arXiv subjects

Murray Brightman

Publications and source records attributed to Murray Brightman.

72 records · Page 4Linked to original sources

arXiv2018

Magnetic field strength of a neutron-star-powered ultraluminous X-ray source

Ultraluminous X-ray sources (ULXs) are bright X-ray sources in nearby galaxies not associated with the central supermassive black hole. Their luminosities imply they are powered by either an extreme accretion rate onto a compact stellar remnant, or an intermediate mass ($\sim100-10^5$ M$_{\odot}$) black hole. The recent detection of coherent pulsations coming from three bright ULXs demonstrates that some of these sources are powered by accretion onto a neutron star, implying accretion rates significantly in excess of the Eddington limit, a high degree of geometric beaming, or both. The physical challenges associated with the high implied accretion rates can be mitigated if the neutron star surface field is high - in the magnetar regime ($10^{14}$ G), since this suppresses the electron scattering cross section, reducing the radiation pressure that chokes off accretion for high luminosities. One of the few ways to determine surface magnetic fields is through the detection of cyclotron resonance scattering features (CRSFs) produced by the transition of charged particles between quantized Landau levels. To date, CRSFs have only been detected in Galactic accreting pulsars. Here we present the detection at 3.8-σ significance of a strong absorption line at a rest-frame energy of 4.5 keV in the Chandra spectrum of a ULX in M51. We find that this feature is likely to be a CRSF produced by the strong magnetic field of a neutron star. Assuming scattering off electrons, the magnetic field strength is implied to be $\sim10^{11}$ G, however the line is narrower than any electron CRSFs previously observed, and assuming thermal broadening, the implied temperature is significantly cooler than the accretion disk or column. The line shape is, however, consistent with a proton resonance scattering feature, implying that the neutron star has a magnetic field near the surface of B$\sim10^{15}$ G.

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arXiv2017

The maximum isotropic energy of gamma-ray bursts

The most energetic gamma-ray bursts (GRBs) are remarkable sources releasing huge amounts of energy on short timescales. Their prompt emission, which usually lasts few seconds, is so bright that it is visible across the whole observable universe. Studying these extreme events may provide clues on the nature of GRB progenitors and on the physical processes at work in relativistic jets. In this paper, we study the bright end of the isotropic energy distribution of long GRBs. We use two samples of long GRBs with redshift detected by Fermi/GBM or Konus-Wind, two instruments which measure the spectral shape and the energetics of the prompt emission accurately. We focus on GRBs within a range of redshifts z = 1 -- 5, a volume that contains a large number of energetic GRBs, and we propose a simple method to reconstruct the bright end of the GRB energy distribution from the observed one. We find that the GRB energy distribution cannot be described by a simple power law but requires a strong cutoff above $1-3 \times 10^{54}$ erg. We attribute this feature to an intrinsic limit on the energy per unit of solid angle radiated by gamma-ray bursts.

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arXiv2016

A broadband X-ray spectral study of the intermediate-mass black hole candidate M82 X-1 with NuSTAR, Chandra and Swift

M82 X-1 is one of the brightest ultraluminous X-ray sources (ULXs) known, which, assuming Eddington-limited accretion and other considerations, makes it one of the best intermediate-mass black hole (IMBH) candidates. However, the ULX may still be explained by super-Eddington accretion onto a stellar-remnant black hole. We present simultaneous NuSTAR, Chandra and Swift/XRT observations during the peak of a flaring episode with the aim of modeling the emission of M82 X-1 and yielding insights into its nature. We find that thin-accretion disk models all require accretion rates at or above the Eddington limit in order to reproduce the spectral shape, given a range of black hole masses and spins. Since at these high Eddington ratios the thin-disk model breaks down due to radial advection in the disk, we discard the results of the thin-disk models as unphysical. We find that the temperature profile as a function of disk radius ($T(r)\propto r^{-p}$) is significantly flatter ($p=0.55^{+ 0.07}_{- 0.04}$) than expected for a standard thin disk ($p=0.75$). A flatter profile is instead characteristic of a slim disk which is highly suggestive of super-Eddington accretion. Furthermore, radiation hydrodynamical simulations of super-Eddington accretion have shown that the predicted spectra of these systems are very similar to what we observe for M82 X-1. We therefore conclude that M82 X-1 is a super-Eddington accretor. Our mass estimates inferred from the inner disk radius imply a stellar-remnant black hole ($M_{\rm BH}=26^{+9}_{-6} M_{\odot}$) when assuming zero spin, or an IMBH ($M_{\rm BH}=125^{+45}_{-30} M_{\odot}$) when assuming maximal spin.

