arXiv subjects

R. Walter

Publications and source records attributed to R. Walter.

At least 109 records · Page 6Linked to original sources

arXiv2009

Suzaku observation of IGR J16318-4848

We report on the first Suzaku observation of IGR J16318-4848, the most extreme example of a new group of highly absorbed X-ray binaries that have recently been discovered by the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The Suzaku observation was carried out between 2006 August 14 and 17, with a net exposure time of 97 ks. The average X-ray spectrum of the source can be well described with a continuum model typical for neutron stars i.e., a strongly absorbed power law continuum with a photon index of 0.676(42) and an exponential cutoff at 20.5(6) keV. The absorbing column is 1.95(3)x10e24 cm-2. Consistent with earlier work, strong fluorescent emission lines of Fe Kalpha, Fe Kbeta, and Ni Kalpha are observed. Despite the large absorbing column, no Compton shoulder is seen in the lines, arguing for a non-spherical and inhomogeneous absorber. Seen at an average 5-60 keV absorbed flux of 3.4x10e-10 erg cm-2 s-1, the source exhibits significant variability on timescales of hours.

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arXiv2009

The 4th IBIS/ISGRI soft gamma-ray survey catalog

In this paper we report on the fourth soft gamma-ray source catalog obtained with the IBIS gamma-ray imager on board the INTEGRAL satellite. The scientific dataset is based on more than 70Ms of high quality observations performed during the first five and a half years of Core Program and public observations. Compared to previous IBIS surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 700 high-energy sources detected in the energy range 17--100 keV, including both transients and faint persistent objects which can only be revealed with longer exposure times. A comparison is provided with the latest Swift/BAT survey results.

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arXiv2009

Extragalactic Hard X-ray Surveys: From INTEGRAL to Simbol-X

We present some results of the deepest extragalactic survey performed by the INTEGRAL satellite. The fraction of very absorbed AGN is quite large. The sharp decrease in the absorption fraction with X-ray luminosity observed at lower-energy X-rays is not observed. The current lack of truly Compton-thick objects, with an upper limit of 14% to the size of this population, is just compatible with recent modeling of the cosmic X-ray background. We also study the prospects for a future hard X-ray serendipitous survey with Simbol-X. We show that Simbol-X will easily detect a large number of serendipitous AGN, allowing us to study the evolution of AGN up to redshifts about 2, opening the door to the cosmological study of hard X-ray selected AGN, which is barely possible with existing satellites like Swift and INTEGRAL.

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arXiv2009

Pulsed thermal emission from the accreting pulsar XMMU J054134.7-682550

Aims. Soft X-ray excesses have been detected in several Be/X-ray binaries and interpreted as the signature of hard X-ray reprocessing in the inner accretion disk. The system XMMU J054134.7-682550, located in the LMC, featured a giant Type II outburst in August 2007. The geometry of this system can be understood by studying the response of the soft excess emission to the hard X-ray pulses. Methods. We have analyzed series of simultaneous observations obtained with XMM-Newton/EPIC-MOS and RXTE/PCA in order to derive spectral and temporal characteristics of the system, before, during and after the giant outburst. Spectral fits were performed and a timing analysis has been carried out. Spectral variability, spin period evolution and energy dependent pulse shapes are analysed. Results. The outburst (L_X = 3* 10^38 erg/s \approx L_EDD) spectrum could be modeled successfully using a cutoff powerlaw, a cold disk emission, a hot blackbody, and a cyclotron absorption line. The magnetic field and magnetospheric radius could be constrained. The thickness of the inner accretion disk is broadened to a width of 75 km. The hot blackbody component features sinusoidal modulations indicating that the bulk of the hard X-ray emission is emitted preferentially along the magnetic equator. The spin period of the pulsar decreased very significantly during the outburst. This is consistent with a variety of neutron star equations of state and indicates a very high accretion rate.

