X-ray Excess Research Articles

Two Quasi-periodic Oscillations in ESO 113-G010

In the narrow-line Seyfert galaxy (NLS1) ESO 113-G010, two quasi-periodic oscillation (QPO) signals are detected in the XMM-Newton data collected in 2005 November. The XMM-Newton telescope has an exposure longer than 100ks in the energy range of 0.3–10 keV. The data are analyzed using the Lomb-Scargle periodogram (LSP) and the weighted wavelet Z-transform (WWZ) methods. It is found that there are ∼2.24 hour and ∼4.09 hour QPOs in the observation with the confidence levels of 7.3σ and 4.8σ, respectively. The two QPO signals have a relationship of approximately 1:2 (1:1.83). The absence of QPO in another observation of the object suggests that it is a transient phenomenon. The relationship between the mass of the ESO 113-G010 central black hole (BH) MBH and its frequency fQPO satisfies the known logarithmic linear relation in highly accreting BH systems, from stellar-mass BHs to supermassive BHs. The energy spectrum analysis reveals that the source has a soft X-ray excess below 1 keV.

Low-dose CT Image Restoration using generative adversarial networks

Computed tomography (CT) images are the prominent modality of medical imaging used in medical diagnosis and are now widely applied in clinical diagnosis. However, the excessive X-ray radiation dose associated with CT scans is a potential risk to patients. In this paper, we propose an algorithm that improves the detail and edge preservation performance of low-dose CT images by applying a generative adversarial network structure to a deep learning noise reduction method based on a convolutional neural network. This approach is performed by competing the discriminator neural network and the denoising filter neural network when learning the neural network. In order to preserve the textural details and edges of low-dose CT image, an optimized generator was derived by minimizing the weighted sum of L2 and structural similarity index (SSIM) losses instead of simply using Mean Square Error (MSE) when optimizing neural network training. According to experimental results, the proposed method can effectively suppress noise and remove artifacts compared to the state-of-the-art methods. Experimental results on low-dose CT images show that the proposed method preserves the details of image, reduces noise and has better visual effects.

Chandra Spectral and Timing Analysis of Sgr A*'s Brightest X-Ray Flares

Abstract We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600× and 245× greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology—it lasts 5.7 ks (∼2 hr), with a rapid rise time of 1500 s and a decay time of 2500 s. The second flare lasts 3.4 ks, with rise and decay times of 1700 and 1400 s. These luminous flares are significantly harder than quiescence: the first has a power-law spectral index Γ = 2.06 ± 0.14 and the second has Γ = 2.03 ± 0.27, compared to Γ = 3.0 ± 0.2 for the quiescent accretion flow. These spectral indices (as well as the flare hardness ratios) are consistent with previously detected Sgr A* flares, suggesting that bright and faint flares arise from similar physical processes. Leveraging the brightest flare’s long duration and high signal-to-noise, we search for intraflare variability and detect excess X-ray power at a frequency of ν ≈ 3 mHz, but show that it is an instrumental artifact and not of astrophysical origin. We find no other evidence (at the 95% confidence level) for periodic or quasi-periodic variability in either flares’ time series. We also search for nonperiodic excess power but do not find compelling evidence in the power spectrum. Bright flares like these remain our most promising avenue for identifying Sgr A*'s short timescale variability in the X-ray, which may probe the characteristic size scale for the X-ray emission region.

The soft X-ray excess: NLS1s versus BLS1s

ABSTRACT The soft X-ray excess – the excess of X-rays below 2 keV with respect to the extrapolation of the hard X-ray spectral continuum model – is a very common feature among type 1 active galactic nuclei (AGNs); yet the nature of the soft X-ray excess is still poorly understood and hotly debated. To shed some light on this issue, we have measured in a model-independent way the soft excess strength in a flux-limited sample of broad- and narrow-line Seyfert 1 galaxies (BLS1s and NLS1s) that are matched in X-ray luminosity but different in terms of the black hole mass and the accretion rate values, with NLS1s being characterized by smaller MBH and larger $\dot{m}$ values. Our analysis, in agreement with previous studies carried out with different AGN samples, indicates that: (1) a soft excess is ubiquitously detected in both BLS1s and NLS1s; (2) the strength of the soft excess is significantly larger in the NLS1 sample, compared to the BLS1 sample; and (3) combining the two samples, the strength of the soft excess appears to positively correlate with the photon index as well as with the accretion rate, whereas there is no correlation with the black hole mass. Importantly, our work also reveals the lack of an anticorrelation between the soft excess strength and the luminosity of the primary X-ray component, predicted by the absorption and reflection scenarios. Our findings suggest that the soft excess is consistent with being produced by a warm Comptonization component. Larger, more complete samples of NLS1s and BLS1s are needed to confirm these conclusions.

Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus.

In the past two decades, high-amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole1,2. X-ray emission from the Seyfert 2 galaxy GSN069 (2MASX J01190869-3411305) at a redshift of z=0.018 was first detected in July 2010 and implies an X-ray brightening by a factor of more than 240 over ROSAT observations performed 16years earlier3,4. The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event5. The X-ray spectrum is ultra-soft and can be described by accretion disk emission with luminosity proportional to the fourth power of the disk temperature during long-term evolution. Here we report observations of quasi-periodic X-ray eruptions from the nucleus of GSN069 over the course of 54days, from December 2018 onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with an event duration of just over an hour and a recurrence time of about nine hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4×105solar masses) with peak X-ray luminosity of about 5×1042erg per second. The warm phase has kT (where T is the temperature and k is the Boltzmann constant) of about 120electronvolts, reminiscent of the typical soft-X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei6-8. If the observed properties are not unique to GSN069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with higher-mass black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years9.

Modelling the spectral energy distribution of super-Eddington quasars

ABSTRACT We develop a broad-band spectral model, agnslim, to describe super-Eddington black hole accretion disc spectra. This is based on the slim disc emissivity, where radial advection keeps the surface luminosity at the local Eddington limit, resulting in L(r) ∝ r−2 rather than the r−3 expected from the Novikov-Thorne (standard, sub-Eddington) disc emissivity. Wind losses should also be important but these are expected to produce a similar radiative emissivity. We assume that the flow is radially stratified, with an outer standard disc, an inner hot Comptonizing region and an intermediate warm Comptonizing region to produce the soft X-ray excess. This gives the model enough flexibility to fit the observed data, but with the additional requirement of energy conservation to give physical constraints. We use this to fit the broad-band spectrum of one of the most extreme Active Galactic Nuclei, the Narrow Line Seyfert 1 RX J0439.6−5311, which has a black hole mass of $(6\sim 9)\times 10^6\, \mathrm{M}_\odot$ as derived from the H β line width. This cannot be fit with the standard disc emissivity at this mass, as even zero spin models overproduce the observed luminosity. Instead, we show that the spectrum is well reproduced by the slim disc model, giving mass accretion rates around (5 ∼ 10) × Eddington limit. There is no constraint on black hole spin as the efficiency is reduced by advection. Such extreme accretion rates should be characteristic of the first Quasars, and we demonstrate this by fitting to the spectrum of a recently discovered super-Eddington Quasar, PSO J006 + 39, at z = 6.6.

Green Peas in X-Rays∗

Abstract Green Peas represent a population of compact, highly star-forming dwarf galaxies at redshifts z ∼ 0.2–0.3 that have recently been found to show signatures of ultraviolet ionizing radiation leakage. They are being considered as analogs to high-redshift star-forming galaxies, possibly responsible for cosmic reionization. Despite intensive studies of Green Peas in the ultraviolet and optical domains, their X-ray properties have only so far been probed by nearby analogs. In this paper, we present the first measurements of Green Peas in the X-ray domain to constrain their spectral properties and fluxes at high energies. We analyzed XMM-Newton observations of three Green Pea sources. For two of them, we found an X-ray luminosity exceeding by a half-order of magnitude its predicted value, derived from the star formation rate and metallicity. Only an upper limit of the X-ray luminosity was derived for the third studied galaxy. Our results indicate that at least some Green Peas produce copious amounts of highly energetic photons, larger than detected in other star-forming galaxies. We discuss possible physical scenarios for the measured X-ray excess, including the presence of a hidden active galactic nucleus, a larger population of X-ray binaries, or ultra-luminous X-ray sources. Future spatially resolved X-ray images will discriminate between the models. Larger Green Pea samples will provide a possible link between the X-ray properties and the leaking ultraviolet radiation.

