| No. 6 - June 8, 2001 | Edited by Thierry Montmerle & Marc Türler |
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The second main event of this spring is the extensive calibration round of INTEGRAL's spectrometer SPI, a major step towards completing the satellite. The articles of this issue give exclusive details about the successful story and first results of these calibrations.
Although SPI is already mounted on the INTEGRAL satellite in Alenia, Turin, the INTEGRAL launch had to be postponed to September 2002. This new and hopefully last delay is due to unforeseen, but now solved, glue problems in the PICsIT detector layer of the IBIS imager.
In parallel with these events, the ISDC is continuing its activities. In order to better inform the community about these activities, we defined a series of topics on different parts of the ISDC data processing, which will be presented in the forthcoming issues of the Newsletter, up to launch. Starting with the description of the data format, the archive and the results of the standard analysis performed at the ISDC, we will finish in September 2002 with details on the operations and the commissioning phase. Stay tuned !
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| ISDC processing of SPI calibration data |
| Volker Beckmann (ISDC Geneva) |
During the calibration of SPI at Bruyères-le-Châtel (see accompanying article) the ISDC installed a PC there to transfer the data to our site. No real time processing was done, but after one week we were able to analyse the first data set completely, including unpacking of the data, pre-processing, energy correction, binning, applying pointing information, image reconstruction and finally spectra derivation.
The spectral results shown here were obtained using the 60Co source
at long distance (125 m).
The two strong emission-lines at 1173 keV and 1333 keV
are clearly seen in the detector spectrum.
For the results shown here, 10 different pointing directions have been used,
thus allowing a real test of SPI's dithering mode.
The spectra are derived from the SPIROS 1.3.1 source extraction
software, which is developed at the University of Birmingham.
With SPIROS, the data are searched for a significant source and the
flux in different energy bands is determined by applying a maximum
likelihood method. As an example, the spectrum
shown on the left
was obtained by applying energy bins of 0.4 keV width around the 1.3
MeV line of 60Co.
The line width is of about 3 keV (FWHM) confirming that the performance of the instrument is
as good as expected. We hope that future work on the response matrices will
provide an even better resolution.
Another way to display this strong emission-line is shown in the image animation on the right. The image size is of 8° × 8° and covers the spectral range from 1165 keV to 1180 keV in a series of 1 keV steps. The intensity of the source in the different energy bins is calculated by using the output from the SPIROS source detection.
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| SPI sees its ``First light'' |
| Gilbert Vedrenne (CESR Toulouse), Volker Schönfelder (MPE Garching) and Bertrand Cordier (CEA Saclay) |
At the beginning of April, until May 4, the INTEGRAL spectrometer SPI was sent to Bruyères-le-Châtel
for its calibration. This campaign, prepared under the responsibility of B.
Cordier at CEA Saclay, was organized thanks to a very fruitful collaboration
with CEA/DAM and CNES. Bruyères-le-Châtel is a largely military research
facility located about 30 km South of Paris, which includes a powerful Van de Graaf linear
accelerator, and provides strict safety measures. The experimental set-up, which was completed in a specially
designed building, made use of gamma-rays not only from radioactive sources but
also induced by accelerated protons bombarding a 13C target.
This allowed to calibrate the instrument in the complete gamma-ray energy range from 20 keV up to 8 MeV.
The figure on the right shows SPI on its rotating calibration bench. The Ge detectors at the ``bottom'' of SPI are visible on the right.
SPI was installed on a 2D-rotating base and cooled to 90 K. Various observing modes could be tested under different orientations, including dithering. In addition to radioactive source exposure, almost 400 hours of accelerator time were necessary to complete the tests.
The figure on the left shows the 4 MV (mega-volt) Van
der Graaf accelerator at Bruyères-le-Châtel (seen here
during maintenance), which was used during the SPI calibration campaign.
