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

We study the X-ray properties of a sample of 14 optically-selected low-mass AGN whose masses lie within the range 1E5 -2E6 M(solar) with XMM-Newton. Only six of these low-mass AGN have previously been studied with sufficient quality X-ray data, thus, we more than double the number of low-mass AGN observed by XMM-Newton with the addition of our sample. We analyze their X-ray spectral properties and variability and compare the results to their more massive counterparts. The presence of a soft X-ray excess is detectable in all five objects which were not background dominated at 2-3 keV. Combined with previous studies, this gives a total of 8 low-mass AGN with a soft excess. The low-mass AGN exhibit rapid, short-term variability (hundreds to thousands of seconds) as well as long-term variability (months to years). There is a well-known anti-correlation between black hole mass and variability amplitude (normalized excess variance). Comparing our sample of low-mass AGN with this relation we find that all of our sample lie below an extrapolation of the linear relation. Such a flattening of the relation at low masses (below about 1E6 M(solar)) is expected if the variability in all AGN follows the same shape power spectrum with a break frequency that is dependent on mass. Finally, we also found two objects that show significant absorption in their X-ray spectrum, indicative of type 2 objects, although they are classified as type 1 AGN based on optical spectra.

ABSTRA C T We study X-ray and soft g-ray spectral properties of nearby broad-line radio galaxies (BLRGs) using data from Ginga, ASCA, OSSE and EXOSAT. The X-ray spectra are well fitted by an intrinsic power-law continuum with an energy index of a , 0:7, moderately absorbed by a cold medium. In addition, the Ginga spectra show fluorescent Fe Ka lines with an average equivalent width of ,100 eV, and, in some cases, Compton reflection humps. However, the latter are significantly weaker than both those seen in radio-quiet Seyfert 1s and those expected if the Fe Ka lines were due to reflection. We find that this weakness of reflection cannot be explained by dilution by another continuum component, e.g. from a jet. Some ASCA and EXOSAT spectra show soft X-ray excesses below ,3 keV. When that component is taken into account, the Fe Ka lines in the ASCA data are found to be unresolved in most cases, and to have equivalent widths &200 eV, consistent with the Ginga data. Multiple observations of 3C 382 and 390.3 show the Fe Ka line approximately constant in flux but accompanied by strong continuum variations. This indicates the bulk of the line is formed by matter at a distance much larger than an accretion-disc scale, consistent with the ASCA line width measurements. The column density of the matter required to account for the observed line fluxes is NH * 10 23 cm π2 . Such a medium is in the line of sight of 3C 445, but it has to be out of it in other objects in which the observed NH are substantially lower. Thus a cold medium with that NH and covering a large solid angle is common in BLRGs, but in most objects it is out of the line of sight, consistent with the unified AGN model. The spectra of BLRGs break and become softer above ,100 keV, as shown by a simultaneous ASCA/OSSE observation of 3C 120 and by the OSSE spectra being on average much softer than the X-ray spectra. Finally, we find the X-ray and g-ray spectral properties of Cen A, a bright narrow-line radio galaxy ‐ a . 0:8, no or weak Compton reflection, NH * 10 23 cm π2 (which is consistent with its Fe Ka line flux), and a high-energy break at ,100 keV ‐ consistent with Cen A being intrinsically very similar to BLRGs studied here, again in agreement with the unified model.

X-ray and optical observations of two AM Her type cataclysmic variables: V388 Peg and V1189 Her

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope Γ with an intrinsic redshifted absorption N H , a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both Γ and N H free. The weighted mean value for the slope of the power law is (F) ≃ 1.75 ± 0.02, and the distribution of best fit values shows an intrinsic dispersion of σ int ≃ 0.30. We do not find hints of a correlation between the spectral index r and the intrinsic absorption column density N H . We then investigate the absorption distribution for the whole sample, deriving the N H values in faint sources by fixing r = 1.8. We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates. By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic N H distribution representative of the whole AGN population, f(N H )dN H , from the observed one. f(N H ) shows a lognormal shape, peaking around log(N H ) ≃ 23.1 and with σ ≃ 1.1. Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN. We find that the fraction of absorbed sources (with N H > 10 22 cm -2 ) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical type I (type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).

