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1.
We have used very deep XMM–Newton observations of the Chandra Deep Field -South to examine the spectral properties of the faint active galactic nucleus (AGN) population. Crucially, redshift measurements are available for 84 per cent (259/309) of the XMM–Newton sample. We have calculated the absorption and intrinsic luminosities of the sample using an extensive Monte Carlo technique incorporating the specifics of the XMM–Newton observations. 23 sources are found to have substantial absorption and intrinsic X-ray luminosities greater than 1044 erg s−1, putting them in the 'type-2' QSO regime. We compare the redshift, luminosity and absorption distributions of our sample to the predictions of a range of AGN population models. In contrast to recent findings from ultradeep Chandra surveys, we find that there is little evidence that the absorption distribution is dependent on either redshift or intrinsic X-ray luminosity. The pattern of absorption in our sample is best reproduced by models in which ∼75 per cent of the AGN population is heavily absorbed at all luminosities and redshifts.  相似文献   

2.
We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10−13 erg cm−2 s−1]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 1042 to  1045 erg s−1  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N H > 1022 cm−2)  and unobscured  ( N H < 1022 cm−2)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.  相似文献   

3.
We explore the nature of X-ray sources with  70 μm  counterparts selected in the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) fields: ELAIS-N1, Lockman Hole and Chandra Deep Field South, for which Chandra X-ray data are available. A total of 28 X-ray/  70 μm  sources in the redshift interval  0.5 < z < 1.3  are selected. The X-ray luminosities and the shape of the X-ray spectra show that these sources are active galactic nuclei (AGN). Modelling of the optical to far-infrared (IR) spectral energy distribution indicates that most of them (27/28) have a strong starburst component  (>50 M⊙ yr−1)  that dominates in the IR. It is found that the X-ray and IR luminosities of the sample sources are broadly correlated, consistent with a link between AGN activity and star formation. Contrary to the predictions of some models for the co-evolution of AGN and galaxies, the X-ray/  70 μm  sources in the sample are not more obscured at X-ray wavelengths compared to the overall X-ray population. It is also found that the X-ray/  70 μm  sources have lower specific star formation rates compared to the general  70 μm  population, consistent with AGN feedback moderating the star formation in the host galaxies.  相似文献   

4.
Chandra and XMM–Newton have resolved the     X-ray background (XRB) into point sources. Many of the fainter sources are obscured active galactic nuclei (AGN) with column densities in the range of     , some of which have quasar-like luminosities. According to obscuration models, the XRB above 8 keV is dominated by emission from Compton-thick AGN, with column densities exceeding     . Here, we consider whether Compton-thick quasars are detectable by Chandra and XMM–Newton by their direct (i.e. not scattered) X-ray emission. Detectability is optimized if the objects individually have a high luminosity and high redshift, so that the direct emission has a significant flux in the observed band. Using a simple galaxy formation model incorporating accreting black holes, in which quasars build most of their mass in a Compton-thick manner before expelling the obscuring matter, we predict that moderately deep 100-ks Chandra and XMM–Newton exposures may contain a handful of detectable Compton-thick quasars. Deep Ms or more Chandra images should contain     distant, optically faint, Compton-thick sources. In passing we show that radiation pressure can be as effective in expelling the obscuring gas as quasars winds, and yields a black hole mass proportional to the velocity dispersion of the host bulge to the fourth power.  相似文献   

5.
We report the results of spectral and temporal variability studies of the ultraluminous X-ray sources (ULXs) contained within the interacting pair of galaxies NGC 4485/4490, combining Chandra and XMM–Newton observations. Each of the four separate observations provide at least modest quality spectra and light curves for each of the six previously identified ULXs in this system; we also note the presence of a new transient ULX in the most recent observation. No short-term variability was observed for any ULX within our sample, but three out of five sources show correlated flux/spectral changes over longer time-scales, with two others remaining stable in spectrum and luminosity over a period of at least 5 yr. We model the spectra with simple power-law and multicolour disc blackbody models. Although the data are insufficient to statistically distinguish models in each epoch, those better modelled (in terms of their  χ2  fit) by a multicolour disc blackbody appear to show a disc-like correlation between luminosity and temperature, whereas those modelled by a power-law veer sharply away from such a relationship. The ULXs with possible correlated flux/spectral changes appear to change spectral form at  ∼2 × 1039 erg s−1  , suggestive of a possible change in spectral state at high luminosities. If this transition is occurring between the very high state and a super-Eddington ultraluminous state, it indicates that the mass of the black holes in these ULXs is around  10–15 M⊙  .  相似文献   