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arXiv2016

The Geometry of the Infrared and X-ray Obscurer in a Dusty Hyperluminous Quasar

We study the geometry of the AGN obscurer in IRAS 09104+4109, an IR-luminous, radio-intermediate FR-I source at $z=0.442$, using infrared data from Spitzer and Herschel, X-ray data from Nustar, Swift, Suzaku, and Chandra, and an optical spectrum from Palomar. The infrared data imply a total rest-frame 1-1000$μ$m luminosity of $5.5\times10^{46}$erg s$^{-1}$ and require both an AGN torus and starburst model. The AGN torus has an anisotropy-corrected IR luminosity of $4.9\times10^{46}$erg s$^{-1}$, and a viewing angle and half opening angle both of approximately $36$ degrees from pole-on. The starburst has a star formation rate of $(110\pm34)$M$_{\odot}$ yr$^{-1}$ and an age of $<50$Myr. These results are consistent with two epochs of luminous activity in 09104: one approximately $150$Myr ago, and one ongoing. The X-ray data suggest a photon index of $Γ\simeq 1.8$ and a line-of-sight column of $N_{\rm H} \simeq 5\times10^{23}$cm$^{-2}$. This argues against a reflection-dominated hard X-ray spectrum, which would have implied a much higher $N_{\rm H}$ and luminosity. The X-ray and infrared data are consistent with a bolometric AGN luminosity of $L_{\rm bol}\sim(0.5-2.5)\times10^{47}$erg s$^{-1}$. The X-ray and infrared data are further consistent with coaligned AGN obscurers in which the line of sight "skims" the torus. This is also consistent with the optical spectra, which show both coronal iron lines and broad lines in polarized but not direct light. Combining constraints from the X-ray, optical, and infrared data suggests that the AGN obscurer is within a vertical height of $20$pc, and a radius of $125$pc, of the nucleus.

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arXiv2015

Spectral and temporal properties of the ultra-luminous X-ray pulsar in M82 from 15 years of Chandra observations and analysis of the pulsed emission using NuSTAR

The recent discovery by Bachetti et al. (2014) of a pulsar in M82 that can reach luminosities of up to 10^40 ergs s^-1, a factor of ~100 the Eddington luminosity for a 1.4 Msol compact object, poses a challenge for accretion physics. In order to better understand the nature of this source and its duty cycle, and in the light of several physical models that have been subsequently published, we conduct a spectral and temporal analysis of the 0.5-8 keV X-ray emission from this source from 15 years of Chandra observations. We fit the Chandra spectra of the pulsar with a power-law model and a disk black body model, subjected to interstellar absorption in M82. We carefully assess for the effect of pile-up in our observations, where 4/19 observations have a pile-up fraction >10%, which we account for during spectral modeling with a convolution model. When fitted with a power-law model, the average photon index when the source is at high luminosity (L_X>10^39 ergs s^-1) is Gamma=1.33+/-0.15. For the disk black body model, the average temperature is T=3.24+/-0.65 keV, consistent with other luminous X-ray pulsars. We also investigated the inclusion of a soft excess component and spectral break, finding that the spectra are also consistent with these features common to luminous X-ray pulsars. In addition, we present spectral analysis from NuSTAR over the 3-50 keV range where we have isolated the pulsed component. We find that the pulsed emission in this band is best fit by a power-law with a high-energy cut-off, where Gamma=0.6+/-0.3 and E_C=14^{+5}_{-3} keV. While the pulsar has previously been identified as a transient, we find from our longer-baseline study that it has been remarkably active over the 15-year period, where for 9/19 (47%) observations that we analyzed, the pulsar appears to be emitting at a luminosity in excess of 10^39 ergs s^-1, greater than 10 times its Eddington limit.