The M 101 galaxy contains the best-known example of an ultraluminous supersoft source (ULS), dominated by a thermal component at kT ≈ 0.1 keV. The origin of the thermal component and the relation between ULSs and standard (broad-band spectrum) ultraluminous X-ray sources are still controversial. We re-examined the X-ray spectral and timing properties of the M 101 ULS using archival Chandra and XMM–Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard-disc emission. The characteristic radius Rbb of the thermal emitter varies from epoch to epoch between ≈10 000 and ≈100 000 km; the colour temperature kTbb varies between ≈50 and ≈140 eV and the two quantities scale approximately as |$R_{\rm bb} \propto T_{\rm bb}^{-2}$|⁠. In addition to the smooth continuum, we also find (at some epochs) spectral residuals well fitted with thermal-plasma models and absorption edges: we interpret this as evidence that we are looking at a clumpy, multitemperature outflow. We suggest that at sufficiently high accretion rates and inclination angles, the supercritical, radiatively driven outflow becomes effectively optically thick and completely thermalizes the harder X-ray photons from the inner part of the inflow, removing the hard spectral tail. We develop a simple, spherically symmetric outflow model and show that it is consistent with the observed temperatures, radii and luminosities. A larger, cooler photosphere shifts the emission peak into the far-UV and makes the source dimmer in X-rays but possibly ultraluminous in the UV. We compare our results and interpretation with those of Liu et al.

We study the properties of SDSS galaxies with and without AGN detection as a function of the local and global environment measured via the local density, the mass of the galaxy host group (parameterised by the group luminosity) and distance to massive clusters. Our results can be divided in two main subjects, the environments of galaxies and their relation to the assembly of their host haloes, and the environments of AGN. (i) For the full SDSS sample, we find indications that the local galaxy density is the most efficient parameter to separate galaxy populations, but we also find that galaxies at fixed local density show some remaining variation of their properties as a function of the distance to the nearest cluster of galaxies (in a range of 0 to 10 cluster virial radii). These differences seem to become less significant if the galaxy samples are additionally constrained to be hosted by groups of similar total luminosity. (ii) In AGN host galaxies, the morphology-density relation is much less noticeable when compared to the behaviour of the full SDSS sample. In order to interpret this result we analyse control samples constructed using galaxies with no detected AGN activity with matching distributions of redshifts, stellar masses, r-band luminosities, g-r colours, concentrations, local densities, host group luminosities, and fractions of central and satellite galaxies. The control samples also show a similar small dependence on the local density indicating an influence from the AGN selection, but their colours are slightly bluer compared to the AGN hosts regardless of local density. Furthermore, even when the local density is held fixed at intermediate or high values, and the distance to the closest cluster of galaxies is allowed to vary, AGN control galaxies away from clusters tend to be bluer than the AGN hosts. (ABRIDGED)

We develop a new diagnostic method to classify galaxies into AGN hosts, star-forming galaxies, and absorption-dominated galaxies by combining the [O III]/Hbeta ratio with rest-frame U-B color. This can be used to robustly select AGNs in galaxy samples at intermediate redshifts (z<1). We compare the result of this optical AGN selection with X-ray selection using a sample of 3150 galaxies with 0.3<z<0.8 and I_AB<22, selected from the DEEP2 Galaxy Redshift Survey and the All-wavelength Extended Groth Strip International Survey (AEGIS). Among the 146 X-ray sources in this sample, 58% are classified optically as emission-line AGNs, the rest as star-forming galaxies or absorption-dominated galaxies. The latter are also known as "X-ray bright, optically normal galaxies" (XBONGs). Analysis of the relationship between optical emission lines and X-ray properties shows that the completeness of optical AGN selection suffers from dependence on the star formation rate and the quality of observed spectra. It also shows that XBONGs do not appear to be a physically distinct population from other X-ray detected, emission-line AGNs. On the other hand, X-ray AGN selection also has strong bias. About 2/3 of all emission-line AGNs at L_bol>10^44 erg/s in our sample are not detected in our 200 ks Chandra images, most likely due to moderate or heavy absorption by gas near the AGN. The 2--7 keV detection rate of Seyfert 2s at z~0.6 suggests that their column density distribution and Compton-thick fraction are similar to that of local Seyferts. Multiple sample selection techniques are needed to obtain as complete a sample as possible.

We present X-ray spectral and long-term variability analyses of an unbiased sample of 361 optically selected radio-loud quasars (RLQs) utilizing sensitive serendipitous X-ray data from the Chandra and XMM-Newton archives. The spectral and temporal properties of RLQs are compared with those of radio-quiet quasars (RQQs) matched in L2500Å and z. The median power-law photon index (Γ) of RLQs is $1.84_{-0.01}^{+0.01}$, which is close to that of matched RQQs ($1.90_{-0.01}^{+0.02}$). No significant correlations between Γ and radio-loudness, Lx/Lx, rqq (the X-ray luminosity over that expected from the Lx–Luv relation for RQQs), redshift, or Eddington ratio are found for our RLQs. The stacked X-ray spectra of our RLQs show strong iron-line emission and a possible Compton-reflection hump. The intrinsic X-ray variability amplitude is ≈40 per cent for RLQs on time-scales of months-to-years in the rest frame, which is somewhat smaller than for the matched RQQs (≈60 per cent) on similar time-scales, perhaps due to the larger black hole masses and lower Eddington ratios in our RLQ sample. The X-ray spectral and variability results for our RLQs generally support the idea that the X-ray emission of typical RLQs is dominated by the disc/corona, as is also indicated by a recent luminosity correlation study.

