X-ray Column Research Articles

X-ray and CO-Derived Column Densities in AGN: A Study of Obscuration Properties in CTAGN and Non-CTAGN

Abstract Obscuration in active galactic nuclei (AGN) provides valuable insights into the nature of the material surrounding the central engine. Comptonthick AGN (CTAGN), characterised by a column density of NH ≥ 1.5 × 1024 cm−2, are heavily obscured by substantial amounts of dust and gas. While X-ray observations are primarily used to determine this column density, our understanding of obscuration properties in the sub-mm regime, particularly for CTAGN, remains limited. In this study, we analyse archival data from the Atacama Large Millimetre/sub-millimetre Array (ALMA) for both CTAGN and non-CTAGN sources, as identified by the 70-month catalogue of the all-sky hard X-ray Swift/Burst Alert Monitor survey and other X-ray surveys. Integrated intensity maps (moment 0) of CO(3–2) emission reveal a concentrated distribution of dense gas around the nucleus. Utilising a constant CO-to-H2 conversion factor, XCO = 2.2 × 1020 cm−2 (K km s−1)−1, we find that the derived molecular hydrogen column densities, NH2, are generally lower than the total hydrogen column densities, NH, obtained from X-ray observations. However, the NH2 values derived in this work are slightly higher than those reported in previous studies due to the adoption of a higher CO-to-H2 conversion factor. This discrepancy between NH and NH2 is consistent with prior findings that X-ray-derived column densities are typically higher, except in the case of non-CTAGN, where NH2 can exceed NH. Statistical analysis using Kendall and Spearman tests reveals a positive monotonic relationship between NH and NH2, although the correlation is not statistically significant. This suggests a complex interplay of factors influencing these properties. The optically thick nature of CO in dense regions may contribute to the observed discrepancies. Our results highlight the importance of adopting an accurate CO-to-H2 conversion factor to derive reliable column densities, which could serve as an alternative method for identifying CTAGN. Further investigations with more comprehensive data sets and refined methodologies are needed to better understand the relationship between sub-millimetre and X-ray properties in AGNs.

A Rapidly Accreting Active Galactic Nucleus Hidden in a Dust-obscured Galaxy at z ∼ 0.8

Dust-obscured galaxies (DOGs) containing central supermassive black holes (SMBHs) that are rapidly accreting (i.e., having high Eddington ratios, λ Edd) may represent a key phase closest to the peak of both the black hole and galaxy growth in the coevolution framework for SMBHs and galaxies. In this work, we present a 68 ks XMM-Newton observation of the high-λ Edd DOG J1324+4501 at z ∼ 0.8, which was initially observed by Chandra. We analyze the XMM-Newton spectra jointly with archival Chandra spectra. In performing a detailed X-ray spectral analysis, we find that the source is intrinsically X-ray luminous with log(LX /erg s−1)=44.71−0.12+0.08 and heavily obscured with log(NH/cm−2)=23.43−0.13+0.09 . We further utilize UV-to-IR archival photometry to measure and fit the source’s spectral energy distribution to estimate its host-galaxy properties. We present a supplementary comparison sample of 21 X-ray luminous DOGs from the XMM-SERVS survey with sufficient (>200) 0.5–10 keV counts to perform a similarly detailed X-ray spectral analysis. Of the X-ray luminous DOGs in our sample, we find that J1324+4501 is the most remarkable, possessing one of the highest X-ray luminosities, column densities, and star formation rates. We demonstrate that J1324+4501 is in an extreme evolutionary stage where SMBH accretion and galaxy growth are at their peaks.

