| No. 1 - July 12, 2000 | Edited by Thierry Montmerle & Marc Türler |
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INTEGRAL is about science, we therefore also wish to centre the dialog of this Newsletter around science. Rather than multiplying papers you will find here abstracts that the editors think are of relevance to the community of users of INTEGRAL. Since this is indeed a dialog, you are invited to also let them know of your abstracts relevant to the general theme.
You will also find here information on the ISDC, its products and its documentation in the form of summaries and, more relevant, of links to notes and documents. In the same spirit of not duplicating information you will find links to ESA and instrument team web sites in addition to articles written specifically for you in the present forum.
The ISDC staff have been working for 5 years now to implement a set of tools to deal with INTEGRAL data. We are working closely with instrument developers to provide you with calibrated data that can be dealt with as comfortably as possible. You will find elsewhere in this first issue a link to a short description of the types of analysis in development and products expected. Future issues will complete the portrait of the ISDC and its products.
The ISDC is made of people at a site, this will also be presented to you starting this time with a list of staff. The functions and profiles of those expected to interact more closely with you will be provided in coming issues. To start with you may not be surprised to learn that Marc Türler is responsible for the user support and that Thierry Montmerle is the ISDC Co-investigator taking a close look to our contacts with the community. Both editors are therefore your priviledged contacts to begin with. Let me take this opportunity to thank them for the work they are making to realise this newsletter.
The ISDC is a service to the community, we hope that you will find this service adequate and count on your help to match our efforts with your needs.
The ISDC-Astrophysics Newsletter is only posted on the ISDC web pages and an archive of past issues will be maintained. The main sections of the newsletter are arranged as follows:
We hope that the ISDC-Astrophysics Newsletter will correspond to your needs of information and that it might become an interesting communication tool within the INTEGRAL community as a whole.
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The ISDC is located in Versoix nearby Geneva, Switzerland since 1996.
The picture on the right shows the ISDC building constructed in the XIXth century, but which is now fully equiped and connected to the international network by optic fibres.
At the beginning of this year, an old barn next to this building was completely renovated (actually nearly reconstructed ...) to host the operation rooms and the data archive.
The ISDC staff is composed of about thirty scientists and engineers coming from and funded by the twelve institutes (see the map) forming the ISDC consortium. The ISDC works in close collaboration with the INTEGRAL intrument teams to ensure that the software developed by these teams is compatible with the data analysis system developed at the ISDC. The ISDC is also in close contact with the INTEGRAL Science Operation Centre (ISOC), located at ESTEC, The Netherlands. It is the ISOC, which will issue the announcement of opportunity (AO) foreseen for November 1st, 2000, select the received proposals and make the observation plan for INTEGRAL.
The ISDC will receive the data from INTEGRAL in real-time and continuously at a rate of 96 kB/sec. The data will be archived immediately as well as formatted and sent for a quick-look analysis (QLA), which consists of comparing the INTEGRAL data with the expected position and flux of known sources. The aim of the QLA is to inform the ISOC within a few hours about the detection of transient sources, in order to give priority to the study of these unexpected events by modifying the foreseen observation plan. An INTEGRAL Burst Alert System (IBAS) is also developed by the ISDC to calculate the location of gamma-ray bursts and to send this information within minutes to the astronomers worldwide.
After the QLA, a standard analysis (SA) will process the data further to provide images of the sky, spectra and light-curves of variable sources. The ISDC archive will include all data from the INTEGRAL satellite as well as the results of their standard analysis. All data will become publicly available after one year. To help the observers in performing their own detailed and specific data analysis, the ISDC will provide some tools for the so called Offline Scientific Analysis (OSA).
A first version of the INTEGRAL Burst Alert System (IBAS) has been sucessfully delivered and integrated. It is capable to detect Gamma-Ray Bursts in the ISGRI detector of the Imager on Board the INTEGRAL Satellite (IBIS), to image them and to translate the burst position to the Optical Monitor Camera (OMC) coordinates and to generate OMC recommanding alerts within one second !
The different teams which are constructing the instruments for INTEGRAL are currently also developing the scientific analysis software. Several components of this software has been delivered to the ISDC or is near to completion.
An observation simulator (OSim) was developed by the ISDC and presented at the INTEGRAL Spring School in April 2000. This simulator is able to simulate SPI, IBIS and JEM-X (the X-ray monitor) scientific data and is now being consolidation and tested in order to be ready for the INTEGRAL AO in November.
This section was written by Marc Türler, for complementary information please have a look at the ISDC public outreach pages.
