arXiv subjects

J. P. D. Mittaz

Publications and source records attributed to J. P. D. Mittaz.

At least 19 recordsLinked to original sources

arXiv2007

The XMM-Newton Serendipitous Survey V. Optical identification of the XMM-Newton Medium sensitivity Survey (XMS)

We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. We report on the optical identification of the XMS samples, complete to 85-95%. At the intermediate flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (~35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

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arXiv2005

Are the WMAP angular magnification measurements consistent with an inhomogeneous critical density Universe?

The propagation of light through a Universe of (a) isothermal mass spheres amidst (b) a homogeneous matter component, is considered. We demonstrate by an analytical proof that as long as a small light bundle passes {\it through} sufficient number of (a) at various impact parameters - a criterion of great importance - its average convergence will exactly compensate the divergence within (b). The net effect on the light is statistically the same as if all the matter in (a) is `fully homogenized'. When applying the above ideas towards understanding the angular size of the primary acoustic peaks of the microwave background, however, caution is needed. The reason is that most (by mass) of (a) are in galaxies - their full mass profiles are not sampled by passing light - at least the inner 20 kpc regions of these systems are missed by the majority of rays, while the rest of the rays would map back to unresolvable but magnified, randomly located spots to compensate for the loss in angular size. Therefore, a scanning pair of WMAP beams finds most frequently that the largest temperature difference occurs when each beam is placed at diametrically opposite points of the Dyer-Roeder collapsed sections. This is the {\it mode} magnification, which corresponds to the acoustic {\it peaks}, and is less than the mean (or the homogeneous pre-clumping angular size). Since space was seen to be Euclidean without taking the said adjustment into account, the true density of the Universe should be supercritical. Our analysis gives $Ω_m =$ 0.278 $\pm$ 0.040 and $Ω_Λ =$ 0.782 $\pm$ 0.040.

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arXiv2005

Thermal and non-thermal nature of the soft excess emission from Sersic 159-03 observed with XMM-Newton

Several nearby clusters exhibit an excess of soft X-ray radiation which cannot be attributed to the hot virialized intra-cluster medium. There is no consensus to date on the origin of the excess emission: it could be either of thermal origin, or due to an inverse Compton scattering of the cosmic microwave background. Using high resolution XMM-Newton data of Sersic 159-03 we first show that strong soft excess emission is detected out to a radial distance of 0.9 Mpc. The data are interpreted using the two viable models available, i.e., by invoking a warm reservoir of thermal gas, or relativistic electrons which are part of a cosmic ray population. The thermal interpretation of the excess emission, slightly favored by the goodness-of-fit analysis, indicates that the warm gas responsible for the emission is high in mass and low in metallicity.

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arXiv2005

Submillimetre photometry of X-ray absorbed QSOs: their formation and evolutionary status

We present an analysis of the submillimetre/X-ray properties of 19 X-ray absorbed, Compton-thin quasi-stellar objects (QSOs) selected to have luminosities and redshifts which represent the peak of cosmic QSO activity. i.e. ~L* objects at 1 1.5, the high star formation rates are consistent with a scenario in which the QSOs evolve to become local luminous elliptical galaxies. Combining these results with previously published data for X-ray unabsorbed QSOs and submillimetre-selected galaxies we propose the following evolutionary sequence: the forming galaxy is initially far-infrared luminous but X-ray weak similar to the sources discovered by the Submillimetre Common-User Bolometer Array; as the black hole and spheroid grow with time a point is reached when the central QSO becomes powerful enough to terminate the star formation and eject the bulk of the fuel supply (the Compton-thin absorbed QSO phase); this transition is followed by a period of unobscured QSO activity which subsequently declines to leave a quiescent spheroidal galaxy.

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arXiv2004

Warm gas in the outskirts of galaxy clusters -- The cluster soft excess phenomenon

The cluster soft excess emission indicates the presence of large amounts of warm gas (T ~ 10^6 K) in the neighborhood of galaxy clusters. Among the clusters that display this phenomenon is the Coma cluster, the nearest rich galaxy cluster. The excess emission is more prominent at the cluster's outskirts than at its center. Detailed studies of its large-scale emission -- up to ~2.6 Mpc from the cluster's center -- reveal that these warm baryons are as massive as, or possibly more massive than, the well-known hot intra-cluster medium (T ~10^8 K). A possible interpretation of the excess emission from the Coma cluster is radiation from low-density filaments located in the neighborhood of the cluster. In this case, the filaments would extend for much larger distances, or feature higher density, than predicted by current cosmological simulations.

