arXiv subjects

G. Hasinger

Publications and source records attributed to G. Hasinger.

At least 163 records · Page 9Linked to original sources

arXiv2004

The Chandra Deep Field South: Optical Spectroscopy I

We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field South (CDFS). 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R-K colours are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external identifications). The R<24 observed sample comprises 161 X-ray objects (181 optical counterparts) and 126 of them have unambiguous spectroscopic identification. We introduce a new classification based solely on X-ray properties, HR and X-ray luminosity, consistent with the unified AGN model. This X-ray classification uncovers a large fraction of optically obscured, X-ray luminous AGNs missed by the classical optical classification.

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arXiv2004

Photometric Redshift of X-Ray Sources in the Chandra Deep Field South

Based on the photometry of 10 near-UV, optical, and near-infrared bands of the Chandra Deep Field South, we estimate the photometric redshifts for 342 X-ray sources, which constitute ~99% of all the detected X-ray sources in the field. The models of spectral energy distribution are based on galaxies and a combination of power-law continuum and emission lines. Color information is useful for source classifications: Type-I AGN show non-thermal spectral features that are distinctive from galaxies and Type-II AGN. The hardness ratio in X-ray and the X-ray-to-optical flux ratio are also useful discriminators. Using rudimentary color separation techniques, we are able to further refine our photometric redshift estimations. Among these sources, 137 have reliable spectroscopic redshifts, which we use to verify the accuracy of photometric redshifts and to modify the model inputs. The average relative dispersion in redshift distribution is ~8%, among the most accurate for photometric surveys. The high reliability of our results is attributable to the high quality and broad coverage of data as well as the applications of several independent methods and a careful evaluation of every source. We apply our redshift estimations to study the effect of redshift on broadband colors and to study the redshift distribution of AGN. Our results show that both the hardness ratio and U-K color decline with redshift, which may be the result of a K-correction. The number of Type-II AGN declines significantly at z>2 and that of galaxies declines at z>1. However, the distribution of Type-I AGN exhibits less redshift dependence. As well, we observe a significant peak in the redshift distribution at z=0.6. We demonstrate that our photometric redshift estimation produces a reliable database for the study of X-ray luminosity of galaxies and AGN.

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arXiv2004

A catalogue of the Chandra Deep Field South with multi-colour classification and photometric redshifts from COMBO-17

We present the COMBO-17 object catalogue of the Chandra Deep Field South for public use, covering a field which is 31.5' x 30' in size. This catalogue lists astrometry, photometry in 17 passbands from 350 to 930 nm, and ground-based morphological data for 63,501 objects. The catalogue also contains multi-colour classification into the categories 'Star', 'Galaxy' and 'Quasar' as well as photometric redshifts. We include restframe luminosities in Johnson, SDSS and Bessell passbands and estimated errors. The redshifts are most reliable at R<24, where the sample contains approximately 100 quasars, 1000 stars and 10000 galaxies. We use nearly 1000 spectroscopically identified objects in conjunction with detailed simulations to characterize the performance of COMBO-17. We show that the selection of quasars, more generally type-1 AGN, is nearly complete and minimally contaminated at z=[0.5,5] for luminosities above M_B=-21.7. Their photometric redshifts are accurate to roughly 5000 km/sec. Galaxy redshifts are accurate to 1% in dz/(1+z) at R<21. They degrade in quality for progressively fainter galaxies, reaching accuracies of 2% for galaxies with R~222 and of 10% for galaxies with R>24. The selection of stars is complete to R~23, and deeper for M stars. We also present an updated discussion of our classification technique with maps of survey completeness, and discuss possible failures of the statistical classification in the faint regime at R>24.

