Search

The existence of a large population of Compton thick (CT, $N_{H}>10^{24} cm^{-2}$) AGN is a key ingredient of most Cosmic X-ray background synthesis models. However, direct identification of these sources, especially at high redshift, is difficult due to the flux suppression and complex spectral shape produced by CT obscuration. We explored the Chandra COSMOS Legacy point source catalog, comprising 1855 sources, to select via X-ray spectroscopy, a large sample of CT candidates at high redshift. Adopting a physical model to reproduce the toroidal absorber, and a Monte-Carlo sampling method, we selected 67 individual sources with >5% probability of being CT, in the redshift range $0.0410^{24} cm^{-2}$, gives a total of 41.9 effective CT, corrected for classification bias. We derive number counts in the 2-10 keV band in three redshift bins. The observed logN-logS is consistent with an increase of the intrinsic CT fraction ($f_{CT}$) from $\sim0.30$ to $\sim0.55$ from low to high redshift. When rescaled to a common luminosity (log(L$_{\rm X}$/erg/s)$=44.5$) we find an increase from $f_{CT}=0.19_{-0.06}^{+0.07}$ to $f_{CT}=0.30_{-0.08}^{+0.10}$ and $f_{CT}=0.49_{-0.11}^{+0.12}$ from low to high z. This evolution can be parametrized as $f_{CT}=0.11_{-0.04}^{+0.05}(1+z)^{1.11\pm0.13}$. Thanks to HST-ACS deep imaging, we find that the fraction of CT AGN in mergers/interacting systems increases with luminosity and redshift and is significantly higher than for non-CT AGN hosts., Comment: 15 pages, 13 figures. Published in MNRAS 31 July 2018

The COSMOS-Legacy survey is a 4.6 Ms Chandra program that has imaged 2.2 deg$^2$ of the COSMOS field with an effective exposure of $\simeq$160 ks over the central 1.5 deg$^2$ and of $\simeq$80 ks in the remaining area. The survey is the combination of 56 new observations, obtained as an X-ray Visionary Project, with the previous C-COSMOS survey. We describe the reduction and analysis of the new observations and the properties of 2273 point sources detected above a spurious probability of 2$\times 10^{-5}$. We also present the updated properties of the C-COSMOS sources detected in the new data. The whole survey includes 4016 point sources (3814, 2920 and 2440 in the full, soft and hard band). The limiting depths are 2.2 $\times$ 10$^{-16}$, 1.5 $\times$ 10$^{-15}$ and 8.9$\times$ 10$^{-16}$ ${\rm erg~cm}^{-2}~{\rm s}^{-1}$ in the 0.5-2, 2-10 and 0.5-10 keV bands, respectively. The observed fraction of obscured AGN with column density $> 10^{22}$ cm$^{-2}$ from the hardness ratio (HR) is $\sim$50$^{+17}_{-16}$%. Given the large sample, we compute source number counts in the hard and soft bands, significantly reducing the uncertainties of 5-10%. For the first time, we compute number counts for obscured (HR$>$-0.2) and unobscured (HR$<$-0.2) sources and find significant differences between the two populations in the soft band. Due to the un-precedent large exposure, COSMOS-Legacy area is 3 times larger than surveys at similar depth and its depth is 3 times fainter than surveys covering similar area. The area-flux region occupied by COSMOS-Legacy is likely to remain unsurpassed for years to come., Comment: 20 pages, 16 figures, accepted for publication on ApJ on December 24, 2015

We present the catalog of optical and infrared counterparts of the Chandra COSMOS-Legacy Survey, a 4.6 Ms Chandra program on the 2.2 square degrees of the COSMOS field, combination of 56 new overlapping observations obtained in Cycle 14 with the previous C-COSMOS survey. In this Paper we report the i, K, and 3.6 micron identifications of the 2273 X-ray point sources detected in the new Cycle 14 observations. We use the likelihood ratio technique to derive the association of optical/infrared (IR) counterparts for 97% of the X-ray sources. We also update the information for the 1743 sources detected in C-COSMOS, using new K and 3.6 micron information not available when the C-COSMOS analysis was performed. The final catalog contains 4016 X-ray sources, 97% of which have an optical/IR counterpart and a photometric redshift, while 54% of the sources have a spectroscopic redshift. The full catalog, including spectroscopic and photometric redshifts and optical and X-ray properties described here in detail, is available online. We study several X-ray to optical (X/O) properties: with our large statistics we put better constraints on the X/O flux ratio locus, finding a shift towards faint optical magnitudes in both soft and hard X-ray band. We confirm the existence of a correlation between X/O and the the 2-10 keV luminosity for Type 2 sources. We extend to low luminosities the analysis of the correlation between the fraction of obscured AGN and the hard band luminosity, finding a different behavior between the optically and X-ray classified obscured fraction., Comment: 28 pages, 14 figures, accepted to ApJ

We construct the rest-frame 2--10 keV intrinsic X-ray luminosity function of Active Galactic Nuclei (AGNs) from a combination of X-ray surveys from the all-sky Swift BAT survey to the Chandra Deep Field-South. We use ~3200 AGNs in our analysis, which covers six orders of magnitude in flux. The inclusion of the XMM and Chandra COSMOS data has allowed us to investigate the detailed behavior of the XLF and evolution. In deriving our XLF, we take into account realistic AGN spectrum templates, absorption corrections, and probability density distributions in photometric redshift. We present an analytical expression for the overall behavior of the XLF in terms of the luminosity-dependent density evolution, smoothed two power-law expressions in 11 redshift shells, three-segment power-law expression of the number density evolution in four luminosity classes, and binned XLF. We observe a sudden flattening of the low luminosity end slope of the XLF slope at z>~0.6. Detailed structures of the AGN downsizing have been also revealed, where the number density curves have two clear breaks at all luminosity classes above log LX>43. The two break structure is suggestive of two-phase AGN evolution, consisting of major merger triggering and secular processes., Comment: 39 Pages, 9 figures. ApJ in press

(Abridged) Stellar mass black hole X-ray binaries exhibit X-ray spectral states which also have distinct and characteristic temporal properties. These states are believed to correspond to different accretion disk geometries. We present analysis of two long XMM-Newton observations of the Ultra-Luminous X-ray source (ULX) NGC 1313 X-1, which reveal that the system was in two different spectral/temporal states. With a count rate of 1.5 counts/s and a fractional variability amplitude of ~15%, the ULX was in a high flux and strongly variable state in March 2006. In October 2006, the count rate of the ULX had reduced by a factor of ~2 and the spectral shape was distinctly different with the presence of a soft component. No strong variability was detected during this low flux state with an upper limit on the amplitude < 3%. Moreover, the spectral properties of the two states implies that the accretion disk geometry was different for them. The low flux state is consistent with a model where a standard accretion disk is truncated at a radius of ~17 Schwarzschild radius around a ~200 Msun black hole. The inner hot region Comptonizes photons from the outer disk to give the primary spectral component. The spectrum of the high flux state is not compatible with such a geometry. Instead, it is consistent with a model where a hot corona covers a cold accretion disk and Comptonizes the disk photons. The variability as a function of energy is also shown to be consistent with the corona model. Despite these broad analogies with Galactic black hole systems, the spectral nature of the ULX is distinct in having a colder Comptonizing temperature (~2 keV) and higher optical depth (~15) than what is observed for the Galactic ones., Comment: 10 pages, MNRAS, submitted