BASS. XLI. The Correlation between Mid-infrared Emission Lines and Active Galactic Nuclei Emission

We analyze Spitzer spectra of 140 active galactic nuclei (AGN) detected in the hard X-rays (14–195 keV) by the Burst Alert Telescope on board Swift. This sample allows us to probe several orders of magnitude in black hole masses (106–109 M ⊙), Eddington ratios (10−3–1), X-ray luminosities (1042–1045 erg s−1), and X-ray column densities (1020–1024 cm−2). The AGN emission is expected to be the dominant source of ionizing photons with energies ≳50 eV, and therefore, high-ionization mid-infrared (MIR) emission lines such as [Ne v] 14.32, 24.32 μm and [O iv] 25.89 μm are predicted to be good proxies of AGN activity, and robust against obscuration effects. We find high detection rates (≳85%–90%) for the MIR coronal emission lines in our AGN sample. The luminosities of these lines are correlated with the 14–150 keV luminosity (with a typical scatter of σ ∼0.4–0.5 dex), strongly indicating that the MIR coronal line (CL) emission is driven by AGN activity. CLs are also tightly correlated to the bolometric luminosity (σ ∼0.2–0.3 dex), calculated from careful analysis of the spectral energy distribution. We find that the relationship between the CL strengths and L 14–150 keV is independent of black hole mass, AGN luminosity, and Eddington ratio, and mostly not affected by high X-ray column densities. This confirms that the MIR CLs can be used as unbiased tracers of the AGN power for X-ray luminosities in the 1042–1045 erg s−1 range.

The Density and Ionization Profiles of Optically Dark and High-Redshift GRBs Probed by X-ray Absorption

The X-ray column density (NHX) of gamma-ray bursts (GRBs) can probe the local environment of their progenitors over a wide redshift range. Previous work has suggested an increasing trend as a function of redshift. The relevance of this current analysis relies on investigating the selection bias method, such as the effect of the X-ray spectrum in high-redshift GRBs, which complicates the measurement of small NHX; this has yet to be fully evaluated or discussed elsewhere. In this work, we evaluated these effects through simulations to define appropriate observational limits in the NHX versus redshift plane. We then applied a one-sided nonparametric method developed by Efron and Petrosian. Within the framework of this method, we investigated the redshift dependence of NHX and the local distribution function. Our results show that the evolution of NHX with redshift firmly exists with a significance of more than four sigma and follows a power law of (1+z)1.39(+0.22,−0.27). Based on these analyses and previous studies, the GRB progenitor mass varies but is more massive in the high-redshift environment and has a higher gas column density. This suggests that part of the luminosity evolution of GRBs, which has been widely reported, may be due to the evolution of the progenitor’s mass. Using the same method, we demonstrate that optically dark GRBs show a consistent evolution: (1+z)1.15(+0.67,−0.83). By applying the Kolmogorov–Smirnov (KS) test, it is shown that optically dark GRBs have statistically identical flux and photon index distributions compared to normal GRBs, but the NHX is systematically larger. This result suggests that the darkness of some GRB populations is not due to an intrinsic mechanism, but rather because a higher density surrounds them.

Comparison of models for the warm-hot circumgalactic medium around Milky Way-like galaxies

ABSTRACT A systematic comparison of the models of the circumgalactic medium (CGM) and their observables is crucial to understanding the predictive power of the models and constraining physical processes that affect the thermodynamics of CGM. This paper compares four analytic CGM models: precipitation, isentropic, cooling flow, and baryon pasting models for the hot, volume-filling CGM phase, all assuming hydrostatic or quasi-hydrostatic equilibrium. We show that for fiducial parameters of the CGM of a Milky Way (MW)-like galaxy ($M_{\rm vir} \sim 10^{12}~{\rm M}_{\odot }$ at $z\sim 0$), the thermodynamic profiles – entropy, density, temperature, and pressure – show most significant differences between different models at small ($r\lesssim 30$ kpc) and large scales ($r\gtrsim 100$ kpc) while converging at intermediate scales. The slope of the entropy profile, which is one of the most important differentiators between models, is $\approx 0.8$ for the precipitation and cooling flow models, while it is $\approx 0.6$ and 0 for the baryon pasting and isentropic models, respectively. We make predictions for various observational quantities for an MW mass halo for the different models, including the projected Sunyaev–Zeldovich effect, soft X-ray emission (0.5–2 keV), dispersion measure, and column densities of oxygen ions (O vi, O vii, and O viii) observable in absorption. We provide Python packages to compute the thermodynamic and observable quantities for the different CGM models.

