXTE) in 2006, when the source was in outburst. The aim of this paper is to test whether a thermal component due to the accretion disc is present in the X-ray spectrum. We fit the data with a range of spectral models, and we find that for all of these models the fits to the X-ray energy spectra significantly require the addition of the disc black-body component. We also find a broad iron emission line at around 6.5 keV, most likely due to iron in the accretion disc. Our results confirm the existence of a cool inner disc extending near or close to the innermost circular orbit (ISCO).We further discovered broad emission lines of NVII and OVIII at ~ 0.52 keV and 0.65 keV, respectively in the RGS spectrum of Swift J1753.5-0127.

Coronae above accretion disks around black holes: the effect of Compton cooling

The geometry of the accretion flow around stellar mass and supermassive black holes depends on the accretion rate. Broad iron emission lines originating from the irradiation of cool matter can indicate that there is an inner disk below a hot coronal flow.These emission lines have been detected in X-ray binaries. Observations with the Chandra X-ray Observatory, XMM Newton and Suzaku have confirmed the presence of these emission lines also in a large fraction of Seyfert-1 active galactic nuclei (AGN). We investigate the accretion flow geometry for which broad iron emission lines can arise in hard and soft spectral state. We study an ADAF-type coronal flow, where the ions are viscously heated and electrons receive their heat only by collisions from the ions and are Compton cooled by photons from an underlying cool disk. For a strong mass flow in the disk and the resulting strong Compton cooling only a very weak coronal flow is possible. This limitation allows the formation of ADAF-type coronae above weak inner disks in the hard state, but almost rules them out in the soft state. The observed hard X-ray luminosity in the soft state, of up to 10% or more of the total flux, indicates that there is a heating process that directly accelerates the electrons. This might point to the action of magnetic flares of disk magnetic fields reaching into the corona. Such flares have also been proposed by observations of the spectra of X-ray black hole binaries without a thermal cut-off around 200 keV.

X-ray spectroscopy of MXB 1728–34 withXMM-Newton

We analyzed an XMM-Newton observation of the low-mass X-ray binary and atoll source MXB 1728–34. The source was in a low-luminosity state during the XMM-Newton observation, corresponding to a bolometric X-ray luminosity of erg s-1. The 1–11 keV X-ray spectrum of the source, obtained combining data from all the five instruments on-board XMM-Newton, is well fitted by a Comptonized continuum. Evident residuals are present at 6−7 keV, which are ascribed to the presence of a broad iron emission line. This feature can be equally well fitted by a relativistically smeared line or by a self-consistent, relativistically smeared reflection model. Under the hypothesis that the iron line is produced by reflection from the inner accretion disk, we can infer important information on the physical parameters of the system, such as the inner disk radius, Rin = 25−100 km, and the inclination of the system, 44° < i < 60°.

Broad iron emission lines in Seyfert galaxies – re-condensation of gas onto an inner disk below the ADAF?

Recent observations of Seyfert 1 AGN with Chandra, XMM-Newton and Suzaku revealed broad iron K_alpha emission lines, some relativistically blurred. For galactic black hole X-ray binaries XMM-Newton spectra during hard state also reveal the presence of a relativistic iron emission line and a thermal component, interpreted as an indication for a weak inner cool accretion disk underneath a hot corona. These thermal components were found after the transition from soft to hard spectral state and can be understood as sustained by re-condensation of gas from an advection-dominated flow (ADAF) onto the disk. In view of the similarity of accretion flows around stellar mass and supermassive black holes we discuss whether the broad iron emission lines in Seyfert 1 AGN can be understood as arising from a similar accretion flow geometry. We derive Eddington-scaled accretion rates for Seyfert galaxies with strong lines in samples of Miller (2007) and Nandra et al. (2007). For the evaluation we use the observed X-ray luminosity, bolometric corrections and black hole masses from literature, most values taken from Fabian and Vasudevan (2009). Rates derived are less than 0.1 of the Eddington rate for more than half of the sources. For 10^7 to 10^8 solar mass black holes in Seyfert 1 AGN this limit corresponds to 0.01 to 0.2 solar masses per year. Our investigation shows that for quite a number of Seyfert AGN in hard spectral state iron emission lines can arise from an inner weak disk surrounded by an ADAF as predicted by the re-condensation model. Some of the remaining sources with higher accretion rates may be in a spectral state comparable to the "very high" state of LMXBs. (abridged)

A strong and broad Fe line in the XMM-Newton spectrum of the new X-ray transient and black hole candidate XTE J1652−453

We observed the new X-ray transient and black-hole candidate XTE J1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTE J1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad iron emission line with an equivalent width of ~ 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disk, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disk is ~ 4 gravitational radii. Assuming that the accretion disk is truncated at the radius of the innermost stable circular orbit, the black hole in XTE J1652-453 has a spin parameter of ~ 0.5. The power spectrum of the RXTE observation has an additional variability component above 50 Hz, which is typical for the hard-intermediate state. No coherent quasi-periodic oscillations at low frequency are apparent in the power spectrum, which may imply that we view the system at a rather low inclination angle.