The COSMOS survey is a multiwavelength survey aimed to study the evolution of galaxies, AGN and the large scale structure. The XMM-COSMOS is a deep X-ray survey over the full 2 deg2 of the COSMOS area. It consists of 55 XMM-Newton pointings for a total exposure of ~1.5 Ms with an average vignetting corrected depth of 40 ks across the field of view and a sky coverage of 2.13 deg2. We present the catalogue of point-like X-ray sources detected with the EPIC CCD cameras, the logN-logS relations and the X-ray colour-colour diagrams. The analysis was performed in the 0.5-2 keV, 2-10 keV and 5-10 keV energy bands. The completeness of the catalogue as well as logN-logS have been calibrated using Monte Carlo simulations. The catalogs contains a total of 1887 unique sources detected in at least one band. The survey, that shows unprecedented homogeneity, has a flux limit of ~1.7x10-15 erg cm-2 s-1, ~9.3x10-15 erg cm-2 s-1 and ~1.3x10-14 erg cm-2 s-1 over 90% of the area (1.92 deg2) in the 0.5-2 keV, 2-10 keV and 5-10 keV energy band, respectively. Thanks to the rather homogeneous exposure over a large area, the derived logN-logS relations are very well determined over the flux range sampled by XMM-COSMOS. These relations have been compared with XRB synthesis models, which reproduce the observations with an agreement of ~10% in the 5-10 keV and 2-10 keV band, while in the 0.5-2 keV band the agreement is of the order of ~20%. The hard X-ray colors confirmed that the majority of the extragalactic sources, in a bright subsample, are actually Type I or Type II AGN. About 20% of the sources have X-ray luminosity typical of AGN (L_X >1042 erg/s) although they do not show any clear signature of nuclear activity in the optical spectrum., Comment: Astronomy and Astrophysics, accepted, language edited. The XMM-COSMOS source catalog and science data products are available at http://irsa.ipac.caltech.edu/data/COSMOS/ by A&A 16 pages, 12 Figures, Color printing suggested

We report results from a Suzaku observation of the narrow-line Seyfert 1 NGC 4051. During our observation, large amplitude rapid variability is seen and the averaged 2--10 keV flux is 8.1x10^-12 erg s^-1 cm^-2, which is several times lower than the historical average. The X-ray spectrum hardens when the source flux becomes lower, confirming the trend of spectral variability known for many Seyfert 1 galaxies. The broad-band averaged spectrum and spectra in high and low flux intervals are analyzed. The spectra are first fitted with a model consisting of a power-law component, a reflection continuum originating in cold matter, a blackbody component, two zones of ionized absorber, and several Gaussian emission lines. The amount of reflection is rather large (R ~ 7, where R=1 corresponds to reflection by an infinite slab), while the equivalent width of the Fe-K line at 6.4 keV is modest (140 eV) for the averaged spectrum. We then model the overall spectra by introducing partial covering for the power-law component and reflection continuum independently. The column density for the former is 1x10^23 cm^-2, while it is fixed at 1x10^24 cm-2 for the latter. By comparing the spectra in different flux states, we identify the causes of spectral variability. (abridged), Comment: 19 pages, 18 figures, accepted for publication in PASJ (Suzaku 3rd special issue)

We present a systematic X-ray study of eight AGNs with intermediate mass black holes (M_BH 8-95x10^4 Msun) based on 12 XMM-Newton observations. The sample includes the two prototype AGNs in this class - NGC4395 and POX52 and six other AGNs discovered with the SDSS. These AGNs show some of the strongest X-ray variability with the normalized excess variances being the largest and the power density break time scales being the shortest observed among radio-quiet AGNs. The excess variance -- luminosity correlation appears to depend on both the BH mass and the Eddington luminosity ratio. The break time scale -- black hole mass relations for AGN with IMBHs are consistent with that observed for massive AGNs. We find that the FWHM of the Hbeta or Halpha line is uncorrelated with the BH mass, but shows strong anticorrelation with the Eddington luminosity ratio. Four AGNs show clear evidence for soft X-ray excess emission (kT_in~150-200eV). X-ray spectra of three other AGNs are consistent with the presence of the soft excess emission. NGC4395 with lowest L/L_Edd lacks the soft excess emission. Evidently small black mass is not the primary driver of strong soft X-ray excess emission from AGNs. The X-ray spectral properties and optical-to-X-ray spectral energy distributions of these AGNs are similar to those of Seyfert 1 galaxies. The observed X-ray/UV properties of AGNs with IMBHs are consistent with these AGNs being low mass extension of more massive AGNs; those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1s while those with low $L/L_{Edd}$ looking more like broad-line Seyfert 1s., Comment: 19 pages, ApJ, Submitted

We present results from a 150 ksec Suzaku observation of the Seyfert 1.5 NGC 3516 in October 2005. The source was in a relatively highly absorbed state. Our best-fit model is consistent with the presence of a low-ionization absorber which has a column density near 5 * 10^{22} cm^{-2} and covers most of the X-ray continuum source (covering fraction 96-100%). A high-ionization absorbing component, which yields a narrow absorption feature consistent with Fe K XXVI, is confirmed. A relativistically broadened Fe K alpha line is required in all fits, even after the complex absorption is taken into account; an additional partial-covering component is an inadequate substitute for the continuum curvature associated with the broad Fe line. A narrow Fe K alpha emission line has a velocity width consistent with the Broad Line Region. The low-ionization absorber may be responsible for producing the narrow Fe K alpha line, though a contribution from additional material out of the line of sight is possible. We include in our model soft band emission lines from He- and H-like ions of N, O, Ne and Mg, consistent with photo-ionization, though a small contribution from collisionally-ionized emission is possible., Comment: Accepted for publication in PASJ (Suzaku second special issue). 36 pages, 10 figures

The origin of soft X-ray excess emission from type 1 active galactic nuclei has remained a major problem for the last two decades. It has not been possible to distinguish alternative models for the soft excess emission despite the excellent data quality provided by XMM-Newton and Chandra. Here we present observations of time lags between the soft and hard band X-ray emission and discuss the implications to the models for the soft excess. We also device a method to distinguish the models for the soft excess using Suzaku's broadband capability., Comment: 4 pages, To appear in the conference proceedings "The Extreme Universe in the Suzaku Era" (Progress of Theoretical Physics, Supplement)

We investigate the origin of the soft X-ray excess emission from narrow-line Seyfert 1 galaxies Akn564 and Mrk1044 using XMM-Newton observations. We find clear evidence for time delays between the soft and hard X-ray emission from Akn564 based on a 100ks long observation. The variations in the 4-10keV band lag behind that in the 0.2-0.5keV band by 1768+/-122s. The full band power density spectrum (PDS) of Akn~564 has a break at ~1.2e-3Hz with power-law indices of ~1 and ~3 below and above the break. The hard (3-10keV) band PDS is stronger and flatter than that in the soft (0.2-0.5keV) band. Based on a short observation of Mrk1044, we find no correlation between the 0.2-0.3keV and 5-10keV bands at zero lag. These observations imply that the soft excess is not the reprocessed hard X-ray emission. The high resolution spectrum of Akn564 obtained with the RGS shows evidence for a highly ionized and another weakly ionized warm absorber medium. The smeared wind and blurred ionized reflection models do not describe the pn data adequately. The spectrum is consistent with a complex model consisting of optically thick Comptonization in a cool plasma for the soft excess and a steep power-law, modified by two warm absorber media as inferred from the RGS data and the foreground Galactic absorption. The smeared wind and optically thick Comptonization models both describe the spectrum of Mrk1044 satisfactorily, but the ionized reflection model requires extreme parameters. The data suggest two component corona -- a cool, optically thick corona for the soft excess and a hot corona for the power-law component. The existence of a break in the soft band PDS suggests a compact cool corona that can either be an ionized surface of the inner disk or an inner optically thick region coupled to a truncated disk., Comment: Accepted for publication in ApJ, 19 pages