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The following contributions describe the instruments:
| IBIS : The High Resolution Imaging Telescope |
| Sandro Mereghetti (IFC/CNR Milano) |
IBIS is the main INTEGRAL instrument devoted to high resolution imaging in the energy range from ~20 keV to ~10 MeV. IBIS(1,2) is developed by a large consortium of Institutes in Europe and USA, under the responsibility of IAS/CNR Roma (P.Ubertini, Principal Investigator). Like the other INTEGRAL high energy instruments, IBIS makes use of a coded mask to obtain images of the gamma-ray sky in this energy range characterised by a small photon interaction cross section and by the difficulty of using focussing techniques. The excellent imaging capabilities of IBIS are obtained with a coded mask, made of 95 x 95 square tungsten elements with a thickness of 15 mm, placed at a distance of ~3 meters from the detection plane. The mask, that accounts for about one third of the total IBIS weight (~650 kg) has been optimised to absorb 70% of the photons at 1.5 MeV, in correspondence of its closed elements, while the mask mechanical support retains an adequate transparency at low energy for the open elements. The good contrast of the aperture pattern is in fact directly related to the instrument sensitivity. The size of the mask elements yields an angular resolution of 12 arcmin. Such a resolution, though clearly not at the level we are used to in the soft X-ray energy range, is unprecedented at the energies covered by IBIS and is appropriate to resolve high energy sources in crowded regions, such as e.g. the galactic bulge. The IBIS field of view is very large: ~30 x 30 degrees (at zero response), and it provides a uniform sensitivity within its central part (~10 x 10 degrees).
The heart of the IBIS instrument is the detection plane, which, in order to effectively cover an extended energy range, is composed of two layers with different characteristics and based on different technologies. The top layer, ISGRI(3), is developed under the responsibility of CEA/Saclay (F.Lebrun, co-Principal Investigator). It consists of an array of 128 x 128 Cadmium Telluride (CdTe) square pixels (4 mm x 4 mm each). This allows to record with high spatial resolution the shadows of the mask cast by the sources present in the field of view. The subsequent analysis of the recorded patterns, performed on ground with appropriate algorithms, allows to reconstruct the observed sky images. ISGRI, with a total sensitive area of ~2600 cm2, covers the energy range from ~20 keV up to a few hundreds keV (given the CdTe thickness of 2 mm, the efficiency is smaller than 10% above 300 keV). The CdTe is a semiconductor that can be used at ambient temperature providing a good energy resolution (~7% at 100 keV) without the need of complex cooling systems. ISGRI will be operated in the so called "photon" mode, in which the complete information of each detected event (position, energy and arrival time) will be transmitted to the ground.
The ISGRI detection plane becomes almost transparent for photons above 150 keV, but such photons will be efficiently stopped by PICsIT, the second layer of the instrument(4). PICsIT, developed by the TESRE/CNR Institute in Bologna, Italy (G.Di Cocco, co-Principal Investigator), is based on an array of 64 x 64 scintillators of Caesium Iodide doped with Thallium (CsI(Tl)). Each element is a small bar of CsI(Tl) with dimensions 8.6 x 8.6 x 30 mm3 coupled with a photodiode. Although the two IBIS layers are in principle operating as independent imaging systems, their data will be used in a combined analysis to study more efficiently the spectral properties of the observed sources over the whole energy range. In addition, a fraction of the detected photons will undergo a Compton interaction in ISGRI, before being detected by PICsIT. For such "Compton" events, the energy and position measurements provided by the two layers (which are separated by ~10 cm), will give some constraints on the possible arrival directions, allowing to increase the signal to noise ratio by appropriate selections aimed at excluding directions not modulated by the mask aperture or not compatible with the source(s) of interest.
IBIS will reach a sensitivity of the order of a few tens of milliCrabs within ~1 hour, thus detecting, in most pointings several sources within its large field of view. Typical observations will in general be much longer, allowing significant detections also in the higher part of the energy range. Numerous INTEGRAL users will thus have the possibility to investigate a variety of topics in the study of different astrophysical objects, ranging from accreting galactic sources containing neutron stars or black holes to active galactic nuclei. The good sensitivity will allow to study in detail the spectra of galactic sources, and in particular to investigate the spectral variability of bright X-ray novae over a broad energy range and on short time scales. It is expected that many extragalactic sources, that were too weak for the sensitivity of previous high energy instruments, will be detectable by IBIS.