The Van de Graaf linear proton accelarator installed at Bruyères-le-Châtel is one of the most powerful of its kind in Europe. It can reach a maximum luminosity of 300 micro-ampère (µA), corresponding to ~ 2 × 1015 proton/s at ~ 1 MeV. (For comparison, more standard accelerators of the ``Saturne'' type yield ~ 1013 p/s at 1 GeV.) Gamma-rays are generated by the reaction
Two years of preparation were required to arrive at a successful procedure. Relatively minor problems, like the cooling of the 13C target (which receives up to 10 W, hence would melt without cooling), had to be solved. But the major issue was that of properly calibrating the accelerator itself ! Contrary to radioactive sources, which have a tagged output, accelerators are notoriously unstable. Using various devices, a precision of ~ 3 to 5 % could be obtained at Bruyères: this is the best that the calibration of SPI in the accelerator range can achieve.
The figure on the right is a schematic view of the SPI calibration installation. A clean room was
specially built to accommodate SPI, with a ``window'' for radioactive sources
placed at a distance of 125 m, outside the accelerator building.
Another very important part of the calibration was the use of strong radioactive sources situated at a large distance (125 m) from SPI. In these conditions the gamma-ray beam reaching the SPI instrument was nearly parallel and the imaging capabilities of SPI could be checked between a few tens of keV and 2.75 MeV. The figures below show the resulting images for gamma-ray "stars" at two energies: 1.17 MeV ( 60Co line) and 2.75 MeV ( 24Na line). The half-life of the latter source (15 h) made this operation quite critical; the sources, produced in one of Saclay's nuclear research reactors, had to be transported in a 550-kg lead container from Saclay to Bruyères (about 20 km apart) on a short time and the measurements with SPI done immediately thereafter, during the night for security reasons.
The figure on the left shows the maximum-entropy image of the 1173 keV line of the 60Co source obtained using
both single and multi-detector (double and triple) events for a single pointing.
There are 19 single and 66 multi-detector combinations used in this image.
The image was made by A. Strong (MPE) using response provided by C. Shrader (GSFC) and the Calibration data at ISDC.
See the dedicated page at MPE for more details.
The figure on the right shows imaging with the 24Na source using
19 dithering pointings.
The image is composed of 101 × 101 pixels with a resolution (pixel size) of 0.2° × 0.2°.
Image courtesy Laurent Bouchet CESR.
For more results see the CESR site (select "Calibration Results" in the margin).
On May 9 SPI was sent to Alenia Spazio in Turin. It was put ON as soon as possible, and on May 21 the detectors have been checked after an outgassing phase and a temperature decrease to 90 K. The 19 Germanium detectors have quite good performances, especially concerning the energy resolution which is nominal for all of them. At this time, SPI is the first major instrument of INTEGRAL to have been delivered to Alenia for the integration and tests at satellite level.
Many thanks from the PIs for the beautiful work done by the calibration and the CNES teams.
Additional information can be found at ESA's science WWW page with useful links to CEA, CESR and MPE.
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| A transition to the soft state in GRS 1758-258 | |
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D. M. Smith1, W. A. Heindl2 & J. H. Swank3 1. Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA 94720 U.S.A. 2. Center for Astrophysics and Space Science, University of California, San Diego, La Jolla, CA 92093 U.S.A. 3. NASA Goddard Space Flight Center, Code 666, Greenbelt, MD 20771 U.S.A. | |
| Accepted for publication in ApJ Letters on April 30, 2001 | |
| Abstract. Near the end of 2001 February, the black-hole candidate (BHC) GRS 1758-258 made an abrupt transition from a standard hard (low) state to a soft state. Unlike Cyg X-1 and other BHCs, whose luminosity increases during this transition, GRS 1758-258 was dimmer after the transition. We present observations with the Proportional Counter Array on the Rossi X-ray Timing Explorer and interpret the phenomenon in the context of a ``dynamical'' soft state model. Using this model we predicted that mass transfer from the companion had ceased, and that the luminosity should decay on a timescale of a few weeks. The most recent data support this prediction, being consistent with a decay time of 28 dy. The current state is consistent with the ``off'' state of GRS 1758-258 reported by GRANAT/Sigma in 1991-1992. | |
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| Spectral differences between the radio-loud and radio-quiet low-hard states of GRS 1915+105: Possible detection of synchrotron radiation in X-rays | |
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S. V. Vadawale1, A. R. Rao1 & S. K.
