Search

Milagrito was an extensive air-shower observatory that served as a prototype for the larger Milagro instrument. Milagrito operated from 1997 February to 1998 May. Although it was designed as a very high energy (few hundred GeV threshold) water-Cerenkov gamma-ray observatory, it could also be used to study solar energetic particles (SEPs). By recording scaler data, which correspond to photomultiplier tube singles rates, it was sensitive to muons and small showers from hadronic primary particles with rigidities above ~4 GV. Milagrito simultaneously recorded air-shower trigger data of primary particles with energies greater than ~100 GeV that provide the data to help reconstruct event directions. The Milagrito scalers registered a ground-level enhancement associated with the 1997 November 6 SEP event and X9 solar flare. At its peak, the enhancement was 22 times the background rms fluctuations. Based on comparisons to neutron monitor and satellite data, we find evidence that the rigidity power-law spectrum for the differential flux of energetic protons steepened above ~4 GV and that the acceleration site was high in the corona (at ~3 R☉ above the photosphere), assuming that a CME-driven shock was responsible for the ground-level enhancement.

Milagrito, a detector sensitive to very high energy gamma rays, monitored the northern sky from 1997 February through 1998 May. With a large field of view and a high duty cycle, this instrument was well suited to perform a search for TeV gamma-ray bursts (GRBs). We report on a search made for TeV counterparts to GRBs observed by BATSE. BATSE detected 54 GRBs within the field of view of Milagrito during this period. An excess of events coincident in time and space with one of these bursts, GRB 970417a, was observed by Milagrito. The excess has a chance probability of 2.8 × 10-5 of being a fluctuation of the background. The probability for observing an excess at least this large from any of the 54 bursts is 1.5 × 10-3. No significant correlations were detected from the other bursts.

Milagro will be the first water-Cerenkov detector specifically built to study extensive air showers. It is being built in an existing man-made pond located near Los Alamos, NM. Milagro will be the only air shower detector to have an energy threshold as low as a few hundred GeV and an angular resolution as good as 0.3°. Milagro will observe the entire overhead sky at all times. We describe here the Milagro detector, it capabilities and its physics goals.

There are currently no experiments, either satellite or ground-based, that are sensitive to astrophysical gamma-rays at energies between 20 and 250 GeV. We are developing the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) to explore this energy range. STACEE will use heliostat mirrors at a solar research facility to collect Cherenkov light from extensive air showers produced by high energy gamma-rays. Here we report on the results of prototype test work at the solar facility of Sandia National Laboratories (Albuquerque, NM). The work demonstrates that the facility is suitable for use as an astrophysical observatory. In addition, using a full scale prototype of part of STACEE, we detected atmospheric Cherenkov radiation at energies lower than any other ground-based experiment to date., Comment: 28 LaTeX pages, 24 PostScript figures, uses psfig.sty submitted to Nucl. Inst. Meth. A

A wide field-of-view, arcsecond imaging, high energy resolution X-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to detect and find counterparts at other wavelengths for /spl gamma/-ray bursts (GRBs). Detection of GRBs requires wide field-of-view (/spl pi/ to 2/spl pi/ field-of-view) and high sensitivity. This will be achieved by using high quantum efficiency CdZnTe pixel detectors with a low energy threshold (few keV) to observe the larger flux levels at lower energies, and large effective area (625 to 1000 cm) per coded aperture imaging module. Counterpart searches can only be done with ultra-high angular resolution detectors (10 to 30" FWHM) which gives 1 to 5" position determination especially for strong GRBs. A few arcsecond size error box is expected to contain at most one object observed at another wavelength. This will be achieved by using ultra-high spatial resolution pixel detectors (50/spl times/50 to 100/spl times/100 /spl mu/m) and a similar resolution coded aperture to achieve the required angular resolution. The low energy range may provide important new information on GRBs and the high energy resolution is expected to help in the observation and identification of emission and absorption lines in the GRB spectrum. The effective energy range is planned to be 2 to 200 keV which is exceptionally wide for such a detector. The major scientific goals of the proposed mission in addition to continuously monitoring GRBs, is to observe AGN, transient phenomena, isolated and binary pulsars, and solar flares. A space-deployed AXGAM detector is expected to observe a few hundred GRBs per year.