The importance of an imaging instrument for such studies should not be undervalued. There are in fact two great advantages with respect to non-imaging detectors operating in the same energy range as IBIS. First, the possibility of contamination from nearby objects, known or unknown, cannot be excluded in collimated instruments. Several cases have shown that this problem is not limited to the crowded regions of the galactic plane, but it also affected a few AGN observations in the past. The second advantage is the capability of simultaneously obtaining the source and background measurements, without the need for on-off pointing techniques and models for the spatial and temporal variation of the instrumental background. For these reasons, IBIS will certainly provide spectra of better quality also for sources already studied in the past.
A large fraction of the INTEGRAL Core Program(5) will be devoted to regular scans of the galactic plane, with the main objectives to discover new bright transients and to monitor the variability of known sources. IBIS, thanks to its very large field of view, good sensitivity and its capability of accurately locating the detected sources will be the primary instrument for such studies. In this respect, it is important to remember that the source location accuracy and the angular resolution, though related, are two different concepts. Despite an angular resolution of 12 arcmin (i.e. the width of the point response function), the sources detected by IBIS can be located with a much better accuracy, of the order of a few arcmin or, for bright sources, even at the subarcmin level. In fact, the source location accuracy is a function of the signal to noise ratio. Both theoretical calculations and Montecarlo simulations show that, for instance, a source with a signal to noise ratio of 30 can be located by ISGRI with an accuracy of 30 arcsec (90% confidence level).
These capabilities are also extremely relevant to the use of IBIS for the detection and localisation of gamma-ray bursts(6). Based on the known GRB LogN-LogS and the IBIS sensitivity, we expect a rate of the order of one GRB per month in the field of view, thus giving the possibility to obtain accurate localisation for follow up observations at other wavelengths. A rapid reaction time is essential for these studies. Exploiting the continuous telemetry downlink of the INTEGRAL satellite, the search for gamma-ray bursts will be carried out in near real time at the INTEGRAL Science Data Center. Here an automatic software system will screen the IBIS data as they arrive (typically within a few seconds from their detection on the satellite) looking for the signatures of gamma-ray bursts, i.e. a significant new point source suddenly appearing in the instrument field of view. After appropriate checks and filters to limit the occurrence of false alerts, the sky coordinates of the GRB will be automatically sent to subscribed clients.
IBIS is currently in an advanced state of its development, to meet the foreseen delivery date to ESA. Representative parts of all the subsystems, and in particular of the two detection layers, both based on a modular design, have been built, assembled and tested. This has given the possibility to timely spot and correct the unavoidable problems that typically are encountered when an innovative frontline instrument is developed. The excellent results obtained so far and the good performances that can be expected for IBIS are exemplified by the two images shown below.
The image on the left is a hard X-ray image obtained with one module
of the ISGRI detector layer. One module consists of 32 x 64 pixels. The final instrument will comprise 8 modules. This image has been obtained without the use of the coded mask, but simply placing the "subject" above
the detector, and illuminating it with a radioactive source.
(Image courtesy of CEA-Saclay DAPNIA)
The image on the right is the first shadowgram obtained with the PICsIT
Qualification Model (32 x 16 pixels).
The module has been covered with a lead mask with a hole
pattern in the shape of the word PICsIT, and then illuminated
using a 203Hg radioactive source (279 keV).
Two images have been obtained: one (upper panel) corresponding
to all events, and one (lower panel) taking into account only
photopeak events (lower panel).
(Image courtesy of ITESRE/CNR Bologna Gamma-Ray Group)
PostScript image [ 32 kB ]
References:
| The OMC : an Optical Monitoring Camera for INTEGRAL |
| Prepared by the OMC Team and communicated by J. Miguel Mas Hesse |
Multiband observations are particularly important in high-energy astrophysics where variability is typically rapid, unpredictable and of large amplitude. Arranging multifrequency observations that are simultaneous for both ground-based and space-borne instruments is extremely difficult, due to weather conditions and scheduling constraints. Thus, the wide band observing opportunity offered by INTEGRAL is of unique importance in providing for the first time simultaneous observations over seven orders of magnitude in photon energy for some of the most energetic objects in the Universe.
The main objectives of the Optical Monitoring Camera can be summarised as follows:
The picture on the right shows the OMC Camera Unit before installation of the thermal insulation blankets.
The optical system and CCD detector are located at the left end of the unit, while a rather large baffle protects from unwanted straylight originated by stars or reflections from the spacecraft.
A cover, visible in the picture at the right, protects the optical system from contamination during ground activities.
