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| Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs |
Context INTEGRAL has two sensitive gamma-ray instruments that have detected and localised 47 gamma-ray bursts (GRBs) from its launch in October 2002 up to July 2007.
Aims. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, and spectrometer, SPI.
Methods. Spectral properties of the GRBs are determined using power-law and, where appropriate, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy gamma-rays with respect to high-energy gamma-rays, are measured for 31 of the GRBs.
Results. The photon index distribution of power-law fits to the prompt emission spectra is presented and is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ~0.15 ph/cm²/s is ~ 1400 per year in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags <0.75s (between 25-50 keV and 50-300 keV) and one with long lags >0.75s . Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies, and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q=-0.225 +/- 0.090 and hence reflect the local large-scale structure of the Universe. |
| Publication date: 10 Oct 2008 |
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| A deep INTEGRAL hard X-ray survey of the 3C 273/Coma region |
| We present an analysis of the deepest hard X-ray survey to date of about 2500 deg² 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, for which we try to find counterparts at other wavelengths. The ratio of Seyfert 1 to Seyfert 2 is significantly more than the ratio found in the optical. This effect may be explained in the framework of the receding-torus model, but could also be due to absorption columns large enough to affect the 20-60 keV band. None of the predicted Compton-thick objects with 1024 < NH < 1025 cm-2 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 1021 <= NH <= 1022 cm-2, which could be expected if absorption has two origins, for instance a torus-like structure and 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 < 1022 cm-2) at a given flux limit compared to the models. In this survey, we resolve about 2.5% of the cosmic X-ray background in the 20-60 keV band.
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| Publication date: 15 Jul 2008 |
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| ISOC Newsletter #20 |
Contents:
- Foreword
- AO-7 Announcement
- AO-6 Programme
- Science Operations
- Contacting ISOC
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| Publication date: 15 Jul 2008 |
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Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL
I. The nature of the companion star |
Context. The INTEGRAL hard X-ray observatory has revealed an emerging population of highly obscured X-ray binary systems through multi-wavelength observations. Previous studies have shown that many of these sources are high-mass X-ray binaries hosting neutron stars orbiting around luminous and evolved companion stars.
Aims. To better understand this newly-discovered population, we have selected a sample of sources for which an accurate localisation is available to identify the stellar counterpart and reveal the nature of the companion star and of the binary system.
Methods. We performed an intensive study of a sample of thirteen INTEGRAL sources, through multi-wavelength optical to NIR photometric and spectroscopic observations, using EMMI and SofI instruments at the ESO NTT telescope. We performed accurate astrometry and identified candidate counterparts for which we give the optical and NIR magnitudes. We detected many spectral lines allowing us to determine the spectral type of the companion star. We fitted with stellar black bodies the mid-infrared to optical spectral energy distributions of these sources. From the spectral analysis and SED fitting we identified the nature of the companion stars and of the binary systems.
Results. Through spectroscopic analysis of the most likely candidates we found the spectral types of IGR J16320-4751, IGR J16358-4726, IGR J16479-4514, IGR J17252-3616, IGR J18027-2016: They all host OB type supergiant companion stars, with IGR J16358-4726 likely hosting an sgB[e]. Our spectra also confirm the supergiant O and B nature of IGR J17391-3021 and IGR J19140+0951. From SED fitting we found that IGR J16418-4532 is a (likely OB supergiant) HMXB, IGR J16393-4643 a (likely BIV-V star) HMXB, and IGR J18483-0311 a likely HMXB system.
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| Publication date: 05 Jun 2008 |
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Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL
II. The environment of the companion star |
Context. The INTEGRAL mission has led to the discovery of a new type of supergiant X-ray binaries (SGXBs), whose physical properties differ from those of previously known SGXBs. Those sources are in the course of being unveiled by means of multi-wavelength X-rays, optical, near- and mid-infrared observations, and two classes are appearing. The first class consists of obscured persistent SGXBs and the second is populated by the so-called supergiant fast X-ray transients (SFXTs).
Aims. We report here mid-infrared (MIR) observations of the companion stars of twelve SGXBs from these two classes in order to assess the contribution of the star and the material enshrouding the system to the total emission.
Methods. We used data from observations we carried out at ESO/VLT with VISIR, as well as archival and published data, to perform broad-band spectral energy distributions of the companion stars and fitted them with a combination of two black bodies representing the star and a MIR excess due to the absorbing material enshrouding the star, if there was any.
Results. We detect a MIR excess in the emission of IGR J16318-4848, IGR J16358-4726, and perhaps IGR J16195-4945. The other sources do not exhibit any MIR excess even when the intrinsic absorption is very high. Indeed, the stellar winds of supergiant stars are not suitable for dust production, and we show that this behaviour is not changed by the presence of the compact object. Concerning IGR J16318-4848 and probably IGR J16358-4726, the MIR excess can be explained by their sgB[e] nature and the presence of an equatorial disk around the supergiant companion in which dust can be produced.