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arXiv2004

XMM-Newton spectra of hard spectrum Rosat AGN: X-ray absorption and optical reddening

We present the XMM-Newton spectra of three low-redshift intermediate Seyferts (one Sy1.5, and two Sy1.8), from our survey of hard spectrum Rosat sources. The three AGN are well fitted by absorbed powerlaws, with intrinsic nuclear photoelectric absorption from column densities between 1.3 and 4.0e21 cm-2. In the brightest object the X-ray spectrum is good enough to show that the absorber is not significantly ionized. For all three objects the powerlaw slopes appear to be somewhat flatter (Gamma~1.3-1.6) than those found in typical unabsorbed Seyferts. The constraints from optical and X-ray emission lines imply that all three objects are Compton-thin. For the two fainter objects, the reddening deduced from the optical broad emission lines in one of them, and the optical continuum in the other, are similar to those expected from the X-ray absorption, if we assume a Galactic gas-to-dust ratio and reddening curve. The broad line region Balmer decrement of our brightest object is larger than expected from its X-ray absorption, which can be explained either by an intrinsic Balmer decrement with standard gas-to-dust ratio, or by a >Galactic gas-to-dust ratio. These >=Galactic ratios of extinction to photoelectric absorption cannot extend to the high redshift, high luminosity, broad line AGN in our sample, because they have column densities >1e22 cm-2, and so their broad line regions would be totally obscured. This means that some effect (e.g., luminosity dependence, or evolution) needs to be present in order to explain the whole population of absorbed AGN.

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arXiv2002

Submillimeter Evidence for the Coeval Growth of Massive Black Holes and Galaxy Bulges

The correlation, found in nearby galaxies, between black hole mass and stellar bulge mass implies that the formation of these two components must be related. Here we report submillimeter photometry of eight x--ray absorbed active galactic nuclei which have luminosities and redshifts characteristic of the sources that produce the bulk of the accretion luminosity in the universe. The four sources with the highest redshifts are detected at 850 microns, with flux densities between 5.9 and 10.1 milliJanskies, and hence are ultraluminous infrared galaxies. Interpreting the submillimeter flux as emission from dust heated by starbursts, these results suggest that the majority of stars in spheroids were formed at the same time as their central black holes built up most of their mass by accretion, accounting for the observed demography of massive black holes in the local universe. The skewed rate of submillimeter detection with redshift is consistent with a high redshift epoch of star formation in radio quiet active galactic nuclei, similar to that seen in radio galaxies.

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arXiv2001

Sersic 159-03: discovery of the brightest soft X-ray excess emitting cluster of galaxies

The soft X-ray excess emission in the southern cluster Sersic 159-03 represents hiterto the strongest effect of its kind. Emission in the ~0.2-0.4 keV passband is detected far in excess of the expected contribution from the hot phase of the intra-cluster medium, and extends to the X-ray signal limit of the cluster. Our analysis of ROSAT PSPC observations reveal that the soft excess can be interpreted as either a thermal or non-thermal effect, and the high data quality allows to place tight constraints on the two currently competing models. However, each model now implies major revisions to our understanding of clusters of galaxies: either `warm' gas masses in similar amounts to the hot gas, or relativistic particles in or above equipartition with the hot phase, appear to be unavoidable.

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arXiv2001

A survey of hard spectrum ROSAT sources 2: optical identification of hard sources

We have surveyed 188 ROSAT PSPC fields for X-ray sources with hard spectra (alpha<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 AGN which show broad lines in their optical spectra (BLAGN), 13 narrow emission line galaxies (NELGs), 5 galaxies with no visible emission lines, 8 clusters and 8 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 1) identified hard sources have broad lines.

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arXiv2000

XMM-Newton observations of the cluster of galaxies Sersic 159-03

The cluster of galaxies Sersic 159-03 was observed with the XMM-Newton X-ray observatory as part of the Guaranteed Time program. X-ray spectra taken with the EPIC and RGS instruments show no evidence for the strong cooling flow derived from previous X-ray observations. There is a significant lack of cool gas below 1.5 keV as compared to standard isobaric cooling flow models. While the oxygen is distributed more or less uniformly over the cluster, iron shows a strong concentration in the center of the cluster, slightly offset from the brightness center but within the central cD galaxy. This points to enhanced type Ia supernova activity in the center of the cluster. There is also an elongated iron-rich structure extending to the east of the cluster, showing the inhomogeneity of the iron distribution. Finally, the temperature drops rapidly beyond 4 arcmin from the cluster center.

