| No. 2 - September 15, 2000 | Edited by Thierry Montmerle & Marc Türler |
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The first issue of the Newsletter presented in detail the IBIS and OMC instruments. This second issue does not include further "INTEGRAL News", but instead a description of the Observation Simulator (OSim) and a presentation of the INTEGRAL Burst Alert System (IBAS). These two contributions fill the "ISDC News" section.
In the "Science News" part, you will find several abstracts and a thesis summary. (There is now a dedicated Web interface to send recently completed Ph.D. thesis summaries). Selected papers from the "e-print archive" are listed on a separated page in order to keep the Newsletter compact.
Finally, you will find several conference announcements and a job offer in the "Community News" section.
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| The ISDC Observation Simulator |
| Antony Bird (University of Southampton), Davide Cremonesi (IFCTR Milano), Jon Lockley (University of Southampton), Ada Paizis (ISDC), Andrew Strong (MPE Garching), Roland Walter (ISDC) and Niels Joergen Westergaard (DSRI Copenhagen) |
The Observation Simulator (OSim) is a software package developed at the ISDC. Its main purpose is to generate accurate simulated data for the high energy instruments on board INTEGRAL to test the scientific (standard and quick-look) analysis pipelines. OSim generates data (at the corrected data level) in a format very close to the one to be used during INTEGRAL operations. All data are stored into FITS files. Those simulated data can then be fed in the analysis software.
The OSim simulates data based on models of the instruments and of the background. The results are therefore only approximation of the reality. OSim may be improved once the ground calibration of the INTEGRAL instrument will be completed and of course once the flight calibration will be performed.
The use of the OSim and of the prototype analysis tools already available allows to simulate an INTEGRAL observation and to derive standard analysis products. Most simulations run 10 to 20 times faster than the time it would take to perform the observation. As the standard analysis chains are not yet available only part of the standard products can be generated. At the time of writing sky images can be generated for the imager (ISGRI detector), the spectrometer and the X-ray monitors and source spectra can be generated for the imager (ISGRI). More standard data products are expected to become available during the next months.
From november 1st 2000, visitors will have access to the OSim and to the prototype analysis tools at ISDC with some support. Based on the experience gained with the users the ISDC may distribute parts of the OSim and prototype analysis software. This software will first be released for the SUN/Solaris platform and possibly then for Linux. As the ISDC staff is now very busy integrating the software necessary for the INTEGRAL launch, user support related to OSim will be very limited.
Please note that, when proposing observation with INTEGRAL, observers should use the Observation Time Estimator (OTE) provided by the INTEGRAL Science Operations Centre (ISOC).
The OSim Web page will continuously reflect the status of the development and contains examples of simulated observations and results. The status of OSim will also be regularly reported in the ISDC-Astrophysics Newsletter.
The Observation Simulator and the prototype analysis tools are composed of several packages
A detailled description is available for all of them, by selecting the relevant hyperlink.
| Rapid Localization of Gamma-Ray Bursts with INTEGRAL |
| Sandro Mereghetti (IFC/CNR Milano) |
The INTEGRAL satellite will play an important role in the study of gamma-ray bursts, thanks to two important characteristics that were never available before in a gamma-ray astronomy mission with wide astrophysical objectives: the excellent imaging capabilities at high energies, and the fact that all the data will be immediately transmitted to the ground.
The IBIS instrument, with a field of view of 30 x 30 degrees, an angular resolution of 12 arcmin, and a source location accuracy of 1-2 arcmin (and even better for bright sources), will be the principal instrument used for the rapid localization of gamma-ray bursts. The chances that a gamma-ray burst take place inside the IBIS field of view are significant. An accurate estimate has been done, based on the BATSE number-flux relations for gamma-ray bursts (properly converted to the lower energy range of IBIS) and taking into account the instrument sensitivity at different off-axis directions within the totally and partially coded field of view. This yields an expected rate of more than 13 gamma-ray bursts per year. Furthermore, since it will also be possible to detect bursts with both shorter and longer duration than those seen by BATSE, the actual number could well be more than a factor two greater.
The high eccentric orbit of INTEGRAL, with a period of 72 hours, allows a nearly continuous contact with the spacecraft. The telemetry data received at the ESA Mission Operation Center in Darmstadt through the Redu and Goldstone ground stations, will be immediately sent at the ISDC on a dedicated line. Here they will be analyzed in real time by an automatic software, thus allowing the gamma-ray bursts detection and localization within at most a few tens of second after they have been observed by the INTEGRAL instruments. The celestial coordinates of the gamma-ray bursts will be automatically distributed through Internet to interested observers within a minimum time delay.