A high sensitivity electronically collimated gamma camera is under development at NOVA. The proposed detector has electronic collimation and will measure the direction of the incident /spl gamma/-ray directly, hence eliminating the need for a physical collimator. This is accomplished by using the Compton double scatter technique. A feasibility study was successfully completed and prototype fabrication started. The new detector is expected to have 3 to 10 times improvement over the present gamma cameras. The high sensitivity is expected to allow fast imaging on time scales of 1 to 2 min and may open the way for dynamic metabolism visualization and Multi Gated Acquisition (MUGA) studies. The prototype is designed to have a high spatial resolution which is 2 to 3 times better compared to present gamma cameras. It is expected to have improved energy resolution by a factor of 2 to 4. Good energy resolution will enable discrimination against and scattered /spl gamma/-ray background with higher effectiveness than presently available. This will improve the signal-to-noise ratio and improve image quality. The detector can be designed as a single gamma camera or formed into a cylindrical detector and used as a tomographic SPECT imager. A modified smaller version may be directly applied to scintimammography.

The field of high energy gamma-ray astronomy grew tremendously in the last decade due to the launch of the EGRET detector on the Compton Gamma-Ray Observatory in 1991 and the proliferation of ground-based air Cherenkov telescopes (ACTs) such as the Whipple 10 meter reflector. Interestingly, the ground-based telescopes only see 4–5 of the over 170 objects detected by EGRET. A simple extrapolation of the EGRET objects' energy spectra up to the energies which the ACTs are sensitive suggests that many of them should have been detected. The key to resolving this lack of detections is to observe these sources in the previously unobserved 20–250 GeV energy range. The Solar Two Observatory collaboration is developing a secondary optics system on the central tower of the world's largest solar energy pilot plant, Solar Two, to observe gamma-ray sources in this energy range. The progress in building the secondary optics system to be used to image ∼64 heliostats at Solar Two located in Barstow, California, is presented. We hope to design and build this detector over the next 2 years.

Milagro is a large field of view (∼ 2 sr), high duty cycle (∼90%), ground‐based observatory sensitive to gamma‐rays above ∼100 GeV. This unique detector is ideal for observing the highest energy gamma‐rays from gamma‐ray bursts. The highest energy gamma rays supply very strong constraints on the nature of gamma‐ray burst sources as well as fundamental physics. Because the highest energy gamma‐rays are attenuated by pair production with the extragalactic infrared background light, Milagro’s sensitivity decreases rapidly for bursts with redshift > 0.5. While only 10 % of bursts have been measured to be within z=0.5, these bursts are very well studied at all wavelengths resulting in the most complete understanding of GRB phenomena. Milagro has sufficient sensitivity in units of E2 dN/dE to detect VHE luminosities lower than the observed luminosities at ∼ 100 keV for these nearby bursts. Therefore, the launch of SWIFT and its ability to localize and measure redshifts of many bursts points to great future poss...

A wide field-of-view, arcsecond imaging, high energy resolution X-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to detect and find counterparts at other wavelengths for gamma ray bursts (GRBs). Detection of GRBs require wide field-of-view (/spl pi/ to 2/spl pi/ field-of-view) and high sensitivity. This will be achieved by using high quantum efficiency CdZnTe pixel detectors, low energy threshold (few keV) to observe larger flux levels that may be possible at lower energies and large effective area (625 to 1,000 cm/sup 2/) per coded aperture imaging module. Counterpart searches can only be done with ultra high angular resolution (10 to 30 arcsecond FWHM) which gives 1 to 5 arcsecond position determination especially for strong GRBs. A few arcsecond resolution error box is expected to contain only one counterpart observed at another wavelength. This will be achieved by using ultra high spatial resolution pixel detectors (50/spl times/50 to 100/spl times/100 micron) and a similar resolution coded aperture to achieve the required angular resolution. AXGAM also has two other important advantages over similar detectors: (1) excellent low energy response (>1 keV) and (2) high energy resolution (

