| No. 8 - December 20, 2001 | Edited by Thierry Montmerle & Marc Türler |
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| INTEGRAL is integral ! |
The latest major event related to INTEGRAL is the delivery of
IBIS to ESA on November 18, and its effortless integration into
the satellite.
The picture on the right shows the integration of IBIS into the INTEGRAL spacecraft. This image was selected as INTEGRAL's Picture of the Month. For the complete story of this integration see ESA's article.
Now, INTEGRAL is integral, with all its experiments on board, ready for new calibration tests at ESTEC, beginning at the end of January. Its massive array of instruments stands atop the satellite bus, for a total of about 4 tons. Very impressive indeed for visitors and scientists alike !
We are now at exactly 300 days from launch, and it is a great satisfaction to see that the many, often serious problems, which arose during the construction of INTEGRAL's instruments, have all been successfully overcome. Congratulations to the Instrument Teams ! This is a wonderful Christmas gift for the whole high-energy astrophysics community.
While INTEGRAL majestically sits on its wheel bench at ESTEC, the activity continues elsewhere. At the ISDC, of course, where software development is progressing successfully in particular with the detection of the first gamma-ray burst at the ISDC ! And elsewhere, as we explain in two snapshots: at MPE Garching on SPI imaging simulations, which show the ability of SPI to distinguish sources as close as 1 degree apart (in ideal S/N conditions at least); and at Southampton, where particle interactions in gamma-ray satellites can now be spectacularly displayed using the Geant 4 Monte-Carlo code. Work being carried out at other places will be described in subsequent issues of this Newsletter.
2002 will be the decisive year for INTEGRAL: let's wish it, and us, a very fruitful year !
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| Overview of activities |
| Roland Walter (ISDC Geneva) |
INTEGRAL will be launched in 300 days and the ISDC is moving towards operations.
At the time of writing the ISDC is receiving and processing for the first time
data from the fully integrated spacecraft.
The image on the right are the very first results of the processing of these data. They show the IBIS/ISGRI (left) and JEM-X 2 (right) detector shadowgrams (i.e. before deconvolving the mask pattern) of a test source observed for the first time simultaneously by both instruments.
Last month the ISDC was involved in a ground segment integration test. We received telemetry (TM) in real-time from the Mission Operation Center (at ESOC, Darmstadt, Germany) during 5 days and were able to exercice many aspects of nominal operations. The test was very successful. It showed that the first part of the system, handling the data in real-time was almost ready for operations. The test got as far as simulating a gamma-ray burst (see the article on IBAS).
Early next year a series of end-to-end tests will occur involving the spacecraft and the ground segment that will be followed by a period of instrument calibration. Data corresponding to about 4 months of operations will be generated.
Those data will be used to perform the final tests of the part of the ISDC system involved with TM and auxiliary data processing, instrument monitoring, configuration/calibration analysis, as well as archiving and distribution.
An archive of calibration data will be built and made available to the INTEGRAL instrument teams for follow-up analysis.
The preparation of the commissioning phase and nominal operations also involve
other aspects. Definition of procedures and activities, hosting and training of new staff, building new office space (see left image) and installing operational hardware.
The image on the left shows the construction of the new additional building of the ISDC.
The part of the system that deals with scientic analysis is also progressing. Consolidated analysis scripts are already available for two of the four instruments. Sky images and spectra could be generated for JEM-X (see article on Standard Analysis products) and for SPI (see article on SPI calibration results). The analysis products are now being checked to make sure they could be used in standard software packages.
We are all impatient to see the data coming from our favorite objects.
| First gamma-ray burst detected by IBAS |
| Sandro Mereghetti (IFC/CNR Milano), Jurek Borkowski (ISDC Geneva) & Diego Goetz (IFC/CNR Milano) |
The INTEGRAL Burst Alert System (IBAS) has successfully detected its first gamma-ray burst (GRB) on November 7, during a Ground Segment Integration Test. Although both INTEGRAL and the GRB were ``virtual'' ones, this activity has been very useful to test the IBAS programs, in particular for what concerns their external interfaces.
The animation on the right shows a series of images produced and analyzed by IBAS during the GRB detection. Each image corresponds to a time interval of 1 second; the GRB lasts about 6 seconds.
Artificial data containing one GRB were produced at IFC/CNR Milano and fed into the INTEGRAL Simulator running at MOC in Darmstadt. The resulting telemetry was transmitted to the ISDC in the same way that will be used during operations. Here the IBAS software analysed in real time the incoming data of the ISGRI instrument, detected the GRB, and sent out an alert message with the derived coordinates. Since the GRB was found to lie within the field of view (5x5 square degrees) of the Optical Monitor Camera (OMC), IBAS also produced the information to reconfigure the CCD windows of the OMC and sent it back to the MOC.