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arXiv2015

Are Compton-Thick AGN the Missing Link Between Mergers and Black Hole Growth?

We examine the host morphologies of heavily obscured active galactic nuclei (AGN) at $z\sim1$ to test whether obscured supermassive black hole growth at this epoch is preferentially linked to galaxy mergers. Our sample consists of 154 obscured AGN with $N_{\rm H}>10^{23.5}$ cm$^{-2}$ and $z<1.5$. Using visual classifications, we compare the morphologies of these AGN to control samples of moderately obscured ($10^{22}$ cm$^{-2}$ $<N_{\rm H}< 10^{23.5}$ cm$^{-2}$) and unobscured ($N_{\rm H}<10^{22}$ cm$^{-2}$) AGN. These control AGN are matched in redshift and intrinsic X-ray luminosity to our heavily obscured AGN. We find that heavily obscured AGN at z~1 are twice as likely to be hosted by late-type galaxies relative to unobscured AGN ($65.3^{+4.1}_{-4.6}\%$ vs $34.5^{+2.9}_{-2.7}\%$) and three times as likely to exhibit merger or interaction signatures ($21.5^{+4.2}_{-3.3}\%$ vs $7.8^{+1.9}_{-1.3}\%$). The increased merger fraction is significant at the 3.8$σ$ level. We also find that the incidence of point-like morphologies is inversely proportional to obscuration. If we exclude all point sources and consider only extended hosts, we find the correlation between merger fraction and obscuration is still evident, however at a reduced statistical significance ($2.5σ$). The fact that we observe a different disk/spheroid fraction versus obscuration indicates that viewing angle cannot be the only thing differentiating our three AGN samples, as a simple unification model would suggest. The increased fraction of disturbed morphologies with obscuration supports an evolutionary scenario, in which Compton-thick AGN are a distinct phase of obscured SMBH growth following a merger/interaction event. Our findings also suggest that some of the merger-triggered SMBH growth predicted by recent AGN fueling models may be hidden among the heavily obscured, Compton-thick population.

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arXiv2015

Obscuration-dependent evolution of Active Galactic Nuclei

We aim to constrain the evolution of AGN as a function of obscuration using an X-ray selected sample of $\sim2000$ AGN from a multi-tiered survey including the CDFS, AEGIS-XD, COSMOS and XMM-XXL fields. The spectra of individual X-ray sources are analysed using a Bayesian methodology with a physically realistic model to infer the posterior distribution of the hydrogen column density and intrinsic X-ray luminosity. We develop a novel non-parametric method which allows us to robustly infer the distribution of the AGN population in X-ray luminosity, redshift and obscuring column density, relying only on minimal smoothness assumptions. Our analysis properly incorporates uncertainties from low count spectra, photometric redshift measurements, association incompleteness and the limited sample size. We find that obscured AGN with $N_{H}>{\rm 10^{22}\, cm^{-2}}$ account for ${77}^{+4}_{-5}\%$ of the number density and luminosity density of the accretion SMBH population with $L_{\rm X}>10^{43}\text{ erg/s}$, averaged over cosmic time. Compton-thick AGN account for approximately half the number and luminosity density of the obscured population, and ${38}^{+8}_{-7}\%$ of the total. We also find evidence that the evolution is obscuration-dependent, with the strongest evolution around $N_{H}\thickapprox10^{23}\text{ cm}^{-2}$. We highlight this by measuring the obscured fraction in Compton-thin AGN, which increases towards $z\sim3$, where it is $25\%$ higher than the local value. In contrast the fraction of Compton-thick AGN is consistent with being constant at $\approx35\%$, independent of redshift and accretion luminosity. We discuss our findings in the context of existing models and conclude that the observed evolution is to first order a side-effect of anti-hierarchical growth.