We present the results of the simultaneous deep XMM and Chandra observations of the bright Seyfert 1.9 galaxy MCG-5-23-16, which is thought to have one of the best known examples of a relativistically broadened iron K-alpha line. The time averaged spectral analysis shows that the iron K-shell complex is best modeled with an unresolved narrow emission component (FWHM < 5000 km/s, EW ~ 60 eV) plus a broad component. This latter component has FWHM ~ 44000 km/s and EW ~ 50 eV. Its profile is well described by an emission line originating from an accretion disk viewed with an inclination angle ~ 40^\circ and with the emission arising from within a few tens of gravitational radii of the central black hole. The time-resolved spectral analysis of the XMM EPIC-pn spectrum shows that both the narrow and broad components of the Fe K emission line appear to be constant in time within the errors. We detected a narrow sporadic absorption line at 7.7 keV which appears to be variable on a time-scale of 20 ksec. If associated with Fe XXVI Ly-alpha this absorption is indicative of a possibly variable, high ionization, high velocity outflow. The variability of this absorption feature appears to rule out a local (z=0) origin. The analysis of the XMM RGS spectrum reveals that the soft X-ray emission of MCG-5-23-16 is likely dominated by several emission lines superimposed on an unabsorbed scattered power-law continuum. The lack of strong Fe L shell emission together with the detection of a strong forbidden line in the O VII triplet is consistent with a scenario where the soft X-ray emission lines are produced in a plasma photoionized by the nuclear emission., Comment: 45 pages, 12 figures, 4 tables, accepted for publication in ApJ

We present the analysis of the source counts in the XMM-COSMOS survey using data of the first year of XMM-Newton observations. The survey covers ~2 deg^2 within the region of sky bounded by 9^h57.5^m

We present the data analysis and the X-ray source counts for the first season of XMM-Newton observations in the COSMOS field. The survey covers ~2 deg^2 within the region of sky bounded by 9^h57.5^m

We present a detailed spectral analysis of point-like X-ray sources in the XMM-COSMOS field. Our sample of 135 sources only includes those that have more than 100 net counts in the 0.3-10 keV energy band and have been identified through optical spectroscopy. The majority of the sources are well described by a simple power-law model with either no absorption (76%) or a significant intrinsic, absorbing column (20%).As expected, the distribution of intrinsic absorbing column densities is markedly different between AGN with or without broad optical emission lines. We find within our sample four Type-2 QSOs candidates (L_X > 10^44 erg/s, N_H > 10^22 cm^-2), with a spectral energy distribution well reproduced by a composite Seyfert-2 spectrum, that demonstrates the strength of the wide field XMM/COSMOS survey to detect these rare and underrepresented sources., Comment: 16 pages, ApJS COSMOS Special Issue, 2007 in press. The full-resolution version is available at http://www.mpe.mpg.de/XMMCosmos/PAPERS/mainieri_cosmos.ps.gz

We present the results of a search for galaxy clusters in the first 36 XMM-Newton pointings on the COSMOS field. We reach a depth for a total cluster flux in the 0.5-2 keV band of 3x10-15 ergs cm-2 s-1, having one of the widest XMM-Newton contiguous raster surveys, covering an area of 2.1 square degrees. Cluster candidates are identified through a wavelet detection of extended X-ray emission. Verification of the cluster candidates is done based on a galaxy concentration analysis in redshift slices of thickness of 0.1-0.2 in redshift, using the multi-band photometric catalog of the COSMOS field and restricting the search to z<1.3 and i_AB < 25. We identify 72 clusters and derive their properties based on the X-ray cluster scaling relations. A statistical description of the survey in terms of the cumulative log(N>S)-lg(S) distribution compares well with previous results, although yielding a somewhat higher number of clusters at similar fluxes. The X-ray luminosity function of COSMOS clusters matches well the results of nearby surveys, providing a comparably tight constraint on the faint end slope of alpha=1.93+/-0.04. For the probed luminosity range of 8x10+42 - 2x10+44 ergs s-1, our survey is in agreement with and adds significantly to the existing data on the cluster luminosity function at high redshifts and implies no substantial evolution at these luminosities to z=1.3., Comment: 15 pages, to appear in the COSMOS Special Issue, ApJS 2007. More information onf XMM-COSMOS project could be found at http://www.mpe.mpg.de/XMMCosmos/

We present the first set of XMM-Newton EPIC observations in the 2 square degree COSMOS field. The strength of the COSMOS project is the unprecedented combination of a large solid angle and sensitivity over the whole multiwavelength spectrum. The XMM-Newton observations are very efficient in localizing and identifying active galactic nuclei (AGN) and clusters as well as groups of galaxies. One of the primary goals of the XMM-Newton Cosmos survey is to study the co-evolution of active galactic nuclei as a function of their environment in the Cosmic web. Here we present the log of observations, images and a summary of first research highlights for the first pass of 25 XMM-Newton pointings across the field. In the existing dataset we have detected 1416 new X-ray sources in the 0.5-2, 2-4.5 and 4.5-10 keV bands to an equivalent 0.5-2 keV flux limit of 7x10-16 erg cm-2 s-1. The number of sources is expected to grow to almost 2000 in the final coverage of the survey. From an X-ray color color analysis we identify a population of heavily obscured, partially leaky or reflecting absorbers, most of which are likely to be nearby, Compton-thick AGN., Comment: 9 pages, ApJS COSMOS Special Issue, 2007 in press. the full-resolution version is available at http://www.mpe.mpg.de/XMMCosmos/PAPERS/grh_cosmos.ps.gz

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$\deg) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution., Comment: 21 pages, 6 figures -- to appear in COSMOS ApJ Suppl. special issue

Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313, named X-1 and X-2, were observed with Suzaku on 2005 September 15. During the observation for a net exposure of 28~ks (but over a gross time span of 90~ks), both objects varied in intensity by about 50~%. The 0.4--10 keV X-ray luminosity of X-1 and X-2 was measured as $2.5 \times 10^{40}~{\rm erg~s^{-1}}$ and $5.8 \times 10^{39}~{\rm erg~s^{-1}}$, respectively, with the former the highest ever reported for this ULX. The spectrum of X-1 can be explained by a sum of a strong and variable power-law component with a high energy cutoff, and a stable multicolor blackbody with an innermost disk temperature of $\sim 0.2$ keV. These results suggest that X-1 was in a ``very high'' state, where the disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1 is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with the innermost disk temperature of 1.2--1.3 keV, and becomes flatter as the source becomes brighter. Hence X-2 is interpreted to be in a slim-disk state. These results suggest that the two ULXs have black hole masses of a few tens to a few hundreds solar masses., Comment: accepted for publication in PASJ