We will provide the instructions to subscribe to the IBAS alerts in a forthcoming issue of this Newsletter.
| Standard Analysis Products |
| Peter Kretschmar (ISDC Geneva) |
Standard Analysis is the automated scientific analysis of INTEGRAL data at ISDC, using essentially the same tools as for interactive analysis but in a standardized fashion with fixed settings of most parameters.
Besides various intermediate data products, e.g. Good Time Intervals, shadowgrams, background model data, ... the main products of Standard Analysis are for the high energy instruments:
The images and spectra produced in this way are supposed to be directly
usable by standard analysis tools like DS9 for images and XSPEC
for spectral analysis.
The image on the left shows the sky image resulting from a simulation of 3 pointings of Vela X-1 with the JEM-X 1 instrument. The graph below is a cut through the image showing clearly the peak corresponding to the Vela X-1 source.
The figure on the right is an example of a JEM-X spectrum obtained after data analysis of a simulated source.
For the Optical Monitoring Camera (OMC) the main products are:
The instruments differ in their treatment of the fact that INTEGRAL's observations usually consist of many individual pointings due the dithering of the satellite. For SPI all results beyond the corrected events are generated for the observation as a whole, combining data across many pointings. In contrast, JEM-X and OMC analyze their data pointing by pointing, combining results only after the actual data reduction, e.g., by building a mosaic image out of the images generated per pointing. IBIS follows an intermediate strategy with some data generated by pointing and some for the observation as a whole. The ISDC will distribute tools to select specific results, e.g. all extracted spectra for a given source in order to make it easier for users to handle the multitude of products produced by the Standard Analysis.
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| SPI resolving power tested with calibration data |
| Andy Strong (MPE Garching) |
One of the main objectives of the SPI calibration at Bruyères-le-Châtel
earlier this year (see the article in a previous issue) was to determine the
instrumental imaging capabilities, including the ability to separarate
sources, i.e. the ``resolving power". Although it was not possible to perform the normal in-flight ``dithering'' strategy during the calibration, nor
to use actual multiple sources, nevertheless each source was observed
at many angles so that it is possible to combine them to synthesize a
limited dithering in one direction.
The images shown were produced
from multiple runs with a 60Co source at 125 m distance, selecting the 1173 keV line. The data were combined in such a way as to emulate a ``dithering'' observation of two sources separated by 2 degrees (image on the left)
and by 1 degree (image on the right).
The images were generated with the maximum entropy
method using the routine ``spiskymax'' at the ISDC, and the SPI
instrumental response matrices generated at the Goddard Space Flight
Centre. They demonstrate that SPI is capable of resolving sources very
well at 2 degrees and also performs well at 1 degree separation;
however it should be pointed out that the calibration conditions are
ideal in that the signal-to-noise is very high, which will not be the
case in orbit. For this reason the result can be seen as a
determination of the purely ``optical'' properties of the instrument
combined with our knowledge of the instrumental response, and as such
is very encouraging.
| The SwiMMing pool |
| Dave Willis and the INTEGRAL group at Southampton |
We show a few examples of Mass Model interactions, used in particular to model cosmic-ray interactions and the resulting pool of secondary particles within the INTEGRAL satellite (TIMM: The INTEGRAL Mass Model). The mass model here is the Swift Mass Model (SwiMM), developed for the Swift mission (rapid detection of gamma-ray bursts) using the latest version of the GEANT Monte Carlo code. The diagrams below show the interactions of a single 20 GeV proton ( thick blue dashed line). Green lines represent photons and red lines represent leptons.
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The image on the left represents the interation of a 20 GeV proton with a Cadmium nucleus in the CZT detector array (The use of Cadmium Zinc Telluride (CZT) is a step up from Cadmium Telluride and an important progress in gamma-ray detection). A cone of mesons and neutrons can be seen from the direct interaction. Electrons and photons can be seen as a result of the proton's interation and the interaction of the secondaries.
In the central image we see the proton interating with the graded-Z shielding. The proton strikes a Pb nucleus and produces numerous secondaries distributed over a range of angles. The secondaries can be seen to be interating with the Radiator (purple square on top of the wide field Burst Alert Telescope) demonstrating the need to include all passive material, even if it is outside of the shielding and the field of view.
In the diagram on the right we see the proton passing almost straight through the detector. A few photons and neutrons may be produced by the slight interaction with the detector plane, but in this case the majority of background inducing secondary particles are produced by an interaction with the aluminium shell of the spacecraft.
These pictures demonstrate the need for Monte Carlo simulations as opposed to statistical analysis. The number and concentration of secondaries would be near impossible to predict by cross-section statistics alone. In these three test cases the secondary particles are produced in different parts of the instrument and spacecraft. The basic paths and interations can be summarised from these diagrams but any proper evaluation would require millions of particles fired into the Swift Mass Model, as it would experience in orbit.