The ISDC software devoted to the rapid detection and localization of gamma-ray bursts is called the INTEGRAL Burst Alert System (IBAS). A first version of IBAS has already been developed, integrated at the ISDC and tested using telemetry data containing simulated gamma-ray bursts. The first step of the gamma-ray burst search is based on the simple monitoring of the incoming count rates, without resorting to more complex image analysis. In practice this is done by looking for significant excesses with respect to a running average, in a way similar to traditional on-board triggering algorithms. In fact any transient source strong enough to appear as a significant new peak in the deconvolved images will also produce a detectable excess in the overall count rate. The search is simultaneously performed in many different time scales and energy ranges, to optimize the sensitivity to bursts with different properties.
When a candidate event is detected, a process of image analysis shall start to verify the origin of the count rate variation and to ensure that the event was not caused by an instrumental malfunctioning or by a background variation. Images shall be accumumulated for different time intervals, deconvolved with very fast algorithms, and compared to the pre-burst reference images in order to detect and localize the new source. If the event is genuine, the satellite attitude information will be applied to derive a sky position that is then automatically transmitted, by e-mail and/or direct UDP/IP socket, to all the subscribed users. (Please note that information on how to subscribe to the automatic distribution of the IBAS will be diffused in a coming issue of the Newsletter.)
Because full event validation and localization might require a longer time, we foresee different levels of alert messages providing increasingly accurate and reliable information. These messages will be configured in such a way to allow an easy filtering by the users in order to react only to the situations that best fit their needs. In addition, if the burst is located in the sky region covered by the OMC, an appropriate telecommand with the definition of a new CCD window centered on the gamma-ray burst error region will be generated and sent to the satellite.
INTEGRAL will be the first mission in which the search for gamma-ray bursts will be done on ground, rather than by a dedicated on-board software. This has the advantage of allowing a greater computing power, and most importantly a large flexibility to change and improve the algorithms in order to optimize the sensitivity of the search. We expect to be able to provide the positions of more than one burst per month with an accuracy generally of the order of a few arcminutes within a delay of only a few tens of seconds from the occurrence of the gamma-ray burst.
More information on the IBAS can be found in the following references:
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| Aspects of the Spectral Evolution of Cosmic Gamma-Ray Bursts | |
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Felix Ryde Stockholm Observatory, SE-133 36 Saltsjöbaden, Sweden | |
| Ph.D. thesis directed by Roland Svensson at the Stockholm University | |
| Ph.D. degree awarded on May, 2000 | ISBN 91-7265-106-7 |
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Summary. Ever since their discovery at the end of the 1960s, the occasional, short flashes of
gamma-rays, denoted gamma-ray bursts (GRBs), have been some of the most enigmatic phenomena to have
been
encountered in astrophysics. Large resources are being put into the quest to understand these objects
and great progress has been made. In particular, during recent years it has become evident that GRBs
lie at large, cosmological distances, which implies, from the measured energies, that they are the
most powerful explosions in the Universe since its creation. They are detected approximately once per
day
and occur in an average galaxy probably once every 10 million years.
This thesis discusses various aspects of the spectral and temporal behaviour of the gamma-ray emission in long and bright pulses of prompt GRBs. This is studied both by analytical derivations and through the study of data from the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO) satellite. A self-consistent formulation of the spectral and temporal evolution during the decay of a GRB pulse is presented and explored. This leads to the finding that the decay of GRB pulses can be described by a particular power-law function and that there is a bimodality in the distribution of the associated power-law index. The importance of studying the temporally resolved spectra during a GRB, and especially during a pulse, is stressed. These spectra have a direct connection with the underlying emission process (possibly affected by relativistic effects due to the outflow emitting the gamma-rays). The time-integrated spectrum, on the other hand, reflects mainly the spectral evolution. Analytical results are given, which connect the properties of the time-integrated spectrum with those of the time-resolved spectra, and are thus useful when studying observed GRB pulse spectra. The correlation between the peak energy of the instantaneous spectrum (as a measure of spectral hardness) and the corresponding intensity during a burst is of great importance and special attention is devoted to it. A new method for studying this relation is introduced, which is advantageous when a large fraction of the bolometric flux lies outside the narrow band over which the spectrum is observed. It is found that this correlation, between the hardness of the spectrum and the intensity, among pulses within a single burst is more similar than that among pulses from different bursts. Also, a characteristic signature in the hardness-intensity correlation is found, which is interpreted as the result of heavily overlapping pulses. Furthermore, there is an indication that this correlation might be the fundamental one, valid for GRB pulses even if their light curves have diverse behaviours. Finally, a new parametric function for fitting spectra is presented. This function is especially useful for broad spectra and it is compared with previously introduced functions. This young and highly active field of astrophysical research will continue to blossom with the help of numerous new satellite missions and will help in unveiling the secrets of GRBs and may eventually provide a way of studying the early Universe. | |
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E-mail contact | Thesis access |
| Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-Ray Tail | |
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J. Martin Laming US Naval Research Laboratory, Washington DC 20375, USA | |
| Accepted for publication in ApJ on August 26, 2000 | |
| Abstract. We propose a model for the hard X-ray (> 10 keV) emission observed from the supernova remnant Cas A. Lower hybrid waves are generated in strong (mG) magnetic fields, generally believed to reside in this remnant, by shocks reflected from density inhomogeneities. These then accelerate electrons to energies of several tens of keV. Around 4% of the x-ray emitting plasma electrons need to be in this accelerated distribution, which extends up to electron velocities of order the electron Alfven speed, and is directed along magnetic field lines. Bremsstrahlung from these electrons produces the observed hard x-ray emission. Such waves and accelerated electrons have been observed in situ at Comet Halley, and we discuss the viability of the extrapolation from this case to the parameters relevant to Cas A. | |
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E-mail contact | Preprint access |