In recent years it has become clear that understanding the absorption present in AGN is essential given its bearing on unification models. We present the most recent XMM-Newton observation of Mrk 6, with the goal of understanding the nature and origin of the complex absorption intrinsic to this source. X-ray spectral fitting shows that a simple warm absorption model provides an equally good statistical representation of the CCD data as a partial covering model. Furthermore, once the RGS data are included in the spectral fitting, the simple warm absorber model provides a very good fit to the data, without increasing the complexity of the model, in contrast with the partial covering model which requires the addition of either a low metalicity (<0.03 solar) thermal plasma or low temperature blackbody emission in order to provide a similar quality fit. The warm absorber is also a considerably more natural way to explain the variability observed in the X-ray absorbing column density between the previous XMM-Newton observation and this one, requiring only a second, higher column density, higher ionisation, absorber to be present during the previous XMM-Newton observation. In comparison, the partial covering models which requires moving, clumpy, material relatively close to the source that result in two distinct lines of sight, with separate absorbing columns that each vary considerably without any associated change in their covering fractions, in order to explain the observed variability. We associate the warm absorber either with an accretion disk wind with densities of ~10^9 /cm^3, or with an ionised `skin' or atmosphere of the molecular torus with densities of ~10^3 - 10^5 /cm^3., Comment: 11 pages, 8 figures. Accepted for publication by MNRAS 05/2006

We report the discovery of a 200mHz quasi-periodic oscillation (QPO) in the X-ray emission from a bright ultra-luminous X-ray source (ULX) Holmberg IX X-1 using a long XMM-Newton observation. The QPO has a centroid at 202.5_{-3.8}^{+4.9}mHz, a coherence Q ~9.3 and an amplitude (rms) of 6% in the 0.2-10keV band. This is only the second detection of a QPO from an ULX, after M82 X-1, and provides strong evidence against beaming. The power spectrum is well fitted by a power law with an index of ~0.7. The total integrated power (rms) is ~9.4% in the 0.001-1Hz range. The X-ray spectrum shows clear evidence for a soft X-ray excess component that is well described by a multicolor disk blackbody (kT_in ~ 0.3keV) and a high energy curvature that can be modeled either by a cut-off power law (Gamma ~ 1; E_cutoff ~9keV) or as a strongly Comptonized continuum in an optically thick (tau ~7.3) and cool (kT_e ~3keV) plasma. Both the presence of the QPO and the shape of the X-ray spectrum strongly suggest that the ULX is not in the high/soft or thermally dominated state. A truncated disk and inner optically thick corona may explain the observed X-ray spectrum and the presence of the QPO., Comment: Submitted to ApJ Letters

ASCA X-ray spectra of many ULXs were described in terms of optically thick emission from hot (kT ~1-2keV) accretion disks, while recent XMM-Newton and Chandra observations have revealed a cool (kT ~0.2keV), soft X-ray excess emission from a number of them. Here we utilize improved calibration and high signal-to-noise XMM-Newton spectra of NGC1313 X-1 to present evidence for a cool (~0.2keV) soft excess and a curved or a cutoff power-law (Gamma ~1-1.5, E_cutoff ~3-8keV). The high energy curvature may also be described by a hot (~1-2.5keV) multicolor disk blackbody. The soft excess emission is unlikely to arise from a cool disk as its blackbody temperature is similar in three XMM-Newton observations, despite a change in the observed flux by a factor of about two. Thus, previous estimates of the black hole mass of 1000Msun for NGC1313 X-1 based on the temperature of the soft excess emission is unlikely to be correct. The power-law cutoff energy is found to decrease from ~8keV to \~3keV when the ULX brightened by a factor of about two. The unusual spectral properties of NGC1313 X-1 are difficult to understand in the framework of the disk/corona models generally adopted for the black hole binaries or active galactic nuclei and may require to invoke super-critical accretion rates., Comment: 5 pages, submitted

Unified models of Seyfert galaxies, based on viewing angles, successfully explain the observed differences between type 1 and 2 Seyferts. The existence of a range in accretion rates relative to the Eddington rate (from broad-line Seyfert 1s to narrow-line Seyfert 1s or NLS1s) and the unification of Seyfert galaxies imply that there must be type 2 counterparts of NLS1 galaxies i.e., Seyfert 2s with high accretion rate or small black hole mass. One such Seyfert 2, NGC5506, has already been unmasked based on near infra-red spectroscopy. Here we confirm the above result, and present evidence for two additional type 2 counterparts of NLS1s based on XMM-Newton observations. The three AGNs NGC7314, NGC7582 and NGC5506, with a type 1.9/2 optical spectrum, show extremely rapid variability by factors >2.4, 1.3, and 1.7 in 200s, 350s and 300s, respectively, and steep 2-12 keV spectrum in their intrinsic X-ray emission, characteristic of NLS1 galaxies. These observations establish the obscured NLS1 galaxies as a subclass of Seyfert 2 galaxies., Comment: 4 pages, To appear in ApJ Letters

We report on XMM-Newton spectroscopy of the low-luminosity active galaxies (LLAGN) M81 and NGC4579 both of which have known black hole masses and well-sampled spectral energy distributions (SED). The iron Kalpha line profiles from both the LLAGN can be described in terms of two components - a narrow line at 6.4 keV and a moderately broad line (FWHM = 20000 km/s) arising from highly ionized, He-like or H-like species (E \sim 6.8 keV). We interpret the broad lines arising from an accretion disk the inner edge of which is restricted to large radii (r_{in} \sim 100 r_g). However, the Eddington ratio, {L}/{L_{Edd}}, of these sources, is 3 - 4 orders of magnitude lower than that required to photo-ionize a cold disk to He-like iron. We suggest that the lines can be explained as collisionally ionized X-ray lines arising from the transition region between a hot (radiatively inefficient) flow in the inner regions and a cold disk outside r \sim 100r_g. The accretion flow geometry probed by our XMM-Newton observations is consistent with the truncated disk models proposed to explain the SED of LLAGNs., Comment: 8 pages, uses emulateapj5, To apear in ApJ

We present a detailed analysis of the complex soft X-ray spectrum of NGC 4151 measured by the RGS instruments aboard XMM-Newton. The XMM-Newton RGS spectra demonstrate that the soft X-ray emission is extremely rich in X-ray emission lines and radiative recombination continua (RRC), with no clear evidence for any underlying continuum emission. Line emission, and the associated RRC, are clearly detected from hydrogen-like and helium-like ionization states of neon, oxygen, nitrogen and carbon. The measured lines are blueshifted with a velocity of between ~100-1000 km/s, with respect to the systemic velocity of NGC 4151, approximately consistent with the outflow velocities of the absorption lines observed in the UV, suggestive of an origin for the UV and soft X-ray emission in the same material. Plasma diagnostics imply a range of electron temperatures of ~1-5x10^4 K and electron densities of between 10^8-10^10 cm^-3. The soft X-ray spectrum of NGC 4151 is extremely similar to that of NGC 1068, suggesting that the soft X-ray excesses observed in many Seyfert galaxies may be composed of similar emission features. Modelling the RGS spectra in terms of emission from photoionized and photoexcited gas in an ionization cone reproduces all of the hydrogen-like and helium-like emission features observed in the soft X-ray spectrum of NGC 4151 in detail and confirms the correspondence between the soft X-ray emission in NGC 4151 and NGC 1068., Comment: 10 pages, 4 figures (2 colour). Accepted for publication in MNRAS