The figures showed here have been produced with GEANT 4. As far as INTEGRAL is concerned, when the TIMM project was started GEANT 3 was used since GEANT 4 was not available. The decision to upgrade TIMM to GEANT 4 has yet to be discussed.
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| A change in the outburst recurrence time of the Rapid Burster | |
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Nicola Masetti Istituto Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, via Gobetti 101, I-40129 Bologna, Italy | |
| Accepted for publication in A&A Letters on November 20, 2001 | |
| Abstract. An apparent sudden change in the outburst recurrence time displayed by the Rapid Burster occurred around the end of 1999. The time between consecutive outbursts shrinked from ~200 to ~100 days somewhere between November 1999 and March 2000. In parallel, the average peak intensity also decreased of a factor ~2 in all outbursts occurred after November 1999 with respect to those occurred before this date. I discuss these results by comparing them with similar cases of changes in the recurrence time of other transient X-ray binaries and with the behaviour of Dwarf Novae. A viable explanation is connected with changes in the value of the quiescence mass transfer rate from the secondary, although the detailed origin of these changes remains at best speculative. Further, this result has important implications for programming time-constrained observations of the Rapid Burster both in outburst and in quiescence. | |
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| X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her | |
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N. Masetti1, D. Dal Fiume1,2, G. Cusumano3, L.
Amati1, et al. 1. Istituto Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, via Gobetti 101, I-40129 Bologna, Italy 2. Deceased 3. Istituto di Fisica Cosmica ed Applicazioni all'Informatica, CNR, via ugo La Malfa 153, I-90146 Palermo, Italy | |
| Accepted for publication in A&A on November 7, 2001 | |
| Abstract. We report on ASCA and BeppoSAX X-ray broad band observations of the galactic low-luminosity X-ray source 4U 1700+24 performed on 1995 and 1998, respectively, and on (quasi-)simultaneous ground observations of its optical counterpart, V934 Her, from the Loiano 1.5-meter telescope. In order to better understand the nature of the source we also analyze public archival ROSAT and RXTE data as well as the RXTE ASM light curve of 4U 1700+24; we also re-analyze a 1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; this allows us to determine a revised distance to the object of ~400 pc. While these spectra do not show either any spectral change between the two epochs or any peculiar feature apart from those observed in normal red giants, the spectroscopic measurements carried out in X-rays reveal a complex and long-term variable spectrum, with a clear soft excess. The X-ray spectral properties of the source are best described by a thermal Comptonization spectrum plus a soft energy (<1 keV) excess, which can be modeled in the form of a blackbody emission with kTBB ~ 1 keV; the latter component is not detected at the lowest source flux levels. The ratio between the two c! omponents varies substantially with the source flux. The X-ray emission from the object appears to become harder as its luminosity increases: indeed, the RXTE data acquired during an outburst occurred in October-November 1997 display a hard tail, detected up to 100 keV and modeled with a comptonizing cloud which is hotter and less opaque than that seen in the low intensity state. Apart from erratic shot-noise variability on timescales of tens to thousands of seconds, no significant properties (such as pulsations or QPOs) are found from the timing analysis of the X-ray light curves extracted from the observations presented here. With the new distance determination, the 2-10 keV X-ray luminosity range spanned in the considered observations lies between ~2x1032 and ~1x1034 erg s-1. All this information, combined with the findings by other authors, allows us to suggest that the scenario which best describes the object consists of a wide binary system in which a neutron star accretes matter from the wind of a M-type giant star. Implications of such a model are discussed. | |
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E-mail contact | Preprint access |
| Toward Resolving the Crab σ-Problem: A Linear Accelerator? | |
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Ioannis Contopoulos1 and Demosthenes Kazanas2 1. ELVAL, Oinofyta, Greece 2. NASA/GSFC, Code 661, Greenbelt, MD 20771 | |
| Accepted for publication in ApJ on October 5, 2001 | |
| Abstract. Using the exact solution of the axisymmetric pulsar magnetosphere derived in a previous publication and the conservation laws of the associated MHD flow, we show that the Lorentz factor of the outflowing plasma increases linearly with distance from the light cylinder. Therefore, the ratio of the Poynting to particle energy flux, generically referred to as σ, decreases inversely proportional to distance, from a large value (typically ≥ 104) near the light cylinder to σ ~ 1 at a transistion distance Rtrans. Beyond this distance the inertial effects of the outflowing plasma become important and the magnetic field geometry must deviate from the almost monopolar form it attains between Rlc and Rtrans. We anticipate that this is achieved by collimation of the poloidal field lines toward the rotation axis, ensuring that the magnetic field pressure in the equatorial region will fall-off faster than R-2 (R being the cylindrical radius). This leads both to a value σ = σs ≪ 1 at the nebular reverse shock at distance Rs (Rs ≫ Rtrans) and to a component of the flow perpendicular to the equatorial component, as required by observation. The presence of the strong shock at R = Rs allows for the efficient conversion of kinetic energy into radiation. We speculate that the Crab pulsar is unique in requiring σs ~ 3x10-3 because of its small translational velocity, which allowed for the shock distance Rs to grow to values ≫ Rtrans. | |
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| BeppoSAX Spectral Survey of BL Lacs - new spectra and results | |
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V. Beckmann1, A. Wolter2, A. Celotti3, L.