astro-ph.HE↗

arXiv2014

NuSTAR Spectroscopy of Multi-Component X-ray Reflection from NGC 1068

We report on observations of NGC1068 with NuSTAR, which provide the best constraints to date on its $>10$~keV spectral shape. We find no strong variability over the past two decades, consistent with its Compton-thick AGN classification. The combined NuSTAR, Chandra, XMM-Newton, and Swift-BAT spectral dataset offers new insights into the complex reflected emission. The critical combination of the high signal-to-noise NuSTAR data and a spatial decomposition with Chandra allow us to break several model degeneracies and greatly aid physical interpretation. When modeled as a monolithic (i.e., a single N_H) reflector, none of the common Compton-reflection models are able to match the neutral fluorescence lines and broad spectral shape of the Compton reflection. A multi-component reflector with three distinct column densities (e.g., N_H~1.5e23, 5e24, and 1e25 cm^{-2}) provides a more reasonable fit to the spectral lines and Compton hump, with near-solar Fe abundances. In this model, the higher N_H components provide the bulk of the Compton hump flux while the lower N_H component produces much of the line emission, effectively decoupling two key features of Compton reflection. We note that ~30% of the neutral Fe Kalpha line flux arises from >2" (~140 pc), implying that a significant fraction of the <10 keV reflected component arises from regions well outside of a parsec-scale torus. These results likely have ramifications for the interpretation of poorer signal-to-noise observations and/or more distant objects [Abridged].

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arXiv2014

CANDELS/GOODS-S, CDFS, ECDFS: Photometric Redshifts For Normal and for X-Ray-Detected Galaxies

We present photometric redshifts and associated probability distributions for all detected sources in the Extended Chandra Deep Field South (ECDFS). The work makes use of the most up-to-date data from the Cosmic Assembly Near-IR Deep Legacy Survey (CANDELS) and the Taiwan ECDFS Near-Infrared Survey (TENIS) in addition to other data. We also revisit multi-wavelength counterparts for published X-ray sources from the 4Ms-CDFS and 250ks-ECDFS surveys, finding reliable counterparts for 1207 out of 1259 sources ($\sim 96\%$). Data used for photometric redshifts include intermediate-band photometry deblended using the TFIT method, which is used for the first time in this work. Photometric redshifts for X-ray source counterparts are based on a new library of AGN/galaxy hybrid templates appropriate for the faint X-ray population in the CDFS. Photometric redshift accuracy for normal galaxies is 0.010 and for X-ray sources is 0.014, and outlier fractions are $4\%$ and $5.4\%$ respectively. The results within the CANDELS coverage area are even better as demonstrated both by spectroscopic comparison and by galaxy-pair statistics. Intermediate-band photometry, even if shallow, is valuable when combined with deep broad-band photometry. For best accuracy, templates must include emission lines.

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arXiv2014

Compton thick active galactic nuclei in Chandra surveys

We present the results from an X-ray spectral analysis of active galactic nuclei (AGN) in the Chandra Deep Field-South, AEGIS-XD and Chandra-COSMOS surveys, focussing on the identification and characterisation of the most heavily obscured, Compton thick (CT, $N_{H}>10^{24} cm^{-2}$) sources. Our sample is comprised of 3184 X-ray selected extragalactic sources, which has a high rate of redshift completeness (97.6 %), and includes improved photometric redshifts over previous studies. We use spectral models designed for heavily obscured AGN which self consistently include all major spectral signatures of heavy absorption. We identify CT sources not selected through our spectral fitting method using X-ray colours, validate our spectral fitting method through simulations, and take considerations for the constraints on $N_{H}$ given the low count nature of many of our sources. After these considerations we identify a total of 100 CT AGN with best-fit $N_{H}>10^{24} cm^{-2}$ and $N_{H}$ constrained to be above $10^{23.5} cm^{-2}$ at 90% confidence. These sources cover an intrinsic $L_{X}$ range of $10^{42}-3x10^{45}$ ergs s$^{-1}$ and a redshift range of z=0.1-4. This sample will enable characterisation of these heavily obscured AGN across cosmic time and to ascertain their cosmological significance. Furthermore, by using the torus models to test different covering factor scenarios, and by investigating the inclusion of the soft scattered emission, we find evidence that the covering factor of the obscuring material decreases with $L_{X}$ for all redshifts, consistent with the receding torus model, and that this factor increases with redshift, consistent with an increase in the obscured fraction towards higher redshifts. We make $N_{H}$, $Γ$ (with uncertainties), observed X-ray fluxes and intrinsic $L_{X}$ for all sources analysed in this work publicly available in an online catalogue.