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arXiv2004

A Huge Drop in X-ray Luminosity of the Non-Active Galaxy RXJ1242.6-1119A, and First Post-Flare Spectrum - Testing the Tidal Disruption Scenario

It has been suggested that an unavoidable consequence of the existence of supermassive black holes, and the best diagnostic of their presence in non-active galaxies, would be occasional tidal disruption of stars captured by the black holes. These events manifest themselves in form of luminous flares powered by accretion of debris from the disrupted star into the black hole. Candidate events among optically non-active galaxies emerged in the past few years. For the first time, we have looked with high spatial and spectral resolution at one of these most extreme variability events ever recorded among galaxies. Here, we report measuring a factor ~200 drop in luminosity of the X-ray source RXJ 1242-1119 with the X-ray observatories Chandra and XMM-Newton, and perform key tests of the favored outburst scenario, tidal disruption of a star by a supermassive black hole. We show that the detected `low-state' emission has properties such that it must still be related to the flare. The power-law shaped post-flare X-ray spectrum indicates a `hardening' compared to outburst. The inferred black hole mass, the amount of liberated energy, and the duration of the event favor an accretion event of the form expected from the (partial or complete) tidal disruption of a star (abstract abbreviated).

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arXiv2003

The X-ray Background and AGNs

Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is largely due to the accretion onto supermassive black holes, integrated over the cosmic time. These surveys have resolved more than 80% of the 0.1-10 keV X-ray background into discrete sources. Optical spectroscopic identifications show that the sources producing the bulk of the X-ray background are a mixture of unobscured (type-1) and obscured (type-2) AGNs, as predicted by the XRB population synthesis models. A class of highly luminous type-2 AGN, so called QSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. The new Chandra AGN redshift distribution peaks at much lower redshifts (z~0.7) than that based on ROSAT data, and the new X-ray luminosity function indicates that the space density of Seyfert galaxies peaks at significantly lower redshifts than that of QSOs. It is shown here, that the low redshift peak applies both to absorbed and unabsorbed AGN and is also seen in the 0.5-2 keV band alone. Previous findings of a strong dependence of the fraction of type-2 AGN on luminosity are confirmed with better statistics here. Preliminary results from an 800 ksec XMM-Newton observation of the Lockman Hole are discussed.

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arXiv2003

SIMBOL-X : a new generation hard X-ray telescope

SIMBOL-X is a hard X-ray mission, operating in the 0.5-70 keV range, which is proposed by a consortium of European laboratories for a launch around 2010. Relying on two spacecraft in a formation flying configuration, SIMBOL-X uses a 30 m focal length X-ray mirror to achieve an unprecedented angular resolution (30 arcsec HEW) and sensitivity (100 times better than INTEGRAL below 50 keV) in the hard X-ray range. SIMBOL-X will allow to elucidate fundamental questions in high energy astrophysics, such as the physics of accretion onto Black Holes, of acceleration in quasar jets and in supernovae remnants, or the nature of the hard X-ray diffuse emission. The scientific objectives and the baseline concepts of the mission and hardware design are presented.

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arXiv2003

Hubble Space Telescope Imaging in the Chandra Deep Field South: III. Quantitative Morphology of the 1Ms Chandra Counterparts and Comparison with the Field Population

We present quantitative morphological analyses of 37 HST/WFPC2 counterparts of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate: 1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF- convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near- neighbor analyses comparing local environments of X-ray sources versus the field control sample. Significant nuclear point-source optical components appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray sources (putative type-1 AGN) in our fields. We infer roughly half of the HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources. The concentration indices of the CDFS counterparts are clearly larger on average than those of the field distribution, at 5-sigma, suggesting that the strong correlation between central black hole mass and host galaxy properties (including concentration index) observed in nearby galaxies is already evident by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved CDFS sources at I<23 is indistinguishable from the I<23 field. Moreover, the frequency of I<23 near neighbors around the CDFS counterparts is not significantly different from the field sample. These results, combined with previous similar findings for local samples, suggest that recent merger/ interaction history is not a good indicator of AGN activity over a substantial range of look-back time.