Evidence of TeV emission from GRB970417a has been previously reported using data from the Milagrito detector. Constraints on the TeV fluence and the energy spectrum are now derived using additional data from a scaler system that recorded the rate of signals from the Milagrito photomultipliers. This analysis shows that if emission from GRB970417a has been observed, it must contain photons with energies above 650 GeV. Some consequences of this observation are discussed., Comment: Submitted to the Astrophysical Journal

The Milagro water Cherenkov detector began full operation in January 2000. This detector is capable of monitoring the Northern sky at energies above 500 GeV for sources of equivalent strength to the Crab Nebula over one year of integration. We report on the current performance and sensitivity of Milagro.

Milagrito, a detector sensitive to gamma-rays at TeV energies, monitored the northern sky during the period February 1997 through May 1998. With a large field of view and high duty cycle, this instrument was used to perform a search for TeV counterparts to gamma-ray bursts. Within the Milagrito field of view 54 gamma-ray bursts at keV energies were observed by the Burst And Transient Satellite Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. This paper describes the results of a preliminary analysis to search for TeV emission correlated with BATSE detected bursts. Milagrito detected an excess of events coincident both spatially and temporally with GRB 970417a, with chance probability $2.8 \times 10^{-5}$ within the BATSE error radius. No other significant correlations were detected. Since 54 bursts were examined the chance probability of observing an excess with this significance in any of these bursts is $1.5 \times 10^{-3}$. The statistical aspects and physical implications of this result are discussed., Comment: 6 pages, 5 figures, to appear in "GeV-TeV Astrophysics: Toward a Major Atmospheric Cherenkov Telescope VI," Snowbird, Utah (August, 1999)

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new instrument for observing astrophysical sources of gamma-rays in the energy range from 50 to 250 GeV. STACEE is currently under construction. The first phase of STACEE, using 32 large heliostat mirrors, was completed in the fall of 1998. We describe the performance of STACEE during the 1998–1999 winter observing season. The instrument is performing very well. We have detected the Crab Nebula with high significance (∼7σ). This result demonstrates that the STACEE concept is sound and that we can expect to make sensitive measurements of gamma-ray sources at energies below 100 GeV. The full STACEE instrument, with 64 heliostats, will be completed during the year 2000. The first three years of observations with the complete instrument will include a range of sources with an emphasis on AGN and supernova remnants.

The Milagro water Cherenkov detector near Los Alamos, New Mexico is the first air shower detector capable of continuously monitoring the sky at energies between 500 GeV and 20 TeV. Preliminary results of the Milagro experiment are presented. A predecessor of the Milagro detector, Milagrito, was operational from February 1997 to May 1998. Milagrito consisted of 228 8″ photomultiplier tubes (PMTs) arranged in a grid with a 2.8 meter spacing and submerged in 1–2 meters of water. During its operation, Milagrito collected in excess of 9 billion events with a median energy of about 3 TeV. The detector’s sensitivity extends below 1 TeV for showers from near zenith. The results of an all sky search for the Milagrito data for both transient and DC sources will be presented, including the Crab Nebula and active galaxies Markarian 501 and 421, which are known sources of TeV gamma-rays. Also presented will be a study of the TeV emission from gamma ray bursts (GRBs) in Milagrito’s field of view detected by the BATSE e...