Context.The eROSITA Final Equatorial Depth Survey (eFEDS), observed with eROSITA ahead of its planned 4-yr all-sky survey, is the largest contiguous-field X-ray survey at present. It yielded a large sample of X-ray sources with very rich multiband photometric and spectroscopic coverage.Aims.We present here the eFEDS active galactic nuclei (AGN) catalog and the eROSITA X-ray spectral properties of the eFEDS sources.Methods.Using a Bayesian method, we performed a systematic X-ray spectral analysis for all the eFEDS sources. We adopted multiple spectral models, including single-component power-law or hot-plasma models and double-component models of a power law plus soft excess. We investigated the capacity of eROSITA X-ray spectra for constraining AGN spectral shapes through a detailed analysis of the posterior parameter probability distribution functions. Hierarchical Bayesian modeling was used to recover the spectral parameter distribution of the sample. The source fluxes and luminosities were measured from the posterior of the spectral fitting.Results.The eFEDS AGN catalog (22 079 sources) comprises ~80% of the eFEDS point sources. Despite a large number of faint sources, our spectral fitting provides reasonable measurements of spectral shapes and intrinsic luminosities for a majority of the sources. Because of sample selection bias, this AGN catalog is dominated by X-ray unobscured sources, with an obscured (logNH&gt; 21.5) fraction of 8%; the power-law emission of the hot corona is also relatively soft, with a typical slope of 2.0. For type-I AGN, the X-ray emission is well correlated with the UV emission with the usual anticorrelation between the X-ray to UV spectral slopeαOXand the UV luminosity. The X-ray spectral properties measured with various models are presented for all the eFEDS sources.

A complete sample of 60 serendipitous hard X-ray sources with flux in the range ~1 × 10-13 ergs cm-2 s-1 to ~4 × 10-12 ergs cm-2 s-1 (2-10 keV), detected in 87 ASCA GIS2 images, was recently presented in the literature. Using this sample it was possible to extend the description of the 2-10 keV log N(> S)- log S down to a flux limit of ~6 × 10-14 ergs cm-2 s-1 (the faintest detectable flux), resolving about a quarter of the cosmic X-ray background (CXB). In this paper we have combined the ASCA GIS2 and GIS3 data of these sources to investigate their X-ray spectral properties using the hardness ratios and the stacked-spectra method. Because of the sample statistical representativeness, the results presented here, which refer to the faintest hard X-ray sources that can be studied with the current instrumentation, are relevant to the understanding of the CXB and of the active galactic nucleus (AGN) unification scheme. The stacked spectra show that the average source's spectrum hardens toward fainter fluxes; it changes from an energy spectral index = 0.87 ± 0.08 for the 20 brightest sources (2-10 keV count rate ≥3.9 × 10-3 counts s-1, the sample) to = 0.36 ± 0.14 for the remaining 40 fainter sources (the sample). The dividing line of 3.9 × 10-3 counts s-1 corresponds to unabsorbed 2-10 keV fluxes in the range ~5.4 × 10-13 to ~3.1 × 10-13 ergs cm-2 s-1 for a source described by a power-law model with energy spectral index between 0.0 and 2.0. It thus seems that we are now beginning to detect those sources that have the correct spectral shape to be responsible for the 2-10 keV CXB. The hardness-ratio analyses indicate that this flattening is due to a population of sources with very hard spectra showing up in the faint sample; about half of the sources in this sample require αE 0.5, while only ~10% of the sources in the bright sample are consistent with an energy spectral index so flat. A number of sources (~30%) in the faint sample seem to be characterized by an apparently inverted X-ray spectrum (i.e., αE 0.0). These objects are probably extremely absorbed sources, as expected from the CXB synthesis models based on the AGN unification scheme, if not a new population of very hard serendipitous sources. The broadband (0.7-10 keV) spectral properties of the selected sources, as inferred from the hardness-ratios diagram, seem to be more complex than is expected from a simple absorbed power-law model. We have thus investigated more complex models, in line with the AGN unification scheme, and we find that these models seem to be able to explain the overall spectral properties of the present sample; this result also seems to be suggested by a comparison of the hardness-ratio diagram of the serendipitous ASCA sources with that obtained using a sample of nearby and well-known Seyfert 1 and Seyfert 2 galaxies observed with ASCA.

