No. 6 - June 8, 2001

Edited by Thierry Montmerle & Marc Türler

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Editorial

Due to the overwhelming response to the first INTEGRAL AO (a factor of nearly 20 in the ratio between requested and available time), the proposal selection process has been a puzzling and difficult task for the Time Allocation Committee (TAC). The TAC members first met from May 14 to 18 at ESTEC in Noorwijk, and the discussions in the different panels often ended only very late in the evening (for more information see the dedicated ESA Science News article). Then the panel Chairs met again on May 31 and June 1st to finalize the proposal selection. The resulting recommendations are now in the hands of ESA's Director of Science for final approval, and should be announced soon.

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 !

Best wishes for a relaxing summer break,
Thierry Montmerle & Marc Türler

This section includes the following articles:

ISDC processing of SPI calibration data

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.

This section includes the following article:

SPI sees its ``First light''
Please note that ESA communicates information about the INTEGRAL mission including status of spacecraft, instruments, and science operations through its own channels, including ESA's science WWW site and the INTEGRAL WWW site at ESTEC.
The aim of this ``INTEGRAL News'' section is to convey informal news about the activities of the INTEGRAL Instrument Teams. For further information, please visit the WWW pages of these teams, which can be reached through the above mentioned ESA sites or through the list of ISDC WWW links.

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 accelerator irradiation experiment

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

p + ¹³C → ¹⁴N* → ¹⁴N + γ To achieve the required range of gamma-ray energies, the ¹³C target irradiation took place with protons accelerated to 550 keV (300 µA) and 1 MeV (100 µA). The resulting gamma-ray flux on a typical run was no less than 15 000 8-MeV photons in 36 h, for each Germanium detector, i.e., a rate of 2 photon/s for SPI as a whole. Isn't that a dream ? This is the reason why a powerful, low-energy accelerator was needed: to obtain excellent statistics in a relatively short period of time.

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.

Calibration with radioactive sources

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).

Current status of SPI

In conclusion, the imaging capabilities of SPI have been checked up to 2.7 MeV; we have obtained a very complete set of data which are now analysed by the Instrument Teams and the ISDC (see accompanying article). We consider that SPI has been quite well calibrated for numerous different energies, with the anticoincidence ON and OFF, and with different anticoincidence thresholds.

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.

Abstracts of recently accepted papers

A transition to the soft state in GRS 1758-258
D.M. Smith, W.A. Heindl & J.H. Swank
Spectral differences between the radio-loud and radio-quiet low-hard states of GRS 1915+105: Possible detection of synchrotron radiation in X-rays
S. V. Vadawale, A. R. Rao & S. K. Chakrabarti
Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000
H. Krawczynski, R. Sambruna, A. Kohnle, P.S. Coppi, The HEGRA Collaboration, F. Aharonian, et al.
Gamma Ray and Hadron Generated Cerenkov Photon Spectra at Various Observation Altitudes
M.A. Rahman, P.N. Bhat, B.S. Acharya, V.R. Chitnis, P. Majumdar et al.
PSR J2229+6114: Discovery of an Energetic Young Pulsar in the Error Box of the EGRET Source 3EG J2227+6122
J.P. Halpern, F. Camilo, E.V. Gotthelf, D.J. Helfand, M. Kramer, A.G. Lyne, et al.
A two-phase model for the Narrow Line Region of NGC 4151
S. Komossa
X-ray outbursts from nearby `normal' and active galaxies. A review, new radio observations, and an X-ray search for further tidal disruption flares
S. Komossa, M. Dahlem

Other papers picked up for you

Below are links to a list of selected papers from the e-print archive accepted in refereed journals and related in a way or another to the science relevant to the INTEGRAL mission.
March 2001
April 2001
May 2001
June 2001 (beginning)

A transition to the soft state in GRS 1758-258
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.
E-mail contact	Preprint access

Spectral differences between the radio-loud and radio-quiet low-hard states of GRS 1915+105: Possible detection of synchrotron radiation in X-rays
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.
E-mail contact	Preprint access

Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000
H. Krawczynski^1,2, 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.
E-mail contact	Preprint access

Gamma Ray and Hadron Generated Cerenkov Photon Spectra at Various Observation Altitudes
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.
E-mail contact

PSR J2229+6114: Discovery of an Energetic Young Pulsar in the Error Box of the EGRET Source 3EG J2227+6122
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.
E-mail contact	Preprint access