6.
We present optical spectra and near-infrared imaging of a sample of 31 serendipitous X-ray sources detected in the field of Chandra observations of the A 2390 cluster of galaxies. The sources have  0.5–7 keV  fluxes of  (0.6–8)×10-14 erg cm-2 s-1  and lie around the break in the  2–10 keV  source counts. They are therefore typical of sources dominating the X-ray Background in that band. 12 of the 15 targets for which we have optical spectra show emission lines at a range of line luminosities, and half of these show broad lines. These active galaxies and quasars have soft X-ray spectra. Including photometric redshifts and published spectra, we have redshifts for 17 of the sources, ranging from   z ∼0.2  up to   z ∼3  , with a peak between   z =1–2  . 10 of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic  2–10 keV  luminosities of  (0.2–3)×1045 erg s-1  , and absorbing columns of   N H>1023 cm-2  . Both of these sources were detected in the mid-infrared by ISOCAM on the Infrared Space Observatory , which when combined with radiative transfer modelling leads to the prediction that the bulk of the reprocessed flux emerges at ∼100 μm.  相似文献   

7.
We present results from XMM–Newton observations of the obscured quasi-stellar object 1SAX J1218.9+2958. We find that the previously reported optical and soft X-ray counterpart positions are incorrect. However, we confirm the spectroscopic redshift of 0.176. The optical counterpart has a K magnitude of 13.5 and an R – K colour of 5.0 and is therefore a bright extremely red object. The X-ray spectrum is well described by a power law  (Γ= 2.0 ± 0.2)  absorbed by an intrinsic neutral column density of  8.2+1.1−0.7× 1022 cm−2  . We find that any scattered emission contributes at most 0.5 per cent to the total X-ray flux. From the optical/near-infrared colour we estimate that the active nucleus must contribute at least 50 per cent of the total flux in the K band and that the ratio of extinction to X-ray absorption is 0.1–0.7 times that expected from a Galactic dust–gas ratio and extinction curve. If 1SAX J1218.9+2958 were 100 times less luminous it would be indistinguishable from the population responsible for most of the 2–10 keV X-ray background. This has important implications for the optical/infrared properties of faint absorbed X-ray sources.  相似文献   

8.
Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C 123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ∼5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission ('synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C 123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C 123, with L X(2–10 keV)=2×1044 erg s−1 and kT ∼5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7×1022 cm−2 and an intrinsic 2–10 keV luminosity of 1044 erg s−1.  相似文献   

9.
We combine Lyman-break colour selection with ultradeep (≳200 ks) Chandra X-ray imaging over a survey area of ∼0.35 deg2 to select high-redshift active galactic nuclei (AGN). Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at   z ∼ 3  . Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at   z = 1  , we find no evidence that the faint slope of the XLF flattens at high z , but we do find significant (factor ∼3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity   L *  . Our data therefore support models of luminosity-dependent density evolution between   z = 1  and   z = 3  . A sharp upturn in the the XLF is seen at the very lowest luminosities  ( L X≲ 1042.5 erg s−1)  , most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.  相似文献   

10.
Recent work by Risaliti, Maiolino & Salvati suggests that more than half of all Seyfert 2 galaxies in the local Universe are Compton-thick ( N H>1024 cm−2). This has implications for AGN synthesis models for the X-ray background, the flexibility of which for the inclusion of large numbers of high- z type 2 sources we examine here. We highlight the importance of Compton down-scattering in determining the individual source spectra and the fit to the X-ray background spectrum, and demonstrate how parameter space 'opens up' considerably if a super-solar iron abundance is assumed for the absorbing material. This is illustrated with a model which satisfies the present constraints, but which predicts substantial numbers of type 2 sources at the faint flux levels soon to be probed for the first time by the Chandra and XMM missions. We demonstrate also how a strong negative K -correction facilitates the detection of sources with 10∼24 N H1025 cm−2 out to the highest redshifts at which they could plausibly exist.  相似文献   

11.
We propose a model for the source of the X-ray background (XRB) in which low-luminosity active nuclei ( L  ∼ 1043 erg s−1) are obscured ( N  ∼ 1023 cm−2) by nuclear starbursts within the inner ∼ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ∼ few × 100 km s−1 and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star-forming cluster itself and the central black hole. A hot ( T  ∼ 106 − 107 K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ∼ 107 M⊙ through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.  相似文献   