Costamante2, G. Ghisellini2 et al. 1. INTEGRAL Science Data Centre, ch. d' Ecogia 16, CH-1290 Versoix, Switzerland 2. Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy 3. SISSA/ISAS, Via Beirut 2-4, I-34014 Trieste, Italy | |
| Accepted for publication in A&A on December 10, 2001 | |
| Abstract. We present BeppoSAX LECS, MECS, and PDS spectra of eleven X-ray selected BL Lacertae objects.Combining these sources with the ones presented elsewhere we have a sample of 21 BL Lacs from the Einstein Medium Sensitivity and Einstein Slew Survey. The sample shows strong correlations of several physical parameters with the peak frequency of the synchrotron branch of the spectral energy distribution. In particular the peak frequency is correlated to the X-ray spectral shape: objects with the peak near to the X-ray band show harder and straighter X-ray spectra than those of the low energy peaked sources. This work shows that the recently proposed unification scenario for different types of blazars can hold also within the class of high frequency peaked BL Lac objects. | |
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| Timing and Spectral Properties of X-Ray Emission from the Converging Flows onto a Black Hole: Monte Carlo Simulations | |
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P. Laurent1, L. Titarchuk2,3 1. CEA/DSM/DAPNIA, 91191 Gif sur Yvette Cedex, France 2. George Mason University/Center for Earth Observing and Space Research, Fairfax, VA 22030-444 3. US Naval Research Laboratory, Space Science Division, 4555 Overlook Avenue, SW, Washington, DC 20375-5352 | |
| Accepted for publication in ApJ Letters on October 11, 2001 | |
| Abstract. We demonstrate that an X-ray spectrum of a converging inflow (CI) onto a black hole is the sum of a thermal (disk) component and the convolution of some fraction of this component with the Comptonization spread (Green's) function. The latter component is seen as an extended power law at energies much higher than the characteristic energy of the soft photons. We show that the high-energy photon production (source function) in the CI atmosphere is distributed with the characteristic maximum at about the photon bending radius, 1.5 rS, independently of the seed (soft) photon distribution. We show that high-frequency oscillations of the soft photon source in this region leads to the oscillations of the high-energy part of the spectrum but not of the thermal component. The high-frequency oscillations of the inner region are not significant in the thermal component of the spectrum. We further demonstrate that Doppler and recoil effects (which are responsible for the formation of the CI spectrum) are related to the hard (positive) and soft (negative) time lags between the soft and hard photon energy channels, respectively. | |
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E-mail contact | |
| Two Different Long-term Behaviors in Black-Hole Candidates: Evidence for Two Accretion Flows? | |
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D. M. Smith1, W. A. Heindl2, & J. H. Swank3 1. Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720, USA 2. Center for Astrophysics and Space Science, University of California, San Diego, USA 3. NASA Goddard Space Flight Center, USA | |
| Accepted for publication in ApJ on December 12, 2001 | |
| Abstract. We discuss the results of long-term hard x-ray monitoring of Galactic black-hole candidates 1E 1740.7-2942, GRS 1758-258, Cyg X-1, GX 339-4, and Cyg X-3 with the Rossi X-Ray Timing Explorer (RXTE). The objects divide into two classes. In the first class, exemplified by Cyg X-1, luminosity and spectral hardness evolve simultaneously. In the second class, the relation is more complicated: the softest spectra occur while the count rate is dropping. Most models of accretion, tailored to Cyg X-1, do not predict the second sort of behavior. One interpretation is a simple model with two simultaneous, independent accretion flows: a thin disk and a hot halo. A drop in the accretion rate affecting both flows would propagate through the halo immediately but might take up to several weeks to propagate through the disk. While the inner halo is thus temporarily depleted compared to the disk, a temporary soft state is expected. This picture is supported by the observation th! at those sources which show delays (1E 1740.7-2942, GRS 1758-258, and GX 339-4) are expected to have low-mass companions, and those which do not (Cyg X-1, Cyg X-3) are known or thought to have high-mass companions. Low-mass companions imply accretion by Roche-lobe overflow, with a high specific angular momentum in the accreting material, and therefore a large disk with a long viscous timescale. Wind accretion from massive companions is expected to result in a much smaller disk, and thus little viscous delay. | |
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