The Optical Monitoring Camera will have a field of view of 5.0 x 5.0 square degrees, as a result of an optimization of the photometric characteristics versus the compatibility with the other instruments, and will be coaligned with the Imager, the Spectrometer and the X-ray monitor. It will provide photometry in the Johnson V band, monitoring simultaneously the optical brightness of all the near-axis mission targets with a sensitivity mv = 19.0 in 1000 s integration time.
The OMC camera is based on a large format CCD (1024 x 2048 pixels) working in frame transfer mode (1024 x 1024 image area) in order to provide continuous sensitivity and to avoid the need for a mechanical shutter (frame transfer time less than 2 ms). The CCD will be cooled by means of a passive radiator to an operational temperature range between -100 °C and -70 °C. An optical baffle affords the necessary reduction of scattered sunlight and also the unwanted stray-light coming from non solar sources outside the FOV. An once-only deployable cover will protect the optics from contamination during ground operations and early operations in orbit. When deployed, it will form part of the baffle. The optics will be based on a refractive system with entrance pupil of 50 mm and baseline field of view of 5.0 x 5.0 degrees square. The complete system will cover the wavelength range between 500 and 850 nm, although a Johnson V filter will be included to allow for photometric calibration in a standard system.
For calibration purposes a LED is installed in the CCD cavity of the camera. This LED will be activated under ground control and will illuminate the image area of the CCD. The differential response of each pixel to this uniform light will be used to build a Flat Field correction matrix which will allow for photometrical calibration of the images once on ground. This system will permit the calibration of the relative quantum efficiency of each CCD pixel to an accuracy of < 1% in a period of < 1000 seconds. It is expected that new Flat Field images will be taken on a monthly (TBC) basis, depending on the stability of the CCD once in orbit. Absolute photometric calibration will be performed continuously by observing photometric reference stars in the target list.
The OMC instrument will be controlled by the Digital Processing Electronics provided by the PLM. The DPE will be a common design for the four instruments onboard INTEGRAL. It will provide the functions of signal processing, housekeeping, packetization, OBDH interface and command decoding. It will interface directly to the OMC (Camera and Electronics units).
During normal operations mode the OMC will take images of the full field of view every 10 to 100 seconds, depending on the integration time for the different targets.
The baseline will be to follow a given sequence of different integration times within these limits in order to monitor both bright and faint sources within the FOV.
The full image will be transferred to the DPE RAM.
A number of sub-windows, typically of 11 x 11 pixels, will be extracted around the positions of objects of interest.
The brightest stars in the FOV will be used for checking the pointing of the OMC, so that any drift will be compensated when extracting the windows.
It is estimated that about 100 such windows will be extracted for integrations of 100 s.
These windows will contain both the targets of interest as well as several reference stars for photometrical calibration and will be transmitted continuously to Earth.
The OMC will provide within its telemetry data the precise pointing of the S/C with a precision of few arcseconds.
These data will be useful for the reconstruction on-ground of the images taken through the coded masks instruments (especially Imager and X-ray monitor Instruments).
The OMC will have the capability to monitor rapidly variable sources at periods down to 1 second, but in this case other functionalities will not be available.
For more information contact the PI Dr. Alvaro Giménez at LAEFF/INTA.
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| Testing Comptonizing coronae on a long BeppoSAX observation of the Seyfert 1 galaxy NGC 5548 | |
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P.O. Petrucci1,2, F. Haardt2, L. Maraschi1,
et al. 1. Osservatorio Astronomico di Brera, Milano, Italy 2. Universita dell'Insubria, Como, Italy | |
| Accepted for publication in ApJ on April 4, 2000 | |
| Abstract. We test accurate models of Comptonization spectra over the high quality data of the BeppoSAX long look at NGC 5548, allowing for different geometries of the scattering region, different temperatures of the input soft photon field and different viewing angles. We find that the BeppoSAX data are well represented by a plane parallel or hemispherical corona viewed at an inclination angle of 30o. For both geometries the best fit temperature of the soft photons is close to 15+3-9 eV. The corresponding best fit values of the hot plasma temperature and optical depth are kTe= 250-260 keV and τ=0.16-0.37 for the slab and hemisphere respectively. These values are substantially different from those derived fitting the data with a power-law + cut off approximation to the Comptonization component (kTe≤ 60 keV, τ= 2.4). In particular the temperature of the hot electrons estimated from Comptonization models is much larger. This is due to the fact that accurate Comptonization spectra in anisotropic geometries show "intrinsic" curvature which reduces the necessity of a high energy cut-off. The Comptonization parameter derived for the slab model is larger than predicted for a two phase plane parallel corona in energy balance, suggesting that a more ``photon-starved" geometry is necessary. The case of a hemispheric corona is consistent with energy balance but requires a large reflection component. The spectral softening detected during a flare which occurred in the central part of the observation corresponds to a decrease of the Comptonization parameter, probably associated with an increase of the soft photon luminosity, the hard photon luminosity remaining constant. The increased cooling fits in naturally with the derived decrease of the coronal temperature kTe in the high state. | |
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E-mail contact | Preprint access |
| On the Origin of the Iron K Line in the Spectrum of the Galactic X-Ray Background | |
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A. Valinia1, V. Tatischeff2, K. Arnaud1, K. Ebisawa1, R.