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arXiv2009

The hard X-ray view of bright infrared galaxies

Aims. The synthesis of the cosmic X-ray background (CXB) requires a large population of Compton-thick active galactic nuclei that have not been detected so far. We probe whether bright infrared galaxies could harbor a population of Compton-thick nuclei and if they could contribute significantly. Methods. We analyzed 112 Msec of INTEGRAL observations obtained on 613 galaxies from the IRAS Revised Bright Galaxy Sample. We derived the average hard X-ray (18-80 keV) emission of Seyfert and various non Seyfert galaxy subsamples to estimate their relative contribution to the locally emitted CXB. Results. The Seyfert 1 & 2 are detected at hard X-rays. None of the other galaxy subsamples were detected. ULIRGs are at least 5 times under-luminous at hard X-rays when compared to Seyferts. The upper limit obtained for the average non Seyfert galaxies is as low as 7E-13 erg/s cm2. On average, these galaxies do not contain active nuclei brighter than 10E41 erg/s at hard X-rays. The total hard X-ray flux detected from the sample is 4.9E-9 erg/s cm2 (about 1% of the CXB), and 64% of this originates in absorbed active nuclei. Local non-Seyfert galaxies contribute for less than 7% and do not harbor the Compton-thick nuclei assumed to synthesize the locally emitted CXB.

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arXiv2008

Broad-Band Spectrum of The Black Hole Candidate IGR J17497-2821 Studied with Suzaku

The broad-band 1-300 keV Suzaku spectrum of IGR J17497-2821, the X-ray transient discovered by INTEGRAL in September 2006, is presented. Suzaku observed IGR J17497-2821 on September 25, eight days after its discovery, for a net exposure of about 53 ksec. During the Suzaku observation, IGR J17497-2821 is very bright, 2 x 10^37 erg/s at 8 kpc in the 1-300 keV range, and shows a hard spectrum, typical of black hole candidates in the low-hard state. Despite the multi-mission X-ray monitoring of the source, only with Suzaku is it possible to obtain a broad-band spectrum in the 1-300 keV range with a very high signal to noise ratio. A sum of a multi-color disc (DISKBB) and a thermal Comptonization component (COMPPS) with mild reflection is a good representation of our IGR J17497-2821 Suzaku spectrum. The spectral properties of the accretion disc as well as the cut-off energy in the spectrum at about 150 keV are clearly detected and constrained. We discuss the implications on the physical model used to interpret the data and the comparison with previous results.

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arXiv2008

INTEGRAL and XMM-Newton observations of AX J1845.0-0433

AX J1845.0-0433 is a transient high-mass X-ray binary discovered by ASCA. The source displays bright and short flares observed recently with INTEGRAL. The transient behaviour and the bright and short flares are studied in order to understand the accretion mechanisms and the nature of the source. Public INTEGRAL data and a pointed XMM-Newton observation are used to study in details the flaring and quiescent phases. AX J1845.0-0433 is a persistent X-ray binary with a O9.5I supergiant companion emitting at a low 0.2-100 keV luminosity of ~1e35 erg/s with seldom flares reaching luminosities of 1e36 erg/s. The most-accurate X-ray position is R.A. (2000) =18h45m01s.4 and Dec. = -04deg33'57''.7 (uncertainty 2''). Variability factors of 50 are observed on time scale as short as hundreds of seconds. The broad-band high-energy spectrum is typical of wind-fed accreting pulsars with an intrinsic absorption of NH=(2.6+/-0.2)e22 cm-2, a hard continuum of Gamma=(0.7-0.9)+/-0.1 and a high-energy cutoff at Ecut=16+/-3 keV. An excess at low energies is also observed fitted with a black body with a temperature of kT=0.18+/-0.05 keV. Optically-thin and highly-ionised iron (Fe XVIII-XIX) located near the supergiant star is detected during the quiescence phase. The spectral shape of the X-ray continuum is constant. The flare characteristics in contrast to the persistent quiescent emission suggest that clumps of mass M~1e22 g are formed within the stellar wind of the supergiant companion.