Population Estimates for Electromagnetically Distinguishable Supermassive Binary Black Holes

Abstract Distinguishing the photon output of an accreting supermassive black hole binary system from that of a single supermassive black hole accreting at the same rate is intrinsically difficult because the majority of the light emerges from near the innermost stable orbits of the black holes. However, there are two possible signals that can distinctively mark binaries, both arising from the gap formed in circumbinary accretion flows inside approximately twice the binary separation. One of these is a “notch” cut into the thermal spectra of these systems in the IR/optical/UV, the other a periodically varying excess hard X-ray luminosity whose period is of order the binary orbital period. Using data from detailed galaxy evolution simulations, we estimate the distribution function in mass, mass ratio, and accretion rate for accreting supermassive binary black holes (SMBBHs) as a function of redshift and then transform this distribution function into predicted source counts for these two potential signals. At flux levels ≳10−13 erg cm−2 s−1, there may be ∼O(102) such systems in the sky, mostly in the redshift range 0.5 ≲ z ≲ 1. Roughly 10% should have periods short enough (≲5 yr) to detect the X-ray modulation; this is also the period range accessible to Pulsar Timing Array observations.

Physics potential of the International Axion Observatory (IAXO)

We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO) . Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as gaγ∼ 10−12 GeV−1, or to electrons gae∼10−13, IAXO has the potential to find the QCD axion in the 1 meV∼1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous “transparency” of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

An integrated physico-chemical approach for explaining the differential impact of FLASH versus conventional dose rate irradiation on cancer and normal tissue responses

For decades the field of radiation oncology has sought to improve the therapeutic ratio through innovations in physics, chemistry, and biology. To date, technological advancements in image guided beam delivery techniques have provided clinicians with their best options for improving this critical tool in cancer care. Medical physics has focused on the preferential targeting of tumors while minimizing the collateral dose to the surrounding normal tissues, yielding only incremental progress. However, recent developments involving ultra-high dose rate irradiation termed FLASH radiotherapy (FLASH-RT), that were initiated nearly 50 years ago, have stimulated a renaissance in the field of radiotherapy, long awaiting a breakthrough modality able to enhance therapeutic responses and limit normal tissue injury. Compared to conventional dose rates used clinically (0.1–0.2 Gy/s), FLASH can implement dose rates of electrons or X-rays in excess of 100 Gy/s. The implications of this ultra-fast delivery of dose are significant and need to be re-evaluated to appreciate the fundamental aspects underlying this seemingly unique radiobiology. The capability of FLASH to significantly spare normal tissue complications in multiple animal models, when compared to conventional rates of dose-delivery, while maintaining persistent growth inhibition of select tumor models has generated considerable excitement, as well as skepticism. Based on fundamental principles of radiation physics, radio-chemistry, and tumor vs. normal cell redox metabolism, this article presents a series of testable, biologically relevant hypotheses, which may help rationalize the differential effects of FLASH irradiation observed between normal tissue and tumors.

X-Ray Observation of a Magnetized Hot Gas Outflow in the Galactic Center Region

Abstract We report the discovery of a 1° scale X-ray plume in the northern Galactic Center (GC) region observed with Suzaku. The plume is located at (l, b) ∼ (0.°2, 0.°6), east of the radio lobe reported by previous studies. No significant X-ray excesses are found inside or to the west of the radio lobe. The spectrum of the plume exhibits strong emission lines from highly ionized Mg, Si, and S that are reproduced by a thin thermal plasma model with kT ∼ 0.7 keV and solar metallicity. There is no signature of non-equilibrium ionization. The unabsorbed surface brightness is 3 × 10−14 erg cm−2 s−1 arcmin−2 in the 1.5–3.0 keV band. Strong interstellar absorption in the soft X-ray band indicates that the plume is not a foreground source but is at the GC distance, giving a physical size of ∼100 pc, density of 0.1 cm−3, thermal pressure of 1 × 10−10 erg cm−3, mass of 600 M ⊙, and thermal energy of 7 × 1050 erg. From the apparent association with a polarized radio emission, we propose that the X-ray plume is a magnetized hot gas outflow from the GC.

