Joshua J. Bloch

Observation of contemporaneous optical radiation from a gamma-ray burst

The origin of γ-ray bursts (GRBs) has been enigmatic since their discovery. The situation improved dramatically in 1997, when the rapid availability of precise coordinates, for the bursts allowed the detection of faint optical and radio afterglows — optical spectra thus obtained have demonstrated conclusively that the bursts occur at cosmological distances. But, despite efforts by several groups, optical detection has not hitherto been achieved during the brief duration of a burst. Here we report the detection of bright optical emission from GRB990123 while the burst was still in progress. Our observations begin 22 seconds after the onset of the burst and show an increase in brightness by a factor of 14 during the first 25 seconds; the brightness then declines by a factor of 100, at which point (700 seconds after the burst onset) it falls below our detection threshold. The redshift of this burst, z ≈ 1.6 (refs 8, 9), implies a peak optical luminosity of 5× 1049 erg s−1. Optical emission from γ-ray bursts has been generally thought to take place at the shock fronts generated by interaction of the primary energy source with the surrounding medium, where the γ-rays might also be produced. The lack of a significant change in the γ-ray light curve when the optical emission develops suggests that the γ-rays are not produced at the shock front, but closer to the site of the original explosion.

Rotse all sky surveys for variable stars I: test fields

The Robotic Optical Transient Search Experiment I (ROTSE-I) experiment has generated CCD photometry for the entire northern sky in two epochs nightly since 1998 March. These sky patrol data are a powerful resource for studies of astrophysical transients. As a demonstration project, we present first results of a search for periodic variable stars derived from ROTSE-I observations. Variable identification, period determination, and type classification are conducted via automatic algorithms. In a set of nine ROTSE-I sky patrol fields covering roughly 2000 deg2, we identify 1781 periodic variable stars with mean magnitudes between mv = 10.0 and mv = 15.5. About 90% of these objects are newly identified as variable. Examples of many familiar types are presented. All classifications for this study have been manually confirmed. The selection criteria for this analysis have been conservatively defined and are known to be biased against some variable classes. This preliminary study includes only 5.6% of the total ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will include more than 32,000 periodic variable stars.

Comparison of GENIE and conventional supervised classifiers for multispectral image feature extraction

The authors have developed an automated feature detection/classification system, called GENetic Imagery Exploitation (GENIE), which has been designed to generate image processing pipelines for a variety of feature detection/classification tasks. GENIE is a hybrid evolutionary algorithm that addresses the general problem of finding features of interest in multispectral remotely-sensed images. The authors describe their system in detail together with experiments involving comparisons of GENIE with several conventional supervised classification techniques, for a number of classification tasks using multispectral remotely sensed imagery.

ALEXIS: An Ultrasoft X-Ray Monitor Experiment Using Miniature Satellite Technology

Los Alamos and Sandia National Laboratories are, building an ultrasoft X-ray monitor experiment. This experiment, called ALEXIS (Array of Low-Energy X-Ray Imaging Sensors), consists of six compact normal-incidence telescopes. ALEXIS will operate in the 70 - 110 eV band. The ultrasoft X-ray/EUV band is nearly uncharted territory for astrophysics. ALEXIS, with its wide fields-of-view and well-defined wavelength bands, will complement the upcoming NASA Extreme Ultraviolet Explorer and ROSAT EUV Wide Field Camera, which are sensitive broadzband survey experiments. The program objectives of ALEXIS are to 1) demonstrate the feasibility of a wide field-of-view, normal incidence ultrasoft X-ray telescope system and 2) to determine ultrasoft X-ray backgrounds in the space environment. As a dividend, ALEXIS will pursue the following scientific objectives: 1) to map the diffuse background, with unprecedented angular resolution, in several emission line bands, 2) to perform a 3-color survey of point sources, 3) to search for transient phenomena in the ultrasoft X-ray band, and 4) to provide synoptic monitoring of variable ultrasoft X-ray sources such as cataclysmic variables and flare stars. The six ALEXIS telescopes are arranged in pairs to cover three 40° fields-of-view. During each spin of the satellite, ALEXIS will monitor more than half the sky. Each telescope consists of a layered synthetic microstructure (LSM) mirror, a curved microchannel plate detector, background-rejecting filters and magnets, and readout electronics. The mirrors will be tuned to 72 eV, 85 eV, and 95 or 107 eV bands, chosen to select and deselect interesting line features in the diffuse background. The geometric area of each ALEXIS telescope will be about 15 cm2. The telescopes employ spherical mirrors with the curved detector at prime focus and are limited by spherical aberration to a resolution of about 1°. Assuming nominal reflectivities, quantum efficiency, and filter transmission, the 5a survey sensitivity will be about 2 x 10-3 photons cm-2 s-1 for line emission at the center of the bandpass. ALEXIS is designed to be flown on a small autonomous payload carrier (a minisat) that could be launched from either a Shuttle Get-Away-Special Can or from an expendable launch vehicle. The experiment weighs 100 pounds, draws 40 watts, and produces 10 kbps of data. It can be flown in any low Earth orbit. Onboard data storage allows operation and tracking from a single ground station at Los Alamos.