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arXiv2000

The NGC4839 group falling into the Coma cluster observed by XMM-Newton

We present here the first analysis of the XMM Newton EPIC-MOS data of the galaxy group around NGC4839, which lies at a projected distance to the Coma cluster center of 1.6Mpc. In our analysis, which includes imaging, spectro-imaging and spectroscopy we find compelling evidence for the sub group being on its first infall onto the Coma cluster. The complex temperature structure around NGC 4839 is consistent with simulations of galaxies falling into a cluster environment. We see indications of a bow shock and of ram pressure stripping around NGC4839. Furthermore our data reveal a displacement between NGC4839 and the center of the hot gas in the group of about 300kpc. With a simple approximation we can explain this displacement by the pressure force originating from the infall, which acts much stronger on the group gas than on the galaxies.

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arXiv2000

X-ray Spectroscopy of the Cluster of Galaxies Abell 1795 with XMM-Newton

The initial results from XMM-Newton observations of the rich cluster of galaxies Abell 1795 are presented. The spatially-resolved X-ray spectra taken by the European Photon Imaging Cameras (EPIC) show a temperature drop at a radius of $\sim 200$ kpc from the cluster center, indicating that the ICM is cooling. Both the EPIC and the Reflection Grating Spectrometers (RGS) spectra extracted from the cluster center can be described by an isothermal model with a temperature of $\sim 4$ keV. The volume emission measure of any cool component ($<1$ keV) is less than a few % of the hot component at the cluster center. A strong OVIII Lyman-alpha line was detected with the RGS from the cluster core. The O abundance and its ratio to Fe at the cluster center is 0.2--0.5 and 0.5--1.5 times the solar value, respectively.

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arXiv2000

A survey of hard spectrum ROSAT sources 1: X-ray source catalogue

We present a catalogue of 147 serendipitous X-ray sources selected to have hard spectra (alpha < 0.5) from a survey of 188 ROSAT fields. Such sources must be the dominant contributors to the X-ray background at faint fluxes. We have used Monte Carlo simulations to verify that our technique is very efficient at selecting hard sources: the survey has > 10 times as much effective area to hard sources as it has to soft sources above a 0.5 - 2 keV flux level of 10^-14 erg/cm^2/s. The distribution of best fit spectral slopes of the hard sources suggests that a typical ROSAT hard source in our survey has a spectral slope alpha ~0. The hard sources have a steep number flux relation (dN/dS propto S^-gamma with a best fit value of gamma = 2.72 +- 0.12) and make up about 15% of all 0.5 - 2 keV sources with S > 10^-14 erg/cm^2/s. If their N(S) continues to fainter fluxes, the hard sources will comprise ~ 40% of sources with 5 10^-15 < S < 10^-14. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S < 10^-14. They probably make a strong contribution to the X-ray background at faint fluxes and could be the solution to the X-ray background spectral paradox.

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arXiv1999

Re-observing the EUV emission from Abell 2199: in-situ measurement of background distribution by offset pointing

The EUV excess emission from the clusters A2199 and A1795 remains an unexplained astrophysical phenomenon. There has been many unsuccessful attempts to `trivialize' the findings. In this Letter we present direct evidence to prove that the most recent of such attempts, which attributes the detected signals to a background non-uniformity effect, is likewise excluded. We address the issue by a re-observation of A2199 which features a new filter orientation, usage of a more sensitive part of the detector and, crucially, includes a background pointing at $\sim$ 2$^o$ offset - the first {\it in situ} measurement of its kind. We demonstrate quantitatively two facts: (a) the offset pointing provides an accurate background template for the cluster observation, while (b) data from other blank fields do not. We then performed point-to-point subtraction of the {\it in situ} background from the cluster field, with appropriate propagation of errors. The resulting cluster radial profile is consistent with that obtained by our original method of subtracting a flat asymptotic background. The emission now extends to a radius of 20 arcmin; it confirms the rising prominence of EUV excess beyond $\sim$ 5 arcmin as previously reported.