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arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

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arXiv2013

Luminosity-dependent unification of Active Galactic Nuclei and the X-ray Baldwin effect

The existence of an anti-correlation between the equivalent width (EW) of the narrow core of the iron Kalpha line and the luminosity of the continuum (i.e. the X-ray Baldwin effect) in type-I active galactic nuclei has been confirmed over the last years by several studies carried out with XMM-Newton, Chandra and Suzaku. However, so far no general consensus on the origin of this trend has been reached. Several works have proposed the decrease of the covering factor of the molecular torus with the luminosity (in the framework of the luminosity-dependent unification models) as a possible explanation for the X-ray Baldwin effect. Using the fraction of obscured sources measured by recent X-ray and IR surveys as a proxy of the half-opening angle of the torus, and the recent Monte-Carlo simulations of the X-ray radiation reprocessed by a structure with a spherical-toroidal geometry by Ikeda et al. (2009) and Brightman & Nandra (2011), we test the hypothesis that the X-ray Baldwin effect is related to the decrease of the half-opening angle of the torus with the luminosity. Simulating the spectra of an unabsorbed population with a luminosity-dependent covering factor of the torus as predicted by recent X-ray surveys, we find that this mechanism is able to explain the observed X-ray Baldwin effect. Fitting the simulated data with a log-linear L_{2-10keV}-EW relation, we found that in the Seyfert regime (L_{2-10keV}< 10^44.2 erg s^-1) luminosity-dependent unification produces a slope consistent with the observations for average values of the equatorial column densities of the torus of log N_H^T > 23.1. In the quasar regime (L_{2-10 keV}> 10^44.2 erg s^-1) a decrease of the covering factor of the torus with the luminosity slower than that observed in the Seyfert regime (as found by recent hard X-ray surveys) is able to reproduce the observations for 23.2 < log N_H^T < 24.2.

astro-ph.CO↗

arXiv2012

Simultaneous X-ray and optical observations of true Type 2 Seyfert galaxies

We present the results of a campaign of simultaneous X-ray and optical observations of `true' Type 2 Seyfert galaxies candidates, i.e. AGN without a Broad Line Region (BLR). Out of the initial sample composed by 8 sources, one object, IC1631, was found to be a misclassified starburst galaxy, another, Q2130-431, does show broad optical lines, while other two, IRAS01428-0404 and NGC4698, are very likely absorbed by Compton-thick gas along the line of sight. Therefore, these four sources are not unabsorbed Seyfert 2s as previously suggested in the literature. On the other hand, we confirm that NGC3147, NGC3660, and Q2131-427 belong to the class of true Type 2 Seyfert galaxies, since they do not show any evidence for a broad component of the optical lines nor for obscuration in their X-ray spectra. These three sources have low accretion rates ($\dot{m}= L_{bol}/L_{Edd} \la0.01$), in agreement with theoretical models which predict that the BLR disappears below a critical value of $L_{bol}/L_{Edd}$. The last source, Mrk273x, would represent an exception even of this accretion-dependent versions of the Unification Models, due to its high X-ray luminosity and accretion rate, and no evidence for obscuration. However, its optical classification as a Seyfert 2 is only based on the absence of a broad component of the Hβ, due to the lack of optical spectra encompassing the Hα band.