view Abstract Citations (93) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS EXOSAT X-Ray Spectra of Quasars Comastri, Andrea ; Setti, Giancarlo ; Zamorani, Giovanni ; Elvis, Martin ; Giommi, Paolo ; Wilkes, Belinda J. ; McDowell, Jonathan C. Abstract We have measured the X-ray spectra in the 0.1-10 keV energy range for 17 AGNs (mostly PG quasars) using the EXOSAT Low and Medium Energy arrays. The sample has been selected on the basis of soft X-ray flux and includes all the AGNs in the 1981 Zamorani et al. and 1986 Tananbaum et al. publications with an Einstein flux greater than 6x10^-12^ ergs cm^-2^ s^-1^. For each object we have determined the best-fit spectral parameters in the medium-energy range (2-10 keV), i.e., the power-law slope, the normalization, and the absorbing column density. In this energy range, all the spectra are well described by a single power law model, with a wide distribution of energy spectral indices in the range 0.4 < α < 1.3. The average spectral index is <α> = 0.89 +/- 0.06, but most of the objects are distributed around α ~ 1.0. The overall distribution of spectral indices is significantly displaced toward steeper spectra, when compared to that found in the same energy band by Turner & Pounds, for a large sample of hard X-ray-selected Seyfert galaxies. This difference can be understood in terms of the different selection criteria of the two samples (soft versus hard X-ray selection), and it is concluded that the true distribution of hard X-ray spectral indices in AGNs is probably wider than previously suggested. Inclusion of lower energy data (0.1-2.0 keV) shows the presence of significant soft excess in six out of 17 cases. This soft excess emission can be modeled by either a steep power law with α ranging from 2.7 to 4.2 or the high-energy tail of a blackbody with temperatures in the range 40-80 eV. The intersection of this soft excess emission with the hard X-ray power law is in the energy interval 0.3-0.75 keV, in the source rest frame. A correlation analysis performed on our data set does not reveal any correlation between the medium-energy spectral properties and other physical parameters of the sources. However, fits performed in the energy range 0.1-1.0 keV (comparable with the Einstein IPC energy range) show a significant correlation between radio-loudness and X-ray spectral properties, with the radio-loud objects having on average, flatter slopes than the radio-quiet ones, in agreement with the findings of Wilkes & Elvis. The fact that the same correlation is not seen in our data at higher energy seems to be explained by a correlation between presence and strength of soft excess and "radio-quietness": the objects with significant soft excess emission tend to be the most radio-quiet quasars in our sample. Publication: The Astrophysical Journal Pub Date: January 1992 DOI: 10.1086/170851 Bibcode: 1992ApJ...384...62C Keywords: Exosat Satellite; Quasars; X Ray Spectra; Galactic Nuclei; Seyfert Galaxies; Astrophysics; GALAXIES: NUCLEI; GALAXIES: QUASARS: GENERAL; GALAXIES: SEYFERT; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (17) NED (17)

In this concluding part of the series of three papers dedicated to theSwift BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and statistical properties of the source sample. Using a dedicated method to extract time-averaged spectra of BATsources, we show that Galactic sources have, generally, softer spectra than extragalactic objects and that Seyfert 2 galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert galaxies is consistent with a power-law with a photon index of 2:00 � 0:07. The cumulative flux-number relation for the extragalacticsourcesinthe 14Y170keV bandisbestdescribed byapower-lawwithaslope � ¼ 1:55 � 0:20 anda normalization of 9:6 � 1:9 ; 10 � 3 AGNs deg � 2 (or 396 � 80 AGNs all-sky) above a flux level of 2 ;10 � 11 ergs cm � 2 s � 1 (� 0.85 mcrab). The integration of the cumulative flux per unit area indicates that BAT resolves 1%Y2% of the X-ray background emission in the 14Y170 keV band. A subsample of 24 extragalactic sources above the 4.5 � detection limitisusedtostudythestatisticalpropertiesof AGNs.Thissampleiscomposedof localSeyfertgalaxies(z ¼ 0:026, median value) and � 10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of NH > 10 22 H atoms cm � 2 but that none is genuinely bona fide Compton thick. This study shows the capabilities of BAT to probe the hard X-ray sky to the millicrab level. Subject headingg galaxies: active — surveys — X-rays: binaries — X-rays: galaxies Online material: color figures