We present results based on an XMM-Newton observation of the prototype narrow-line Seyfert 1 galaxy I Zw 1 performed in June 2002. The 0.6-12 keV spectrum is well described by a steep power-law (Gamma\sim 2.3) and a weak soft blackbody component (kT \sim 200 eV) below 2 kev. The soft X-ray excess emission is featureless and contributes only about 10% to the total X-ray emission in the $0.6-2\kev$ band. There are tentative evidences for an iron iron Kalpha line and an iron K edge, both from high ionized He-like iron. The 0.6-12kev continuum of I Zw 1 can be explained in terms of a blackbody (kT \sim 40eV), thermal comptonization and its reflection off the surface of an ionized accretion disk. We note that characterizing the soft excess copmonent `above a power law continuum' may not be appropriate due to the low energy curvature of a comptonized spectrum. X-ray emission from I Zw 1 was highly variabile during the observation. I Zw 1 showed a large, symmetric X-ray flare during which the X-ray luminosity increased by about 6e43 erg/s in an interval of 2800s only. The X-ray spectral variability was unusual during the flare. In contrast to the general observation of spectral steepening with flux from Seyfert galaxies, there is a clear evidence for hardening of the 0.6-10kev spectrum of I Zw 1 with increasing flux. This hardening is due to a stronger and slightly flatter power-law component at higher flux without any change in the soft X-ray excess component. The large flare, the accompanying spectral flattening, and the steady nature of the soft excess component may be due to an additional flatter power-law component during the flare possibly arising as a result of ejection of electron plasma and the SSC process., Comment: 14 pages, uses emulateapj5, submitted to ApJ

Recent XMM-Newton observations have measured the hard (2.5-12 keV) X-ray spectrum of the well-known Seyfert galaxy NGC 4151 with a signal-to-noise unprecedented for this source. We find that a spectral model, developed to fit previous Beppo-SAX and ASCA observations of NGC 4151, provides an excellent description of the XMM-Newton EPIC data. The results support the view that it is the level of the continuum that is the main driver of the complex spectral variability exhibited by NGC 4151. We focus on the iron K features in the NGC 4151 spectrum. There is no requirement for a relativistically broadened iron Ka line, in contrast to several earlier studies. The iron Ka line profile is well modelled by a narrow Gaussian, the intensity of which varies by ~25% on timescales of about a year. There is also a strong suggestion that the cold media present in the active nucleus of NGC 4151 have an iron abundance that is at least twice the solar value., Comment: 7 Pages, 3 figures, Accepted for publication in MNRAS

XMM-Newton observations of the bright Seyfert 1.9 galaxy MCG-5-23-16 have revealed a broad FeKalpha emission line which is nearly symmetric in contrast to the broad and red-shifted asymmetric FeKalpha line sometimes observed from Seyfert 1 galaxies. The FeKalpha line has two distinct components - a narrow unresolved component with equivalent width of 40 eV and a broad component with full width at half maximum of 40000 km/s and equivalent width of 120 eV. An absorption feature at 7.1 keV has also been observed. The energies of the emission and absorption features are consistent with those arising from neutral iron. The broad component is consistent with an FeKalpha emission line expected from a relativistic accretion disk around a Schwarzschild or a Kerr black hole. Alternatively, most of the flux in the broad component could also be modeled as reflection emission which mimics emission line like feature due to the presence of iron K-shell edge at 7.1eV, however, the reflection fraction is much higher than that inferred from the BeppoSax observations. The disk inclination angle of about 47 deg, inferred from the disk-line fits, and the absorption column of about 10^{22} cm^{-2}, inferred from the low-energy spectral curvature due to photoelectric absorption, suggest that our line of sight passes through the outer edge of a putative torus and are consistent with those expected for a Seyfert 1.9 galaxy falling within the unification scheme. The strength of the narrow iron Kalpha emission and the optical depth of the iron K absorption edge suggest their origin in the putative torus with N_H about 10^{24} cm^{-2} in the inner regions and N_H about 10^{22} cm^{-2} in the outer edges., Comment: 14 pages including 7 figures, To appear in ApJ

By combining complementary monitoring observations spanning long, medium and short time scales, we have constructed power spectral densities (PSDs) of six Seyfert~1 galaxies. These PSDs span $\gtrsim$4 orders of magnitude in temporal frequency, sampling variations on time scales ranging from tens of minutes to over a year. In at least four cases, the PSD shows a "break," a significant departure from a power law, typically on time scales of order a few days. This is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass compact systems with breaks on time scales of seconds. NGC 3783 shows tentative evidence for a doubly-broken power law, a feature that until now has only been seen in the (much better-defined) PSDs of low-state XRBs. It is also interesting that (when one previously-observed object is added to make a small sample of seven), an apparently significant correlation is seen between the break time scale $T$ and the putative black hole mass $M_{\rm BH}$, while none is seen between break time scale and luminosity. The data are consistent with the linear relation $ T = M_{\rm BH}/10^{6.5} \Msun$; extrapolation over 6--7 orders of magnitude is in reasonable agreement with XRBs. All of this strengthens the case for a physical similarity between Seyfert~1s and XRBs., Comment: 27 pages, 13 figures. Accepted for publication in ApJ. Typo corrected

We present XMM-Newton observations of AXJ0341.4-4453, a mildly reddened A_V<7 QSO at a redshift of z=0.672. The XMM-Newton spectrum shows a large obscuring column N_H~10^23 cm^{-2} corresponding to A_V~70, in agreement with previous results based on the lower limit of the ASCA hardness ratio. The X-ray spectrum is represented by a 'scattering' model with Gamma~2.0 with the scattered power-law normalization being a few per cent of the hard component. No FeK line is detected with a 90 per cent upper limit on its equivalent width of ~360eV. The large discrepancy between the column density observed in X-rays and that inferred from the Balmer decrement can be explained by dust sublimation near the nucleus. Then, the X-ray and the optical obscuration come from two different regions: the X-ray close to the accretion disk while the optical at much larger >0.25pc scales., Comment: To appear in MNRAS

We report the progress on our analysis of two deep XMM observations in the areas of Hubble Deep Field-North and the Groth-Westphal Strip field. We present the source detection method, Monte-Carlo simulation on source detection, and our preliminary Log N -- Log S relations. Comparing these two fields and other fields in literature, we find up to 30% variations in number counts near our flux limits, which is most likely to be due to cosmic variance. This serves as a pilot study for a more systematic investigation. The nature of the X-ray sources in the Groth-Westphal field is also discussed in terms of hardness ratios, morphology and spectroscopic properties., Comment: Replaced Fig. 3 which had an incorrect data point. Astron. Nachr. issue dedicated to the proceedings of the workshop "X-ray Surveys in the light of New Observatories", Sep. 2002, Santander, Spain

We present a detailed analysis of the hard X-ray (>2.5 keV) EPIC spectra from the first observations of NGC 4151 made by XMM-Newton. We fit the spectra with a model consisting of a power-law continuum modified by line-of-sight absorption (arising in both partially photoionized and neutral gas) plus additional iron-K emission and absorption features. This model provides an excellent overall fit to the EPIC spectra. The iron K-alpha line is well modelled as a narrow Gaussian component. In contrast to several earlier studies based on data from ASCA, a relativistically broadened iron K-alpha emission feature is not required by the XMM-Newton data. The upper limit on the flux contained in any additional broad line is ~8% of that in the narrow line. The measured intrinsic line width (sigma =32+_7 eV) may be ascribed to (i) the doublet nature of the iron K-alpha line and (ii) emission from low ionization states of iron, ranging from neutral up to ~FeXVII. The additional iron absorption edge arises in cool material and implies factor ~2 overabundance of iron in this component., Comment: Submitted for publication in the proceeding of `New Visions of the Universe in the XMM-Newton and Chandra Era' conference, 4 pages