Cygnus A obscuring torus: ionized, atomic, or molecular?

ABSTRACT The prototypical powerful FR II radio galaxy Cygnus A fits extremely well into the quasar/radio galaxy unified model: high polarization with an angle almost perpendicular to the radio jet and polarized flux showing broad permitted lines. It has been claimed that ionized gas in the torus reveals a very clear torus shape via Bremmstrahlung emission. We rule out the later with an energetic argument, and we constrain the molecular and atomic gas properties with existing observations. The atomic absorption against the core has been shown to match the X-ray column only if the spin temperature is an implausible Ts = 1 × 106 K. This points to a molecular medium for the X-ray column $\log (N_{\rm H} ~[\rm {cm^{-2}}]) \sim 23.5$. Yet not low-J CO absorption is detected to sensitive limits. The non-detection is surprising given that this powerful radio galaxy hosts a luminous, dust-obscured active nucleus and copious warm molecular hydrogen. These conditions suggest a detectable level of emission. Furthermore, the torus X-ray column density suggests detectable absorption. We explore various possibilities to explain the lack of a signature from warm CO (200–250 K). Specifically, that the radiative excitation by the radio core renders low-J CO absorption below current sensitivities, and that high-J levels are well populated and conducive to producing absorption. We test this hypothesis using archival Hershel/SPIRE FTS observations of Cygnus A of high-J CO lines (14 ≥ J ≥ 4 transitions). Still high-J CO lines are not detected. We suggest that ALMA observations near its high frequency limit can be critical to obtain the signature of molecular line of the torus of Cygnus A.

Constraints on pre-SN outbursts from the progenitor of SN 2023ixf using the large binocular telescope

ABSTRACT The progenitor of SN 2023ixf was an ∼104.8 to $10^{5.0}\, \text{L}_\odot$ star (∼9 to $14\, \text{M}_\odot$ at birth) obscured by a dusty $\dot{M} \simeq 10^{-5}\, \text{M}_\odot \rm \, yr^{-1}$ wind with a visual optical depth of τV ≃ 13. This is required by the progenitor spectral energy distribution, the post-SN X-ray and H α luminosities, and the X-ray column density estimates. In Large Binocular Telescope (LBT) data spanning 5600 to 400 d before the supernova (SN), there is no evidence for optical variability at the level of $\sim 10^3\, \text{L}_\odot$ in R band, roughly three times the predicted luminosity of the obscured progenitor. This constrains direct observation of any pre-SN optical outbursts where there are LBT observations. However, models of the effects of any pre-SN outburst on the dusty wind show that an outburst of essentially any duration exceeding ∼5 times the luminosity of the progenitor would have detectable effects on the dust optical depth for decades. While the dust obscuration here is high, all red supergiants have dusty winds, and the destruction (or formation) of dust by even short-lived transients will always have long-term effects on the observed brightness of the star because changes in the dust optical depths after a luminous transient occur very slowly.

Obscuring Environment and X-Ray Variability of Compact Symmetric Objects Unveiled with XMM-Newton and NuSTAR

Compact symmetric objects (CSOs) show radio features such as jets, lobes, and hot spots, which are contained within their host galaxies, and likely represent a recent radio activity. A subpopulation of CSOs with high intrinsic X-ray column densities has been inferred from shallow, soft X-ray band exposures, and observed to cluster in the linear radio size versus 5 GHz radio power plane, which suggests that a dense circumnuclear medium may dramatically influence the growth of compact radio structures. Here, we report on the first detection of two CSOs, 2021+614 and J1511+0518, at energies above 10 keV with NuSTAR. We model the NuSTAR data jointly with the new XMM-Newton data of J1511+0518, and with the archival XMM-Newton data of 2021+614. A toroidal reprocessor model fits the data well and allows us to robustly confirm the X-ray properties of the CSO absorbers and continuum. In both sources, we find intrinsic X-ray absorbing column densities in excess of 1023 cm−2, hard photon indices of the primary emission, Γ ∼ 1.4–1.7, Fe Kα line emission, and variability of the intrinsic X-ray flux density on the timescale of years. The studied X-ray continua are dominated by the primary power-law emission at energies above 3 keV, and by the scattered component at energies below 3 keV. An additional soft X-ray component, modeled with a hot, collisionally ionized plasma with temperature kT ∼ 1 keV, is required by the XMM-Newton data in J1511+0518, which is corroborated by the tentative evidence for the extension in the archival Chandra image of the source.

