Search

We report on the results of rapid responses to GRB 080330 and GRB 080413A by the ROTSE-IIIb and ROTSE-IIIc telecopes. In both cases, optical follow-up started during the prompt emission phase and recorded optical flares peaking at similar times as the last hard X-ray pulses detected by the Swift BAT. For both events, the XRT observations started at the end of the prompt BAT detection and displayed an evolution in time opposite to the optical lightcurve. These very early observations provide us an opportunity to probe the onset of afterglow for these two events.

We report the successful performance of the Robotic Optical Transient Search Experiment (ROTSE) to promptly‐disseminated Swift GRB triggers. ROTSE‐III is a worldwide network of 4 unfiltered 0.45m optical telescopes. The telescopes operate robotically, automatically responding to GRB position triggers with a preset observations sequence. Including weather and other downtime, ROTSE‐III can immediately respond to ∼40% of Swift trigger positions, with shutter open within approximately 6–8 seconds from the trigger dissemination. We discuss improvements made possible in the automated response the the small, accurate Swift error boxes. We report ROTSE‐III’s general results, including OTs discovered or confirmed, the distribution of imaging start times relative to the GRB duration, and an overview of OT lightcurves.

The RAPTOR system (RAPid Telescopes for Optical Response) is an array of several distributed robotic telescopes that automatically respond to GCN localization alerts. Raptor‐S is a 0.4‐m telescope with 24 arc min. field of view employing a 1k × 1k Marconi CCD detector, and has already detected prompt optical emission from several GRBs within the first minute of the explosion. We present a real‐time data analysis and alert system for automated identification of optical transients in Raptor‐S GRB response data down to the sensitivity limit of ∼ 19 mag. Our custom data processing pipeline is designed to minimize the time required to reliably identify transients and extract actionable information. The system utilizes a networked PostgreSQL database server for catalog access and distributes email alerts with successful detections.

With the coming launch of the Swift satellite, there will be many new opportunities to study the physics of the prompt optical emission from gamma ray bursts with robotic ground‐based telescopes. We discuss a new imaging system under development at Los Alamos National Laboratory that will provide simultaneous multicolor photometry of the rapidly evolving prompt optical emission in the first minutes after a burst trigger. This next generation system employs state‐of‐the‐art photon‐counting imaging technology at the focal plane of a rapidly slewing telescope. The imaging sensor is composed of an S‐20 photocathode, stacked microchannel plates, and crossed delay line readout electronics that together are capable of measuring the time of arrival and positions for individual optical photons with 200 picosecond time resolution. The imager is coupled with electronically tunable liquid‐crystal filters to provide essentially simultaneous linear polarization and multicolor photometric measurements of the prompt opti...

While it has been known for centuries that the optical sky is variable, monitoring the sky for optical transients with durations as short as a minute is an area of astronomical research that remains largely unexplored. Prompt follow‐up observations of Gamma Ray Bursts have shown that bright, explosive, optical transients exist. However, there are many reasons to suspect the existence of explosive optical transients that cannot be located through sky monitoring by high‐energy satellites. The RAPTOR sky monitoring system is an autonomous system of telescope arrays at Los Alamos National Laboratory that identifies fast optical transients as short as a minute and makes follow‐up observations in real time. The core of the RAPTOR system is composed of two arrays of telescopes, separated by 38 kilometers, that stereoscopically monitor a field of about 1300 square degrees for transients down to about 12.5th magnitude in 30 seconds. Both arrays are coupled to real‐time data analysis pipelines that are designed to identify transients on timescales of seconds. Each telescope array also contains a more sensitive higher resolution “fovea” telescope, capable of both measuring the light curve at a faster cadence and providing color information. In a manner analogous to human vision, each array is mounted on a rapidly slewing mount so that the “fovea” of the array can be rapidly directed for real‐time follow‐up observations of any interesting transient identified by the wide‐field system. We discuss the first results from RAPTOR and show that stereoscopic imaging and the absence of measurable parallax is a powerful tool for distinguishing real celestial transients in the “forest” of false positives.

With the launch of HETE‐2 and the coming launch of the Swift satellite, there will be many new opportunities to study the physics of the prompt optical emission with robotic ground‐based telescopes. Time‐resolved spectrophotometry of the rapidly varying optical emission is likely to be a rich area for discovery. We describe a program to apply state‐of‐the‐art photon‐counting imaging technology to the study of prompt optical emission from gamma‐ray bursts. The Remote Ultra‐Low Light Imaging (RULLI) project at Los Alamos National Laboratory has developed an imaging sensor which employs stacked microchannel plates and a crossed delay line readout with 200 picosecond photon timing to measure the time of arrival and positions for individual optical photons. RULLI detectors, when coupled with a transmission grating having 300 grooves/mm, can make photon‐counting spectroscopic observations with spectral resolution that is an order of magnitude greater and temporal resolution three orders of magnitude greater than the most capable photon‐counting imaging detectors that have been used for optical astronomy.