Chakrabarti2 1. Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai - 400 005 INDIA 2. S. N. Bose National Center for Basic Sciences, Salt Lake, Calcutta - 700 091 INDIA | |
| Accepted for publication in A&A on March 20, 2001 | |
| Abstract. The Galactic microquasar GRS 1915+105 exhibits several episodes of steady X-ray emission characterized by a hard power-law spectrum and intense Quasi Periodic Oscillations. It is known that there are two types of such low-hard states, one with steady radio emission and the other without any significant radio emission. We present the results of a detailed X-ray spectroscopic study of GRS 1915+105, using data from the Rossi X-ray Timing Explorer obtained during various episodes of the low-hard states of the source. We show that there are distinct X-ray spectral differences between the radio-quiet and radio-loud low-hard states of the source. The X-ray spectra of the radio-quiet low-hard state is best described by a model consisting of a multicolor disk-blackbody and a Comptonized component, whereas the X-ray spectra of radio-loud low-hard state requires a model consisting of three components: a multicolor disk-blackbody, a Comptonized component and a power-law, for statistically and physically acceptable fits. We attempt to model the presence of this additional power-law component as due to synchrotron radiation which is responsible for the radio and infrared radiation from the source. We show that a simple adiabatically expanding jet model for the synchrotron radiation can account for the observed X-ray flux for reasonable values of the magnetic field and the mass outflow rate. This is the first report of detection of the synchrotron radiation in the X-ray band for this source. | |
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| Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000 | |
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H. Krawczynski1,2, R. Sambruna3, A. Kohnle1, P.S.
Coppi2, The HEGRA Collaboration, F. Aharonian2, et al. 1. Yale University, P.O. Box 208101, New Haven, CT 06520-8101, USA 2. Max-Planck-Institut fuer Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany 3. George Mason University, 4400 University Drive, M/S 3F3, Fairfax, VA 22030, USA | |
| Accepted for publication in ApJ on May 18, 2001 | |
| Abstract. In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively. The source was monitored from February 2nd to February 16th and from May 3rd to May 8th, 2000. We discuss in detail the temporal and spectral properties of the source. Remarkably, the TeV observations of February 7th/8th showed statistically significant evidence for substantial TeV flux variability on 30 min time scale. We show the results of modeling the data with a time dependent homogeneous Synchrotron Self-Compton (SSC) model. The X-ray and TeV gamma-ray emission strengths and energy spectra together with the rapid flux variability strongly suggest that the emission volume is approaching the observer with a Doppler factor of 50 or higher. The different flux variability time scales observed at X-rays and TeV Gamma-rays indicate that a more detailed analysis will require inhomogeneous models with several emission zones. | |
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| Gamma Ray and Hadron Generated Cerenkov Photon Spectra at Various Observation Altitudes | |
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M.A. Rahman, P.N. Bhat, B.S. Acharya, V.R. Chitnis, P. Majumdar et al. Tata Institute of Fundamental Research, Mumbai 400 005, India. | |
| Accepted for publication in Experimental Astronomy on March 30, 2001 | |
| Abstract. We study the propagation of Cerenkov photons generated by Very High Energy γ-rays and hadrons in the atmosphere. The photon production height distributions are estimated from semi-empirical methods and compared with those derived by standard simulation techniques. Incident spectra at various observation altitudes are then derived after applying wavelength dependent corrections due to photon attenuation in the atmosphere during the propagation of photons from the height of production to the height of observation. These are generated both for γ- and hadron primaries of various energies. The derived production height distributions agree very well with those generated by the simulation package `CORSIKA' at all energies and for both γ-ray and proton primaries. The incident photon spectra are found to be both altitude dependent and primary energy dependent. The peak of the incident spectrum shifts towards the shorter wavelength with increasing altitude of observation for a given primary. Also the peak of the photon spectrum shifts towards the shorter wavelength with increasing energy of the primary at given altitude. The fraction of the UV component in the incident Cerenkov spectrum is estimated both for γ-ray and hadronic primaries at various observation altitudes and energies. Hadron generated Cerenkov spectra are marginally richer in UV light and the difference increases slightly at higher altitudes. The fraction of the UV to the visible light in the Cerenkov spectrum could be a useful parameter to separate γ-rays from cosmic ray background only if one can measure this fraction very accurately. | |
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| PSR J2229+6114: Discovery of an Energetic Young Pulsar in the Error Box of the EGRET Source 3EG J2227+6122 | |
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J.P. Halpern1, F. Camilo1, E.V. Gotthelf1, D.J.