Multiwavelength study of NGC 1365: The obscured active nucleus and off-nuclear compact X-ray sources

ABSTRACTWe present a multiwavelength study of the active nucleus and the off-nuclear X-ray sources in the nearby spiral galaxy, NGC 1365 using three simultaneous UV/X-ray observations by AstroSat over a two months period and archival IR observations performed with Spitzer and Herschel. Utilizing the data from the Soft X-ray Telescope (SXT) onboard AstroSat, we find spectral variability mainly caused by the variation in the X-ray column density, (NH ∼ 1022–1023 cm−2). With the accurate spatial resolution of the UVIT onboard AstroSat, we separate the intrinsic AGN flux from the host galaxy emission and then correct for the Galactic and the internal reddening. We detect no significant variation in the NUV emission over the observation period. The AGN in FUV band is undetectable due to heavy intrinsic extinction. Further, the multiwavelength IR/UV/X-ray AGN SED reveals that the AGN is in a low-luminosity phase with accretion rate ∼0.01 LEdd. The steady UV emission and strong X-ray absorption variability suggest that the obscuring clouds are likely compact and affect the compact X-ray source only and do not possibly cover the extended UV emitting region. In addition, the UVIT is able to resolve two bright spots at a radius of 7 arcsec (∼6.3 Kpc) from the central nucleus in the South-West (SW) direction. In the UVIT image of the entire galaxy, we identify UV counterparts to four Chandra identified bright X-ray sources. One well-known ultra-luminous X-ray source (ULX) NGC 1365 X2 is identified with its UV counterpart at 86 arcsec from the nucleus in the North-East (NE) direction from the active nucleus.

Absence of nuclear polycyclic aromatic hydrocarbon emission from a compact starburst: The case of the type-2 quasar Mrk 477

Mrk 477 is the closest type-2 quasar, at a distance of 163 Mpc. This makes it an ideal laboratory for studying the interplay between nuclear activity and star formation with a great level of detail and signal-to-noise. In this Letter we present new mid-infrared (mid-IR) imaging and spectroscopic data with an angular resolution of 0.4″ (∼300 pc) obtained with the Gran Telescopio Canarias instrument CanariCam. TheN-band (8–13 μm) spectrum of the central ∼400 pc of the galaxy reveals [S IV]λ10.51 μm emission, but no 8.6 or 11.3 μm polycyclic aromatic hydrocarbon (PAH) features, which are commonly used as tracers of recent star formation. This is in stark contrast with the presence of a nuclear starburst of ∼300 pc in size, an age of 6 Myr, and a mass of 1.1×108 M⊙, as constrained from ultravioletHubbleSpace Telescope observations. Considering this, we argue that even the more resilient, neutral molecules that mainly produce the 11.3 μm PAH band are most likely being destroyed in the vicinity of the active nucleus despite the relatively large X-ray column density, logNH = 23.5 cm−2, and modest X-ray luminosity, 1.5×1043erg s−1. This highlights the importance of being cautious when using PAH features as star formation tracers in the central region of galaxies to evaluate the impact of feedback from active galactic nuclei.

