| No. 9 - March 28, 2002 | Edited by Thierry Montmerle & Marc Türler |
 |
 |
 |
| << Previous issue | Next issue >> |
| |||
| |||
|
| INTEGRAL: real and virtual |
In parallel, imaging simulations of astrophysical sources, based on data obtained with previous-generation instruments like Sigma and OSSE, continue to be developed. As an example, Andy Strong and Paul Connell have obtained Core Program "virtual" images of the Galactic Center Deep Exposure, emphasizing both point sources and diffuse emission as seen by SPI.
The ESTEC calibrations have been followed continuously by the ISDC staff and the first "real" gamma-ray burst was successfully detected by the IBAS system. The ISDC Quick-Look Analysis is another tool to detect rapidly unforeseen events in the gamma-ray sky such as new sources or known sources in flaring state. The analysis scripts for this tool as well as for the standard analysis are based on a C++ interpreter called Root.
All these developments around INTEGRAL make us more and more impatient to look at the "real" gamma-ray sky: we're now at less than 200 days from launch !
 |
 |
|
| Overview of activities |
| Roland Walter (ISDC Geneva) |
A 3 weeks calibration of the INTEGRAL payload took place in ESTEC in February. The high energy instruments were irradiated by a number of sources of X-rays and gamma-rays in various configuration. The data are currently analyzed in the instrument teams to understand the instrument behavior and to build or test the instrument calibration and response files (see the INTEGRAL News section below). Those data are also being processed at the ISDC to test the ground system and will be archived in the same format than the flight data.
Several gamma-ray bursts (GRB) have been simulated during the calibration
campaign, which were successfully detected in real-time at the ISDC with a
software specially tuned for sources located nearby the instruments rather
than at infinity.
The image on the right shows the detection (image, spectrum and light curve) of a "real" gamma-ray burst. "Real" means here that unlike the virtual GRB reported in the last Newsletter the photons of the simulated GRB were produced by an X-ray source switched on and off and were detected by the INTEGRAL instruments. For more information please look at the INTEGRAL picture of the month of March 2002.
A few days before the calibrations an end-to-end test took place involving all the elements of the ground segment and the spacecraft. Most operations taking place along the orbit have been exercised including science observations. The test ran successfully and was used to evaluate most aspects of the ground system from observation planning to scientific analysis. This test was also used to simulate operational activities, in particular members of the instrument teams were present to train the set-up foreseen for the commissioning phase.
The processing of those data has been used to finalize the software needed for the technical processing and archiving. This part of the ISDC system will be ready soon for the simulation phase prior to launch.
The scientific analysis software is also maturing for all instruments. The executables performing the basic block of the analysis (ftools like) have been integrated into analysis scripts written in the Root C++ framework (see the Root article below). Those scripts will be used by the standard analysis and will be distributed to the community. During the coming weeks the technical and scientific validation of the analysis software will be finalized based on simulated and calibration data.
Procedures for the release of the analysis software have been sorted out. The documentation is being written. Effort to port the distributed software to Linux have started and will be pursued. The aim is of course to be able to distribute data and software early in the mission.
We still have 200 days to become ready for operations and community support. The status will be reviewed at the Ground Segment Readiness Review that will take place next June.
| The Quick-Look Analysis |
| Ken Ebisawa (ISDC Geneva) |
The Quick-Look Analysis (QLA) performed at the ISDC, is intended to search for important transient astronomical phenomena in the INTEGRAL data, and alert the scientific community as quickly as possible.
Basically, the same scientific scripts to be used by Guest Observers are run on all the data automatically, and all the point sources are searched for. The resultant source list and source fluxes are compared with the standard X-ray/gamma-ray catalog (which we are currently compiling), and new sources, significant increases or decreases of the source fluxes, or significant spectral changes will be spotted. Particular attention will be paid for the Targets of Opportunity (TOO) to check if their triggering criteria are satisfied.
The QLA will run on the IBIS and JEM-X data of each Science Window lasting about 30 minutes. This so called "ScW QLA" will be able to issue alerts for bright sources within one or two hours after the observation. To reach higher sensitivities, however with a longer delay, the QLA will also be run on observation groups of several Science Windows (Obs QLA) for the IBIS, JEM-X and also SPI data.
Approximate QLA 5-sigma detection sensitivities are the following: For the ScW QLA (~30 min exposure), 4.2 mCrab (at 6 keV) or 11.8 mCrab (at 20 keV) with JEM-X, and 40 mCrab (at 100 keV) and 1.8 Crab (at 1 MeV) with IBIS. For the Obs QLA combining 5 x 5 Science Windows, i.e. with ~5 times better sensitivity, we obtain 0.8 mCrab (at 6 keV) or 2.2 mCrab (at 20 keV) with JEM-X and 8.4 mCrab (at 100 keV) or 0.4 Crab (at 1 MeV) with IBIS.
The improvement of the QLA will be done continuously in parallel to the improvement of the standard scientific analysis. As an example, it is foreseen to implement timing analysis (such as pulsation search) into the QLA.
| The use of Root at the ISDC |
| Bruce O'Neel (ISDC Geneva) |
ISDC Scientific Analysis procedures will be in the form of Root
analysis scripts. Root
is an object-oriented scientific analysis package used for particle physics
at CERN, Geneva.
It consists of a C++ interpreter which can be easily extended to include
user functions. It includes a large number of C++ scientific analysis
classes as well as both 2D and 3D plotting and a graphical user interface (GUI).
Root continues to be actively developed both for a number of high-energy
physics experiments as well as for the GLAST (Gamma-Ray Large Area Space
Telescope) mission.
The image illustrates the graphical 2D and 3D display capabilities of Root. The characteristics of SPI events are displayed here as part of the Interactive Operation Status Monitoring (IOSM) performed at the ISDC.
In the case of ISDC, all of the ISDC libraries (eg: the Data Access Libraries) will be available from the Root command line. In addition, work continues on using C++ features to allow easer an easier syntax for scientific analysis using ISDC libraries and ISDC tools. All of the ISDC distributed analysis tools will run within Root and Ftools like tools are also easy to run. The ISDC scientific analysis scripts run from within Root and will be easily extendable and changable in order to allow scientists to do custom INTEGRAL analyses.
|
|
 |
|
| The INTEGRAL payload calibrations successfully completed at ESTEC |
| Philippe Laurent & Jacques Paul (CEA Saclay) |
From January 23 to February 6, 2002, an international team has fulfilled a very
important calibration campaign on the whole INTEGRAL payload, in one of the
clean rooms of ESTEC. The satellite was in its in-flight configuration, with
the flight models of the two gamma-ray telescopes IBIS and SPI, and the two
X-ray monitors JEM-X.
The INTEGRAL satellite flight model in the clean room of ESTEC at Noordwijk (Netherlands) where the calibration campaign took place. The payload, mounted on a service module similar to the XMM one, is composed of the SPI spectrometer (right), the IBIS telescope (left), and the two X-ray monitors JEM-X in between (Image courtesy S. Schanne).
A calibration device, developed and built by CEA/Saclay, has allowed the positioning of different types of calibration sources with the position accuracy required by the alignment process. This device is composed of an X-ray generator, a carrousel of fluorescence targets, and a source holder in order to accommodate the 14 radioactive sources used during the campaign. CEA/DAPNIA has also built for this campaign a Fast Acquisition System which has allowed to accelerate the recording of IBIS events, and thus to shorten the calibration duration.
 |
Illustration of the calibration source change procedure above the INTEGRAL payload in the clean room of ESTEC in Noordwijk. |
 |
The calibration device built by CEA/DAPNIA to accommodate, with a high mechanical precision, the calibration sources. From left to right we see the carrousel for the fluorescence target, the X-ray generator (in blue), and the source holder and collimator. |
At the end of the campaign, over two billion photons were recorded and analyzed by the different calibration teams. These photons had energies between 17 keV and 2750 keV, and were emitted at several different incidences. The capability of the IBIS telescope to make images was measured from 17 keV (with the ISGRI detector), up to 2,75 MeV (with the PICsIT detector).
