Astron. Astrophys. 332, L13-L16 (1998)

2. Plateau de Bure Observations

For the observations (see Table 1) with the PdBI (Guilloteau et al. 1992), we used the GRB's position derived at 8.46 GHz by Frail et al. (1997a), which is , (Eq. 2000.0, error in both coordinates ). The SIS dual frequency receivers were operated with bandwidths of 460 MHz using the tuning of the previously scheduled projects to get more time on-source. During clear sky conditions the atmospheric phase correction was used to improve the signal to noise ratio.

Table 1. Overview of decimal UT dates, frequencies, fluxes, offsets, beams (major axis, minor axes, PA), applied decorrelation factors from UV plane point source fits, configurations and weather conditions during the PdBI observations of GRB 970508. Fluxes are the maxima of UV plane point source fits with map r.m.s. Calibrators: (1) 3C273  Jy, (2) MWC349  Jy, (3) 3C454.3  Jy, (4) 2200+420  Jy, (5) NRAO530  Jy, (6) 0923+392  Jy (from Dutrey and Ungerechts 1997)

The flux of GRB 970508 was estimated relative to the phase calibrator 0716+714 (  Jy) for comparable atmospheric phase decorrelations, whose flux was derived relative to MWC 349 and 3C273 during the observing dates with the lowest amounts of precipitable water. As 0716+714 is known for rapid variability in the optical and cm range (10-20% over 4-12 hours, see Wagner et al. 1996, Quirrenbach et al. 1991), special care was taken to verify the flux calibration each day relative to one or more other sources. Between May 18 and 28, its maximum day-to-day variation was below 7%, and no significant source-intrinsic variation was detected during individual observations. Only on May 17, a 14% increase was derived relative to 3C454.3, but the conditions for this calibration were marginal so that the observations are consistent with a constant flux for 0716+714 during May 17-28. On August 12, the flux of 0716+714 was found to have increased to 1.24 Jy relative to MWC349 and NRAO530.

Observations at 232 GHz were done in parallel but resulted only in upper limits for GRB 970508. Calibration was relative to a MWC 349 flux of 1.56 Jy and a 3C273 flux of 20.5 Jy at this frequency.

Individual dates were checked for variations in the GRB flux level to assure that the detections did not come from high fluxes during short time intervals, which might have been subsequently "spread" over the observing time. No such variations were detected within the error limits, neither in the 86.2 GHz flux levels, nor in the non-detections at 86.2, 87.76, 86.0 and 232 GHz. The decorrelation factors of the May 17-22 observations are similar, which allows to combine their UV tables (without position shifting) after the appropriate adjustments for the slightly different frequencies. Because central positions of low- detections move around statistically, the UV fit positions were released for the flux determination. The result is position offsets: , ;  mJy.

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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