Polar dust obscuration in broad-line active galaxies from the XMM-XXL field

Aims. Dust is observed in the polar regions of nearby active galactic nuclei (AGN) and it is known to contribute substantially to their mid-IR emission and to the obscuration of their UV to optical emission. We aim to carry out a statistical test to check whether this component is a common feature based on an analysis of the integrated spectral energy distributions of these composite sources. Methods. We selected a sample of 1275 broad-line AGN in the XMM-XXL field, with optical to infrared photometric data. These AGN are seen along their polar direction and we expect a maximal impact of dust located around the poles when it is present. We used X-CIGALE, which introduces a dust component to account for obscuration along the polar directions, modeled as a foreground screen, and an extinction curve that is chosen as it steepens significantly at short wavelengths or is much grayer. By comparing the results of different fits, we are able to define subsamples of sources with positive statistical evidence in favor of or against polar obscuration (if present) and described using the gray or steep extinction curve. Results. We find a similar fraction of sources with positive evidence for and against polar dust. Applying statistical corrections, we estimate that half of our sample could contain polar dust and among them, 60% exhibit a steep extinction curve and 40% a flat extinction curve; although these latter percentages are found to depend on the adopted extinction curves. The obscuration in the V-band is not found to correlate with the X-ray column density, while AV/NH ratios span a large range of values and higher dust temperatures are found with the flat, rather than with the steep extinction curve. Ignoring this polar dust component in the fit of the spectral energy distribution of these composite systems leads to an overestimation of the stellar contribution. A single fit with a polar dust component described with an SMC extinction curve efficiently overcomes this issue but it fails at identifying all the AGN with polar dust obscuration.

The variable absorption in the X-ray spectrum of GRB 190114C

Gamma-ray burst (GRB) 190114C was a bright burst that occurred in the local Universe (z = 0.425). It was the first GRB ever detected at teraelectronvolt (TeV) energies, and this was thanks to MAGIC. We characterize the ambient medium properties of the host galaxy through the study of the absorbing X-ray column density. Using a combination of Swift, XMM-Newton, and NuSTAR observations, we find that the GRB X-ray spectrum is characterized by a high column density that is well in excess of the expected Milky Way value and decreases, by a factor of ∼2, around ∼105 s. Such a variability is not common in GRBs. The most straightforward interpretation of the variability in terms of the photoionization of the ambient medium is not able to account for the decrease at such late times, when the source flux is less intense. Instead, we interpret the decrease as due to a clumped absorber, denser along the line of sight and surrounded by lower-density gas. After the detection at TeV energies of GRB 190114C, two other GRBs were promptly detected. These two also have high intrinsic column density values, and there are hints for a decrease in their column densities as well. We speculate that a high local column density might be a common ingredient of TeV-detected GRBs.

An XMM–Newton study of active–inactive galaxy pairs

ABSTRACT While theory and simulations indicate that galaxy mergers play an important role in the cosmological evolution of accreting black holes and their host galaxies, samples of active galactic nuclei (AGNs) in galaxies at close separations are still small. In order to increase the sample of AGN pairs, we undertook an archival project to investigate the X-ray properties of an SDSS-selected sample of 32 galaxy pairs with separations ≤150 kpc containing one optically identified AGN, which were serendipitously observed by XMM–Newton. We discovered only one X-ray counterpart among the optically classified non-active galaxies, with a weak X-ray luminosity (≃ 5 × 1041 erg s−1). 59 per cent (19 out of 32) of the AGNs in our galaxy pair sample exhibit an X-ray counterpart, covering a wide range in absorption-corrected X-ray luminosity (5 × 1041–2 × 1044 erg s−1). More than 79 per cent of these AGNs are obscured (column density NH > 1022 cm−2), with more than half thereof (i.e. about 47 per cent of the total AGN sample) being Compton-thick. AGN/no-AGN pairs are therefore more frequently X-ray obscured (by a factor ≃1.5) than isolated AGNs. When compared to a luminosity and redshift-matched sample of bona fide dual AGN, AGN/no-AGN pairs exhibit one order-of-magnitude lower X-ray column density in the same separation range (>10 kpc). A small sample (4 objects) of AGN/no-AGN pairs with sub-pc separation is all heavily obscured, driving a formal anticorrelation between the X-ray column density and the galaxy pair separation in these systems. These findings suggest that the galactic environment has a key influence on the triggering of nuclear activity in merging galaxies.