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arXiv2003

Chandra Observations of SNR 1987A

We report on the results of our monitoring program of the X-ray remnant of supernova 1987A with the {\it Chandra X-Ray Observatory}. We have performed two new observations during the {\it Chandra} Cycle 3 period, bringing the total to six monitoring observations over the past three years. These six observations provide a detailed time history of the birth of a new supernova remnant in X-rays. The high angular resolution images indicate that soft X-ray bright knots are associated with the optical spots, while hard X-ray features are better correlated with radio images. We interpret this in terms of a model in which fast shocks propagating through the circumstellar HII region produce the hard X-ray and radio emission, while the soft X-ray and optical emission arise in slower shocks entering into dense knots in the circumstellar inner ring. New observations begin to show changes in the morphology that may herald a new stage in the development of this incipient supernova remnant. The observed X-ray fluxes increase by nearly a factor of three over the last 30 months. The X-ray remnant is expanding at a velocity of $\sim$5000 km s$^{-1}$.

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arXiv2003

XMM-Newton observation of the ULIRG NGC 6240: The physical nature of the complex Fe K line emission

We report on an XMM-Newton observation of the ultraluminous infrared galaxy NGC 6240. The 0.3-10 keV spectrum can be successfully modelled with: (i) three collisionally ionized plasma components with temperatures of about 0.7, 1.4, and 5.5 keV; (ii) a highly absorbed direct power-law component; and (iii) a neutral Fe K_alpha and K_beta line. We detect a significant neutral column density gradient which is correlated with the temperature of the three plasma components. Combining the XMM-Newton spectral model with the high spatial resolution Chandra image we find that the temperatures and the column densities increase towards the center. With high significance, the Fe K line complex is resolved into three distinct narrow lines: (i) the neutral Fe K_alpha line at 6.4 keV; (ii) an ionized line at about 6.7 keV; and (iii) a higher ionized line at 7.0 keV (a blend of the Fe XXVI and the Fe K_beta line). While the neutral Fe K line is most probably due to reflection from optically thick material, the Fe XXV and Fe XXVI emission arises from the highest temperature ionized plasma component. We have compared the plasma parameters of the ultraluminous infrared galaxy NGC 6240 with those found in the local starburst galaxy NGC 253. We find a striking similarity in the plasma temperatures and column density gradients, suggesting a similar underlying physical process at work in both galaxies.

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arXiv2003

Tracing the large scale structure in the Chandra Deep Field South

We report the discovery of large scale structures of X-ray sources in the 1Msec observation of the Chandra Deep Field South. Two main structures appear as narrow (Delta_z < 0.02) spikes in the source redshift distribution at z=0.67 and z=0.73, respectively. Their angular distribution spans a region at least ~ 17 arcmin wide, corresponding to a physical size of 7.3 h_{70}^{-1} Mpc at a redshift of z ~ 0.7 (Omega_m=0.3, Omega_{Lambda}=0.7). These spikes are populated by 19 sources each, which are mainly identified as Active Galactic Nuclei (AGN). Two sources in each spike are extended in X-rays, corresponding to galaxy groups/clusters embedded in larger structures. The X-ray source redshift distribution shows other spikes, the most remarkable at z=1.04, 1.62 and 2.57. This is one of the first evidences for large scale structure traced by X-ray sources and for spatial clustering of X-ray selected AGN. The X-ray data have been complemented with the spectroscopic data from the K20 near infrared survey (Cimatti et al. 2002), which covers ~1/10 of the X-ray field. Also in this survey the source redshift distribution shows several spikes. Two narrow structures at z=0.67 and z=0.73 (again with Delta_z ~ 0.02) are the most significant features, containing 24 and 47 galaxies, respectively. While the K20 structure at z=0.73 is dominated by a standard galaxy cluster with a significant concentration around a central cD galaxy and morphological segregation, the galaxies at z=0.67 constitute a loose structure rather uniformly distributed along the K20 field.[abridged]

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arXiv2003

A broad absorption feature in the X-ray spectrum of the isolated neutron star RBS1223 (1RXS J130848.6+212708)

