R. Starr

Science applications of the Mars Observer gamma ray spectrometer

The Mars Observer gamma ray spectrometer will return data related to the elemental composition of Mars. The instrument has both a gamma ray spectrometer and several neutron detectors. The gamma ray spectrometer will return a spectrum nominally every 20 s from Mars permitting a map of the elemental abundances to be made. The gamma rays are emitted from nuclei involved in radioactive decay, from nuclei formed by capture of a thermal neutron, and from nuclei put in an excited state by a fast-neutron interaction. The gamma rays come from an average depth of the order of a few tens of centimeters. The spectrum will show sharp emission lines whose intensity determines the concentration of the element and whose energy identifies the element. The neutron detectors, using the fact that the orbital velocity of the Mars Observer spacecraft is similar to the velocity of thermal neutrons, determine both the thermal and epithermal neutron flux. These parameters are particularly sensitive to the concentration of hydrogen in the upper meter of the surface. By combining the results from both techniques it is possible to map the depth dependence of hydrogen in the upper meter as well. These data permit a variety of Martian geoscience problems to be addressed including the crust and mantle composition, weathering processes, volcanism, and the volatile reservoirs and processes. In addition, the instrument is also sensitive to gamma ray and particle fluxes from non-Martian sources and will be able to address problems of astrophysical interest including gamma ray bursts, the extragalactic background, and solar processes.

The Very Red Afterglow of GRB 000418: Further Evidence for Dust Extinction in a Gamma-Ray Burst Host Galaxy*

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts

Proton-induced radiation damage in germanium detectors

High-purity germanium (HPGe) detectors will be used in future space missions for gamma-ray measurements and will be subject to interactions with energetic particles. To simulate this process, several large-volume n-type HPGe detectors were incrementally exposed to a particle fluence of up to 10/sup 8/ protons cm/sup -2/ (proton energy: 1.5 GeV) at different operating temperatures (90 to 120 K) to induce radiation damage. Basic scientific and engineering data on detector performance were collected. During the incremental irradiation, the peak shape produced by the detectors showed a significant change from a Gaussian shape to a broad complex structure. After the irradiation, all detectors were thoroughly characterized by measuring many parameters. To remove the accumulated radiation damage, the detectors were stepwise-annealed at temperatures of T >

The status and future of the third interplanetary network

The 3rd interplanetary network (IPN), which has been in operation since 1990, presently consists of 9 spacecraft: AGILE, RHESSI, Suzaku, and Swift, in low Earth orbit; INTEGRAL, in eccentric Earth orbit with apogee 0.5 light‐seconds; Wind, up to ∼7 light‐seconds from Earth; MESSENGER, en route to Mercury; and Mars Odyssey, in orbit around Mars. Ulysses and HETE have ceased operations, and the Fermi GBM is being incorporated into the network. The IPN operates as a full‐time, all‐sky monitor for transients down to a threshold of about 6 × 10^(−7) erg cm^(−2) or 1 photon cm^(−2) s^(−1). It detects about 275 cosmic gamma‐ray bursts per year. These events are generally not the same ones detected by narrower field of view imaging instruments such as Swift, INTEGRAL IBIS, and SuperAGILE; the localization accuracy is in the several arcminute and above range.

GRB 080407: an ultra-long burst discovered by the IPN

We present observations of the extremely long GRB 080704 obtained with the instruments of the Interplanetary Network (IPN). The observations reveal two distinct emission episodes, separated by a approx.1500 s long period of quiescence. The total burst duration is about 2100 s. We compare the temporal and spectral characteristics of this burst with those obtained for other ultra-long GRBs and discuss these characteristics in the context of different models.

The very red afterglow of GRB 000418: Further evidence for dust extinction in a gamma-ray burst host galaxy

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts

The very red afterglow of GRB 000418 - further evidence for dust extinction in a GRB host galaxy

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts