arXiv subjects

H. Brunner

Publications and source records attributed to H. Brunner.

51 records · Page 3Linked to original sources

arXiv2006

The XMM-Newton wide-field survey in the COSMOS field: I. Survey description

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.

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arXiv2004

The unresolved hard X-ray background: the missing source population implied by the Chandra and XMM-Newton deep fields

We extend our earlier work on X-ray source stacking in the deep XMM-Newton observation of the Lockman Hole, to the 2 Ms Chandra Deep Field North and the 1 Ms Chandra Deep Field South. The XMM-Newton work showed the resolved fraction of the X-ray background to be ~80-100 per cent at <2 keV but this decreased to only ~50 per cent above ~8 keV. The CDF-N and CDF-S probe deeper, and are able to fill-in some of the missing fraction in the 4-6 keV range, but the resolved fraction in the 6-8 keV band remains only ~60 per cent, confirming the trend seen with XMM-Newton. The missing X-ray background component has a spectral shape that is consistent with a population of highly obscured AGN at redshifts \~0.5-1.5 and with absorption column densities of ~10^23 - 10^24 cm^-2.

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arXiv2004

XMM-Newton observation of the Chandra Deep Field-South: Statistical treatment of faint source spectra

We present first results of the X-ray spectral analysis of the 500 ksec deep survey obtained with XMM-Newton on the Chandra Deep Field South (CDFS). Statistical distributions of spectral index and intrinsic absorption are derived for a sample containing 70 sources with a count limit of 100 (flux limit in the [2-10] keV band of 8.9 $\times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$), of which 44 have redshift identification. We observe a separation between the type-1 and the type-2 AGN in diagnostics involving different X-ray parameters. Using the subsample with known $z$, we show that this separation between the AGN populations is a consequence of different absorption column densities. The two populations have the same average spectral index, $<Γ> = 2 \pm 0.1$. We present integrated spectrum for the most distant type-2 QSO whith strong X-ray absorption and a clear soft excess; we obtained the best fit for these objects with two difference models: a scattering model and a double power law model. We also confirm a progressive hardening for the combined integrated spectra for faint objects which at first was noted by \cite{toz01a}. Our results shown a clear evolution of decrease of $<Γ>$ with decreasing flux in the hard 2-10 keV band. However, we detect not only a regular increase of $<Γ>$ for different subsamples of fluxes in comparison with Chandra results, but also an internal discrepancy of the values, if we fitted in the different energy bands.

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arXiv2004

XMM-Newton spectroscopy of an X-ray selected sample of RL AGNs

This paper presents the X-ray spectroscopy of an X-ray selected sample of 25 radio-loud (RL) AGNs extracted from the XBSS sample. The main goal is to study the origin of the X-ray spectral differences usually observed between radio-loud and radio-quiet (RQ) AGNs. To this end, a comparison sample of 53 RQ AGNs has been also extracted from the same XBSS sample and studied together with the sample of RL AGNs. We have focused the analysis on the distribution of the X-ray spectral indices of the power-law component that models the large majority of the spectra in both samples. We find that the mean X-ray energy spectral index is very similar in the 2 samples and close to alpha_X~1. However, the intrinsic distribution of the spectral indices is significantly broader in the sample of RL AGNs. In order to investigate the origin of this difference, we have divided the RL AGNs into blazars and ``non-blazars'', on the basis of the available optical and radio information. We find strong evidence that the broad distribution observed in the RL AGN sample is mainly due to the presence of the blazars. Furthermore, within the blazar class we have found a link between the X-ray spectral index and the value of the radio-to-X-ray spectral index suggesting that the observed X-ray emission is directly connected to the emission of the relativistic jet. This trend is not observed among the ``non-blazars'' RL AGNs. This favours the hypothesis that, in these latter sources, the X-ray emission is not significantly influenced by the jet emission and it has probably an origin similar to the RQ AGNs. Overall, the results presented here indicate that the observed distribution of the X-ray spectral indices in a given sample of RL AGNs is strongly dependent on the amount of relativistic beaming present in the selected sources.