 |
Energy spectrum recorded by the spectrometer SPI of the fluorescence emission of molybdenum at 17 keV. |  |
Energy spectrum recorded by SPI of the 2,75 MeV radiation resulting from the decay of a 24Na radioactive source. |
 |
Shadowgram image of the 17 keV molybdenum radiation projecting the IBIS mask on the ISGRI detector plane. |  |
Shadowgram image of a 2.75 MeV 24Na source projecting the IBIS mask on the PICsIT detector plane. |
The analysis of this huge amount of data - equivalent to one year of source photons in flight - will enable the calibration teams to harmonize the spectral response of the different instruments over the whole INTEGRAL energy range, and also to measure the perturbations induced by the surrounding material on each of these instruments.
 |
Image of the SPI coded mask projected onto the IBIS/PICSIT detector by a 2.75 MeV source at a large incidence angle with respect to the IBIS telescope axis. |  |
Image of the structure holding the two JEM-X coded masks, as seen by IBIS/PICSIT at 2.75 MeV. One can see on top of the image a part of the IBIS coded mask. |
| SPI Data Analysis Simulations: the example of the Galactic Center Deep Exposure |
| Andy Strong (MPE Garching) |
A presentation of SPI Data Analysis was given by A. Strong at the Moriond Workshop "The Gamma-Ray Universe" in March 10-15. This was intended to give a very broad overview of the SPI possibilities from the user's point of view. We give here an introductory selection from the talk, focused on simulated data from the Galactic Center Deep Exposure (GCDE), which is part of the INTEGRAL Core Program.
There are two main methods to analyse SPI data: "spiros" and "spiskymax". The main goals of spiros are source detection, location and spectral extraction, while spiskymax is aimed mainly at imaging diffuse emission. spiros has also imaging and time variability functions, and can provide inputs to XSPEC for spectral model fitting. spiros is developed by Paul Connell at the University of Birmingham and spiskymax by A. Strong at MPE in Garching.
First, to illustrate the spiros method in the source mode, here is a simulation of the GCDE, based on five sources from the SIGMA catalogue (see left image),and a simulation of spectral extraction for 4 sources with different spectral shapes, observed for 106 s (see right image).
 |
spiros simulated reconstruction of 5 sources from the SIGMA catalogue, in the 70-150 keV band. Data are for a Core Program Galactic Centre Deep Exposure pattern with total time 4.8 x 106 seconds. |  |
spiros simulated spectral extraction for 4 sources with different spectra. An 11 x 11 degree dithering pattern with total exposure 106 seconds was used. The sources are first detected, then accurately located, and finally the spectra fitted simultaneously. |
Second, to illustrate spiskymax, here are simulations of the GCDE observations of the 200-400 keV diffuse continuum and the diffuse 511 keV line emssion. The inputs for the two simulations is based on OSSE results.
 |
 |
Simulations of spiskymax images of the Galactic diffuse 200-400 keV continuum (left image) and 511 keV line emission (right image). Data were simulated for one year of GCDE (2 cycles of 4.8 x 106 sec). The models are based on OSSE results, and the obtained longitude-latitude profiles are consistent with the models within the error bars. (Images by A. Strong).