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| Publication date: 05 Jun 2008 |
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| ISOC Newsletter #19 |
Contents:
- Foreword
- AO-6 Announcement
- Key Programmes in AO-6
- Recent Scientific Highlights
- Science Operations
- Long Term Observation Planning
- Changes at ISOC
- Contacting ISOC
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| Publication date: 15 Mar 2008 |
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| Recent activity of the Rapid Burster (MXB 1730-335) |
| Observations taken as part of the INTEGRAL Galactic bulge monitoring program (see ATel #1385) on 2008, February 23 13:32-17:13 (UT) showed bursting activity from the X-ray transient, the Rapid Burster (MXB 1730-335). During one of the 1800-sec pointings when the source was in the field-of-view of JEM-X a series of 6 X-ray bursts were observed, once every 250-350 sec, with durations of 30-60 sec and peak fluxes of about 0.6-0.9 Crab (3-10 keV). |
| Publication date: 26 Feb 2008 |
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| Hard X-ray emission from eta Carinae |
Context. If relativistic particle acceleration takes place in colliding-wind binaries, hard X-rays and gamma-rays are expected through inverse Compton emission, but to date these have never been unambiguously detected.
Aims. To detect this emission, observations of eta Carinae were performed with INTEGRAL, leveraging its high spatial resolution.
Methods. Deep hard X-ray images of the region of eta Car were constructed in several energy bands.
Results. The hard X-ray emission previously detected by BeppoSax around eta Car originates from at least 3 different point sources. The emission of eta Car itself can be isolated for the first time, and its spectrum unambiguously analyzed. The X-ray emission of eta Car in the 22-100 keV energy range is very hard (Gamma ~ 1±0.4 ) and its luminosity is 7x1033erg s-1.
Conclusions. The observed emission is in agreement with the predictions of inverse Compton models, and corresponds to about 0.1% of the energy available in the wind collision. Eta Car is expected to be detected in the GeV energy range. |
| Publication date: 20 Jan 2008 |
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| An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays |
| Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis accreting compact objects and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (~10-50° from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 1041 positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter. |
| Publication date: 10 Jan 2008 |
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| INTEGRAL/IBIS all-sky survey in hard X-rays |
| We present results of an all-sky hard X-ray survey based on almost four years of observations with the IBIS telescope onboard the INTEGRAL observatory. The dead time-corrected exposure of the survey is ~33 Ms. Approximately 12% and 80% of the sky has been covered to limiting fluxes lower than 1 and 5 mCrab, respectively. Our catalog of detected sources includes 403 objects, 316 of which exceed a 5-sigma detection threshold on the time-averaged map of the sky, and the rest were detected in various subsamples of exposures. Among the identified sources, 219 are Galactic (90 low-mass X-ray binaries, 76 high-mass X-ray binaries, 21 cataclysmic variables, 6 coronally active stars, and other types) and 137 are extragalactic, including 130 active galactic nuclei (AGNs) and 3 galaxy clusters. We derived number-flux functions of AGNs and Galactic sources. The log N-log S relation of non-blazar AGNs is based on 68 sources located at Galactic latitudes |b| > 5°, where the survey is characterized by high identification completeness, with fluxes higher than Slim = 1.1 × 10-11 erg s-1 cm-2 (~0.8 mCrab) in the 17-60 keV energy band. The cumulative AGN number-flux function can be described by a power law with a slope of 1.62 ± 0.15 and normalization of (5.7±0.7) × 10-3 sources per deg² at fluxes > 1.43 × 10-11 erg s-1 cm-2 (>1 mCrab). Those AGNs with fluxes higher than Slim make up ~1% of the cosmic X-ray background at 17-60 keV. We present evidence of strong inhomogeneity in the spatial distribution of nearby (<70 Mpc) AGNs, which reflects the large-scale structure in the local Universe. |
| Publication date: 15 Nov 2007 |
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| ISOC Newsletter #18 |
Contents:
- Foreword
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Five Years of INTEGRAL
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AO-6 KP Announcement and AO-5 KP Update
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Recent Scientific Highlights
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Science Operations
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ISDA and INVITE
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Changes at ISOC
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Contacting ISOC
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| Publication date: 15 Oct 2007 |
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| HESS J1616-508: likely powered by PSR J1617-5055 |
| HESS J1616-508 is one of the brightest emitters in the TeV sky. Recent observations with the IBIS/ISGRI telescope on board the INTEGRAL spacecraft have revealed that a young, nearby and energetic pulsar, PSR J1617-5055, is a powerful emitter of soft gamma-rays in the 20-100 keV domain. In this paper we present an analysis of all available data from the INTEGRAL, Swift, BeppoSAX and XMM-Newton telescopes with a view to assessing the most likely counterpart to the HESS source. We find that the energy source that fuels the X/gamma-ray emissions is derived from the pulsar, both on the basis of the positional morphology, the timing evidence and the energetics of the system. Likewise, the 1.2% of the pulsar's spin down energy loss needed to power the 0.1-10 TeV emission is also fully consistent with other HESS sources known to be associated with pulsars. The relative sizes of the X/gamma-ray and VHE sources are consistent with the expected lifetimes against synchrotron and Compton losses for a single source of parent electrons emitted from the pulsar. We find that no other known object in the vicinity could be reasonably considered as a plausible counterpart to the HESS source. We conclude that there is good evidence to assume that the HESS J1616-508 source is driven by PSR J1617-5055 in which a combination of synchrotron and inverse Compton processes combine to create the observed morphology of a broad-band emitter from keV to TeV energies.