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arXiv2004

Exploring the X-ray Sky with the XMM-Newton Bright Serendipitous Survey

ABRIDGED. We present "The XMM-Newton Bright Serendipitous Survey", two flux-limited samples with flux limit fx ~7E-14 cgs in the 0.5-4.5 keV (BSS) and 4.5-7.5 keV (HBSS) energy band, respectively. After discussing the survey strategy, we present the basic data on a complete sample of 400 X-ray sources derived from the analysis of 237 XMM-Newton fields. The survey covers an area of 28.10 (25.17 for the HBSS) sq. deg. Up to now ~71% (~90%) of the sources have been spectroscopically identified making the BSS (HBSS) the sample with the highest number of identified XMM-Newton sources published so far. We find that: a) the optical counterpart in the majority (~90%) of cases has a magnitude brighter than the POSS II limit; b) the majority of the objects identified so far are broad line AGN. No obvious trend of the source spectra (as deduced from the Hardness Ratios analysis) as a function of the count rate is seen and the average spectra of the "extragalactic" population corresponds to an X-ray energy spectral index of ~0.8 (~0.64) for the BSS (HBSS) sample. About 13% (40%) of the sources are described by an energy spectral index flatter than that of the CXB. We measure a surface density of optically type 1 and type 2 AGN of 1.63+/-0.25 deg-2 and 0.83+/-0.18$ deg-2, respectively; optically type 2 AGN represent 34+/-9% of the total AGN population. Finally, we find a clear separation, in the hardness ratio diagram and in the (hardness ratio) vs. (X-ray to optical flux ratio) diagram, between "coronal emitting" stars and extragalactic sources.

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arXiv2004

The (un)resolved X-ray background in the Lockman Hole

Most of the soft and a growing fraction of the harder X-ray background has been resolved into emission from point sources, yet the resolved fraction above 7 keV has only been poorly constrained. We use ~700 ks of XMM-Newton observations of the Lockman Hole and a photometric approach to estimate the total flux attributable to resolved sources in a number of different energy bands. We find the resolved fraction of the X-ray background to be ~90 per cent below 2 keV but it decreases rapidly at higher energies with the resolved fraction above ~7 keV being only ~50 per cent. The integrated X-ray spectrum from detected sources has a slope of Gamma~1.75, much softer than the Gamma=1.4 of the total background spectrum. The unresolved background component has the spectral signature of highly obscured AGN.

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arXiv2002

The XMM-Newton Serendipitous Source Catalogue

We describe the production, properties and scientific potential of the XMM-Newton catalogue of serendipitous X-ray sources. The first version of this catalogue is nearing completion and is planned to be released before the end of 2002.

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arXiv2002

The census of cataclysmic variables in the ROSAT Bright Survey

We give an identification summary and results of polarimetric, photometric and spectroscopic follow-up observations of new, X-ray bright cataclysmic variables. These were identified as optical counterparts of high galactic latitude sources in the ROSAT All-Sky Survey. This optical identification programme is termed the ROSAT Bright Survey (RBS) and represents the first complete soft X-ray selected, flux-limited sample of CVs at high galactic latitude (survey area ~20400 sq.deg.). The systems described here escaped previous identification programmes since these surveys were designed to identify even brighter than ours or particularly soft X-ray sources. Among the 11 new RBS-CVs we find 6 magnetic systems of AM Herculis type, 4 dwarf novae (among them one candidate), and one particularly bright system of uncertain nature, tentatively identified as dwarf nova or symbiotic binary. Orbital periods could be determined for all magnetic systems which range from 87.1 min to 187.7 min. Three of the new dwarf novae have moderate to high inclination and two of them might be eclipsing. Using non-magnetic systems only we derive a space density of CVs of ~3 10^{-5} pc^{-3}. This limit rests on the two new nearby, low-luminosity systems RBS0490 and RBS1955, with estimated distances of 30pc only and luminosities below 10^{30} erg s^{-1}.