Helfand1, M. Kramer2, A.G. Lyne2, et al. 1. Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027 2. University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL, UK | |
| Accepted for publication in ApJ Letters on April 5 2001 | |
| Abstract. We report the detection of radio and X-ray pulsations at a period of 51.6 ms from the X-ray source RX/AX J2229.0+6114 in the error box of the EGRET source 3EG J2227+6122. An ephemeris derived from a single ASCA observation and multiple epochs at 1412 MHz from Jodrell Bank indicates steady spin-down with dP/dt = 7.83 · 10-14 s/s. From the measured P and dP/dt we derive spin-down power dE/dt = 2.2 · 1037 erg/s, magnetic field B = 2.0 · 1012 G, and characteristic age P/(2dP/dt) = 10,460 yr. An image from the Chandra X-ray Observatory reveals a point source surrounded by centrally peaked diffuse emission that is contained within an incomplete radio shell. We assign the name G106.6+2.9 to this new supernova remnant, which is evidently a pulsar wind nebula. For a distance of 3 kpc estimated from X-ray absorption, the ratio of X-ray luminosity to spin-down power is ~8 · 10-5, smaller than that of most pulsars, but similar to the Vela pulsar. If PSR J2229+6114 is the counterpart of 3EG J2227+6122 then its efficiency of gamma-ray production, if isotropic, is 0.016 (d/3 kpc)2. It obeys an established trend of gamma-ray efficiency among known gamma-ray pulsars which, in combination with the demonstrated absence of any other plausible counterpart for 3EG J2227+6122, makes the identification compelling. If confirmed, this identification bolsters the pulsar model for unidentified Galactic EGRET sources. | |
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| A two-phase model for the Narrow Line Region of NGC 4151 | |
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Stefanie Komossa Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr., D-85748 Garching | |
| Accepted for publication in A&A on March 16, 2001 | |
| Abstract. NGC 4151 is one of the brightest and best-studied Seyfert galaxies. Here, we present a two-phase model of the narrow-line region (NLR) of NGC 4151. This study is motivated by (i) the fact that the X-ray spectrum of NGC 4151 is among the flattest known for Seyferts, and (ii) the recent Chandra detection of an X-ray narrow-line region in this galaxy. X-ray spectra as flat as that of NGC 4151 (ΓX = -1.5) are expected to favor the presence of two gas phases in pressure equilibrium (Krolik, McKee, Tarter 1981). In the present study, we show that a pronounced two-phase equilibrium develops in the extended emission-line region if we use the observed multi-wavelength spectrum of NGC 4151 to ionize the clouds. The material is stable to isobaric perturbations over a wide range of temperatures. We therefore propose that such a condition has arisen in the NLR of NGC 4151, and that it explains the detection of hot, extended X-ray gas which we identify as the NLR-cloud confining medium. | |
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| X-ray outbursts from nearby `normal' and active galaxies. A review, new radio observations, and an X-ray search for further tidal disruption flares | |
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S. Komossa, M. Dahlem 1. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr., D-85748 Garching 2. European Southern Observatory, Casilla 19001, Santiago 19, Chile | |
| Conf. proceedings of MAXI workshop on AGN variability | |