Reconciling X-Ray and λ21 cm H I Absorption Gas Column Densities toward Obscured AGN

Abstract Hydrogen column densities inferred from X-ray absorption are typically 5–30 times larger than the neutral atomic hydrogen column densities derived from λ21 cm H I absorption toward radio-loud active galactic nuclei (AGN). Some part of the difference is ascribed to uncertainty in the spin temperature T sp = 100 K that is often used to convert λ21 cm H I absorption to N(H I). Here we propose another way to infer the gas column from H I absorption. In our Galaxy there is a nearly linear correlation between the inteferometrically measured integrated λ21 cm absorption ϒH I and reddening, ϒH I ∝ E(B–V)1.10 for ϒH I ≳ 0.7 km s−1 or E(B–V) ≳ 0.04 mag. Scaling E(B–V) then provides the total gas column density N(H) from the same dust column that is responsible for optical obscuration and X-ray absorption, without calculating N(H I). Values of N(H) so derived typically exceed N(H I) by a factor 4 because the ubiquitous Galactic λ21 cm H I absorption samples only a portion of the interstellar gas. If the well-studied case of Hydra-A is a guide, even very large disparities in X-ray and λ21 cm gas column densities can be explained by resolving the core radio continuum and inferring N(H) from λ21 cm absorption. Milky Way conditions are often invoked in discussion of obscured AGN, so the empirical relationship seen in the Milky Way should be a relevant benchmark.

How Do Supernovae Impact the Circumgalactic Medium? I. Large-scale Fountains around a Milky Way–like Galaxy

Abstract Feedback is indispensable in galaxy formation. However, lacking resolutions, cosmological simulations often use ad hoc feedback parameters. Conversely, small-box simulations, while they better resolve the feedback, cannot capture gas evolution beyond the simulation domain. We aim to bridge the gap by implementing small-box results of supernovae-driven outflows into dark matter halo-scale simulations and studying their impact on large scales. Galactic outflows are multiphase, but small-box simulations show that the hot phase (T ≈ 106–7 K) carries the majority of energy and metals. We implement hot outflows in idealized simulations of the Milky Way halo, and examine how they impact the circumgalactic medium. In this paper, we discuss the case when the star formation surface density is low and therefore the emerging hot outflows are gravitationally bound by the halo. We find that outflows form a large-scale, metal-enriched atmosphere with fountain motions. As hot gas accumulates, the inner atmosphere becomes “saturated.” Cool gas condenses, with a rate balancing the injection of the hot outflows. This balance leads to a universal density profile of the hot atmosphere, independent of mass outflow rate. The atmosphere has a radially decreasing temperature, naturally producing the observed X-ray luminosity and column densities of O vi, O vii, and O viii. The self-regulated atmosphere has a baryon and a metal mass of (0.5–1.2) × 1010 M ⊙ and (0.6–1.4) × 108 M ⊙, respectively, small compared to the “missing” baryons and metals from the halo. We conjecture that the missing materials reside at even larger radii, ejected by more powerful outflows in the past.

The relationship between X-ray and optical absorbers in active galactic nuclei

ABSTRACT We present a large compilation of reddening estimates from broad-line Balmer decrements for active galactic nuclei (AGNs) with measured X-ray column densities. The median reddening is E(B − V) ≈ 0.77 ± 0.10 for type-1 to type-1.9 AGNs with reported column densities. This is notably higher than the median reddening of AGNs from the SDSS. We attribute this to the selection bias of the SDSS towards blue AGNs. For other AGNs, we find evidence of a publication bias against reporting low column densities. We find a significant correlation between NH and E(B − V) but with a large scatter of ±1 dex. On average, the X-ray columns are consistent with those predicted from E(B − V) for a solar neighbourhood dust-to-gas ratio. We argue that the large scatter of column densities and reddenings can be explained by X-ray column density variability. For AGNs with detectable broad-line regions (BLRs) that have undergone significant changes of Seyfert type (‘changing-look’ AGNs), we do not find any statistically significant differences in NH or E(B − V) compared to standard type-1 to type-1.9 AGNs. There is no evidence for any type-1 AGNs being Compton thick. We also analyse type-2 AGNs and find no significant correlation between NH and narrow-line region reddening. We find no evidence for a previously claimed anticorrelation. The median column density of LINERs is 22.68 ± 0.75 compared to a column density of 22.90 ± 0.28 for type-2 AGNs. We find the majority of low column density type-2 AGNs are LINERs, but NH is probably underestimated because of scattered light.