- Accepted for publication in MNRAS - |
| Publication date: 05 Jul 2007 |
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| 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. |
| Publication date: 02 May 2007 |
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| SPI observations of the diffuse 60Fe emission in the Galaxy |
| Gamma-ray line emission from radioactive decay of 60Fe provides constraints on nucleosynthesis in massive stars and supernovae. The spectrometer SPI on board INTEGRAL has accumulated nearly three years of data on gamma-ray emission from the Galactic plane. We have analyzed these data with suitable instrumental-background models and sky distributions to produce high-resolution spectra of Galactic emission. We detect the gamma-ray lines from 60Fe decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9 sigma significance for the combination of the two lines. The average flux per line is (4.4 +- 0.9) x 10-5 ph cm-2 s-1 rad-1 for the inner Galaxy region. Deriving the Galactic 26Al gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of 60Fe/26Al gamma-rays as 0.148 +- 0.06. The current theoretical predictions are still consistent with our result. |
| Publication date: 30 Apr 2007 |
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| ISOC Newsletter #17 |
Contents
- Foreword
- 5th Announcement of Opportunity (AO-5)
- Key Programmes in AO-5
- Science Highlights
- Science Operations - Highlights
- The Fastest INTEGRAL TOO
- ISOC Science Data Archive Version 2.7
- INTEGRAL Visualisation Tool & Explorer
- INTEGRAL Publication Status
- Changes in ISOC
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| Publication date: 15 Mar 2007 |
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| Cannibalism in space: A star eats its companion |
| The work of ESA's INTEGRAL high-energy observatory usually follows a long-term plan that is established every year by selecting only the very best of the numerous observing proposals from the scientific community. However, nature does not always follow the same plan, so INTEGRAL and the people who keep it running have to react to unforeseen and sudden 'Targets of Opportunity'. The case here is neutron star IGR J00291+5934, an incredibly dense object with a mass similar to that of our Sun's compressed into a rapidly spinning sphere only a few kilometres across. Not only that, but it is busy swallowing its stellar companion. |
| Publication date: 15 Nov 2006 |
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| Scientific highlights from INTEGRAL |
| The gamma-ray observatory INTEGRAL was launched in October 2002 and produces since then a wealth of discoveries and important new results. I will present a selection of scientific highlights obtained during the first 2.5 years of the mission. |
| Publication date: 15 Oct 2006 |
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| ISOC Newsletter #16 |
Contents
- Foreword
- INTEGRAL AO-4 Results
- INTEGRAL AO-5 Key Programme
- INTEGRAL Publication Status
- Science Highlights
- Science Operations - Highlights
- The ISOC Science Data Archive
- The 6th INTEGRAL Workshop
- Changes at ISOC
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| Publication date: 15 Sep 2006 |
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| Imaging extended sources with coded mask telescopes: application to the INTEGRAL IBIS/ISGRI instrument |
Context. In coded mask techniques, reconstructed sky images are pseudo-images: they are maps of the correlation between the image recorded on a detector and an array derived from the coded mask pattern. Aims. The INTEGRAL/IBIS telescope provides images where the flux of each detected source is given by the height of the local peak in the correlation map. As such, it cannot provide an estimate of the flux of an extended source. What is needed is intensity sky images giving the flux per solide angle as typically done at other wavelengths.
Methods. In this paper, we present the response of the INTEGRAL IBIS/ISGRI coded mask instrument to extended sources. We develop a general method based on analytical calculations in order to measure the intensity and the associated error of any celestial source and validated with Monte-Carlo simulations.
Results. We find that the sensitivity degrades almost linearly with the source extent. Analytical formulae are given as well as an easy-to-use recipe for the INTEGRAL user. We check this method on IBIS/ISGRI data but these results are general and applicable to any coded mask telescope. |
| Publication date: 15 Sep 2006 |
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| The Signature of 44Ti in Cassiopeia A Revealed by IBIS/ISGRI on INTEGRAL |
| We report the detection of both the 67.9 and 78.4 keV 44Sc gamma-ray lines in Cassiopeia A with the INTEGRAL IBIS/ISGRI instrument. Besides the robustness provided by spectroimaging observations, the main improvements compared to previous measurements are a clear separation of the two 44Sc lines together with an improved significance of the detection of the hard X-ray continuum up to 100 keV. These allow us to refine the determination of the 44Ti yield and to constrain the nature of the nonthermal continuum emission. By combining COMPTEL, BeppoSAX PDS and ISGRI measurements, we find a line flux of (2.5+/-0.3)×10-5 cm-2s-1 leading to a synthesized 44Ti mass of 1.6+0.6-0.3×10-4 Msolar. This high value suggests that Cas A is peculiar in comparison to other young supernova remnants, from which so far no line emission from 44Ti decay has been unambiguously detected. |
| Publication date: 15 Aug 2006 |
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