Search

Data acquired with the COBE Differential Microwave Radiometer (DMR) provide a unique opportunity to observe simultaneous emission from cosmic gamma-ray bursts in the previously unexplored microwave region of the spectrum. We have searched the COBE DMR time-ordered data sets for instances when one of the DMR horns (FWHM ~ 7°) was pointing in the direction of a gamma-ray burst at the time of burst occurrence. During the overlap period 1991 April-December corresponding to the first public release of COBE data, 210 Compton Gamma Ray Observatory (CGRO)/BATSE gamma-ray bursts listed in the Third BATSE (3B) Catalog were viewable by the COBE DMR. For five of these events the DMR was pointing within 7° of the burst positions at the exact moment of burst occurrence. For another four events the DMR was pointed within 2° of the BATSE positions within 10 s of the burst trigger time. No obvious microwave emission (at 31.5, 53, or 90 GHz), with upper limits in the 10-100 kJy range, can be associated with any of these events. The COBE DMR has a relatively low sensitivity for the detection of point sources within its field of view. A positive detection of a gamma-ray burst by the COBE DMR would imply that the integrated microwave flux must be of the same order as the energy observed in gamma rays. By extending an acceptance window in time of up to 20 minutes before and after a gamma-ray burst another 60 bursts are sampled by the DMR, whose signals are analyzed statistically. We conclude that the "average" gamma-ray burst produces less than about 7-42 kJy in simultaneous microwave radiation.

We are investigating the COBE DMR data at instances of known γ-ray bursts (GRBs) when any of the six DMR horn directions was coincident with the direction of a burst. The BATSE instrument on board GRO has detected 207 bursts during the eight-month period of overlap corresponding to the current release of COBE data. The odds of a GRB occurring within the DMR field of view are near one coincidence per year. Here we report on one such serendipitous observation in 1991, GRB 911226, for which a detailed analysis is currently in progress.

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...

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...

Some years ago, Willems & de Jong (1988) noticed that many carbon stars display an excess of emission at 60 µm and explained it by the presence of a fossil dust shell, containing only cold dust. This detached dust shell would be the result of an interruption of the mass loss, consequence of a thermal pulse. Detached shells around C stars have actually been mapped in the CO lines (Olofsson et al. 1992), and at 60µm (Waters et al. 1994). In 1992, Zijlstra et al. found about 100 M stars displaying an excess of emission at 60 µm, and proposed that interruptions of the mass loss due to thermal pulses is a general phenomenon on the AGB. This assumption is now supported by the theoretical calculations of Vassiliadis & Wood (1993). Here we present a detailed study of the 100 M stars of Zijlstra et al. in order to test the previous assumption.