X-ray spectra of the isolated neutron star RBS1223 obtained with the instruments on board XMM-Newton in December 2001 and January 2003 show deviations from a Planckian energy distribution at energies below 500 eV. The spectra are well fit when a broad, Gaussian-shaped absorption line with sigma = 100 eV and centered at an energy of 300 eV is added to an absorbed blackbody model. The resulting equivalent width of the line is -150 eV. However, the spectral resolution at these low energies of the EPIC detectors and the lower statistical quality and restricted energy band of the RGS instruments are not sufficient to exclude even broader lines at energies down to 100 eV or several unresolved lines. The most likely interpretation of the absorption feature is a cyclotron absorption line produced by protons in the magnetic field of the neutron star. In this picture line energies of 100-300 eV yield a magnetic field strength of 2-6x10^13 G for a neutron star with canonical mass and radius. Folding light curves from different energy bands at a period of 10.31 s, which implies a double peaked pulse profile, shows different hardness ratios for the two peaks. This confirms that the true spin period of RBS1223 is twice as long as originally thought and suggests variations in cyclotron absorption with pulse phase. We also propose that changes in photo-electric absorption seen in phase resolved spectra of RX J0720.4-3125 by Cropper et al. (2001), when formally fit with an absorbed blackbody model, are caused instead by cyclotron absorption varying with pulse phase.

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arXiv2003

The nature of X-ray selected EROs

We report on the X-ray, optical, near-infrared, submillimetre and radio properties of five Extremely Red Objects (EROs) selected at X-ray wavelengths by XMM-Newton in the Lockman Hole field. They all have enough counts in the X-ray band to allow spectral fitting: four are most probably obscured, Compton-thin AGN with redshift dependent absorbing column densities of 10^22 - 10^24 cm-2, whilst the fifth is best fitted by a thermal spectrum and is likely to be a massive elliptical galaxy in a deep gravitational potential. Their optical/near-infrared colours and sizes suggest that X-ray selected EROs comprise a mixture of dusty 'starburst' galaxies and non-dusty galaxies that are dominated by either star-light or light from an active nucleus. The colour diagnostics are supported by the submillimetre and radio data: the two AGN with 'starburst' colours have submillimetre or radio flux densities that imply large star-formation rates, whilst those with 'elliptical' colours do not. The one source detected in the submillimetre waveband has narrow emission lines at a redshift of 1.45. Although the bulk of its radio emission originates from processes other than star formation, it is most probably a radio-quiet ultraluminous infrared galaxy.

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arXiv2002

A deep VLA survey at 6 cm in the Lockman Hole

We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is about 11 microJy. From these data we have extracted a catalogue of 63 radio sources. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to about 21 per cent of the radio sample. Most of these associations (8 out of 13) are classified as Type II AGN. Using optical CCD (V and I) and K' band data we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of about 92 per cent, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift (z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. We also find evidence that at these faint radio limits a large fraction (about 60 per cent) of the faintest optical counterparts (i.e. sources in the magnitude range 22.5 4.

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arXiv2002

Missing baryons and the soft X-ray background

The X-ray background intensity around Lick count galaxies and rich clusters of galaxies is investigated in three ROSAT energy bands. It is found that the X-ray enhancements surrounding concentrations of galaxies exhibit significantly softer spectrum than the standard cluster emission and the average extragalactic background. The diffuse soft emission accompanying the galaxies is consistent with the thermal emission of the hot gas postulated first by the Cen & Ostriker hydrodynamic simulations. Our estimates of the gas temperature - although subject to large uncertainties - averaged over several Mpc scales are below 1 keV, which is substantially below the temperature of the intra-cluster gas, but consistent with temperatures predicted for the local intergalactic medium. It is pointed out that the planned ROSITA mission would be essential for our understanding of the diffuse thermal component of the background.