Other simulations have been done for multiple sources, the galactic 1809 keV 26Al diffuse emission, and the 44Ti 1157 keV line emission from Cas A.
|
|
 |
|
| Double-Head Photon-Counting Photometer for Fast Timing Observations | |
|
A. Piccioni1, F. Giovannelli2,C. Bartolini1, I.
Bruni1, et al. 1. Dipartimento di Astronomia, Universit\`a di Bologna, via Ranzani 1, I 40127 Bologna, Italy 2. Istituto di Astrofisica Spaziale, CNR, Area di Ricerca di Roma Tor Vergata, Via del Fosso del Cavaliere 100, I 00133 Roma, Italy | |
| Accepted for publication in Astronomy Reports on October 23, 2001 | |
| Abstract. The structure of the electronics of the two head, high speed photometer, up to now operating at the 152 cm G.D. Cassini telescope of the Bologna University, has been fully redesigned to perform high-time-resolution observations of fast variable cosmic sources, and in particular of optical counterparts of X-ray binary systems. The fast photometer allows now sampling times from 0.1 ms up to 100 s. The system clock handling and the I/O configuration have been modified taking into account the need of a rigorous synchronization of the sampling time with an external high stability clock and to allow a continuous acquisition of an uninterrupted stream of data. The continuous acquisition process, driven by the external clock, is limited only by the capacity of the hard disk and is supported by a double buffer SRAM memory designed to overcome any discontinuity in the asynchronous communication with the computer. Real time monitoring of data allows a continuous evaluation of the weather conditions; the light curve, directly displayed, suggests the best management of the observations. | |
|
E-mail contact | Preprint access |
| Observational Signatures of Black Holes: Spectral and Temporal Features of XTE J1550-564 | |
|
L.Titarchuk1,2, C. Shrader2,3 1. George Mason University/ CEOSR 2. US Naval Research Laboratory 3. NASA/Goddard Space Flight Center | |
| Accepted for publication in ApJ on November 13 2001 | |
| Abstract. The theoretical predictions of the converging inflow, or Bulk-Motion Comptonization model are discussed and some predictions are compared to X- and gamma-ray observations of the high/soft state of Galactic black hole candidate XTE J1550+564. The hundred Hz QPO phenomenon tends to be detected in the high-state at times when the bolometric luminosity surges and the hard-power-law spectral component is dominant. Furthermore, the power in these features increases with energy. We offer interpretation of this phenomenon, as oscillations of the innermost part of the accretion disk, which in turn supplies the seed photons for the converging inflow where the hard power-law is formed through Bulk Motion Comptonization (BMC). We further argue that the noted lack of coherence between intensity variations of the high-soft-state low and high energy bands is a natural consequence of our model, and that a natural explanation for the observed hard and soft lag phenomenon is offered. In addition, we address some criticisms of the BMC model supporting our claims with observational results. | |
|
E-mail contact | Preprint access |
| Spectra of the Expansion Stage of X-ray Bursts | |
|
N. Shaposhnikov1, L.Titarchuk1,2 1. George Mason University/CEOSR 2. NASA/Goddard Space Flight Center | |
| Accepted for publication in ApJ on November 9, 2001 | |
| Abstract. We present an analytical theory of thermonuclear X-ray burst atmosphere structure. Newtonian gravity and diffusion approximation are assumed. Hydrodynamic and thermodynamic profiles are obtained as a numerical solution of the Cauchy problem for the first-order ordinary differential equation. We further elaborate a combined approach to the radiative transfer problem which yields the spectrum of the expansion stage of X-ray bursts in analytical form where Comptonization and free-free absorption-emission processes are accounted for. Relaxation method on an energy opacity grid is used to simulate radiative diffusion process in order to match analytical form of spectrum, which contains free parameter, to energy axis. Numerical and analytical results show high similarity. All spectra consist of a power-law soft component and diluted black-body hard tail. We derive simple approximation formulae usable for mass-radius determination by observational spectra fitting. | |