The ~1 Ms Chandra Deep Field North observation is used to study the extended X-ray sources in the region surrounding the Hubble Deep Field North (HDF-N), yielding the most sensitive probe of extended X-ray emission at cosmological distances to date. A total of six such sources are detected, the majority of which align with small numbers of optically bright galaxies. Their angular sizes, band ratios, and X-ray luminosities -- assuming they lie at the same distances as the galaxies coincident with the X-ray emission -- are generally consistent with the properties found for nearby groups of galaxies. One source is notably different and is likely to be a poor-to-moderate X-ray cluster at high redshift (i.e., z > 0.7). We are also able to place strong constraints on the optically detected cluster of galaxies ClG 1236+6215 at z=0.85 and the wide-angle-tail radio galaxy VLA J123725.7+621128 at z~1-2. With rest-frame 0.5--2.0 keV X-ray luminosities of <(3-15)e42 ergs s^{-1}, the environments of both sources are either likely to have a significant deficit of hot intra-cluster gas compared to local clusters of galaxies, or they are X-ray groups. We find the surface density of extended X-ray sources in this observation to be 167 (+97,-67) deg^{-2} at a limiting soft-band flux of approximately 3e-16 ergs s^{-1} cm^{-2}. No evolution in the X-ray luminosity function of clusters is needed to explain this value. (Abridged), Comment: 16 pages, 14 figures (8 color), LaTeX emulateapj5.sty, accepted for publication by the Astronomical Journal. Manuscript with full resolution embedded images available at http://www.astro.psu.edu/users/niel/hdf/hdf-chandra.html

An extremely deep X-ray survey (about 1 Ms) of the Hubble Deep Field North and its environs (about 450 arcmin^2) has been performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory. This is one of the two deepest X-ray surveys ever performed; for point sources near the aim point it reaches 0.5-2.0 keV and 2-8 keV flux limits of 3 x 10^{-17} erg/cm^2/s and 2 x 10^{-16} erg/cm^2/s, respectively. Here we provide source catalogs along with details of the observations, data reduction, and technical analysis. Observing conditions, such as background, were excellent for almost all of the exposure. We have detected 370 distinct point sources: 360 in the 0.5-8.0 keV band, 325 in the 0.5-2.0 keV band, 265 in the 2-8 keV band, and 145 in the 4-8 keV band. Two new Chandra sources in the HDF-N itself are reported and discussed. Source positions are accurate to within 0.6-1.7 arcsec (at 90% confidence) depending mainly on the off-axis angle. We also detect two highly significant extended X-ray sources and several other likely extended X-ray sources. We present basic number count results for sources located near the center of the field. Source densities of 7100^{+1100}_{-940} deg^{-2} (at 4.2 x 10^{-17} erg/cm^2/s) and 4200^{+670}_{-580} deg^{-2} (at 3.8 x 10^{-16} erg/cm^2/s) are observed in the soft and hard bands, respectively., Comment: 41 pages, AJ, in press; tables 3 and 6, images, and version with full-resolution figures at http://www.astro.psu.edu/users/niel/hdf/hdf-chandra.html

We provide constraints on the nature of the optically faint (I>=24) X-ray source population from a 1 Ms Chandra exposure of a 8.4x8.4 arcmin region containing the Hubble Deep Field North. We detect 47 (2,400 deg^-2) optically faint X-ray sources. The fraction of optically faint X-ray sources is approximately constant (at approx 35%) for 0.5-8.0 keV fluxes from 3x10^-14 cgs down to the X-ray flux limit. A considerable fraction (approx 30%) of the sources are Very Red Objects (I-K>=4). Analysis of the optical and X-ray properties suggests a large number of optically faint X-ray sources are likely to host obscured AGN activity at z= 1-3. From these results we calculate that a significant fraction (approx 5-45%) could be obscured QSOs at z<=3. Given the number of X-ray sources without I-band counterparts, there are unlikely to be more than approx 15 sources at z>6. We provide evidence that the true number of z>6 sources is considerably lower. Nine optically faint X-ray sources have micro-Jy radio counterparts; approx 53% of the optically faint micro-Jy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detected, optically faint micro-Jy radio sources is obscured AGN activity. However, two of these sources have been previously detected at sub-millimeter wavelengths and the X-ray emission from these sources could be due to luminous star formation activity. Assuming the SED of NGC 6240, we estimate the 175 micron flux of a typical optically faint X-ray source to be <10 mJy. As expected for sources with AGN activity, the two optically faint X-ray sources within the most sensitive area of the ISOCAM region have faint (approx 50 micro-Jy) 15 micron counterparts. (ABRIDGED)., Comment: AJ accepted, 22 pages, includes emulateapj5.sty. This version includes a black and white Figure 1. Figure 1 is available in color from http://www.astro.psu.edu./users/davo/alexander.fig1-color.ps.gz The color figure caption is commented out in the tex file. This version is a replacement of an earlier version and includes correction of typos and re-calculation of the VRO fraction

We present the first results of the XMM-Newton observation on the Groth-Westphal Strip Field. The scheduled exposure time was 70 [ks]. We first analyzed about 50 [ks] of calibrated and cleaned EPIC-PN data, which were the only ones we could successfully process with an early version of the SAS software applicable to these observations. A total of about 110 sources have been detected in the central 10 arcmin of the PN field of view (FOV). A cross-correlation with the Hubble Space Telescope (HST) Medium Deep Survey (MDS) database shows a range of morphological properties of host galaxies of the X-ray source. Spectra of hard X-ray sources in our data indeed show absorption in the range N_H~10^{22}-10^{23} [cm^{-2}] at z~1., Comment: 5 page, 4 postscript figures, to appear in the proceedings of the XXIst Moriond Astrophysics Meeting "Galaxy Clusters and the High Redshift Universe Observed in X-rays" held in Les Arcs, France, March 2001

Using 100ks of XMM-Newton data on the Lockman Hole, we show how the X-ray background 'spectral paradox' is being resolved. We find the summed spectra of the Type I AGN, Type II AGN, and the unidentified objects. We conclude that the hard slope of the XRB is caused predominantly by latter sources, i.e. the Type II AGN and unidentified objects., Comment: 2 pages, 1 figure, to appear in the proceedings of the conference "New Century of X-ray Astronomy" held in Yokohama, Japan, March 2001

We present an analysis of the \rosat and \asca spectra of 21 broad line AGN (QSOs) with $z\sim 1$ detected in the 2-10 keV band with the \asca \gis. The summed spectrum in the \asca band is well described by a power-law with $\Gamma=1.56\pm0.18$, flatter that the average spectral index of bright QSOs and consistent with the spectrum of the X-ray background in this band. The flat spectrum in the \asca band could be explained by only a moderate absorption ($\sim 10^{22} \rm cm^{-2}$) assuming the typical AGN spectrum ie a power-law with $\Gamma$=1.9. This could in principle suggest that some of the highly obscured AGN, required by most X-ray background synthesis models, may be associated with normal blue QSOs rather than narrow-line AGN. However, the combined 0.5-8 keV \asca-\rosat spectrum is well fit by a power-law of $\Gamma=1.7\pm0.2$ with a spectral upturn at soft energies. It has been pointed out that such an upturn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless if real, it could imply that the above absorption model suggested by the \asca data alone is ruled out. Then a large fraction of QSOs could have ``concave'' spectra ie they gradually steepen towards softer energies. This result is in agreement with the \bepposax hardness ratio analysis of $\sim$ 100 hard X-ray selected sources., Comment: 7 pages, 4 figures, to be published in MNRAS