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arXiv2001

The XMM-Newton Serendipitous Survey. II. First results from the AXIS high galactic latitude medium sensitivity survey

We present the first results on the identifications of a medium sensitivity survey (X-ray flux limit 2E-14 erg/cm2/s in the 0.5-4.5 keV band) at high galactic latitude (|b|>20 deg) carried out with the XMM-Newton X-ray observatory within the AXIS observing programme. This study is being conducted as part of the XMM-Newton Survey Science Centre activities towards the identification of the sources in the X-ray serendipitous sky survey. The sample contains 29 X-ray sources in a solid angle of 0.26deg2 (source density 113 +/- 21 sources/deg2), out of which 27 (93%) have been identified. The majority of the sources are broad-line AGN (19), followed by narrow emission line X-ray emitting galaxies (6, all of which turn out to be AGN), 1 nearby non-emission line galaxy (NGC 4291) and 1 active coronal star. Among the identified sources we find 2 broad-absorption line QSOs (z~1.8 and z~1.9), which constitute ~10% of the AGN population at this flux level, similar to optically selected samples. Identifications of a further 10 X-ray sources fainter than our survey limit are also presented.

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arXiv2000

XMM-Newton observation of the Lockman Hole; I. The X-ray Data

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.

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arXiv2000

The variable XMM-Newton spectrum of Markarian 766

The narrow-line Seyfert 1 galaxy Markarian 766 was observed for 60 ks with the XMM-Newton observatory. The source shows a complex X-ray spectrum. The 2-10 keV spectrum can be adequately represented by a power law and broad Fe Ka emission. Between 0.7 and 2 keV the spectrum is harder and exhibits a flux deficit with respect to the extrapolated medium energy slope. Below 0.7 keV, however, there is a strong excess of emission. The RGS spectrum shows an edge-like feature at 0.7 keV; the energy of this feature is inconsistent with that expected for an OVII edge from a warm absorber. Markarian 766 varies by a factor of ~ 2 in overall count rate in the EPIC and RGS instruments on a timescale of a few thousand seconds, while no significant flux changes are observed in the ultraviolet with the OM. The X-ray variability is spectrally dependent with the largest amplitude variability occurring in the 0.4-2 keV band. The spectral variability can be explained by a change in flux and slope of the medium energy continuum emission, superimposed on a less variable (or constant) low energy emission component.

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arXiv2000

The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1

We present a summary of an identification program of the more than 2000 X-ray sources detected during the ROSAT All-Sky Survey (Voges et al. 1999) at high galactic latitude, |b| > 30 degr, with countrate above 0.2 s-1. This program, termed the ROSAT Bright Survey RBS, is to more than 99.5% complete. A sub-sample of 931 sources with countrate above 0.2 s-1 in the hard spectral band between 0.5 and 2.0 keV is to 100% identified. The total survey area comprises 20391 sq.deg at a flux limit of 2.4 \times 10^{-12} erg/cmsq/s in the 0.5 -- 2.0 keV band. About 1500 sources of the complete sample could be identified by correlating the RBS with SIMBAD and the NED. The remaining 500 sources were identified by low-resolution optical spectroscopy and CCD imaging utilizing telescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apart from completely untouched sources, catalogued clusters and galaxies without published redshift as well as catalogued galaxies with unusual high X-ray luminosity were included in the spectroscopic identification program. Details of the observations with an on-line presentation of the finding charts and the optical spectra will be published separately. Here we summarize our identifications in a table which contains optical and X-ray information for each source. As a result we present the most massive complete sample of X-ray selected AGNs with a total of 669 members and a well populated X-ray selected sample of 302 clusters of galaxies with redshifts up to 0.70. Three fields studied by us remain without optical counterpart (RBS0378, RBS1223, RBS1556). While the first is a possible X-ray transient, the two latter are isolated neutron star candidates (Motch et al. 1999, Schwope et al. 1999).