| Abstract. In the last few years, giant-amplitude, non-recurrent X-ray flares have been observed from several non-active galaxies (NGC 5905, RXJ1242-11, RXJ1624+75, RXJ1420+53, RXJ1331-32). All of them share similar properties, namely: extreme X-ray softness in outburst, huge peak luminosity (up to about 1044 erg/s), and the absence of optical signs of Seyfert activity. Tidal disruption of a star by a supermassive black hole is the favored explanation of these unusual events. We present a review of the previous results, a search for radio emission from all outbursters, based on the NVSS database, and dedicated radio observations of NGC 5905 carried out with the VLA. These provide important constraints on the presence of an (obscured) active nucleus (AGN) at the center of each flaring galaxy. We rigorously explore AGN scenarios to account for the unusual X-ray outbursts from the optically `normal' galaxies and find AGN-related models highly unlikely. We conclude that the previously favored scenario - tidal disruption of a star by a supermassive black hole at the center of each of the outbursters - provides the best explanation for the X-ray observations. Finally, we present results from our on-going search for further X-ray flares from a sample of ~140 nearby active and non-active galaxies, using the ROSAT data base. While we do not find another X-ray flaring normal galaxy among this sample - entirely consistent with the predictions of the tidal disruption scenario - several highly variable active galaxies are detected. Their variability is not linked to tidal disruption, but best explained in terms of absorption or accretion-disk-related models. | |
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HESSI was built to perform x-ray and gamma-ray imaging and spectroscopy of solar flares, but will also perform a number of non-solar observations. Its gamma-ray spectrometer consists of 9 very large germanium detectors, for a total detector volume comparable to INTEGRAL/SPI, but the detectors are unshielded; they will therefore have a much higher background than SPI, but will view the whole sky every day, providing a complement to INTEGRAL science. Planned non-solar projects include high-resolution spectra of gamma-ray bursts, studies of cyclotron lines and period variations in accreting pulsars, measurements of the line shapes of the 511 keV and 1809 keV lines from the Galactic Center, and detection of prompt annihilation radiation from novae. In addition, the Crab Nebula will enter HESSI's imaged solar field-of-view once per year, allowing 2-arcsecond imaging of the nebula up to 100 keV or higher.
HESSI's primary goal, however, is to understand the mechanisms for the acceleration, interaction, and energy loss of energetic particles associated with solar flares. The germanium detectors sit under pairs of fine grids, which will provide unprecedentedly high-resolution imaging (down to 2 arcsec) and high-resolution spectroscopy (down to 1 keV resolution) over a range of > 3 orders of magnitude in photon energy (3 keV to 17 MeV). The 15-rpm spacecraft spin will cause rapid modulations in the count rate in each detector due to the transmission of the associated grid pair. These fast modulations can be deconvolved by a variety of techniques to reproduce the image of the flare on the Sun.
HESSI is a joint project of the Space Sciences Laboratory (SSL) at the University of California, Berkeley, NASA's Goddard Space Flight Center, the Paul Scherrer Institute in Switerland, and other institutions. Professor Robert P. Lin, U. C. Berkeley, is the Principal Investigator.
The launch will take place from Cape Canaveral Air Force Station following a captive-carry flight from Vandenberg Air Force Base of the Pegasus XL rocket aboard its dedicated L-1011 aircraft. HESSI's circular orbit, 38 degrees inclination at 600 kilometers altitude, will carry it over a new ground station at Berkeley several times a day for command uplink and data downlink.
All HESSI data will be public from the time of collection, and participation from the community in solar and non-solar analyses is encouraged. Please see the HESSI web page for information on solar data access and analysis, and please contact David Smith if you are interested in participating in the non-solar mission.
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