The Solar Two Gamma-Ray Observatory is designed to close the energy gap between 20–300 GeV that is inaccessible by current instruments, such as the satellite-borne EGRET detector and the ground-based air Cherenkov telescopes. Utilizing the facilities of the Solar Two Power Plant in Barstow, CA, the observatory will detect the Cherenkov light generated as high-energy gamma rays and charged cosmic-ray particles interact with the atmosphere. With over 2000 heliostats available, Solar Two has the largest heliostat mirror area in the world and, thus, the potential to be the most sensitive gamma-ray detector at these energies. Construction of a secondary mirror system capable of imaging 32 heliostats is nearing completion with plans for the first observations of the Crab Nebula in late November. We report on the design, status and testing of this secondary mirror system including the optics, electronics, and heliostat field.

FiberGLAST is one of several instrument concepts being developed for possible inclusion as the primary Gamma-ray Large Area Space Telescope (GLAST) instrument. The predicted FiberGLAST effective area is more than 12,000 cm2 for energies between 30 MeV and 300 GeV, with a field of view that is essentially flat from 0°–80°. The detector will achieve a sensitivity more than 10 times that of EGRET. We present results of simulations that illustrate the sensitivity of FiberGLAST for the detection of gamma-ray bursts.

The Solar Two Gamma-Ray Observatory is designed to close the energy gap between 20–300 GeV that is inaccessible by current instruments, such as the satellite-borne EGRET detector and the ground-based air Cherenkov telescopes. Utilizing the facilities of the Solar Two Power Plant in Barstow, CA, the observatory will detect the Cherenkov light generated as high-energy gamma rays and charged cosmic-ray particles interact with the atmosphere. With over 2000 heliostats available, Solar Two has the largest heliostat mirror area in the world and, thus, the potential to be the most sensitive gamma-ray detector at these energies. Construction of a secondary mirror system capable of imaging 32 heliostats is nearing completion with plans for the first observations of the Crab Nebula in late November. We report on the design, status and testing of this secondary mirror system including the optics, electronics, and heliostat field.

The Solar Two Gamma-Ray Observatory is a ground-based instrument designed to detect 20-300 GeV gamma rays by sampling the Cherenkov light generated as gamma rays and cosmic rays interact with the atmosphere. The observatory utilizes the solar two pilot power plant in Barstow, California which has the largest heliostat mirror area in the world. It has over 2,000 heliostats each with about 41 m 2 mirror are. the total active are is over 75,000 m 2 . Thus, Solar Two Observatory has the potential to be the most sensitive ground-based gamma-ray detector between 20-300 GeV. The secondary mirror system, each capable of viewing 32 heliostats has been designed. The secondary mirror systems also include the photomultiplier tube camera, electronics, and heliostat field. The first secondary camera system is now being manufactured and tested.

The FARXITE balloon payload concept contains 10 co-aligned, hard X-ray telescopes, each containing a set ofnested multilayer mirror modules and an actively shielded CZT strip detector at each focal plane. The 500 micronstrip pitch provides 26 arcsecond pixels at the 4m focal length of FARJ(ITE. The active shielding and advancedCZT detector techniques reduce the background at float altitudes to a few times iO counts/cm2 s keY. We describethese advanced detectors and how they allow us to meet the scientific objectives of the FAR3UTE program.Keywords: X-ray astronomy, CZT detectors, telescopes, balloons 1. INTRODUCTION The Fine Angular Resolution X-ray Imaging TelescopE (FARJ(ITE, pronounced "far sight") combines state-of-the-art hard X-ray mirrors and room temperature semiconductor (CZT) detectors, which are designed to fly on along duration balloon gondola with an advanced pointing system. One scientific goal of the FARXITE mission isimaging with sub-arcminute resolution deeper than ever before with iO ergs/s sensitivity in the 20-90 keV rangeat the Galactic center. This goal requires both spatial and spectral resolution at hard X-ray energies. FAR.XITE ischaracterized by