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arXiv2008

AGN population in the deepest hard X-ray extragalactic survey

We present the results of the analysis of the AGN population in the deepest extragalactic hard X-ray survey. The survey is based on INTEGRAL observation of the 3C 273/Coma cluster region, and covers 2500 deg2 with a 20-60 keV flux limit 1.5 times lower than other surveys at similar energies, resolving about 2.5% of the cosmic hard X-ray background. Using this survey, we can constrain in an unbiased way the distribution of hydrogen column absorption up to Nh=10^25 cm-2. We put an upper limit of 24% to the fraction of Compton-thick objects. Compared to models of the AGN population selected in the 2-10 keV band, the Log N-Log S diagram is generally in good agreement, but the Nh distribution is significantly different, with significantly less unabsorbed sources (Nh<10^22 cm-2) at a given flux limit compared to the models. We also study the local hard X-ray luminosity function (LF), which is compatible with what is found in other recent hard X-ray surveys. The extrapolation of the 2-10 keV LF is lower than the hard X-ray LF. The discrepancy is resolved if AGN spectra typically present reflection humps with reflection fraction R~1. Finally, we use the population properties of this survey to show that a future ultra-deep INTEGRAL extragalactic survey can result in a quite large AGN sample with enough objects at redshifts larger than z=0.05 so that we can detect evolution in the hard X-ray LF.

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arXiv2008

The multiwavelength variability of 3C 273

We present an update of 3C 273's database hosted by the ISDC, completed with data from radio to gamma-ray observations over the last 10 years. We use this large data set to study the multiwavelength properties of this quasar,especially focussing on its variability behaviour. We study the amplitude of the variations and the maximum variability time scales across the broad-band spectrum and correlate the light curves in different bands, specifically with the X-rays, to search for possible connections between the emission at different energies. 3C 273 shows variability at all frequencies, with amplitudes and time scales strongly depending on the energy and being the signatures of the different emission mechanisms. The variability properties of the X-ray band imply the presence of either two separate components (possibly a Seyfert-like and a blazar-like) or at least two parameters with distinct timing properties to account for the X-ray emission below and above ~20 keV. The dominant hard X-ray emission is most probably not due to electrons accelerated by the shock waves in the jet as their variability does not correlate with the flaring millimeter emission, but seems to be associated to long-timescale variations in the optical. This optical component is consistent with being optically thin synchrotron radiation from the base of the jet and the hard X-rays would be produced through inverse Compton processes (SSC and/or EC) by the same electron population. We show evidence that this synchrotron component extends from the optical to the near-infrared domain, where it is blended by emission of heated dust that we find to be located within about 1 light-year from the ultraviolet source.

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arXiv2008

A deep INTEGRAL hard X-ray survey of the 3C 273/Coma region

(abridged) We present an analysis of the deepest hard X-ray survey to date of about 2500 deg^2 performed by the IBIS instrument on board INTEGRAL in the 20-60 keV band, with a total exposure time of 4 Ms. We find 34 candidate sources. The ratio of Seyfert 1 to Seyfert 2 is significantly more than the ratio found in the optical, which may be evidence for a receding torus, but could also be due to absorption columns > 10^{25} cm^{-2}. None of the predicted Compton-thick objects is detected unambiguously; when taking lower limits on Nh into account, the fraction of these objects is found to be lower than 24%. We do not see, but cannot exclude, a relationship between absorption and luminosity similar to what is seen in the 2-10 keV band. Our data suggests the possibility of a lack of objects with 10^{21} < Nh < 10^{22} cm^{-2}, as expected if absorption originates either in a torus-like structure or in the host galaxy. We find that the Log N-Log S diagram of our sources is compatible with those obtained in other surveys in hard X-rays. Compared to models of the AGN population selected in the 2-10 keV band, the Log N-Log S diagram is generally in good agreement, but the Nh distribution is significantly different, with significantly less unabsorbed sources (Nh<10^{22} cm^{-2}) at a given flux limit compared to the models. We resolve here about 2.5% of the cosmic X-ray background. We also study the local hard X-ray luminosity function, which is compatible with what is found in other recent hard X-ray surveys. The characteristic luminosity Log L*_{20-60 keV}=43.66 is found to be a factor about 5 lower than the value observed in the 2-10 keV band. We find a space density of 10^{-3} AGN with L>10^{41} per Mpc^3 and a corresponding luminosity density of 0.9 10^{39} erg s^{-1} Mpc^{-3}.

