R. Walter

The Third IBIS/ISGRI Soft Gamma-Ray Survey Catalog

In this paper we report on the third soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific data set is based on more than 40 Ms of high-quality observations performed during the first 3.5 yr of Core Program and public IBIS/ISGRI observations. Compared to previous IBIS/ISGRI surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 400 high-energy sources detected in the energy range 17-100 keV, including both transients and faint persistent objects that can only be revealed with longer exposure times.

XMM-Newton and INTEGRAL observations of new absorbed supergiant high-mass X-ray binaries

Context. During the first year in operation, INTEGRAL, the European Space Agency’s γ-ray observatory, has detected more than 28 new bright sources in the galactic plane which emit the bulk of their emission above 10 keV. Aims. To understand the nature of those sources we have obtained follow-up observations in the X-ray band with XMM-Newton. Methods. We derive accurate X-ray positions, propose infrared counterparts and study the source high energy long and short term variability and spectra. Results. 70% of the sources are strongly absorbed (NH ≥ 10 23 atom cm −2 ). More than half of these absorbed sources show pulsations with periods ranging from 139 to 1300 s, i.e., they are slow X-ray pulsars. The candidate infrared counterparts are not as strongly absorbed demonstrating that part of the absorbing matter is local to the sources. Conclusions. Many of these new sources are supergiant high-mass X-ray binaries (HMXB) in which the stellar wind of the companion star is accreted onto the compact object. The large local absorption in these new sources can be understood if the compact objects are buried deep in their stellar winds. These new objects represent half of the population of supergiant HMXB.

Probing clumpy stellar winds with a neutron star

Context. INTEGRAL, the European Space Agency’s γ-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. Aims. 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. Methods. Hard X-ray flares and quiescent emission of SFXT systems have been characterized and used to derive wind clump parameters. Results. 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 10 36 erg/s. Such flares are most probably emitted by the interaction of a compact object orbiting at ∼10 R∗ with wind clumps (10 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 10 2−4 in SFXT systems. Conclusions. 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.

Unveiling the nature of INTEGRAL objects through optical spectroscopy. V. Identification and properties of 21 southern hard X-ray sources

Optical spectroscopic identification of the nature of 21 unidentified southern hard X-ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL . Our results show that 5 of these objects are magnetic Cataclysmic Variables (CVs), 4 are High-Mass X-ray Binaries (HMXBs; one of which is in the Large Magellanic Cloud) and 12 are Active Galactic Nuclei (AGNs). When feasible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in the study of the hard X-ray spectrum of AGNs, HMXBs and CVs, and the usefulness of a strategy of catalogues cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of the X-ray sources detected with INTEGRAL .

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

Aims. We study the spectrum of the cosmic X-ray background (CXB) in energy range ∼5−100 keV. Methods. Early in 2006 the INTEGRAL observatory performed a series of four 30 ks 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. Results. 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 ∼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.

Design and Operation of FACT -- The First G-APD Cherenkov Telescope

The First G-APD Cherenkov Telescope (FACT) is designed to detect cosmic gamma-rays with energies from several hundred GeV up to about 10 TeV using the Imaging Atmospheric Cherenkov Technique. In contrast to former or existing telescopes, the camera of the FACT telescope is comprised of solid-state Geiger-mode Avalanche Photodiodes (G-APD) instead of photomultiplier tubes for photo detection. It is the first full-scale device of its kind employing this new technology. The telescope is operated at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain) since fall 2011. This paper describes in detail the design, construction and operation of the system, including hardware and software aspects. Technical experiences gained after one year of operation are discussed and conclusions with regard to future projects are drawn.

The INTEGRAL Burst Alert System

We describe the INTEGRAL Burst Alert System (IBAS): the automatic software for the rapid distribution of the coordinates of the Gamma-Ray Bursts detected by INTEGRAL. IBAS is implemented as a ground based system, working on the near-real time telemetry stream. During the first six months of operations, six GRB have been detected in the field of view of the INTEGRAL instruments and localized by IBAS. Positions with an accuracy of a few arcminutes are currently distributed by IBAS to the community for follow-up observations within a few tens of seconds of the event.

INTEGRAL IBIS Extragalactic Survey: Active Galactic Nuclei Selected at 20-100 keV*