A wide field-of-view, arcsecond imaging, high energy resolution x-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to monitor and study gamma ray bursts (GRBs) with high energy and angular resolution and also find counterparts at other wavelengths. Detection of GRBs requires wide field-of-view ((pi) to 2 (pi) field-of-view) and high sensitivity. This is achieved by using high quantum efficiency CdZnTe pixel detectors with a low energy threshold (few keV) to observe the larger flux levels at lower energies, and large effective area (625 to 1,000 cm2) per coded aperture imaging module. Counterpart searches can only be done with ultra high angular resolution detectors (10 to 30 arcsecond FWHM) which gives 1 to 5 arcsecond position determination especially for strong GRBs. A few arcsecond size error box is expected to contain at most one object observed at another wavelength. This will be achieved by using ultra high spatial resolution pixel detectors (100 by 100 microns) and a similar resolution coded aperture to achieve the required angular resolution. AXGAM also has two other important advantages over similar detectors: (1) excellent low energy response (greater than 1 keV) and (2) high energy resolution (less than 6% at 5.9 keV, less than 3% at 14 keV, less than 4% at 122 keV). The low energy range may provide important new information on GRBs and the high energy resolution is expected to help in the observation and identification of emission and absorption lines in the GRB spectrum. The effective energy range is planned to be 2 to 200 keV which is exceptionally wide for such a detector. AXGAM will be built in the form of a 'bucky ball' using a coded aperture mask in a semi-geodesic dome arrangement placed over a two-dimensional, high resolution CdZnTe pixel detector array using newly developed p-i-n detector technology. The p-i-n structure decreases the electron and hole trapping effect and increases energy resolution significantly. The major scientific goals of the proposed mission in addition to continuously monitoring gamma- ray bursts, is to observe AGNs, transient phenomena, isolated and binary pulsars, and solar flares. A space deployed AXGAM detector is expected to observe several hundred gamma ray bursts per year.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We propose a new astrophysics space mission for a low energy gamma-ray-burst observatory (LEGO) that will fit the envelope of a small-explorer (SMEX) type mission. The LEGO instrument combines silicon pixel detectors with ultra-high energy resolution and a novel cost effective fine-pitch coded mask, to image the sky with sub-arcminute accuracy in the 0.3 - 30 keV range with a wide field-of-view. LEGO is well adapted to study hundreds of short transients such as gamma-ray bursts and soft gamma repeaters in the unexplored energy range below 5 keV. LEGO takes one of the next logical steps in GRB studies in the post-BeppoSAX era by attacking the astrophysics questions raised by recent discoveries of variable radio, optical, and x-ray counterparts to burst sources. In addition to monitoring the sky for gamma-ray bursts, LEGO would provide a first all-sky monitor in the 0.3 - 30 keV range. LEGO will be sensitive to all mCrab sources in the sky in a day and to 0.1 mCrab sources in a year, and thus, may provide daily light curves and sensitive spectral measurements on about 103 objects and yearly data on an order of magnitude more sources.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present a space mission concept for a low energy gamma-ray telescope, ATHENA, which is under investigation as the next major advance in gamma-ray spectroscopy following the current COMPTON Gamma Ray Observatory and the planned INTEGRAL missions. The instrument covers the nuclear line emission energy domain with dramatically improved sensitivity and spectral resolution. The baseline configuration combines a high resolution Compton telescope constructed from Ge planar strip detectors for the 0.3 - 10 MeV energy region with a coded-aperture system for the 10 - 200 keV domain. The Ge Compton telescope provides a broad field of view with exceptional spectral and imaging resolution. The requirements, capabilities and simulations of ATHENA are discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Tracking and Imaging Gamma-Ray Experiment (TIGRE) uses multilayers of silicon strip detectors both as a gamma-ray converter and to track Compton recoil electrons and positron-electron pairs. The silicon strip detectors also measure the energy losses of these particles. For Compton events, the direction and energy of the Compton scattered gamma ray are measured with arrays of small CsI(TI)-photodiode detectors so that an unique direction and energy can be found for each incident gamma ray. The incident photon direction for pair events is found from the initial pair particle directions. TIGRE is the first Compton telescope with a direct imaging capability. With a large (pi) -steradian field-of-view, it is sensitive to gamma rays from 0.3 to 100 MeV with a typical energy resolution of 3% (FWHM) and a 1-(sigma) angular resolution of 40 arc-minutes at 2 MeV. A small balloon prototype instrument is being constructed that has a high absolute detection efficiency of 8% over the full energy range and a sensitivity of 10 milliCrabs for an exposure of 500,000 s. TIGRE's innovative design also uses the polarization dependence of the Klein-Nishina formula for gamma-ray source polarization measurements. The telescope will be described in detail and new results from measurements at 0.5 MeV and Monte Carlo calculations from 1 to 100 MeV will be presented.

