J. Gregory Stacy

High-Energy Gamma Rays from the BL Lacertae Object PKS 2155–304

We present the first detection of gamma-ray emission from the extragalactic source PKS 2155-304, which is often considered to be a prototype of X-ray-selected BL Lacertae objects. The 30 MeV-10 GeV photon spectrum derived from measurements taken with EGRET on the Compton Observatory during 1994 November 15-29 indicates a very hard spectrum with a power-law spectral index of 1.71 ± 0.24. This spectral hardness when combined with the proximity of the source (z = 0.116) makes it an excellent candidate for detection at TeV gamma-ray energies. We briefly discuss the implications of these gamma-ray measurements for models of BL Lac objects.

The Gamma-Ray Blazar PKS 0208–512 from MeV to GeV Energies

We present a comprehensive report on the high-energy properties of the γ-ray blazar PKS 0208-512 as observed with EGRET and the Imaging Compton Telescope (COMPTEL) of the Compton Gamma Ray Observatory (CGRO), spanning the entire nine-year mission (1991-2000). More high-significance detections were recorded with EGRET of PKS 0208-512 (nine at greater than 6 σ confidence) than for any other γ-ray blazar. On timescales of weeks to months, PKS 0208-512 is one of the five most variable of the γ-ray blazars. We find a statistically significant correlation between intensity and spectral index for PKS 0208-512, and this source constitutes the best example of spectral hardening with intensity within the EGRET database of blazar observations. Under the assumption of isotropic Eddington-limited emission, we infer a black hole mass of approximately 6 × 107 M☉ at the nucleus of PKS 0208-512. Gamma-ray transparency arguments, however, clearly support the hypothesis of relativistic beaming in PKS 0208-512, with a minimum relativistic Doppler factor for the jet of δ 3.2 at GeV energies. We have also carried out a comprehensive reanalysis of the COMPTEL data associated with PKS 0208-512 at MeV energies and reexamined the earlier reports of excess 1-3 MeV emission from this source. For individual CGRO viewing periods, we find upper limits only for MeV emission from PKS 0208-512. Our reanalysis of the original COMPTEL data for the period 1993 May-June leads us to conclude that the significance of the original reported detection is marginal at best and that there is no compelling evidence at present for the existence of a distinct flaring state for PKS 0208-512 at MeV energies.

Coded-aperture x-ray/gamma-ray telescope for arc- minute localization of gamma-ray bursts

MARGIE will be a large-area, wide field-of-view, hard x- ray/gamma-ray imaging telescope capable of providing accurate positions for faint gamma-ray bursts in near-real- time and of performing a sensitive survey of both steady and transient cosmic sources. The instrument is designed to image faint bursts at the low-intensity end of the log N - log S distribution. MARGIE was recently selected by NASA for a mission-concept study for an Ultra Long Duration Balloon flight. We describe a program to develop an instrument based on the new detector technology of either cadmium zinc telluride room-temperature semiconductors or pixelated cesium iodide scintillators viewed by fast timing charge- coupled devices.

Wavelength-shifting fiber readout of LaCl 3 and LaBr 3 scintillators

Inorganic scintillators such as NaI(Tl) and CsI(Na) have been used extensively in hard x-ray and low-energy gamma-ray imaging systems. Recently, a new generation of scintillators has been developed with properties that could greatly enhance the performance of such imaging systems. In particular, the lanthanum halides show great promise with increased light yield and peak emission at shorter wavelengths compared to NaI or CsI. Since these scintillators emit at relatively short wavelengths, wavelength-shifting fibers can be used which re-emit at wavelengths around 420 nm, providing a good match to bialkali photocathode response. Multi-anode photomultiplier tubes can be used to read out individual fibers from orthogonal layers to provide x-y position information, while energy measurements can be made by large area photomultiplier tubes. Such an arrangement potentially provides improved overall position and energy resolution and lower thresholds compared to imaging systems configured as standard NaI or CsI gamma cameras. We present measurements of the energy resolution obtained from lanthanum chloride (LaCl3) and lanthanum bromide (LaBr3) scintillators viewed both perpendicular to the axis and down the length of square multi-clad wavelength-shifting fibers. These results are compared to a standard NaI detector with wavelength-shifting fibers. The implications of these results for gamma-ray imaging will then be discussed.

Ultraviolet observations of the Gamma‐Ray Blazar 3C 279 following the gamma‐ray flare of June 1991

Ultraviolet observations of the gamma‐ray blazar 3C 279 were carried out in July 1991 with the International Ultraviolet Explorer (IUE) satellite, 28 days after the outburst of intense gamma‐ray emission detected from this source with the high‐energy EGRET instrument aboard the Compton Gamma Ray Observatory (CGRO). IUE observations were conducted over the wavelength range 1200–3200 A (5–10 eV), and are compared with archival UV measurements spanning the period 1988 to 1991. This set of observations was analyzed with a uniform, standarized data‐reduction procedure. No significant variability in the UV spectrum of 3C 279 is noted over timescales of hours, though variations at the 3σ level or higher exist on longer timescales of months to years. The UV observations which most closely bracket the γ‐ray flare detected by EGRET show ∼3σ differences in flux and spectral index, consistent with (but not definitive proof of) variations in the UV flux during the period of the γ‐ray flare. Statistically mariginal evi...