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arXiv1999

X-ray Spectra of the RIXOS source sample

We present results of an extensive study of the X-ray spectral properties of sources detected in the RIXOS survey, that is nearly complete down to a flux limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing sources with low count rate spectral slopes estimated using simple hardness ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray data using statistical techniques appropriate to the Poisson regime which removes the effects of this bias. We have then applied this technique to the RIXOS survey to study the spectral properties of the sample. For the AGN we find an average energy index of 1.05+-0.05 with no evidence for spectral evolution with redshift. Individual AGN are shown to have a range of properties including soft X-ray excesses and intrinsic absorption. Narrow Emission Line Galaxies also seem to fit to a power-law spectrum, which may indicate a non-thermal origin for their X-ray emission. We infer that most of the clusters in the sample have a bremsstrahlung temperature >3 keV, although some show evidence for a cooling flow. The stars deviate strongly from a power-law model but fit to a thermal model. Finally, we have analysed the whole RIXOS sample containing 1762 sources. We find that the mean spectral slope of the sources hardens at lower fluxes in agreement with results from other samples. However, a study of the individual sources demonstrates that the hardening of the mean is caused by the appearance of a population of very hard sources at the lowest fluxes. This has implications for the nature of the soft X-ray background.

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arXiv1999

Soft X-ray spectral variations of the narrow line Seyfert 1 galaxy Markarian 766

The X-ray variability of the narrow line Seyfert 1 galaxy Markarian 766 is studied using 9 ROSAT PSPC datasets. The spectrum is well described by a power law combined with a blackbody (kT~ 70 eV) soft excess. Examination of flux ratio changes and variability amplitude in 3 X-ray bands, shows that the power law component varies continuously on time-scales of ~5000s and is steeper when it is brighter. In contrast, variability of the soft excess is not detected. Spectral modeling of 31 spectra from different observations and at a range of count rates is also consistent with a picture in which the power law is steeper when it is brighter, and in which the soft excess component does not vary. The power law variability can be explained if the power law is produced by variable thermal or non-thermal Comptonization of soft photons. This behaviour is similar to that of Galactic black hole candidates in the low state. The X-ray and multiwavelength properties of Markarian 766 are shown to be very similar to those of other narrow line Seyfert 1s. This may mean that the rapid X-ray variability seen in other narrow line Seyfert 1s may also not originate in their strong soft excess components.

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arXiv1998

Clustering of X-ray selected Active Galactic Nuclei

A total of 235 Active Galactic Nuclei (AGN) from two different soft X-ray surveys (the ROSAT Deep Survey -DRS- and the ROSAT International X-ray Optical Survey -RIXOS-) with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2 sigma significant detection of clustering of such objects is found on scales <40-80/h Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5<~ r_0 <~ 3.3/h Mpc is implied for comoving evolution, while 1.9 <~ r_0 <~ 4.8 for stable clustering and 2.2 <~ r_0 <~ 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent UV and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be <~1.7 on scales <~ 10/h Mpc, a significantly smaller value than is inferred from local large-scale dynamical studies.

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arXiv1997

Optical and X-ray properties of the RIXOS AGN: II - Emission lines

We present the optical and UV emission line properties of 160 X-ray selected AGN taken from the RIXOS survey (including Halpha, Hbeta, [OIII]5007, MgII2798 and CIII]1909). This sample is believed to contain a mixture of absorbed and unabsorbed objects, with column densities up to 4e21 cm-2. Although the distribution of the [OIII] EW for the RIXOS AGN is typical of optically selected samples, the Balmer line EWs are relatively low. This is consistent with the presence of a dust absorber between the broad and narrow line regions (eg. a molecular torus), and intrinsically weak optical line emission. We find Baldwin effects in CIII] and MgII, and a positive response of the MgII line to its ionizing continuum. There is a strong correlation between the EW and FWHM of MgII, which may be similar to that seen in other samples for Hbeta. We demonstrate that this is consistent with models which suggest two line-emitting zones, a `very broad line region' (VBLR) and an `intermediate line region' (ILR). The correlation between EW and FWHM in MgII may be a physical characteristic of the ILR or it may reflect a geometric dependence. We found no correlation between the Hbeta FWHM and the slope of the X-ray spectrum, however this may be due to the effects of dust absorption which suppresses the broad Hbeta component, masking any relationship. The Halpha FWHM does tend to be narrow when alpha_X is soft, and broadens as alpha_X hardens, although the formal probability for this correlation is low (91 per cent). If the distribution of alpha_X in the RIXOS sample reflects the level of intrinsic absorption in these AGN, the data suggest a possible link between the velocity of the Balmer line-emitting region and the amount of absorbing material beyond.

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