Is CT bulletproof? On the use of CT for characterization of bullets in forensic radiology

PurposeForensic investigations could benefit from non-invasive in situ characterization of bullets. Therefore, the use of CT imaging was explored for the analysis of bullet geometry and composition. Bullet visualization with CT is challenging as the metal constituents suffer from excessive X-ray attenuation due to their high atomic number, density, and geometry.MethodsA metal reference phantom was developed containing small discs of various common metals (aluminum, iron, stainless steel, copper, brass, tungsten, and lead). CT images were acquired with 70–150 kVp and 200–400 mAs and were reconstructed using an extended Hounsfield unit (HU) scale (− 10,240 to + 30,710). For each material, the mean CT number (HU) was measured to construct a metal database. Different bullets (n = 11) were scanned in a soft tissue-mimicking phantom. Bullet size and shape were measured, and composition was evaluated by comparison with the metal database. Also, the effect of bullet orientation within the CT scanner was evaluated.ResultsIn the reference phantom, metals were classified into three groups according to their atomic number (Z): low (Z ≤ 13; HU < 3000), medium (Z = 25–30; HU = 13,000–20,000), and high (Z ≥ 74; HU > 30,000). External bullet contours could be accurately delineated. Internal interfaces between jacket and core could not be identified. Cross-sectional spatial profile plots of the CT number along bullets’ short axis revealed beam hardening and photon starvation effects that depended on bullet size, shape, and orientation within the CT scanner. Therefore, the CT numbers of bullets were unreliable and could not be used for material characterization by comparison with the reference phantom.ConclusionCT-based characterization of bullets was feasible in terms of size and shape but not composition.

AGN Feedback in Galaxy Groups: A Detailed Study of X-Ray Features and Diffuse Radio Emission in IC 1262

Abstract This paper reports a systematic search of X-ray cavities, density jumps, and shocks in the intergalactic environment of the galaxy group IC 1262 using Chandra, GMRT, and Very Large Array archival observations. The X-ray imaging analysis reveals a pair of X-ray cavities to the north and south of the X-ray peak, at projected distances of 6.48 kpc and 6.30 kpc respectively. Total mechanical power contained in both of these cavities is found to be ∼12.37 × 1042 erg s−1, and compares well with the X-ray luminosity, within the cooling radius, measured to be ∼3.29 × 1042 erg s−1, suggesting that the mechanical power injected by the central AGN efficiently balances the radiative loss. We detect a previously unknown X-ray cavity at the position of the southern radio lobe in the intra-group medium and find a loop of excess X-ray emission extending ∼100 kpc southwest from the central galaxy. The X-ray cavity at the position of the southern radio lobe probably represents a first generation X-ray cavity. Two surface brightness edges are evident to the west and northeast of the center of this group. The radio galaxy at the core of the IC 1262 group is a rare low-redshift ultra-steep radio galaxy, its spectral index being α ∼ −1.73 (including the central AGN) and α ∼ −2.08 (excluding the central AGN). We detect a radio phoenix embedded within the southern radio lobe, for the first time in a poor group, with a spectral index (α ≤ −1.92). The spectral index distribution across the phoenix steepens with increasing distance from its intensity peak.

A Study of the Merger History of the Galaxy Group HCG 62 Based on X-Ray Observations and Smoothed Particle Hydrodynamic Simulations

We choose the bright compact group HCG 62, which was found to exhibit both excess X-ray emission and high Fe abundance to the southwest of its core, as an example to study the impact of mergers on chemical enrichment in the intragroup medium. We first reanalyze the high-quality Chandra and XMM-Newton archive data to search for the evidence for additional SN II yields, which is expected as a direct result of the possible merger-induced starburst. We reveal that, similar to the Fe abundance, the Mg abundance also shows a high value in both the innermost region and the southwest substructure, forming a high-abundance plateau, meanwhile all the SN Ia and SN II yields show rather flat distributions in $>0.1r_{200}$ in favor of an early enrichment. Then we carry out a series of idealized numerical simulations to model the collision of two initially isolated galaxy groups by using the TreePM-SPH GADGET-3 code. We find that the observed X-ray emission and metal distributions, as well as the relative positions of the two bright central galaxies with reference to the X-ray peak, can be well reproduced in a major merger with a mass ratio of 3 when the merger-induced starburst is assumed. The `best-match' snapshot is pinpointed after the third pericentric passage when the southwest substructure is formed due to gas sloshing. By following the evolution of the simulated merging system, we conclude that the effects of such a major merger on chemical enrichment are mostly restricted within the core region when the final relaxed state is reached.