We present results from a 479.7 ks Chandra exposure of the Hubble Deep Field North (HDF-N) and its immediate vicinity. In our X-ray image, the deepest ever reported with a 0.5-2.0 keV flux limit of about 4.9 x 10^{-17} erg/cm^2/s, we detect four new HDF-N X-ray sources bringing the total number of such sources to 12. The new sources include two optically bright (R=18.3-18.8), low-redshift (z<0.15) galaxies, a Fanaroff-Riley I radio galaxy, and an edge-on spiral hosting either a powerful starburst or a low-luminosity active galactic nucleus (AGN). Notably, we have now detected X-ray emission from all luminous galaxies (M_V<-18) with z<0.15 known in the HDF-N. We have also detected the remarkable microJy radio source VLA J123642.09+621331.4, which is located just outside the HDF-N and has a likely redshift of z=4.424. We have detected X-ray variability from two of the previously known HDF-N X-ray sources, and spectral fitting shows clear evidence for X-ray absorption in the brightest X-ray source in the HDF-N, a z=0.960 broad-line AGN with associated Mg II absorption. Stacking analyses of optically bright HDF-N galaxies not individually detected in X-rays have provided estimates of their average X-ray fluxes, and we find that the X-ray luminosities of `normal' spirals at z~0.5 are not more than a factor of \~2 larger (per unit B-band luminosity) than those of spirals in the local Universe (z<0.01). This constrains models for the evolution of low-mass X-ray binary populations in galaxies in response to the declining cosmic star-formation rate. Monte-Carlo simulations support the validity of the stacking analyses and show that the Chandra Advanced CCD Imaging Spectrometer (ACIS) performs source detection well even with effective exposure times of ~8 Ms. (Abridged), Comment: 40 pages, AJ, in press, improved and expanded to address referee Comments, also available at http://www.astro.psu.edu/users/niel/hdf/hdf-chandra.html

We report on the first deep X-ray survey with the XMM-Newton observatory during the performance verification phase. The field of the Lockman Hole, one of the best studied sky areas over a very wide range of wavelengths, has been observed. A total of ~100 ksec good exposure time has been accumulated. Combining the images of the European Photon Imaging Camera (EPIC) detectors we reach a flux limit of 0.31, 1.4 and 2.4 X 10^{-15} erg cm^{-2} s^{-1}, respectively in the 0.5-2, 2-10, and 5-10 keV band. Within an off-axis angle of 10 arcmin we detect 148, 112 and 61 sources, respectively. The log(N)-log(S) relation in the three bands is compared with previous results. In particular in the 5-10 keV band these observations present the deepest X-ray survey ever, about a factor 20 more sensitive than the previous BeppoSAX observations. Using X-ray spectral diagnostics and the set of previously known, spectroscopically identified ROSAT sources in the field, the new sources can be classified. XMM-Newton detects a significant number (~40%) of X-ray sources with hard, probably intrinsically absorbed X-ray spectra, confirming a prediction of the population synthesis models for the X-ray background., Comment: 6 pages, based on the new A&A style file (included), to be published in A&A 365 (2001, special XMM issue), one page contains color images

Using a sample of 165 X-ray selected QSOs from seven deep ROSAT fields we investigate the X-ray spectral properties of an ``average''radio-quiet broad line QSO as a function of redshift. The QSO stacked spectra, in the observers 0.1-2 keV band, in five redshift bins over the range 0.11.0) the single power-law model gives an acceptable fit to the data over the full energy band, at lower redshifts the spectra need a second component at low energies, a `soft excess'. Inclusion of a simple model for the soft excess, i.e. a black-body component (kT~100 eV), results in a significant improvement to the model fit, and yields power-law slopes of 1.8-1.9, for all redshift bins. This power-law is not inconsistent, within the error bars, with those of nearby AGN in the 2-10 keV band, suggesting that the same intrinsic power-law slope may continue from 10 keV down to below 0.5 keV. We caution that there is a possibility that the spectral upturn observed may not represent a real physical component but could be due to co-adding spectra with a large dispersion in spectral indices. Regardless of the origin of the soft excess, the average QSO spectrum has important consequences for the origin of the X-ray background: the average spectra of the typical, faint, high redshift QSO are significantly steeper than the spectrum of the X-ray background extending the spectral paradox into the soft 0.1-2 keV X-ray band., Comment: 7 pages, to appear in the MNRAS

We present an analysis of X-ray variability in a flux limited sample of QSOs. Selected from our deep ROSAT survey, these QSOs span a wide range in redshift ($0.10.5$) do not show the anti-correlation with luminosity seen in local AGN. We speculate on the implications of these results for physical models of AGN and their evolution. Finally, we find evidence for X-ray variability in an object classified as a narrow emission-line galaxy, suggesting the presence of an AGN., Comment: Accepted for publication in MNRAS. 11 pages, 12 encapsulated figures. Final proof version, with corrections to Equation A5, Figure 10 and references. Also available at http://www.roe.ac.uk/~omar/astro.html

A total of 10 good candidates for gravitational lensing have been discovered in the WFPC2 images from the HST Medium Deep Survey (MDS) and archival primary observations. These candidate lenses are unique HST discoveries, i.e. they are faint systems with sub-arcsecond separations between the lensing objects and the lensed source images. Most of them are difficult objects for ground-based spectroscopic confirmation or for measurement of the lens and source redshifts. Seven are ``strong lens'' candidates which appear to have multiple images of the source. Three are cases where the single image of the source galaxy has been significantly distorted into an arc. The first two quadruply lensed candidates were reported in Ratnatunga et al 1995 (ApJL, 453, L5) We report on the subsequent eight candidates and describe them with simple models based on the assumption of singular isothermal potentials. Residuals from the simple models for some of the candidates indicate that a more complex model for the potential will probably be required to explain the full structural detail of the observations once they are confirmed to be lenses. We also discuss the effective survey area which was searched for these candidate lens objects., Comment: 26 pages including 12 figures and 10 tables. AJ Vol. 117, No. 5

We have constructed a morphologically divided redshift distribution of faint field galaxies using a statistically unbiased sample of 196 galaxies brighter than I = 21.5 for which detailed morphological information (from the Hubble Space Telescope) as well as ground-based spectroscopic redshifts are available. Galaxies are classified into 3 rough morphological types according to their visual appearance (E/S0s, Spirals, Sdm/dE/Irr/Pec's), and redshift distributions are constructed for each type. The most striking feature is the abundance of low to moderate redshift Sdm/dE/Irr/Pec's at I < 19.5. This confirms that the faint end slope of the luminosity function (LF) is steep (alpha < -1.4) for these objects. We also find that Sdm/dE/Irr/Pec's are fairly abundant at moderate redshifts, and this can be explained by strong luminosity evolution. However, the normalization factor (or the number density) of the LF of Sdm/dE/Irr/Pec's is not much higher than that of the local LF of Sdm/dE/Irr/Pec's. Furthermore, as we go to fainter magnitudes, the abundance of moderate to high redshift Irr/Pec's increases considerably. This cannot be explained by strong luminosity evolution of the dwarf galaxy populations alone: these Irr/Pec's are probably the progenitors of present day ellipticals and spiral galaxies which are undergoing rapid star formation or merging with their neighbors. On the other hand, the redshift distributions of E/S0s and spirals are fairly consistent those expected from passive luminosity evolution, and are only in slight disagreement with the non-evolving model., Comment: 11 pages, 4 figures (published in ApJ)