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arXiv2012

The evolution of the Compton thick fraction and the nature of obscuration for AGN in the Chandra Deep Field South

(Abridged) We present the results from the X-ray spectral analysis of high-z AGN in the CDFS, making use of the new 4Ms data set and new X-ray spectral models from Brightman & Nandra, which account for Compton scattering and the geometry of the circumnuclear material. Our goals are to ascertain to what extent the torus paradigm of local AGN is applicable at earlier epochs and to evaluate the evolution of the Compton thick fraction (f_CT) with z, important for XRB synthesis models and understanding the accretion history of the universe. In addition to the torus models, we measure the fraction of scattered nuclear light, f_scatt known to be dependant on covering factor of the circumnuclear materal, and use this to aid in our understanding of its geometry. We find that the covering factor of the circumnuclear material is correlated with NH, and as such the most heavily obscured AGN are in fact also the most geometrically buried. We come to these conclusions from the result that f_scatt decreases as NH increases and from the prevalence of the torus model with the smallest opening angle as best fit model in the fits to the most obscured AGN. We find that a significant fraction of sources (~ 20%) in the CDFS are likely to be buried in material with close to 4 pi coverage having been best fit by the torus model with a 0\degree opening angle. Furthermore, we find 41 CTAGN in the CDFS using the new torus models, 29 of which we report here for the first time. We bin our sample by z in order to investigate the evolution of f_CT. Once we have accounted for biases and incompleteness we find a significant increase in the intrinsic f_CT, normalised to LX= 10^43.5 erg/s, from \approx 20% in the local universe to \approx 40% at z=1-4.

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arXiv2012

X-ray colour-colour selection for heavily absorbed AGN

We present a method for the identification of heavily absorbed AGN (NH>10^23 cm^-2) from X-ray photometric data. We do this using a set of XMM-Newton reference spectra of local galaxies for which we have accurate NH information, as described in Brightman & Nandra. The technique uses two rest-frame hardness ratios which are optimised for this task, which we designate HR1 (2-4/1-2 keV) and HR2 (4-16/2-4 keV). The selection method exploits the fact that while obscured AGN appear hard in HR2 due to absorption of the intrinsic source flux below ~4 keV, they appear soft in HR1 due to excess emission originating from scattered source light, thermal emission, or host galaxy emission. Such emission is ubiquitous in low redshift samples. The technique offers a very simple and straight forward way of estimating the fraction of obscured AGN in samples with relatively low signal-to-noise ratio in the X-ray band. We apply this technique to a moderate redshift (z~1) sample of AGN from the Chandra Deep Field North, finding that 61% of this sample has NH> 10^23 cm^-2. A clear and robust conclusion from our analysis, is that in deep surveys the vast majority of sources do not show hardness ratios consistent with a simple absorbed power-law. The ubiquity of complex spectra in turn shows that simple hardness ratio analysis will not yield reliable obscuration estimates, justifying the more complex colour-colour analysis described in this paper. While this method does very well at separating sources with NH> 10^23 cm^-2 from sources with lower NH, only X-ray spectroscopy can identify Compton thick sources, through the detection of the Fe Ka line. This will be made possible with the high throughput X-ray spectral capabilities of ATHENA.

astro-ph.HE↗

arXiv2011

An XMM-Newton spectral survey of 12 micron selected galaxies. II. Implications for AGN selection and unification

We present a multi-waveband analysis of a 126 galaxy sub-sample of the 12 micron galaxy sample (12MGS), for which we have carried out a detailed X-ray spectral analysis of in a previous paper. We present optical activity classes and we characterise these by their X-ray, 12 um and [OIII] luminosities and by their X-ray spectral properties. Our most interesting results are: Seyfert (Sy) 1s and Sy 2s show significantly different LX distributions from each other; Sy 2s with a detection of an HBLR show a significantly higher LX than those without a detection; Sy 1s also present a significantly different L(12um) distribution from both Sy1.2-1.9s and Sy 2s; the Sy 2 fraction decreases towards high LX; X-ray indications of AGN power agree well with the optical classifications; There is X-ray evidence for the presence of an AGN in 17% of HII/AGN composites and 40% of LINERs; we advocate the use of a LX >10^41 erg/s in the X-ray selection of AGN, which gives a contamination rate of only 3% from star-forming galaxies; Sy 1s and Sy 2 have the same intrinsic Γ; in 24% of cases the absorption measured in X-rays does not correspond directly with that implied in the optical band from the visibility of the BLRs; the obscured fraction in AGN is a strong function of LX peaking at 10^42 erg/s; The average obscured and Compton thick fractions for this sample are 62\pm5% and 20\pm4% respectively, which are higher than hard X-ray and optically selected samples; we assess the use of the T ratio (FX/F[OIII]) for selecting Compton thick candidates. We conclude here that this quantity can often be unreliable due uncertainties in the extinction corrections to the [OIII] flux. These results have important impacts on AGN selection and unification and the results from the 12MGS are particularly useful as a local analogue to z=1 24 um samples.