Resolving the X-Ray Obscuration in a Low-flux Observation of the Quasar PDS 456

Abstract Simultaneous XMM-Newton, NuSTAR, and HST observations, performed in 2017 March, of the nearby (z = 0.184) luminous quasar PDS 456 are presented. PDS 456 had a low X-ray flux compared to past observations, where the first of the two new XMM-Newton observations occurred during a pronounced dip in the X-ray light curve. The broadband X-ray spectrum is highly absorbed, attenuated by a soft X-ray absorber of column density N H = 6 × 1022 cm−2. An increase in obscuration occurs during the dip, which may be due to an X-ray eclipse. In addition, the persistent, fast Fe K outflow is present, with velocity components of −0.25c and −0.4c. The soft absorber is less ionized ( ) compared to the iron K outflow ( ) and is outflowing with a velocity of approximately −0.2c. A soft X-ray excess is present below 1 keV against the highly absorbed continuum and can be attributed to the re-emission from a wide-angle wind. The complex X-ray absorption present in PDS 456 suggests that the wind is inhomogeneous, whereby the soft X-ray absorber originates from denser clumps or filaments that may form farther out along the outflow. In contrast to the X-ray observations, the simultaneous UV spectrum of PDS 456 is largely unabsorbed, where only a very weak broad absorption trough is present blueward of Lyα, compared to a past observation in 2000 when the trough was significantly stronger. The relative weakness of the UV absorption may be due to the soft X-ray absorber being too highly ionized and almost transparent in the UV band.

Theoretical X-Ray Light Curves of Young SNe. II. The Example of SN 2013ej

The X-ray signal from hydrogen-rich supernovae (SNe II) in the first tens to hundreds of days after the shock breakout encodes important information about the circumstellar material (CSM) surrounding their progenitors before explosion. In this study, we describe a way to generate the SN II X-ray light curves from hydrodynamical simulations performed with the code Athena++, using the X-ray package XSPEC. In addition, we employ a radiation diffusion hydrodynamic code SNEC for generating the optical light curves in different bands. In this numerical setup, we model the X-ray and optical emission from a set of progenitor models, consisting of either two (red supergiant + low density steady wind), or three (red supergiant + dense CSM + low density steady wind) components. We vary the density in the wind and the slope in the CSM to see how these parameters influence the resulting X-ray and optical light curves. Among our models, we identify one that is able to roughly reproduce both optical and X-ray data of the well observed SN 2013ej. In order to achieve this, the slope of the dense CSM in this model should be steeper than the one of a steady wind ($\rho\propto r^{-2}$), and closer to $\rho\propto r^{-5}$. On the other hand, we show that too steep and extended CSM profiles may produce excessive X-ray emission in the first few tens of days, up to a few orders of magnitude larger than observed. We conclude that ability to reproduce the observed X-ray signal from SNe~II together with their optical light curves is crucial in establishing the validity of different CSM models.

Balance lead in solution-processed CH3NH3PbBrxCl(3−x) single crystals for high performance X-ray detection

Organometallic perovskites MAPbX3 (MA = CH3NH3 and X = I, Br or Cl) are considered as potential materials for X-ray and gamma ray detection due to its high Z atom, large mobility-lifetime product and growth from solution. However, perovskites with one kind halide own numbers of free charges and yielded unbearable noise current. In this work, we grow perovskite single crystals (PSC) with different halide ratio and find the MAPbBr2.5Cl0.5 PSC shows outstanding performance in electric properties. Through energy dispersive X-ray spectrum, vacancy and excess of Pb is thought as the main cause of free charges. The atomic ratio between halide and Pb as 3 in MAPbBr2.5Cl0.5 PSC and its large resistivity 5.6 × 108 Ω cm and 160 cm2 V−1 s−1 high mobility are measured. Thanks to the smaller noise current, higher electric field could be established for higher charges collect efficiency. As a result, compared with pristine perovskite single crystals, improved X-ray detection performance is demonstrated. This work provides useful information to achieve high performance X-ray detectors based on organometallic perovskites.