Ramaty1 1. NASA's GSFC, Code 662, Greenbelt, MD 20771, USA 2. CSNSM, IN2P3-CNRS, 91405 Orsay, France | |
| Accepted for publication in ApJ on June 14, 2000 | |
| Abstract. We propose a mechanism for the origin of the Galactic ridge X-ray background that naturally explains the properties of the Fe K line, specifically the detection of the centroid line energy below 6.7 keV and the apparent broadness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/gamma-ray background, we consider a model that is a mixture of thermal plasma components of perhaps supernova origin and nonthermal emission from the interaction of low energy Cosmic ray electrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomic collisions of fast electrons produce characteristic nonthermal, narrow X-ray emission lines that can explain the complex Galactic background spectrum. Using the ASCA GIS archival data from the Scutum arm region, we show that a two-temperature thermal plasma model with kT~0.6 and ~2.8 keV, plus a LECRe component models the data satisfactorily. Our analysis rules out a purely nonthermal origin for the emission. It also rules out a significant contribution from low energy Cosmic ray ions, because their nonthermal X-ray production would be accompanied by a nuclear gamma-ray line diffuse emission exceeding the upper limits obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate. The proposed model naturally explains the observed complex line features and removes the difficulties associated with previous interpretations of the data which evoked a very hot thermal component (kT~7 keV). | |
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E-mail contact | Preprint access |
| Aluminum 26 production in asymptotic giant branch stars | |
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N. Mowlavi1,2, G. Meynet1 1. Geneva Observatory, CH-1290 Sauverny, Switzerland 2. INTEGRAL Science Data Centre, ch. d' Ecogia 16, CH-1290 Versoix, Switzerland | |
| Accepted for publication in A&A on July 5, 2000 | |
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Abstract. The production of 26Al in asymptotic giant branch (AGB) stars is studied based on evolutionary stellar models of different masses
(1.5 ≤ M/M⊙ ≤ 6) and metallicities (0.004 ≤ Z ≤ 0.02).
It is confirmed that 26Al is efficiently produced by hydrogen burning, but destruction of that nuclei by n-capture reactions during the interpulse and pulse phases becomes increasingly more efficient as the star evolves on the AGB.
The amount of 26Al available in the intershell region follows, at a given metallicity, a very well defined pattern as a function of the H-burning shell temperature TH. Two zones must be distinguished. The first one comprises those He-rich layers containing H-burning ashes which escape pulse injection. The amount of 26Al in that zone (1-2·10-7 M⊙ at the first pulse in 1.5-3 M⊙ Z=0.02 stars) steadily decreases with pulse number. Its contribution to the surface 26Al enhancement can only be important during the first pulses if dredge-up occurs at that stage. The second zone consists of the C-rich material emerging from the pulses. The amount of 26Al available in that zone is higher than that in the first zone (3-4·10-7 M⊙ at the first pulse in 1.5-3 M⊙ Z=0.02 stars), and keeps constant during about the first dozen pulses before decreasing when TH >~ 55·106 K. This zone is thus an important potential reservoir for surface 26Al enrichment. Using third dredge-up (3DUP) efficiencies from model calculations, the surface 26Al abundance is predicted to reach 1-2·10-7 mass fractions in our low-mass solar metallicity stars, with an uncertainty factor of about three. It decreases with increasing stellar mass, being about three times lower in a 4 M⊙ than in 2-3 M⊙ stars. In massive AGB stars, however, hot bottom burning enables to easily reach surface 26Al mass fractions above 10-6. The 26Al / 27Al ratios measured in meteoritic SiC and oxide grains are discussed, as well as that possibly measured in the nearby C-star IRC+10216. We also adress the contribution of AGB stars to the 2-3 M⊙ present day mass of 26Al detected in the Galaxy. Finally, we discuss the possibility of directly detecting an AGB star or a planetary nebula as a single source at 1.8 MeV with the future INTEGRAL satellite. | |
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E-mail contact | Preprint access |
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