A self-consistent approach to the hard and soft states of 4U 1705-44

We analyzed two XMM-Newton observations of the bright atoll source 4U 1705-44, which can be considered a prototype of the class of the persistent NS LMXBs showing both hard and soft states. The first observation was performed when the source was in a hard low flux state, the second during a soft, high-flux state. Both the spectra show broad iron emission lines. We fit the spectra using a two-component model, together with a reflection model specifically suited to the case of a neutron star, where the incident spectrum has a blackbody shape. In the soft state, the reflection model, convolved with a relativistic smearing component, consistently describes the broad features present in the spectrum, and we find a clear relation between the temperature of the incident flux and the temperature of the harder X-ray component that we interpret as the boundary layer emission. In this state we find converging evidence that the boundary layer outer radius is ~ 2 times the neutron star radius. In the low flux state, we observe a change in the continuum shape of the spectrum with respect to the soft state. Still, the broad local emission features can be associated with a disk reflecting matter, but in a lower ionization state, and possibly produced in an accretion disk truncated at greater distance. Our analysis provides strong evidence that the reflection component in soft states of LMXBs comes from to hard X-ray thermal irradiation, which we identify with the boundary layer emission, also present in the continuum model. In the hard state, the broad iron line if also produced by reflection, and the continuum disk emission can be self-consistently accounted if the disk is truncated at a greater distance than the soft state.

An intermediate black hole spin in the NLS1 galaxy SWIFT J2127.4+5654: chaotic accretion or spin energy extraction?

We have observed the hard X-ray selected Narrow-Line Seyfert 1 galaxy SWIFT J2127.4+5654 with Suzaku. We report the detection of a broad relativistic iron emission line from the inner accretion disc. By assuming that the inner edge of the accretion disc corresponds to the innermost stable circular orbit of the black hole spacetime, the line profile enables us to measure a black hole spin a= 0.6\pm0.2. However, a non-rotating Schwarzschild spacetime is excluded at just above the 3 sigma level, while a maximally spinning Kerr black hole is excluded at the ~5 sigma level. The intermediate spin we measure may indicate that accretion-driven black hole growth in this source proceeds through short-lived episodes with chaotic angular momentum alignment between the disc and the hole rather than via prolonged accretion. The measured steep emissivity index (q~5) constrains the irradiating X-ray source to be very centrally concentrated. Light bending may help focus the primary emission towards the innermost accretion disc, thus steepening the irradiation profile. On the other hand, steep profiles can also be reached if magnetic extraction of the hole rotational energy is at work. If this is the case, the interplay between accretion (spinning up the black hole) and rotational energy extraction (spinning it down) forces the hole to reach an equilibrium spin value which, under standard assumptions, is remarkably consistent with our measurement. Rotational energy extraction would then be able to simultaneously account for the intermediate spin and steep emissivity profile we infer from our spectral analysis without the need to invoke chaotic accretion episodes. (abridged)

STELLAR-MASS BLACK HOLE SPIN CONSTRAINTS FROM DISK REFLECTION AND CONTINUUM MODELING

Accretion disk reflection spectra, including broad iron emission lines, bear the imprints of the strong Doppler shifts and gravitational red-shifts close to black holes. The extremity of these shifts depends on the proximity of the innermost stable circular orbit to the black hole, and that orbit is determined by the black hole spin parameter. Modeling relativistic spectral features, then, gives a means of estimating black hole spin. We report on the results of fits made to archival X-ray spectra of stellar-mass black holes and black hole candidates, selected for strong disk reflection features. Following recent work, these spectra were fit with reflection models and disk continuum emission models (where required) in which black hole spin is a free parameter. Although our results must be regarded as preliminary, we find evidence for a broad range of black hole spin parameters in our sample. The black holes with the most relativistic radio jets are found to have high spin parameters, though jets are observed in a black hole with a low spin parameter. For those sources with constrained binary system parameters, we examine the distribution of spin parameters versus black hole mass, binary mass ratio, and orbital period. We discuss the results within the context of black hole creation events, relativistic jet production, and efforts to probe the innermost relativistic regime around black holes.

Discovery of a broad iron line in the black hole candidate Swift J1753.5−0127, and the disc emission in the low/hard state revisited

We analyzed simultaneous archival XMM-Newton and RXTE observations of the X-ray binary and black hole candidate Swift J1753.5-0127. In a previous analysis of the same data a soft thermal component was found in the X-ray spectrum, and the presence of an accretion disk extending close to the innermost stable circular orbit was proposed. This is in contrast with the standard picture in which the accretion disk is truncated at large radii in the low/hard state. We tested a number of spectral models and we found that several of them fit the observed spectra without the need of a soft disk-like component. This result implies that the classical paradigm of a truncated accretion disk in the low/hard state can not be ruled out by these data. We further discovered a broad iron emission line between 6 and 7 keV in these data. From fits to the line profile we found an inner disk radius that ranges between ~6-16 gravitational radii, which can be in fact much larger, up to ~250 gravitational radii, depending on the model used to fit the continuum and the line. We discuss the implications of these results in the context of a fully or partially truncated accretion disk.

Broad Iron‐Kα Emission Lines as a Diagnostic of Black Hole Spin

We address the ability of broad iron emission lines from black hole accretion disks to diagnose the spin of the black hole. Using a high-resolution three-dimensional MHD simulation of a geometrically thin accretion disk in a pseudo-Newtonian potential, we show that both the mid-plane density and the vertical column density of the accretion flow drop dramatically over a narrow range of radii close to the innermost stable circular orbit (ISCO). We argue that this drop of density is accompanied by a sharp increase in the ionization parameter of the X-ray photosphere, and that the resulting imprint of the ISCO on the X-ray reflection spectrum can be used to constrain spin. Motivated by this simulation, we construct a simplified toy model of the accretion flow within the ISCO of a Kerr black hole, and use this model to estimate the systematic error on inferred black hole spin that may result from slight bleeding of the iron line emission to the region inside of the ISCO. We find that these systematic errors can be significant for slowly spinning black holes but become appreciably smaller as one considers more rapidly rotating black holes.