astro-ph.HE↗

arXiv2010

An XMM-Newton spectral survey of 12 micron selected galaxies. I. X-ray data

We present an X-ray spectral analysis of 126 galaxies of the 12 micron galaxy sample. We pay particular attention to Compton thick AGN with the help of new spectral fitting models that we have produced, which are based on Monte-Carlo simulations of X-ray radiative transfer, using both a spherical and torus geometry, and taking into account Compton scattering and Fe fluorescence. We use this data to show that with a torus geometry, unobscured sight lines can achieve a maximum EW of the Fe Kαline of ~150 eV, originally shown by Ghisellini, Haardt & Matt (1994). In order for this to be exceeded, the line of sight must be obscured with N_H>10^23 cm^-2, as we show for one case, NGC 3690. We also calculate flux suppression factors from the simulated data, the main conclusion from which is that for N_H>10^25 cm^-2, the X-ray flux is suppressed by a factor of >10 in all X-ray bands and at all redshifts, revealing the biases present against these extremely heavily obscured systems inherent in all X-ray surveys. Furthermore, we confirm previous results from Murphy & Yaqoob (2009) that show that the reflection fraction determined from slab geometries is underestimated with respect to toroidal geometries. For the 12 micron selected galaxies, we investigate the distribution of X-ray power-law indices, finding that the mean $<Γ>=1.90_{-0.07}^{+0.05}$ and $σ_Γ= 0.31_{-0.05}^{+0.05}$) is consistent with previous works, and that the distribution of Γfor obscured and unobscured sources is consistent with the source populations being the same, in general support of unification schemes. We determine a Compton thick fraction for the X-ray AGN in our sample to be 18+/-5% which is higher than the hard X-ray (>10 keV) selected samples. Finally we find that the obscured fraction for our sample is a strong function of X-ray luminosity, peaking at L_X~10^42-43 ergs s^-1.

astro-ph.HE↗

arXiv2008

On the nature of unabsorbed Seyfert 2 galaxies

We present an analysis of six 12 um selected Seyfert 2 galaxies that have been reported to be unabsorbed in the X-ray. By comparing the luminosities of these galaxies in the mid-IR (12um), optical ([O III]) and hard X-ray (2-10 keV), we show that they are all under-luminous in the 2-10 keV X-ray band. Four of the objects exhibit X-ray spectra indicative of a hard excess, consistent with a heavily obscured X-ray component and hence a hidden nucleus. In these objects the softer X-rays may be dominated by a strong soft scattered continuum or contamination from the host galaxy, which is responsible for the unabsorbed X-ray spectra observed, and accounts for the anomalously low 2-10 keV X-ray luminosity. We confirm this assertion in NGC4501 with a Chandra observation, which shows hard X-ray emission coincident with the nucleus, consistent with heavy absorption, and a number of contaminating softer sources which account for the bulk of the softer emission. We point out that such 'Compton thick' sources need not necessarily present iron Ka emission of high EW. An example in our sample is IRASF01475-0740, which we know must host an obscured AGN as it hosts a HBLR seen in scattered light (Tran 2003). The X-ray spectrum is nonetheless relatively unobscured and the iron Ka line only moderate in strength. These observations can be reconciled if the hidden nuclear emission is dominated by transmitted, rather than reflected X-rays, which can then be weak compared to the soft scattered light or galactic emission. Despite these considerations, we conclude that NGC3147 & 3660 may intrinsically lack a BLR, confirming the recent results of Bianchi et al. (2008) for NGC3147. Neither X-ray spectrum shows signs of hidden hard emission and both sources exhibit X-ray variability leading us to believe we are viewing the nucleus directly.

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