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arXiv2008

Probing Clumpy Stellar Winds in SFXTs

Quantitative constraints on the wind clumping of massive stars can be obtained from the study of the hard X-ray variability of SFXTs. In these systems, a large fraction of the hard X-ray emission is emitted in the form of flares with typical duration of 3 ksec, frequency of 7 days and luminosity of $10^{36}$ ergs/s. Such flares are most probably emitted by the interaction of a compact object orbiting at $\sim10$ R$_*$ with wind clumps ($10^{22-23}$ g). The density ratio between the clumps and the inter-clump medium is $10^{2-4}$ . The parameters of the clumps and of the inter-clump medium are in good agreement with macro-clumping scenario and line-driven instability simulations.

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arXiv2008

The broad-band XMM-Newton and INTEGRAL spectra of bright type 1 Seyfert galaxies

The 0.5-150 keV broad-band spectra of a sample of nine bright type 1 Seyfert galaxies are analyzed here. These sources have been discovered/detected by INTEGRAL and subsequently observed with XMM--Newton for the first time with high sensitivity below 10 keV. The sample, although small, is representative of the population of type 1 AGN which are now being observed above 20 keV. The intrinsic continuum has been modeled using three different parameterizations: a power-law model, an exponential cut-off power-law and an exponential cut-off power-law with a Compton reflection component. In each model the presence of intrinsic absorption, a soft component and emission line reprocessing features has also been tested. A simple power-law model is a statistically good description of most of the spectra presented here; an FeK line, fully and/or partial covering absorption and a soft spectral component are detected in the majority of the sample sources. The average photon index (< Gamma > = 1.7 $\pm$ 0.2) is consistent, within errors, with the canonical spectral slope often observed in AGN although the photon index distribution peaks in our case at flat Gamma (~ 1.5) values. For four sources, we find a significantly improved fit when the power-law is exponentially cut-off at an energy which is constrained to be below ~ 150 keV. The Compton reflection parameter could be estimated in only two objects of the sample and in both cases is found to be R > 1.

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arXiv2008

Unveiling the nature of INTEGRAL objects through optical spectroscopy. VI. A multi-observatory identification campaign

Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of still unidentified hard X-ray sources.

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arXiv2008

An X-ray view of absorbed INTEGRAL AGN

Aims. We present a 0.2--200 keV broad-band study of absorbed AGN observed with INTEGRAL, XMM-Newton, Chandra and ASCA to investigate the continuum shape and the absorbing/reflecting medium properties. Methods. The sources are selected in the INTEGRAL AGN sample to have a 20--100 keV flux below 8$\times10^{-11}$ $\flux$ (5 mCrab), and are characterized by a 2--10 keV flux in the range (0.8--10)$\times10^{-11}$ $\flux$. The good statistics allow us a detailed study of the intrinsic and reflected continuum components. In particular, the analysis performed on the combined broad-band spectra allow us to investigate the presence of Compton reflection features and high energy cut-off in these objects. Results. The column density of the absorbing gas establishes the Compton thin nature for three sources in which a measure of the absorption was still missing. The Compton thin nature of all the sources in this small sample is also confirmed by the diagnostic ratios F$x/F[OIII]. The Compton reflection components we measure, reflection continuum and iron line, are not immediately compatible with a scenario in which the absorbing and reflecting media are one and the same, i.e. the obscuring torus. A possible solution is that the absorption is more effective than reflection, e.g. under the hypothesis that the absorbing/reflecting medium is not uniform, like a clumpy torus, or that the source is observed through a torus with a very shallow opening angle. The high energy cut-off (a lower limit in two cases) is found in all sources of our sample and the range of values is in good agreement with that found in type 1 Seyfert galaxies. At lower energies there is clear evidence of a soft component (reproduced with a thermal and/or scattering model), in six objects.