HST/COS observations of the newly discovered obscuring outflow in NGC 3783

Aims.To understand the nature of transient obscuring outflows in active galactic nuclei, we use simultaneous multiwavelength observations withXMM-Newton, NuSTAR, theHubbleSpace Telescope (HST), and the Max Planck Gesellschaft/European Southern Observatory (ESO) 2.2 m telescope triggered by soft X-ray absorption detected bySwift.Methods.We obtained ultraviolet spectra on 2016-12-12 and 2016-12-21 using the Cosmic Origins Spectrograph (COS) on HST simultaneously with X-ray spectra obtained withXMM-NewtonandNuSTAR. We modeled the ultraviolet spectra to measure the strength and variability of the absorption, and used photoionization models to obtain its physical characteristics.Results.We find new components of broad, blue-shifted absorption associated with Lyα, N V, Si IV, and C IVin our COS spectra. The absorption extends from near-zero velocities in the rest-frame of the host galaxy to −6200 km s−1. These features appear for the first time in NGC 3783 at the same time as heavy soft X-ray absorption seen in theXMM-NewtonX-ray spectra. The X-ray absorption has a column density of ∼1023cm−2, and it partially covers the X-ray continuum source. Combining the X-ray column densities with the UV spectral observations yields an ionization parameter for the obscuring gas of logξ= 1.84−0.2+0.4erg cm s−1. Despite the high intensity of the UV continuum in NGC 3783,F(1470 Å) = 8 × 10−14erg cm−2s−1Å−1>, the well known narrow UV absorption lines are deeper than in earlier observations in unobscured states, and low ionization states such as C IIIappear, indicating that the narrow-line gas is more distant from the nucleus and is being shadowed by the gas producing the obscuration. Despite the high continuum flux levels in our observations of NGC 3783, moderate velocities in the UV broad line profiles have substantially diminished.Conclusions.We suggest that a collapse of the broad line region has led to the outburst and triggered the obscuring event.

Spectral evolution of the supergiant HMXB IGR J16320–4751 along its orbit using XMM-Newton

Context. The INTEGRAL satellite has revealed a previously hidden population of absorbed high-mass X-ray binaries (HMXBs) hosting supergiant stars. Among them, IGR J16320–4751 is a classical system intrinsically obscured by its environment, with a column density of ∼1023 cm−2, more than an order of magnitude higher than the interstellar absorption along the line of sight. It is composed of a neutron star rotating with a spin period of ∼1300 s, accreting matter from the stellar wind of an O8I supergiant star, with an orbital period of ∼9 days. Aims. We investigated the geometrical and physical parameters of both components of the binary system IGR J16320–4751. Since in systems of this type the compact object is usually embedded in the dense and powerful wind of an OB supergiant companion, our main goal here was to study the dependence of the X-ray emission and column density along the full orbit of the neutron star around the supergiant star. Methods. We analyzed all existing archival XMM-Newton and Swift/BAT observations collected between 2003 and 2008, performing a detailed temporal and spectral analysis of the X-ray emission of the source. We then fitted the parameters derived in our study, using a simple model of a neutron star orbiting a supergiant star. Results. The XMM-Newton light curves of IGR J16320–4751 display high-variability and flaring activity in X-rays on several timescales, with a clear spin period modulation of ∼1300 s. In one observation we detected two short and bright flares where the flux increased by a factor of ∼10 for ∼300 s, with similar behavior in the soft and hard X-ray bands. By inspecting the 4500-day light curves of the full Swift/BAT data, we derived a refined period of 8.99 ± 0.01 days, consistent with previous results. The XMM-Newton spectra are characterized by a highly absorbed continuum and an Fe absorption edge at ∼7 keV. We fitted the continuum with a thermally comptonized COMPTT model, and the emission lines with three narrow Gaussian functions using two TBABS absorption components, to take into account both the interstellar medium and the intrinsic absorption of the system. For the whole set of observations we derived the column density at different orbital phases, showing that there is a clear modulation of the column density with the orbital phase. In addition, we also show that the flux of the Fe Kα line is correlated with the N H column, suggesting a clear link between absorbing and fluorescent matter that, together with the orbital modulation, points towards the stellar wind being the main contributor to both continuum absorption and Fe Kα line emission. Conclusions. Assuming a simple model for the supergiant stellar wind we were able to explain the orbital modulation of the absorption column density, Fe Kα emission and the high-energy Swift/BAT flux, allowing us to constrain the geometrical parameters of the binary system. Similar studies applied to the analysis of the spectral evolution of other sources will be useful to better constrain the physical and geometrical properties of the sgHMXB class.