Analysis of INTEGRAL Core Program and public Open Time observations performed up to April 2005 provides a sample of 62 active galactic nuclei in the 20-100 keV band above a flux limit of ∼ 1.5×10 erg cm s. Most(42) of the sources in the sample are Seyfert galaxies, almost equally divided between type 1 and 2 objects, 6 are blazars and 14 are still unclassified. Excluding the blazars, the average redshift of our sample is 0.021 while the mean luminosity is Log(L) = 43.45. We find that absorption is present in 65% of the objects with 14% of the total sample due to Compton thick active galaxies. In agreement with both Swift/BAT team results and 2-10 keV studies, the fraction of absorbed objects decreases with the 20-100 keV luminosity. All Seyfert 2s in our sample are absorbed as are 33% of Seyfert 1s. The present data highlight the capability of INTEGRAL to probe the extragalactic gamma-ray sky and to find new and/or absorbed active galaxies. Subject headings: surveys — galaxies: active — gamma rays: observations Based on observations obtained with the ESA science mission INTEGRAL IASF-Bologna/INAF, via P. Gobetti 101, 40129 Bologna, Italy; bassani@iasfbo.inaf.it, malizia@iasfbo.inaf.it, stephen@iasfbo.inaf.it School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK; molinam@astro.soton.ac.uk, ajb@astro.soton.ac.uk, ajd@astro.soton.ac.uk IASF-Roma/INAF, via del Fosso del Cavaliere 100, I-00133 Roma, Italy; bazzano@rm.iasf.cnr.it, derosa@rm.iasf.cnr.it, ubertini@rm.iasf.cnr.it CEA Saclay/DSM/DAPNIA/Sap, 91191 Gif Sur Yvette, France; flebrun@cea.fr, fil@discovery.saclay.cea.fr, belanger@hep.saclay.cea.fr INTEGRAL Science Data Center, Chemin D’Écogia 16, 1290 Versoix, Switzerland; roland.walter@obs.unige.chAnalysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) Core Programme and public open-time observations performed up to 2005 April provides a sample of 62 active galactic nuclei in the 20-100 keV band above a flux limit of ~1.5 × 10-11 ergs cm-2 s-1. Most (42) of the sources in the sample are Seyfert galaxies, almost equally divided between type 1 and type 2 objects; six are blazars, and 14 are still unclassified. Excluding the blazars, the average redshift of our sample is 0.021, while the mean luminosity is log L = 43.45. We find that absorption is present in 65% of the objects, with 14% of the total sample due to Compton-thick active galaxies. In agreement with both Swift BAT team results and 2-10 keV studies, the fraction of absorbed objects decreases with the 20-100 keV luminosity. All Seyfert 2s in our sample are absorbed, as are 33% of Seyfert 1s. The present data highlight the capability of INTEGRAL to probe the extragalactic gamma-ray sky and to find new and/or absorbed active galaxies.

INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

Aims. We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the � 20‐200 keV range from the data of the IBIS instrument aboard the INTEGRALsatellite obtained during the four dedicated Earthoccultation observations in early 2006. Methods. We analyze the modulation of the IBIS/ISGRI detector counts induced by the passage of the Earth through the field of view of the instrument. Unlike previous studies, we do not fix the spectral shape of the various contributions, but model i nstead their spatial distribution and derive for each of them the expected modulation of the detector counts. The spectra of the diffuse emission components are obtained by fitting the normalizations of the model lightcurves to the observed modulation in different energy bins. Because of degeneracy, we guide the fits with a realistic choi ce of the input parameters and a constraint for spectral smoothness. Results. The obtained CXB spectrum is consistent with the historic HEAO-1results and falls slightly below the spectrum derived with Swift/BAT. A 10 % higher normalization of the CXB cannot be completely excluded, but it would imply an unrealistically high albedo of the Earth. The derived spectrum of the GRXE confirms the pre sence of a minimum around 80 keV with improved statistics and

Unveiling the nature of INTEGRAL objects through optical spectroscopy IV. A study of six new hard X-ray sources

INTEGRAL Science Data Centre, Chemin d’Ecogia 16, CH-1290 Versoix, SwitzerlandReceived 28 February 2006; accepted 21 April 2006Abstract. We present further results from our onging optical spectrophotometric campaign at the AstronomicalObservatory of Bologna in Loiano (Italy) on unidentified hard X–ray sources detected by INTEGRAL. We ob-served spectroscopically the putative optical counterparts of the INTEGRAL sources IGR J00234+6141, IGRJ01583+6713, IGR J06074+2205, IGR J13091+1137 and IGR J20286+2544. We find that the first two areGalactic objects, namely a Cataclysmic Variable at a distance d ∼ 300 pc and a Be/X transient High-MassX–ray Binary (HMXB) located at ∼6.4 kpc, respectively, whereas the last one is identified with MCG +04-48-002, a Starburst/H ii galaxy at redshift z = 0.013 hiding a Seyfert 2 nucleus. We identify IGR J13091+1137 asthe (likely Seyfert 2 type) active nucleus of galaxy NGC 4992, which we classify as an X–ray Bright, OpticallyNormal Galaxy; this is the first example of this type of object to be detected by INTEGRAL, and one of theclosest of this class. We moreover confirm the possible Be/X nature of IGR J06074+2205, and we estimate itto be at a distance of ∼1 kpc. We also reexamine the spectrum of the z = 0.087 elliptical radio galaxy PKS0352−686, the possible counterpart of the INTEGRAL source IGR J03532−6829, and we find that it is a BL Lac.Physical parameters for these sources are also evaluated by discussing our findings in the context of the availablemultiwavelength information. These identifications further stress the importance of INTEGRAL in the study ofthe hard X–ray spectrum of Active Galactic Nuclei, HMXBs and Cataclysmic Variables.Key words. Galaxies: Seyfert — BL Lacertae objects: individual: PKS 0352−686 — Stars: novae, cataclysmicvariables — X–rays: binaries — Techniques: spectroscopic — X–rays: individuals: IGR J00234+6141; IGRJ01583+6713; IGR J03532−6829 (=PKS 0352−686); IGR J06074+2205; IGR J13091+1137 (=NGC 4992); IGRJ20286+2544 (=MCG +04-48-002)