We report ASCA GIS and SIS observations of the nearby (D = 11.6 Mpc), nearly edge-on, starburst galaxy NGC 2146. These X-ray spectral data complement ROSAT PSPC and HRI imaging discussed by Armus et al., 1995. The broad band (0.6-9 keV) X-ray spectrum of NGC 2146 is best described by a two component model: the soft X-ray emission with a Raymond-Smith thermal plasma model having a temperature of kT $\sim 0.8$ keV; the hard X-ray emission with a thermal plasma model having kT $\sim 8$ keV or a power-law model having a photon index of $\sim 1.7$. We do not find compelling evidence of substantial excess absorption above the Galactic value. The soft (hard) thermal component provides about 30% (70%) of the total luminosity in the 0.5 - 2.0 keV energy band, while in the 2-10 keV energy range only the hard component plays a major role. The spectral results allow us to set tighter constraints on the starburst-driven superwind model, which we show can satisfactorily account for the luminosity, mass, and energy content represented by the soft X-ray spectral component. We estimate that the mass outflow rate ($\sim$ 9 M$_{\odot}$ per year) is about an order of magnitude greater than the predicted rate at which supernovae and stellar winds return mass into the interstellar medium and, therefore, argue that the flow is strongly "mass-loaded" with material in and around the starburst. The estimated outflow velocity of the hot gas is close to the escape velocity from the galaxy, so the fate of the gas is not clear. We suggest that the hard X-ray spectral component is due to the combined emission of X-ray binaries and/or young supernovae remnants associated with the starburst., Comment: 26 pages plus 4 figures, LaTex manuscript, Accepted for publication in the Astrophysical Journal

We use UKIRT and ASCA observations to determine the nature of a high redshift (z=2.35) narrow-line AGN, previously discovered by Almaini et al. (1995). The UKIRT observations show a broad Halpha line while no Hbeta line is detected. This together with the red colour (B-K=5.4) suggest that our object is a moderately obscured QSO (Av>3), in optical wavelengths. The ASCA data suggest a hard spectrum, probably due to a large obscuring column, with a photon index of 1.93 (+0.62, -0.46), NH~10^23. The combined ASCA and ROSAT data again suggest a heavily obscured spectrum (NH~10^23 or Av~100). In this picture, the ROSAT soft X-ray emission may arise from electron scattering, in a similar fashion to local Seyfert 1.9. Then, there is a large discrepancy between the moderate reddening witnessed by the IR and the large X-ray absorbing column. This could be possibly explained on the basis of e.g. high gas metallicities or assuming that the X-ray absorbing column is inside the dust sublimation radius. An alternative explanation can be obtained when we allow for variability between the ROSAT and ASCA observations. Then the best fit spectrum is still flat, 1.35 (+0.16, -0.14), but with low intrinsic absorption in better agreement with the IR data, while the ROSAT normalization is a factor of two below the ASCA normalization. This object may be one of the bright examples of a type-II QSO population at high redshift, previously undetected in optical surveys. The hard X-ray spectrum of this object suggests that such a population could make a substantial contribution to the X-ray background., Comment: 7 pages, to appear in MNRAS

We present a new, objectively selected, sample of galaxy overdensities detected in the Hubble Space Telescope Medium Deep Survey. These clusters/groups were found using an automated procedure which involved searching for statistically significant galaxy overdensities. The contrast of the clusters against the field galaxy population is increased when morphological data is used to search around bulge-dominated galaxies. In total, we present 92 overdensities above a probability threshold of 99.5%. We show, via extensive Monte Carlo simulations, that at least 60% of these overdensities are likely to be real clusters and groups and not random line-of-sight superpositions of galaxies. For each overdensity in the MDS cluster sample, we provide a richness and the average of the bulge-to-total ratio of galaxies within each system. This MDS cluster sample potentially contains some of the most distant clusters/groups ever detected, with about 25% of the overdensities having estimated redshifts z > 0.9. We have made this sample publicly available to facilitate spectroscopic confirmation of these clusters and help more detailed studies of cluster and galaxy evolution. We also report the serendipitous discovery of a new cluster close on the sky to the rich optical cluster CLl0016+16 at z=0.546. This new overdensity, HST 001831+16208, may be coincident with both an X-ray source and a radio source. HST 001831+16208 is the third cluster/group discovered near to CL0016+16 and appears to strengthen the claims of Connolly et al. (1996) of superclustering at high redshift., Comment: 25 pages including 7 figures and 9 tables. AJ Vol. 116, No. 6

We report on the discovery of a narrow-emission-line object at z=0.672 detected in a deep ASCA survey. The object, AXJ0341.4-4453, has a flux in the 2--10keV band of 1.1+/-0.27 E-13 erg/s/cm**2, corresponding to a luminosity of 1.8E44 erg/s (q0=0.5, $H0 = 50km/s/Mpc. It is also marginally detected in the ROSAT 0.5-2keV band with a flux 5.8E-15 erg/s/cm**2. Both the ASCA data alone and the combined ROSAT/ASCA data show a very hard X-ray spectrum, consistent with either a flat power law (alpha < 0.1) or photoelectric absorption with a column of nH > 4E22 cm**-2 (alpha = 1). The optical spectrum shows the high-ionisation, narrow emission lines typical of a Seyfert 2 galaxy. We suggest that this object may be typical of the hard sources required to explain the remainder of the X-ray background at hard energies., Comment: MNRAS accepted, 5 pages, 2 figures

We summarise our recent work on the faint galaxy contribution to the cosmic X-ray background (XRB). At bright X-ray fluxes (in the ROSAT pass band), broad line QSOs dominate the X-ray source population, but at fainter fluxes there is evidence for a significant contribution from emission-line galaxies. Here we present statistical evidence that these galaxies can account for a large fraction of the XRB. We also demonstrate that these galaxies have significantly harder X-ray spectra than QSOs. Finally we present preliminary findings from infra-red spectroscopy on the nature of this X-ray emitting galaxy population. We conclude that a hybrid explanation consisting of obscured/Type 2 AGN surrounded by starburst activity can explain the properties of these galaxies and perhaps the origin of the entire XRB., Comment: Proceedings of the X-ray Surveys Workshop. To appear in Astron Nachr., 4 pages, 7 encapsulated figures, uses xsurv_article macro (included). Revised version with correct figure annotation, plus other minor changes. Also available from http://www.ast.cam.ac.uk/~omar/astro.html

We present additional optical spectroscopic identifications of sources identified in three deep ASCA GIS fields which also form part of a deep ROSAT survey. In total, 26 ASCA sources have been detected down to a 2-10keV flux limit of S = 5e-14 erg/cm/s. LDSS observations have increased the spectroscopic completeness of the survey to between 65 and 85 per cent, with identifications for up to 13 QSOs with broad emission lines and 6 objects with narrow emission lines. Combining these objects with the AGN identified in the LMA/HEAO-1 sample by Grossan, we find evidence for significant cosmological evolution in the 2-10keV band (=0.63+/-0.03), fit by a pure luminosity evolution model; L \propto (1+z)^k, k=2.04^{+0.16}_{-0.22}$. The present-epoch 2-10keV AGN X-ray luminosity function, \Phi(L_X), is best represented by a two power law function: \Phi(L_X) \propto L^{-3.0}, L*>10^{44.1}erg/s; $\Phi (L_X) \propto L^{-1.9}$, L*<10^{44.1}erg/s. Depending on the extent to which we extrapolate the z=0 AGN luminosity function, we predict a total contribution to the 2-10keV X-ray background from QSOs and narrow-emission-line galaxies which ranges from 48 per cent (for AGN with L>10^{42}erg/s) to 80 per cent (L>10^{39}erg/s)., Comment: 10 pages, TeX, 5 figures, MNRAS in press (significant revision at proof stage)