Coordinated millimeter-wave observations of bright, variable gamma-ray blazars with the Haystack radio telescope

We present results of an ongoing program to monitor at millimeter wavelengths a select sample of bright, variable gamma-ray blazars with the 37-m Haystack radio telescope. Our primary objective during the 1996–1997 observing season was to follow the blazars 3C 279 and PKS 0528+134, each the subject of intense multiwavelength observing campaigns in parallel with observations carried out with the Compton Gamma Ray Observatory (CGRO). Our secondary objective was to monitor those active galactic nuclei (AGN) identified as particularly strong candidates for gamma-ray flares based on their CGRO detection history. Quasi-weekly monitoring sessions were carried at frequencies of 43 and 86 GHz with the Haystack telescope using new beam-switching instrumentation. In particular, we have applied a promising new technique (termed “multiple drift scanning”) that has been demonstrated to reliably and repeatedly measure continuum source fluxes to levels of a few hundred milliJanskys at 43 GHz, and to ∼1–1.3 Jy at 86 GHz. ...

A continued search for transient events in the COBE DMR database simultaneous with cosmic gamma-ray bursts

We report on the status of our ongoing project to search the database of the COBE Differential Microwave Radiometer (DMR) experiment for transient signals at microwave wavelengths simultaneous with cosmic gamma-ray bursts (GRBs). To date we have carried out a complete search of the DMR database using burst positions taken from the original BATSE 1B catalog for the eight-month period of overlap (May–December 1991) corresponding to the first public release of COBE data. We are currently repeating our original search of the COBE DMR database using the revised burst positions of the newly-released BATSE 3B catalog. Using BATSE 1B positions, at least two apparent simultaneous observations of GRBs by the COBE DMR occurred in 1991, along with a number of “near misses” within 30 seconds in time. At present, only upper limits to burst microwave emission are indicated. Even in the event of a non-detection of a GRB by the COBE DMR, unprecedented observational limits will still be obtained, constraining the predictio...

A Program To Search For Transient Microwave Emission From GRBs And Other High‐Energy Sources Using Archival WMAP Datasets

We report on a new program to search the public time‐ordered datasets acquired with the Wilkinson Microwave Anisotropy Probe (WMAP) for transient signals associated with gamma‐ray bursts (GRBs) and other high‐energy sources. This program is an extension of earlier work in which we established the first limits on prompt microwave emission from GRBs using archival datasets from the Differential Microwave Radiometers (DMR) aboard the COBE satellite. The increased sensitivity and angular resolution of the WMAP radiometers compared to the COBE/DMR lead to a factor of ∼10,000 improvement in overall point‐source sensitivity. Such limits approach the signal levels predicted in the microwave band for the peak prompt emission arising from reverse shocks in GRBs. In the first phase of our program we are verifying our analysis software and assessing sensitivity limits by searching for microwave transients or flaring signals from known blazars and similar sources that are detected in the cumulative WMAP data as “foreg...

An extended search for transient events in the COBE/DMR database associated with cosmic gamma-ray bursts

We report on an extension of earlier work to search the archival database of the Differential Microwave Radiometers (DMR) aboard the COBE satellite for evidence of transient events at microwave wavelengths associated with cosmic gamma-ray bursts (GRBs). Over its four-year lifetime the DMR experiment repeatedly surveyed the sky to measure fluctuations in the cosmic microwave background. On a number of occasions at least one of the horns of the DMR was also serendipitously pointing in the direction of a cosmic gamma-ray burst at the moment of burst occurrence. Our original investigation covered the eight-month period April–December 1991 when a number of GRBs were observed by the COBE/DMR at or within a few seconds of the CGRO/BATSE trigger time. Upper limits only were obtained for any simultaneous microwave flux from these events, in the range 7–42 kJy. Most recently, we have extended our search using the additional 24 months of COBE DMR data now publicly available, covering the time period January 1992 thr...

Gamma-Ray Astronomy

The project has progressed successfully during this period of performance. The highlights of the Gamma Ray Astronomy teams efforts are: (1) Support daily BATSE data operations, including receipt, archival and dissemination of data, quick-look science analysis, rapid gamma-ray burst and transient monitoring and response efforts, instrument state-of-health monitoring, and instrument commanding and configuration; (2) On-going scientific analysis, including production and maintenance of gamma-ray burst, pulsed source and occultation source catalogs, gamma-ray burst spectroscopy, studies of the properties of pulsars and black holes, and long-term monitoring of hard x-ray sources; (3) Maintenance and continuous improvement of BATSE instrument response and calibration data bases; (4) Investigation of the use of solid state detectors for eventual application and instrument to perform all sky monitoring of X-Ray and Gamma sources with high sensitivity; and (5) Support of BATSE outreach activities, including seminars, colloquia and World Wide Web pages. The highlights of this efforts can be summarized in the publications and presentation list.