The O vi Mystery: Mismatch between X-Ray and UV Column Densities

Abstract The UV spectra of Galactic and extragalactic sightlines often show O vi absorption lines at a range of redshifts, and from a variety of sources from the Galactic circumgalactic medium to active galactic nuclei (AGN) outflows. Inner shell O vi absorption is also observed in X-ray spectra (at Å), but the column density inferred from the X-ray line was consistently larger than that from the UV line. Here we present a solution to this discrepancy for the z = 0 systems. The O ii Kβ line at 562.40 eV (≡22.04 Å) is blended with the O vi Kα line in X-ray spectra. We estimate the strength of this O ii line in two different ways, and show that in most cases the O ii line accounts for the entire blended line. The small amount of O vi equivalent width present in some cases has column density entirely consistent with the UV value. This solution to the O vi discrepancy, however, does not apply to high column-density systems like AGN outflows. We discuss other possible causes to explain their UV/X-ray mismatch. The O vi and O ii lines will be resolved by gratings on board the proposed mission Arcus and the concept mission Lynx, and would allow the detection of weak O vi lines not just at z = 0, but also at higher redshift.

The NuSTAR Extragalactic Survey: Average Broadband X-Ray Spectral Properties of the NuSTAR-detected AGNs

Abstract We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the Extended Chandra Deep Field South (E-CDFS), Extended Groth Strip (EGS), and the Cosmic Evolution Survey (COSMOS). The sample includes 182 NuSTAR sources (64 detected at 8–24 keV), with 3–24 keV fluxes ranging between f 3 – 24 keV ≈ 10 − 14 and 6 × 10−13 erg cm−2 s−1 ( f 8 – 24 keV ≈ 3 × 10 − 14 – 3 × 10 − 13 erg cm−2 s−1) and redshifts in the range of z = 0.04 – 3.21 . We produce composite spectra from the Chandra + NuSTAR data ( E ≈ 2 – 40 keV , rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) active galactic nuclei (AGNs), and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is Γ = 1.65 − 0.03 + 0.03 , flatter than the Γ ≈ 1.8 typically found for AGNs. While the spectral slope of BL and X-ray unabsorbed AGNs is consistent with the typical values ( Γ = 1.79 − 0.01 + 0.01 ), a significant flattening is seen in NL AGNs and heavily absorbed sources ( Γ = 1.60 − 0.05 + 0.08 and Γ = 1.38 − 0.12 + 0.12 , respectively), likely due to the effect of absorption and to the contribution from the Compton reflection component to the high-energy flux ( E > 10 keV). We find that the typical reflection fraction in our spectra is R ≈ 0.5 (for Γ = 1.8 ), with a tentative indication of an increase of the reflection strength with X-ray column density. While there is no significant evidence for a dependence of the photon index on X-ray luminosity in our sample, we find that R decreases with luminosity, with relatively high levels of reflection ( R ≈ 1.2 ) for L 10 – 40 keV < 10 44 erg s−1 and R ≈ 0.3 for L 10 – 40 keV > 10 44 erg s−1 AGNs, assuming a fixed spectral slope of Γ = 1.8 .