Giselher G. Lichti

The Fermi gamma-ray burst monitor

The Gamma-Ray Burst Monitor (GBM) will significantly augment the science return from the Fermi Observatory in the study of gamma-ray bursts (GRBs). The primary objective of GBM is to extend the energy range over which bursts are observed downward from the energy range of the Large Area Telescope (LAT) on Fermi into the hard X-ray range where extensive previous data sets exist. A secondary objective is to compute burst locations onboard to allow re-orienting the spacecraft so that the LAT can observe delayed emission from bright bursts. GBM uses an array of 12 sodium iodide scintillators and two bismuth germanate scintillators to detect gamma rays from ~8 keV to ~40 MeV over the full unocculted sky. The onboard trigger threshold is ~0.7 photons cm–2 s–1 (50-300 keV, 1 s peak). GBM generates onboard triggers for ~250 GRBs per year.

SPI: The spectrometer aboard INTEGRAL

SPI is a high spectral resolution gamma-ray telescope on board the ESA mission INTEGRAL (International Gamma Ray Astrophysics Laboratory). It consists of an array of 19 closely packed germanium detectors surrounded by an active anticoincidence shield of BGO. The imaging capabilities of the instrument are obtained with a tungsten coded aperture mask located 1.7 m from the Ge array. The fully coded field-of-view is 16degrees, the partially coded field of view amounts to 31degrees, and the angular resolution is 2.5degrees. The energy range extends from 20 keV to 8 MeV with a typical energy resolution of 2.5 keV at 1.3 MeV. Here we present the general concept of the instrument followed by a brief description of each of the main subsystems. INTEGRAL was successfully launched in October 2002 and SPI is functioning extremely well.

An exceptionally bright flare from SGR 1806-20 and the origins of short-duration γ-ray bursts

Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1 s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380 s) giant flare from SGR 1806–20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic γ-ray burst. At least a significant fraction of the mysterious short-duration γ-ray bursts may therefore come from extragalactic magnetars.

Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1σ) between 1° and 2° within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm 2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons

Radioactive 26Al from massive stars in the Galaxy.

Gamma-rays from radioactive 26Al (half-life ∼7.2 × 105 years) provide a ‘snapshot’ view of continuing nucleosynthesis in the Galaxy. The Galaxy is relatively transparent to such γ-rays, and emission has been found concentrated along its plane. This led to the conclusion that massive stars throughout the Galaxy dominate the production of 26Al. On the other hand, meteoritic data show evidence for locally produced 26Al, perhaps from spallation reactions in the protosolar disk. Furthermore, prominent γ-ray emission from the Cygnus region suggests that a substantial fraction of Galactic 26Al could originate in localized star-forming regions. Here we report high spectral resolution measurements of 26Al emission at 1808.65 keV, which demonstrate that the 26Al source regions corotate with the Galaxy, supporting its Galaxy-wide origin. We determine a present-day equilibrium mass of 2.8 (± 0.8) solar masses of 26Al. We use this to determine that the frequency of core collapse (that is, type Ib/c and type II) supernovae is 1.9 (± 1.1) events per century.

Emission from 44Ti associated with a previously unknown Galactic supernova

Nearly 400 years have passed since a supernova was last observed directly in the Milky Way (by Kepler, in 1604). Numerous Galactic supernovae are expected to have occurred since then, but only one (Cassiopeia A) may have been seen. The historical record of supernovae is therefore incomplete, as demonstrated by the spatial distribution of young supernova remnants. The discovery, of γ-ray emission from the decay of 44Ti nuclei associated with Cassiopeia A, the youngest known remnant, has revealed a new way to search for the remnants of other relatively recent supernovae (less than ∼1,000 years old). Here we report the discovery of 44Ti line emission from a previously unknown young supernova remnant, in the direction of the Vela remnant. We estimate a distance of ∼200 parsecs and an age of ∼680 years for the remnant, making it the closest young remnant to the Earth. Why it was not recorded historically remains unknown.

SPI/INTEGRAL in-flight performance

The SPI instrument has been launched on-board the INTEGRAL observatory on October 17, 2002. SPI is a spectrometer devoted to the sky observation in the 20 keV-8 MeV energy range using 19 germanium detectors. The performance of the cryogenic system is nominal and allows to cool the 19 kg of germanium down to 85 K with a comfortable margin. The energy resolution of the whole camera is 2.5 keV at 1.1 MeV. This resolution degrades with time due to particle irradiation in space. We show that the annealing process allows the recovery of the initial performance. The anticoincidence shield works as expected, with a low threshold at 75 keV, reducing the GeD background by a factor of 20. The digital front-end electronics system allows the perfect alignement in time of all the signals as well as the optimisation of the dead time (12%). We demonstrate that SPI is able to map regions as complex as the galactic plane. The obtained spectrum of the Crab nebula validates the present version of our response matrix. The 3sigma sensitivity of the instrument at 1 MeV is 8x10(-7) ph cm(-2) s(-1) keV(-1) for the continuum and 3x10(-5) ph cm(-2) s(-1) for narrow lines.

The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C

Context. The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on accurate knowledge of the distance to the events. Aims. Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at energies >0.1 GeV. Methods. Observations were done with the 7-channel “Gamma-Ray Optical and Near-infrared Detector” (GROND) at the 2.2 m MPI/ESO telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4 m telescope in South Africa, and the GMOS instrument at Gemini-S. Results. The afterglow photometric redshift of z = 4.35 ± 0.15, based on simultaneous 7-filter observations with GROND, places GRB 080916C among the top 5% most distant GRBs and makes it the most energetic GRB known to date. The detection of GeV photons from such a distant event is unexpected because of the predicted opacity due to interaction with the extragalactic background light. The observed gamma-ray variability in the prompt emission, together with the redshift, suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of Γ > 1090. This value rivals any previous measurements of Γ in GRBs and strengthens the extreme nature of GRB 080916C.

New high energy |[gamma]|-ray sources observed by COS B

LOCALISED γ-ray sources contribute to the overall galactic emission; some of these sources have been identified with known astronomical objects1,2, while several unidentified γ-ray sources have also been reported3,4. We describe here a search for γ-ray sources using data from the ESA γ-ray satellite COS B which revealed 10 new unidentified sources. These sources seem to be galactic with typical γ-ray luminosities above 100 MeV in excess of 1035 erg s−1.

The Fermi GBM Gamma-Ray Burst Catalog : The First Two Years

The Fermi Gamma-ray Burst Monitor (GBM) is designed to enhance the scientific return from Fermi in studying gamma-ray bursts (GRBs). In its first two years of operation GBM triggered on 491 GRBs. We summarize the criteria used for triggering and quantify the general characteristics of the triggered GRBs, including their locations, durations, peak flux, and fluence. This catalog is an official product of the Fermi GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.