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arXiv2002

XMM-Newton observation of the Lockman Hole II. Spectral analysis

We present the results of the X-ray spectral analysis of the first deep X-ray survey with the XMM-Newton observatory during Performance Verification. We restrict the analysis to the sample of 98 sources with more than 70 net counts (flux limit in the [0.5-7] keV band of 1.6 10^{-15} erg cm^{-2} s^{-1}) of which 61 have redshift identification. We find no correlation between the spectral index Gamma and the intrinsic absorption column density N_H and, for both the Type-1 and Type-2 AGN populations, we obtain ~2. The progressive hardening of the mean X-ray source spectrum with decreasing flux is essentially due to an increase in intrinsic absorption. The marked separation between the two AGN populations in several diagnostics diagrams, involving X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is also a consequence of different absorption column densities and enables the classification of optically faint obscured AGN. About 27% of the subsample with R-K' colour are EROs (R-K>5) and most of these 18 X-ray selected EROs contain anobscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios. There are six sources in our sample with L_X[0.5-10]>10^44 erg s^{-1} and log(N_H)>10^22 cm^{-2}: which are likely Type-2 QSOs and we thus derive a density of ~69 objects of this class per square degree.

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arXiv2002

Discovery of an ionized Fe-K edge in the z=3.91 Broad Absorption Line Quasar APM 08279+5255 with XMM-Newton

Recent XMM-Newton observations of the high-redshift, lensed, broad absorption line (BAL) quasi-stellar object APM 08279+5255, one of the most luminous objects in the universe, allowed the detection of a high column density absorber ($N_H \approx 10^{23}$ cm$^{-2}$) in the form of a K-shell absorption edge of significantly ionized iron (Fe XV - XVIII) and corresponding ionized lower-energy absorption. Our findings confirm a basic prediction of phenomenological geometry models for the BAL outflow and can constrain the size of the absorbing region. The Fe/O abundance of the absorbing material is significantly higher than solar (Fe/O = 2-5), giving interesting constraints on the gas enrichment history in the early Universe.

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arXiv2002

XMM-Newton observations of the Lockman Hole : Spectral analysis

We present the results of the X-ray spectral analysis of the deep survey obtained with the XMM-Newton observatory on the Lockman Hole. The X-ray data and the cumulative source counts were reported by Hasinger et al. (2001). Our sample contains 104 sources with a count limit of 70 of which 55 have redshift identification. The redshift distribution peaks at z ~ 0.8, with a strong excess of low z AGN and a deficiency of sources at z > 2 compared to population synthesis models for the X-ray background. The type 2 (obscured) AGN have weaker soft X-ray and optical fluxes. They cluster around z ~ 1. There is a clear separation between the classical/type 1 AGN and the obscured/type 2 ones in several diagnostics involving X-ray colour, X-ray flux, optical/near IR colour and optical brightness. Using the z subsample, 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, Gamma ~ 1.9. At the 70 count detection limit, there is also a strong overlap between the two populations in hard X-ray flux and near IR brightness. These diagnostics should enable the classification of obscured/type 2 AGN very faint optically.

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arXiv2002

The XMM-Newton Bright Serendipitous Source Sample (XMM-Newton BSS)

The XMM-Newton Survey Science Center is constructing a large (~1000 sources) and complete sample of bright serendipitous XMM-Newton sources at high galactic latitude, so as to allow both the discovery of sources of high individual interest as well as statistical population studies. The sample, known as ``The XMM-Newton Bright Serendipitous Source Sample" (XMM-Newton BSS), has a flux limit of ~10^(-13) cgs in the 0.5--4.5 keV energy band and will be fundamental in complementing other medium and deep survey programs (having fluxes 10 to 100 times fainter) in order to: a) characterize the X-ray sky and its constituents, b) understand the evolution of the population selected and c) define statistical identification procedures to select rare and interesting populations of X-ray sources (e.g. high z clusters of galaxies, BL Lacs, type 2 QSOs, etc..). We discuss here the scientific rationale and the details of the project and we present some preliminary results.

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