|
E-mail contact | Preprint access |
| COMPTEL Observations of the Gamma-Ray Blazar PKS 1622-297 | |
|
S. Zhang1, W. Collmar1, K. Bennett2, H.
Bloemen3, W. Hermsen3 1. Max-Planck-Institut fur Extraterrestrische Physik, Postfach 1603, C-85740 Garching, Germany 2. Astrophysics Division, Space Science Department of ESA/ESTEC, NL-2200 AG Noordwijk, The Netherland 3. SRON National Institute for Space Research, Sorbonnelaan 2, NL-3584 CA Utrecht, The Netherland | |
| Accepted for publication in A&A on February 19, 2002 | |
| Abstract. We report results of observations and analyses on the gamma-ray blazar PKS 1622-297, with emphasis on the COMPTEL data (0.75 - 30 MeV) collected between April 1991 to November 1997. PKS 1622-297 was detected as a source of gamma-rays by the EGRET experiment aboard CGRO in 1995 during a gamma-ray outburst at energies above 100 MeV lasting for five weeks. In this time the blazar was significantly (~ 5.9 sigma) detected by COMPTEL at 10-30 MeV. At lower COMPTEL energies the detection is marginal, resulting in a hard MeV spectrum. The combined COMPTEL/EGRET energy spectrum shows a break at MeV energies. The broad-band spectrum (radio to gamma-rays) shows that the gamma-ray emission dominates the overall power output. On top of the 5-week gamma-ray outburst, EGRET detected a huge flare lasting for > 1 day. Enhanced MeV emission (10 - 30 MeV) is found near the time of this flare, suggesting a possible time delay with respect to the emission above 100 MeV. Outside the 5-week flaring period in 1995, we do not detect MeV emission from PKS 1622-297. | |
|
E-mail contact | |
| Energy dependence of a low frequency QPO in GRS 1915+105 | |
|
J. Rodriguez, et al. DSM/DAPNIA/Service d'Astrophysique (CNRS URA 2052), CEA Saclay, F-91191 Gif sur Yvette, France | |
| Accepted for publication in A&A on February 5, 2002 | |
| Abstract. We analyze a set of three RXTE Target of Opportunity observations of the Galactic microquasar GRS 1915+105, observed on April 2000, during a multi-wavelength campaign. During the three observations, a strong, variable low frequency (2-9 Hz) quasi periodic oscillation (hereafter QPO), often referred to as the ubiquitous QPO, is detected together with its first harmonic. We study the spectral properties of both features, and show that : 1) their frequency variations are better correlated with the soft X-ray flux (2-5 keV), favoring thus the location of the QPO in the accretion disk; 2) the QPO affects more the hard X-rays, usually taken as the signature of an inverse compton scattering of the soft photons in a corona; 3) the fundamental and its harmonic do not behave in the same manner: the fundamental sees its power increase with the energy up to 40 keV, whereas the harmonic increases up to ~10 keV. The results presented here could find an explanation in the context of the Accretion-Ejection Instability, which could appear as a rotating spiral or hot point located in the disk, between its innermost edge and the corotation radius. The presence of the harmonic could then be a signature of the non-linear behavior of the instability. The high-energy (>40 keV) decrease of the fundamental would favor an interpretation where most or all of the quasi-periodic modulation at high energies comes, not from the comptonized corona as usually assumed, but from a hot point in the optically thick disk. | |
|
E-mail contact | Preprint access |
| Three Type I X-ray bursts from Cyg X-2: Application of Analytical Models for Neutron Star Mass and Radius Determination | |
|
L. Titarchuk, N. Shaposhnikov 1. George Mason University/CEOSR 2. US NRL Code 7655, Washington, DC 20375-5352; lev@xip.nrl.navy.mil | |
| Accepted for publication in ApJ Letters on March, 22 2002 | |
| Abstract. We study the spectral and temporal properties of three Type I X-ray bursts observed from Cyg X-2 with the Rossi X-ray Timing Explorer. Despite the short time durations (~ 5 sec) these bursts show radial expansion of order of several neutron star (NS) radii. We apply the analytical models of spectral formation during the expansion and contraction stages to derive physical conditions for the matter in the burning zone close to the surface of the NS as well as the NS mass-radius relation. Our results, combined with statistical errors, show that the central object is a compact star with mass ~1.4 solar masses and radius ~9 km. Our results favor the softer equation of state for neutron star matter. | |
|
E-mail contact | Preprint access |
|
|
 |
|
|
|
 |
 |