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arXiv1998

The ROSAT BRIGHT survey: I. Identification of an AGN sample with hard ROSAT X-ray spectra

The ROSAT Bright Survey (RBS) aims to completely optically identify the more than 2000 brightest sources detected in the ROSAT all-sky survey at galactic latitudes |b| > 30 degr (excluding LMC, SMC, Virgo cluster). This paper presents a subsample of 66 bright point-like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is > 0.5 for most of the sources. The subsample could be nearly completely identified by low-resolution optical spectroscopy with the following breakdown into object classes: 31 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies 1 cataclysmic variable, and 5 bright stars. Only one object remained unidentified and one X-ray source was a spurious detect ion. The redshift distribution peaks around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. Observations with medium spectral resolution were obtained for most of the new S eyfert galaxies. A large fraction (20 objects) are type 1 Seyfert galaxies, the other fraction includes Seyfert galaxies of type 1.5 - 1.8 (5 objects), two LINERs, and 4 possible narrow-line Seyfert 1 galaxies (NLS1). About one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. The large fraction of interacting galaxies among our sample suggests a scenario where interaction is the main trigger of AGN activity.

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arXiv1997

UV to X-ray Spectra of Radio-Quiet Quasars. Comparison With Accretion Disk Models

We present UV to soft X-ray spectra of 31 radio-quiet quasars, comprising data from the IUE ULDA database and the ROSAT pointed observation phase. 90 % of the sample members show a soft X-ray excess above an underlying hard X-ray power law spectrum. Particularly for the steep X-ray spectrum (alpha_energy > 1.5), low redshift subsample (17 objects) the X-ray spectral power law index is strongly correlated with the optical to X-ray broad-band spectral index alpha_ox, indicating that the main contribution to the soft X-ray and optical emission is due to the same emission component. We model the UV/soft X-ray spectra in terms of thermal emission from a geometrically thin alpha-accretion disk. The structure and radiation field of the disk is calculated self-consistently and Compton scattering is treated by the Kompaneets equation. All relativistic effects on the disk structure and the emergent disk spectrum are included. Satisfactory spectral fits of the UV and soft X-ray continuum spectra are achieved when additional non-thermal hard X-ray and IR power law emission components are taken into account. The UV and soft X-ray spectra are well described by emission resulting from accretion rates in the range ~0.1 to ~0.3 times the Eddington accretion rate. Low mass/low redshift objects are found to accret at < 0.15 Mdot_Eddington. Correlations of the accretion disk parameters with alpha_ox are discussed.

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arXiv1996

ROSAT Observations of BL Lacertae Objects

We present soft X-ray spectra of 74 BL Lacertae objects observed with the PSPC detector on board of the ROSAT satellite. The sample contains all BL Lac objects detected during the pointed observation phase as a target or serendipitously. We have investigated the soft X-ray and broad band spectral properties and discuss the consequences for the X-ray emission processes. For the first time a clear dependence of the X-ray spectral steepness on the radio to X-ray spectral energy distribution is found: {alpha}_{rx} and {alpha}_{x} are correlated in the X-ray selected (XBL) subsample and anticorrelated in the radio selected (RBL) subsample. The objects with intermediate {alpha}_{rx} values thus do have the steepest soft X-ray spectra. Simulated PSPC spectra based on a set of simple two component multifrequency spectra are in good agreement with the measurements and suggest a broad range of synchrotron cutoff energies. We have calculated synchrotron self-Compton beaming factors for a subsample of radio bright objects and find a correlation of the beaming factors {delta}_{IC} with {alpha}_{rx} and {alpha}_{x}. The most extreme RBL objects are very similar to flat spectrum radio quasars in all their broad band and X-ray properties.

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