Observations of the Optical Counterpart to XTE J1118+480 during Outburst by the Robotic Optical Transient Search Experiment I Telescope

The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of 2000. As detected by the Rossi X-Ray Timing Explorer (RXTE), the first outburst began in early January and the second began in early March. Routine imaging of the northern sky by the Robotic Optical Transient Search Experiment (ROTSE) shows the optical counterpart to XTE J1118+480 during both outbursts. These data include over 60 epochs from January to June 2000. A search of the ROTSE data archives reveal no previous optical outbursts of this source in selected data between 1998 April and 2000 January. While the X-ray-to-optical flux ratio of XTE J1118+480 was low during both outbursts, we suggest that they were full X-ray novae and not minioutbursts based on comparison with similar sources. The ROTSE measurements taken during the 2000 March outburst also indicate a rapid rise in the optical flux that preceded the X-ray emission measured by the RXTE by approximately 10 days. Using these results, we estimate a preoutburst accretion disk inner truncation radius of ~1.2 × 104 Schwarzschild radii.

Rapid optical follow-up observations of SGR events with ROTSE-I

In order to observe nearly simultaneous emission from Gamma-ray Bursts (GRBs), the Robotic Optical Transient Search Experiment (ROTSE) receives triggers via the GRB Coordinates Network (GCN). Since beginning operations in March, 1998, ROTSE has also taken useful data for 10 SGR events: 8 from SGR 1900+14 and 2 from SGR 1806-20. We have searched for new or variable sources in the error regions of these SGRs and no optical counterparts were observed. Limits are in the range m_ROTSE ~ 12.5 - 15.5 during the period 20 seconds to 1 hour after the observed SGR events.

The ALEXIS small satellite project: better, faster, cheaper faces reality

ALEXIS, one of the most sophisticated miniature satellites developed to date and the first satellite project led by Los Alamos National Laboratory, carries both soft X-ray astrophysics and ionospheric physics experiments. It is an example for experimenters who desire better, faster, and cheaper access to space. The satellite was launch-ready 3 1/2 years after concept. The soft X-ray experiment, ALEXIS, is a novel set of wide-angle, normal incidence telescopes which scan half the sky every satellite rotation. BLACKBEARD is a broadband receiver and digitizer designed to study ionospheric propagation in the 25-175 MHz band. The spin-stabilized spacecraft is compact and efficient; for example, it provides 50 W to the payload while consuming 10 W itself. ALEXIS will fly on a Pegasus air-launched booster. >

ALEXIS Lunar Observations

The ALEXIS small satellite was designed as a large area monitor operating at extreme ultraviolet wavelengths (130 – 190 A). At these energies, the moon is the brightest object in the night sky and was the first source identified in the ALEXIS data. Due to the design of ALEXIS and the lunar orbit, the moon is observed for two weeks of every month. Since lunar emissions in the extreme ultraviolet are primarily reflected solar radiation these observations may be useful as a solar monitor in the extreme ultraviolet. The data show distinct temporal and spectral variations indicating similar changes in the solar spectrum. We will present a preliminary dataset of lunar observations and discussions covering the variations observed and how they relate to the solar spectrum.