Spitzer IRAC selection is a powerful tool for identifying luminous AGN. For deep IRAC data, however, the AGN selection wedges currently in use are heavily contaminated by star-forming galaxies, especially at high redshift. Using the large samples of luminous AGN and high-redshift star-forming galaxies in COSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys. The new IRAC criteria are designed to be both highly complete and reliable, and incorporate the best aspects of the current AGN selection wedges and of infrared power-law selection while excluding high redshift star-forming galaxies selected via the BzK, DRG, LBG, and SMG criteria. At QSO-luminosities of log L(2-10 keV) (ergs/s) > 44, the new IRAC criteria recover 75% of the hard X-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN candidates have X-ray counterparts, a fraction that rises to 52% in regions with Chandra exposures of 50-160 ks. X-ray stacking of the individually X-ray non-detected AGN candidates leads to a hard X-ray signal indicative of heavily obscured to mildly Compton-thick obscuration (log N_H (cm^-2) = 23.5 +/- 0.4). While IRAC selection recovers a substantial fraction of luminous unobscured and obscured AGN, it is incomplete to low-luminosity and host-dominated AGN.

Using the data of 5 years of RXTE observations we investigate the X-ray spectral and timing properties of GRS 1915+105 during the hard steady states. According to the results of our simultaneous X-ray spectral and timing analysis the behavior the source during the hard steady states can be reduced to a couple of major distinct types. i) Type I states: The dominant hard component of the energy spectrum has characteristic quasi- exponential cut-off at 50-120 keV. The broad-band power density spectrum of the source shows significant high frequency noise component with a cut-off at 60-80 Hz. ii) Type II states: The hard spectral component has a break in its slope at ~12-20 keV. The high frequency part of the power density spectrum fades quickly lacking significant variability at frequencies higher than ~30 Hz. These two types of the X-ray hard states are also clearly distinguished by their properties in the radio band: while during the type I observations the source tends to be 'radio-quiet', the type II observations are characterized by high level of radio flux ('plateau' radio states). In this work we demonstrate aforementioned differences using the data of 12 representative hard steady state observations. We conclude that the difference between these two types can be probably explained in terms of different structure of the accretion flow in the immediate vicinity of the compact object due to presence of relativistic outflow of matter.

view Abstract Citations (195) References (85) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The X-Ray Spectra of High-Luminosity Active Galactic Nuclei Observed by Ginga Williams, O. R. ; Turner, M. J. L. ; Stewart, G. C. ; Saxton, R. D. ; Ohashi, T. ; Makishima, K. ; Kii, T. ; Inoue, H. ; Makino, F. ; Hayashida, K. ; Koyama, K. Abstract Results are presented on the X-ray emission from 13 objects, observed by Ginga as part of a spectral survey of bright quasars in the energy range 2-20 keV. The distribution of the power-law energy index for this sample has a mean of α = 0.81 and shows significant intrinsic dispersion, σ = 0.31, values which are broadly compatible with those from samples of lower luminosity active galactic nuclei. The mean spectral slope is clearly too steep to fit the spectrum of the cosmic diffuse X-ray background (CXB) at energies less than 20 keV, confirming the "spectral paradox" in the discrete-source explanation of the CXB. We have searched for correlations between the X-ray spectral properties and those in other frequency bands, and have found a possible connection between the near-infrared spectral indices and the X-ray properties of quasars. There is also evidence that quasars with steep X-ray spectra are radio-quiet, in support of the previous results obtained for the 0.3-3.5 keV band; our results, however, are compatible with a single power-law component in the 0.3-10 keV range, in contrast to some previous results. The first evidence for departures from a simple power-law continuum in Mrk 205 is revealed; this, together with previously reported evidence for iron lines in other quasars, suggests that an appreciable fraction of quasars may possess iron lines with equivalent widths ~100 eV. Models involving reprocessing of continuum flux in cool material near the core, successful in the case of Seyfert galaxies, fit well for the lower luminosity objects in our sample; for the high-luminosity objects the fit is not as good, and other processes, such as beaming, may be implicated. Publication: The Astrophysical Journal Pub Date: April 1992 DOI: 10.1086/171194 Bibcode: 1992ApJ...389..157W Keywords: Active Galactic Nuclei; Active Galaxies; Astronomical Satellites; Galactic Radiation; Quasars; X Ray Spectra; Accretion Disks; Cosmic X Rays; Emission Spectra; Near Infrared Radiation; Seyfert Galaxies; X Ray Astronomy; Astrophysics; GALAXIES: ACTIVE; GALAXIES: NUCLEI; GALAXIES: QUASARS: GENERAL; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (16) NED (15)