A rich, but relatively unexplored, region in optical astronomy is the study of transients with durations of less than a day. We describe a wide‐field optical monitoring system, RAPTOR, which is designed to identify and make follow‐up observations of optical transients in real‐time. The system is composed of an array of telescopes that continuously monitor about 1500 square degrees of the sky for transients down to about 12th magnitude in 60 seconds and a central fovea telescope that can reach 16th magnitude in 60 seconds. Coupled to the telescope array is a real‐time data analysis pipeline that is designed to identify transients on timescales of seconds. In a manner analogous to human vision, the entire array is mounted on a rapidly slewing robotic mount so that the fovea of the array can be rapidly directed at transients identified by the wide‐field system. The goal of the project is to develop a ground‐based optical system that can reliably identify transients in real‐time and ultimately generate alerts with source locations to enable follow‐up observations with other, larger, telescopes.

The giant outbursts of GeV emission observed from some blazars pose a significant challenge to our understanding of blazar emission mechanisms. Unfortunately, the giant outbursts of GeV emission from blazars are not always predictable from observations in other energy bands. Here we describe a strategy that will be employed during Cycle 9 of the CGRO observing program which will allow us to search the sky for intense outbursts of GeV emission while having a minimal impact on the CGRO observing schedule and remaining spark chamber gas.

We have developed a design for a Compton scatter polarimeter to measure the polarization of hard X-rays (50–300 keV) from solar flares. The modular design is based on an annular array of scintillating fibers coupled to a 5-inch position-sensitive PMT. Incident photons scatter from the fiber array into a small array of NaI detectors located at the center of the annulus. The location of the interactions in both the fiber array and in the NaI array can be used to measure the linear polarization of the incident flux. This compact design may be well-suited to a variety of astrophysical applications. An extensive series of Monte Carlo simulations has been performed to characterize this design.

We describe a technique for measuring the polarization of hard x-rays from γ-ray bursts based on the angular distribution of that portion of the flux which is scattered off the top of the Earth’s atmosphere. The scattering cross section depends not only on the scatter angle itself, but on the orientation of the scatter angle with respect to the incident polarization vector. Consequently, the distribution of the observed albedo flux will depend on the direction and the polarization properties (i.e., the level of polarization and polarization angle) of the source. Although the BATSE design (with its large field-of-view for each detector) is not optimized for albedo polarimetry, we have nonetheless investigated the feasibility of this technique using BATSE data.

We describe a technique for measuring the polarization of hard X‐rays from solar flares based on the angular distribution of that portion of the flux which is scattered off the top of the Earth’s atmosphere. The scattering cross section depends not only on the scatter angle itself, but on the orientation of the scatter angle with respect to the incident polarization vector. Consequently, the distribution of the observed albedo flux will depend on the direction and the polarization properties (i.e., the level of polarization and polarization angle) of the source. Since the albedo component can represent a relatively large fraction (up to 40%) of the direct source flux, there will generally be sufficient signal for making such a measurement. The sensitivity of this approach is therefore dictated by the effective area and the ability of a detector system to ‘image’ the albedo flux. The 4π coverage of the BATSE detectors on the Compton Gamma‐Ray Observatory provides an opportunity to measure both the direct a...

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) provides solar‐flare observation capabilities that represent a significant improvement over previous experiments. In this paper, we discuss the OSSE solar observation modes, give an overview of the solar‐flare observations to date, and discuss in some detail the observations obtained during the June 1991 CGRO solar Target of Opportunity. We show evidence for time structure of flare hard X‐ray (≳100 keV) emission as short as tens of msec. Several nuclear deexcitation lines along with emission ≳10 MeV were seen from the four GOES X10+ flares observed by OSSE. Neutrons were observed from two of the flares. Preliminary results from a search of the OSSE flare database for nuclear emission from weak flares are presented and compared to results from a similar search of the Solar Maximum Mission Gamma‐Ray Spectrometer database.

The study of the polarization properties of the gamma‐ray bursts is the one remaining unexplored avenue of research which may help to answer some of the fundamental problems regarding the nature of these mysterious objects. We have designed an instrument to measure linear polarization in cosmic gamma‐ray bursts at energies ≳50 keV. Here we describe the design of this instrument, which we call the Gamma‐ray Burst Polarimeter Experiment (GRAPE).