A large international collaboration from the high energy astrophysics community has proposed the Tracking and Imaging Gamma Ray Experiment (TIGRE) for future space observations. TIGRE will image and perform energy spectroscopy measurements on celestial sources of gamma rays in the energy range from 1 to 100 MeV. This has been a difficult energy range experimentally for gamma ray astronomy but is vital for the future considering the recent exciting measurements below 1 and above 100 MeV. TIGRE is both a double scatter Compton and gamma ray pair telescope with direct imaging of individual gamma ray events.Multi‐layers of Si strip detectors are used as Compton and pair converters CsI(Tl) scintillation detectors are used as a position sensitive calorimeter. Alternatively, thick GE strip detectors may be used for the calorimeter. The Si detectors are able to track electrons and positrons through successive Si layers and measure their directions and energy losses. Compton and pair events are completely reconstr...

The UCSD, WU, UCR and Nova collaboration has made significant progress on the necessary techniques for coded mask imaging of gamma-ray bursts: position sensitive CZT detectors with good energy resolution, ASIC readout, coded mask imaging, and background properties at balloon altitudes. Results on coded mask imaging techniques appropriate for wide field imaging and localization of gamma-ray bursts are presented, including a shadowgram and deconvolved image taken with a prototype detector/ASIC and MURA mask.

The Solar Two Gamma-Ray Telescope is planned as an extension to the STACEE Sandia experiment that is being developed for ground-based gamma-ray astronomy using the atmospheric Cherenkov detection technique in the energy range of 20 to 250 GeV. Because of its large effective light collection area, the STACEE and Solar Two Observatory will be able to reach low energy thresholds with very high angular resolution. A series of tests and observations has been made at Solar Two at Barstow, California and the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories Albuquerque, New Mexico. These measurements demonstrate the suitability of the site for astronomical observations and verify the performance of the telescope concept. A 48 heliostat detector is under construction at Sandia Laboratories and is expected to take data soon. A larger detector with /spl ges/80 heliostats is planned at the Solar Two, Barstow, California. If funded, the Solar Two Observatory is expected to be operational possibly within two to three years.

Results are presented from the Crab Pulsar PSR 0531 + 21 for energies of 0.3-30 MeV. For energies of 1-30 MeV, the absolute phase and separation of the first and second pulses 12.9 + or - 0.3 ms, widths of the pulses 2.2 + or - 0.5 ms FWHM, and 1.6 + or - 0.4 ms FWHM, ratio of the counts in the second to the first pulse 0.64 + or - 0.33 and ratio of counts in the interpulse region to the total pulsed counts 0.17 + or - 0.30 are compared to te results vor energies, E greater than 50 MeV and their variations with time. Values from the phase plots for E greater than 0.3 MeV are compared with low-energy gamma rays during 1970 through 1980 from several experiments with energies from about 20 to 360 keV. The above suggest different production mechanisms for low- and high-energy gamma rays with a transition at about 1 MeV. Derived fluxes from 0.3 to 30 MeV confirm a previously derived power law.