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arXiv2007

Probing clumpy stellar winds with a neutron-star

INTEGRAL tripled the number of super-giant high-mass X-ray binaries (sgHMXB) known in the Galaxy by revealing absorbed and fast transient (SFXT) systems. Quantitative constraints on the wind clumping of massive stars can be obtained from the study of the hard X-ray variability of SFXT. A large fraction of the hard X-ray emission is emitted in the form of flares with a typical duration of 3 ksec, frequency of 7 days and luminosity of 1E36 ergs/s. Such flares are most probably emitted by the interaction of a compact object orbiting at ~ 10 R* with wind clumps (1E(22-23) g) representing a large fraction of the stellar mass-loss rate. The density ratio between the clumps and the inter-clump medium is 1E(2-4) . The parameters of the clumps and of the inter-clump medium, derived from the SFXT flaring behavior, are in good agreement with macro-clumping scenario and line-driven instability simulations. SFXT are likely to have larger orbital radius than classical sgHMXB.

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arXiv2007

Probing clumpy stellar winds with a neutron star

INTEGRAL, the European Space Agency's gamma-ray observatory, tripled the number of super-giant high-mass X-ray binaries (sgHMXB) known in the Galaxy by revealing absorbed and fast transient (SFXT) systems. In these sources, quantitative constraints on the wind clumping of the massive stars could be obtained from the study of the hard X-ray variability of the compact accreting object. Hard X-ray flares and quiescent emission of SFXT systems have been characterized and used to derive wind clump parameters. A large fraction of the hard X-ray emission is emitted in the form of flares with a typical duration of 3 ks, frequency of 7 days and luminosity of 1E36 erg/s. Such flares are most probably emitted by the interaction of a compact object orbiting at ~10 R* with wind clumps (1E(22-23) g) representing a large fraction of the stellar mass-loss rate. The density ratio between the clumps and the inter-clump medium is 1E(2-4) in SFXT systems. The parameters of the clumps and of the inter-clump medium, derived from the SFXT flaring behavior, are in good agreement with macro-clumping scenario and line driven instability simulations. SFXT have probably a larger orbital radius than classical sgHMXB.

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arXiv2007

IGR J17254-3257, a new bursting neutron star

The study of the observational properties of uncommonly long bursts from low luminosity sources with extended decay times up to several tens of minutes is important when investigating the transition from a hydrogen-rich bursting regime to a pure helium regime and from helium burning to carbon burning as predicted by current burst theories. IGR J17254-3257 is a recently discovered X-ray burster of which only two bursts have been recorded: an ordinary short type I X-ray burst, and a 15 min long burst. An upper limit to its distance is estimated to about 14.5 kpc. The broad-band spectrum of the persistent emission in the 0.3-100 keV energy band obtained using contemporaneous INTEGRAL and XMM-Newton data indicates a bolometric flux of 1.1x10^-10 erg/cm2/s corresponding, at the canonical distance of 8 kpc, to a luminosity about 8.4x10^35 erg/s between 0.1-100 keV, which translates to a mean accretion rate of about 7x10^-11 solar masses per year. The low X-ray persistent luminosity of IGR J17254-3257 seems to indicate the source may be in a state of low accretion rate usually associated with a hard spectrum in the X-ray range. The nuclear burning regime may be intermediate between pure He and mixed H/He burning. The long burst is the result of the accumulation of a thick He layer, while the short one is a prematurate H-triggered He burning burst at a slightly lower accretion rate.

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arXiv2007

INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth

We study the spectrum of the cosmic X-ray background (CXB) in energy range $\sim$5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is $\sim$10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin.

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