A two-phase model for the Narrow Line Region of NGC 4151
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 (Γ_X = -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.
E-mail contact	Preprint access

X-ray outbursts from nearby `normal' and active galaxies. A review, new radio observations, and an X-ray search for further tidal disruption flares
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.
E-mail contact	Preprint access

This section includes:

Upcoming conferences around the astrophysics of INTEGRAL
Contributed News

Upcoming conferences around the astrophysics of INTEGRAL

Contributed announcements
- The High-Energy Universe at Sharp Focus: Chandra Science (Symposium at the ASP)
  16-18 July 2001, St. Paul, Minnesota, USA
  The first symposium dedicated to results from the Chandra X-ray Observatory will be held 16-18 July 2001 in St. Paul, Minnesota, as part of the Astronomical Society of the Pacific's 113th Annual Meeting. The program of this international symposium will include all major science areas covered by Chandra observations and will be highlighted by numerous invited review talks by noted experts in each area. Ample time has been included in the schedule for discussions of posters and participants' on-going research activities.
  Abstract submission deadline: 18 May 2001
- Workshop on X-ray spectroscopy of active galactic nuclei with Chandra and XMM-Newton
  3-6 December 2001, MPE, Garching, Germany
  The high throughput and excellent spectral capabilities of XMM-Newton and Chandra have already revealed many new and unexpected observational results. However, some of these have led to controversial discussions and require a better understanding; e.g.
  - the nature of the soft X-ray excess: Line or continuum emission?
  - new spectral features and their interpretations, e.g. the nature of the unresolved transition array,
  - the presence of warm absorber edges,
  - the relationship between X-ray and ultraviolet absorption,
  - grating spectroscopy of the broad and narrow emission lines,
  - relativistic disk lines at cosmological redshifts,
  - theoretical interpretations and modeling issues
  The aim of the workshop is to provide a platform for intense discussions of these topics. Future X-ray observations with XEUS and Constellation-X, in light of the recent XMM-Newton and Chandra findings, will also be included.
  Registration and abstract submission deadline: May 1st, 2001
Other conferences picked-up for you
- Gamma-Ray Bursts in the Afterglow Era
  10 June - 1st July 2001, Aspen Center for Physics, CO, USA
- IAU Colloquium No. 184: AGN Surveys
  18-22 June 2001, Byurakan, Armenia
- Stellar Coronae in the Chandra and XMM-Newton Era, 35th ESLAB Symposium
  25-29 June 2001 ESTEC, Noordwijk, The Netherlands
- AGN variability across the Electromagnetic Spectrum
  25-29 June 2001 Sydney, Australia
- IAU Symposium 208: Astrophysical Supercomputing using Particle Simulations
  10-13 July 2001 Tokyo, Japan
- International Europhysics Conference on High Energy Physics
  12-18 July 2001, Budapest, Hungary
- Statistical Challenges in Modern Astronomy III
  18-21 July 2001, Penn State University, Pennsylvania, USA
- Neutron Stars in Supernova Remnants
  14-17 August 2001, Boston, Massachusetts, USA
- European school of high-energy physics
  26 August - 8 September 2001, Beatenberg, Switzerland
- Galactic Black Hole 2001 - DPG School for Physics
  26-31 August 2001, Physikzentrum Bad Honnef near Bonn, Germany
- Two Years of Science with Chandra (Symposium)
  5-7 September 2001, Washington DC, USA
- Multi-Frequency Observations of Variable Stars and AGNs: First Italian-Jordanian Conference in Astrophysics
  17-20 September 2001, Mafraq, Jordan
- The Chemical Composition of Stars
  19-22 September 2001, Seattle, WA, USA
- European Summer School on Experimental Nuclear Astrophysics
  1-6 October 2001, Santa Tecla, Sicily, Italy
- The evolution of galaxies. II. Basic Building Blocks (EuroConference)
  16-21 October 2001, St Denis, la Réunion, France
- Gamma Ray Burst and Afterglow Astronomy
  5-9 November 2001, Woods Hole, Massachusetts, USA
- New Visions of the X-ray Universe in the XMM-Newton and Chandra era
  26-30 November 2001, ESTEC, Noordwijk, The Netherlands

Contributed News

The High Energy Solar Spectroscopic Imager (HESSI), a NASA spacecraft carrying a germanium gamma-ray spectrometer, is scheduled for launch on June 12, 2001.
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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