12.
We present a study of the X-ray emission from the nuclei of galaxies observed in the core of the Perseus cluster in a deep exposure with Chandra . Point sources are found coincident with the nuclei of 13 early-type galaxies, as well as the central galaxy NGC 1275. This corresponds to all galaxies brighter than M B > −18 in the Chandra field. All of these sources have a steep power-law spectral component and four have an additional thermal component. The unabsorbed power-law luminosities in the 0.5–7.0 keV band range from 8 × 1038 to 5 × 1040 erg s−1. We find no simple correlations between the K -band luminosity, or the FUV and NUV AB magnitudes of these galaxies and their X-ray properties. We have estimated the black hole masses of the nuclei using the K -band   M BH– L K bol  relation and again find no correlation between black hole mass and the X-ray luminosity. Bondi accretion on to the black holes in the galaxies with minihaloes should make them much more luminous than observed.  相似文献   

13.
Extensive measurements of the X-ray background (XRB) yield a reasonably reliable characterization of its basic properties. Having resolved most of the cosmic XRB into discrete sources, the levels and spectral shapes of its main components can be used to probe both the source populations and also alternative cosmological and large-scale structure models. Recent observations of clusters seem to provide evidence that clusters formed earlier and are more abundant than predicted in the standard Λ cold dark matter (ΛCDM) model. This motivates interest in alternative models that predict enhanced power on cluster scales. We calculate predicted levels and spectra of the superposed emission from groups and clusters of galaxies in ΛCDM and in two viable alternative non-Gaussian  (χ2)  and early dark energy models. The predicted levels of the contribution of clusters to the XRB in the non-Gaussian models exceed the measured level at low energies and levels of the residual XRB in the 2–8 keV band; these particular models are essentially ruled out. Our work demonstrates the diagnostic value of the integrated X-ray emission from clusters, by considering also its dependences on different metallicities, gas and temperature profiles, Galactic absorption, merger scenarios and on a non-thermal pressure component. We also show that the XRB can be used for an upper limit for the concentration parameter value.  相似文献   

14.
We have surveyed 188 ROSAT Position Sensitive Proportional Counter (PSPC) fields for X-ray sources with hard spectra ( α <0.5); such sources must be major contributors to the X-ray background at faint fluxes. In this paper we present optical identifications for 62 of these sources: 28 active galactic nuclei (AGN) which show broad lines in their optical spectra (BLAGN), 13 narrow emission line galaxies (NELGs), five galaxies with no visible emission lines, eight clusters and eight Galactic stars.
The BLAGN, NELGs and galaxies have similar distributions of X-ray flux and spectra. Their ROSAT spectra are consistent with their being AGN obscured by columns of 20.5< log( N H/cm−2)<23 . The hard spectrum BLAGN have a distribution of X-ray to optical ratios which is similar to that found for AGN from soft X-ray surveys (1< α OX<2) . However, a relatively large proportion (15 per cent) of the BLAGN, NELGs and galaxies are radio loud. This could be because the radio jets in these objects produce intrinsically hard X-ray emission, or if their hardness is caused by absorption, it could be because radio-loud objects are more X-ray luminous than radio-quiet objects. The eight hard sources identified as clusters of galaxies are the brightest, and softest group of sources and hence clusters are unlikely to be an important component of the hard, faint population.
We propose that BLAGN are likely to constitute a significant fraction of the faint, hard, 0.5–2 keV population and could be important to reproducing the shape of the X-ray background, because they are the most numerous type of object in our sample (comprising almost half the identified sources), and because all our high redshift ( z >1) identified hard sources have broad lines.  相似文献   

15.
We present results of a Chandra survey of the ultraluminous X-ray sources (ULX) in 13 normal galaxies, in which we combine source detection with X-ray flux measurement. 22 ULX were detected, i.e. with   L x > 1 × 1039 erg s−1 ( L 10)  and 39 other sources were detected with   L x > 5 × 1038 erg s−1 ( L 5)  . We also use radial intensity profiles to remove extended sources from the sample. The majority of sources are not extended, which for a typical distance constrains the emission region size to less than 50 pc. X-ray colour–colour diagrams and spectral fitting results were examined for indicators of the ULX nature. In the case of the brighter sources, spectral fitting generally requires two-component models. In only a few cases do colour–colour diagrams or spectral fitting provide evidence of a black hole nature. We find no evidence of a correlation with stellar mass, however, there is a strong correlation with star formation as indicated by the 60-μm flux as found in previous studies.  相似文献   