The broad-band spectrum of the narrow-line Seyfert 1 NGC 4748: from UV to hard X-ray

We report the results obtained by a broad-band (0.5-500 keV) data analysis of narrow-line Seyfert 1 galaxy NGC 4748 observed with an XMM-Newton/PN, INTEGRAL/ISGRI and SWIFT/BAT telescopes. This galaxy has a soft X-ray excess that is typical for the class of narrow-line Seyfert 1. The question of the origin of soft excess in such objects is still unclear. We tested and compared two spectral models for the soft X-ray spectra based on the different physical scenarios. The first one is based on the Done & Nayakshin model of two-phase accretion disc in a vertical direction, which includes two reflection zones with different ionization levels. According to this model, we found that a highly ionized reflection has the value of ionization $\xi\sim3000$ $erg~s^{-1}~cm$ and is mostly responsible for the soft excess. This reflection becomes comparable with a low ionized one ($\xi\sim30$ $erg~s^{-1}~cm$) in moderate X-ray range. However, this model requires also an additional component at soft energies with $kT\sim300$ eV. The second model is an energetically self-consistent model and assumes that a soft excess arises from optically thick thermal Comptonization of the disc emission. Combination of the UV (from XMM/Optical monitor) and X-ray data in the latter model allowed us to determine a mass of the central black hole of $6.9\times10^6 M_{\odot}$ and Eddington ratio $log_{L/L_{Edd}}\simeq-0.57$. Also, we were not able to rule out one of competing models using only X-ray spectra of NGC 4748.

Low-luminosity AGN and X-Ray Binary Populations in COSMOS Star-forming Galaxies

Abstract We present an X-ray stacking analysis of ∼75,000 star-forming galaxies between 0.1 &lt; z &lt; 5.0 using the Chandra COSMOS-Legacy survey to study the X-ray emission of low-luminosity active galactic nuclei (AGN) and its connection to host galaxy properties. The stacks at z &lt; 0.9 have luminosity limits as low as 1040–1041 erg s−1, a regime in which X-ray binaries (XRBs) can dominate the X-ray emission. Comparing the measured luminosities to established XRB scaling relations, we find that the redshift evolution of the luminosity per star formation rate (SFR) of XRBs depends sensitively on the assumed obscuration and may be weaker than previously found. The XRB scaling relation based on stacks from the Chandra Deep Field South overestimates the XRB contribution to the COSMOS high specific SFR stacks, possibly due to a bias affecting the CDF-S stacks because of their small galaxy samples. After subtracting the estimated XRB contribution from the stacks, we find that most stacks at z &gt; 1.3 exhibit a significant X-ray excess indicating nuclear emission. The AGN emission is strongly correlated with stellar mass but does not exhibit an additional correlation with SFR. The hardness ratios of the high-redshift stacks indicate that the AGN are substantially obscured (N H ∼ 1023 cm−2). These obscured AGN are not identified by IRAC color selection and have L X ∼ 1041–1043 erg s−1, consistent with accretion at an Eddington rate of ∼10−3 onto 107–108 M ⊙ black holes. Combining our results with other X-ray studies suggests that AGN obscuration depends on stellar mass and an additional variable, possibly the Eddington rate.

Large-amplitude Rapid X-Ray Variability in the Narrow-line Seyfert 1 Galaxy PG 1404+226

Abstract We present the first results from a detailed analysis of a new, long (∼100 ks) XMM-Newton observation of the narrow-line Seyfert 1 galaxy PG 1404+226, which showed a large-amplitude, rapid X-ray variability by a factor of ∼7 in ∼10 ks with an exponential rise and a sharp fall in the count rate. We investigate the origin of the soft X-ray excess emission and rapid X-ray variability in the source through time-resolved spectroscopy and fractional rms spectral modeling. The strong soft X-ray excess below 1 keV observed in both the time-averaged and time-resolved spectra is described by the intrinsic disk Comptonization model as well as the relativistic reflection model where the emission is intense merely in the inner regions ( ) of an ionized accretion disk. We detected no significant UV variability, while the soft X-ray excess flux varies together with the primary power-law emission (as ), although with a smaller amplitude, as expected in the reflection scenario. The observed X-ray fractional rms spectrum is approximately constant with a drop at ∼0.6 keV and is described by a non-variable emission line component with the observed energy of ∼0.6 keV and two variable spectral components: a more variable primary power-law emission and a less variable soft excess emission. Our results suggest the “lamppost geometry” for the primary X-ray-emitting hot corona, which illuminates the innermost accretion disk due to strong gravity and gives rise to the soft X-ray excess emission.