| Modelling 20 years of synchrotron flaring in the jet of 3C 273 | |
|
M. Türler1,2, T. J.-L. Courvoisier1,2, S.
Paltani1,2 1. INTEGRAL Science Data Centre, ch. d' Ecogia 16, CH-1290 Versoix, Switzerland 2. Geneva Observatory, ch. des Maillettes 51, CH-1290 Sauverny, Switzerland | |
| Accepted for publication in A&A on July 5, 2000 | |
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Abstract. We present a phenomenological jet model which is able to reproduce well the observed
variations of the submillimetre-to-radio emission of the bright quasar 3C 273 during the last 20 years.
It is a generalization of the original shock model of Marscher & Gear (1985), which is now able to describe an accelerating or decelerating shock wave, in a curved, non-conical and non-adiabatic jet.
The model defines the properties of a synchrotron outburst which is expected to be emitted by the jet material in a small region just behind the shock front.
By a proper parameterization of the average outburst's evolution and of the peculiarities of individual outbursts, we are able to decompose simultaneously thirteen long-term light-curves of 3C 273 in a series of seventeen distinct outbursts.
It is the first time that a model is so closely confronted to the long-term multi-wavelength variability properties of a quasar. The ability of the model to reproduce the very different shapes of the submillimetre-to-radio light curves of 3C 273 gives strong support to the shock model of Marscher & Gear (1985). Indirectly, it also reinforces the idea that the outbursts seen in the light-curves are physically linked to the distinct features observed to move along the jet with apparently superluminal velocities. | |
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E-mail contact | Preprint access |
| On the Hardness-Intensity Correlation in Gamma-Ray Burst Pulses | |
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L. Borgonovo & F. Ryde Stockholm Observatory, SE-133 36 Saltjäbaden, Sweden | |
| Accepted for publication in ApJ on September 8, 2000 | |
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Abstract. We study the hardness-intensity correlation (HIC) in gamma-ray bursts
(GRBs). In particular, we analyze the decay phase of pulse structures
in their light curves. The study comprises a sample of 82 long
pulses selected from 66 long bursts observed by the Burst And Transient
Source Experiment (BATSE) on the Compton Gamma-Ray Observatory.
We find that at least 57 % of these pulses have HICs that can be well
described by a power law. A number of the other cases can still be
explained with the power law model if various limitations of the
observations are taken into account.
The distribution of the power law indices γ, obtained by modeling the HIC of pulses from different bursts, is broad with a mean of 1.9 and a standard deviation of 0.7. We also compare indices among pulses from the same bursts and find that their distribution is significantly narrower. The probability p of a random coincidence is shown to be very small ( p < 2 10-5). In most cases, the indices are equal to within the uncertainties. These results demand a physical model to be able to reproduce multiple pulses with similar characteristics for an individual burst, but with a large diversity for pulses from an ensemble of bursts. This is particularly relevant when comparing the external versus the internal models. In our analysis, we also use a new method for studying the hardness-intensity correlation, in which the intensity is represented by the peak value of the E F(E) spectrum, where E is the energy and F (E) is the energy flux spectrum. We compare it to the traditional method in which the intensity over a finite energy range is used instead, which may be an incorrect measure of the bolometric intensity. This new method gives stronger correlations and is useful in the study of various aspects of the HIC. In particular, it produces a better agreement between indices of different pulses within the same burst. Also, we find that some pulses exhibit a track jump in their HICs, in which the correlation jumps between two power laws with the same index. We discuss the possibility that the track jump is caused by strongly overlapping pulses. Based on our findings, the constancy of the index is proposed to be used as a tool for pulse identification in overlapping pulses and examples of its application are given. | |
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E-mail contact | Preprint access |
| Flash Heating of Circumstellar Clouds by Gamma-Ray Bursts | |
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Charles D. Dermer1 & Markus Böttcher2 1. Naval Research Laboratory, Code 7653, Washington, DC 20375-5352 USA 2. Department of Space Physics and Astronomy, Rice University, Houston, TX 77005-1892 | |
| Accepted for publication in ApJ on March 31, 2000 (ApJ 534, L155) | |
| Abstract. The blast-wave model for gamma-ray bursts (GRBs) has been called into question by observations of spectra from GRBs that are harder than can be produced through optically thin synchrotron emission. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense γ radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, hardening the intrisic spectrum. This effect resolves the "line-of-death" objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions. | |
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E-mail contact | |
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