16.
We report the results of a study of X-ray point sources coincident with the high-velocity system (HVS) projected in front of NGC 1275. A very deep X-ray image of the core of the Perseus cluster, made with the Chandra X-ray Observatory , has been used. We find a population of ultraluminous X-ray sources [ULXs; seven sources with   L X(0.5 − 7.0  keV) > 7 × 1039 erg s-1  ]. As with the ULX populations in the Antennae and Cartwheel galaxies, those in the HVS are associated with a region of very active star formation. Several sources have possible optical counterparts found on the Hubble Space Telescope ( HST ) images, although the X-ray brightest one does not. Absorbed power-law models fit the X-ray spectra, with most having a photon index between 2 and 3.  相似文献   

17.
The precision of intensity measurements of the extragalactic X-ray background (XRB) on an angular scale of about a degree is dominated by spatial fluctuations caused by source confusion noise. X-ray source counts at the flux level responsible for these fluctuations, ∼10−12 erg cm−2 s−1, will soon be accurately measured by new missions, and it will then be possible to detect the weaker fluctuations caused by the clustering of the fainter, more distant sources which produce the bulk of the XRB. We show here that measurements of these excess fluctuations at the level of (Δ I/I )∼2×10−3 are within reach, improving by an order of magnitude on present upper limits. Since it is likely that most (if not all) of the XRB will be resolved into sources by AXAF , subsequent optical identification of these sources will reveal the X-ray volume emissivity in the Universe as a function of redshift. With these ingredients, all-sky observations of the XRB can be used to measure the power spectrum (PS) of the density fluctuations in the Universe at comoving wavevectors k c∼0.01–0.1 Mpc−1 at redshifts where most of the XRB is likely to originate ( z ∼1–2) with a sensitivity similar to, or better than, the predictions from large-scale structure theories. A relatively simple X-ray experiment, carried out by a large-area proportional counter with a 0.5–2 deg2 collimated field of view scanning the whole sky a few times, would be able to determine the PS of the density fluctuations near its expected peak in wavevector with an accuracy better than 10 per cent.  相似文献   

18.
We present a measurement of the cluster X-ray luminosity–temperature ( L – T ) relation out to high redshift ( z ∼0.8). Combined ROSAT PSPC spectra of 91 galaxy clusters detected in the Wide Angle ROSAT Pointed Survey (WARPS) are simultaneously fitted in redshift and luminosity bins. The resulting temperature and luminosity measurements of these bins, which occupy a region of the high-redshift L – T relation not previously sampled, are compared with existing measurements at low redshift in order to constrain the evolution of the L – T relation. We find the best fit to low-redshift ( z <0.2) cluster data, at T >1 keV, to be L ∝ T 3.15±0.06. Our data are consistent with no evolution in the normalization of the L – T relation up to z ∼0.8. Combining our results with ASCA measurements taken from the literature, we find η =0.19±0.38 (for Ω0=1, with 1 σ errors) where L Bol∝(1+ z ) η T 3.15, or η =0.60±0.38 for Ω0=0.3. This lack of evolution is considered in terms of the entropy-driven evolution of clusters. Further implications for cosmological constraints are also discussed.  相似文献   

19.
We report the identification of a possible optical counterpart to the super-Eddington X-ray source NGC 5204 X-1. New Chandra data show that the X-ray source is point-like, with a luminosity of 5.2×1039 erg s−1 (0.5–8 keV) . It displays medium- and long-term X-ray variability in observations spanning a period of 20 yr. The accurate Chandra position allows us to identify a blue optical continuum source ( m v =19.7) at the position of NGC 5204 X-1, using newly obtained optical data from the INTEGRAL instrument on the William Herschel Telescope. The X-ray and optical source properties are consistent with the scenario in which we are observing the beamed X-ray emission of a high-mass X-ray binary in NGC 5204, composed of an O star with either a black hole or neutron star companion.  相似文献   

20.
Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as active galactic nuclei (AGN), and sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have therefore carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5–10 keV band, finding 180 sources in ∼50 deg2 of sky with flux≳5×10−14 erg cm−2 s−1. After correction for the non-uniform sky coverage this corresponds to resolving about 30 per cent of the hard cosmic X-ray background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA – ROSAT surveys (at 95–99 and 90 per cent confidence levels respectively), thus supporting the scenario in which a significant fraction of the XRB is created by obscured AGN.  相似文献   

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