The data consisted of 6 exposures (3 of 600 seconds each and 3 of 500 seconds each), which were "dithered" to move the transient's location to a different region of the detector on each exposure. James Rhoads reduced the images following mostly standard procedures for Mosaic data. Calibration frames from preceding nights (April 1 and 3 UT) were used, and we used dome flats. We estimate residual flatfielding errors for relative photometry at less than 3% ; this could be improved using dark sky flats, but we see no compelling reason to do so.
The final coadded image shows no source at the location of the optical transient. The flux obtained in a 5 pixel (1.30 arcsecond) radius aperture and calibrated by comparison to Groot et al's reference star 1 ( IAU Circular 6852) corresponds to -0.44 +- 0.33 microJansky at R band. The corresponding 3 sigma limiting magnitude is R > 24.3 ; the 2 sigma limit is R > 25.3 . For a 7 pixel aperture, the flux is -0.52 +- 0.38 microJansky.
We also placed an aperture on the location of Grossan et al's nearby source (GRB Coordinate Network notice no. 35). The fluxes in 5 and 7 pixel apertures are 0.14 +- 0.32 microJansky and 0.51 +- 0.37 microJansky respectively. Corresponding 3 sigma limits are R > 23.6 and 23.2 ; two sigma limits are R > 24.0 and 23.4 respectively. These results are just consistent with Grossan et al's magnitude measurement of R=24.1 or 24.3 for the neighboring source.
A 104 by 91 arcsecond section of the final image can be obtained in FITS format by anonymous FTP from ftp://gemini.kpno.noao.edu/pub/grb/980326. Note that the instrument's field of view is 36 arcminutes, and so our data cover the entire BeppoSAX error box (16 arcminute diameter). We can make the raw data covering this full region available to anyone who would like to see them. However, note that some reprocessing of the data would be required to obtain a cosmetically clean image throughout this region. The present chips in the instrument have numerous and large cosmetic defects. We placed the transient's location on a clean region of the detector in all exposures, and so did not pay much attention to mitigating the effects of bad pixels on other regions of the image during data processing.
If you use this data in a publication, please at least cite our upcoming communication to the GCN (Gamma Ray Burst Coordinate Network). A standard issue Kitt Peak acknowledgement may also be in order in some cases, e.g. if you wish to reproduce the image. That acknowledgement is something like: This work uses data obtained by F. Valdes, B. Jannuzi, and J. Rhoads on behalf of the NOAO Deep Widefield survey and the KPNO followup team using the KPNO 4 meter telescope. Kitt Peak National Observatory, National Optical Astronomy Observatories, is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.
General comments: The pixel scale is approximately 0.26 arcsec per pixel. The instrument used was the Kitt Peak CCD Mosaic camera. North is to the left, and East is at the bottom. There is a world coordinate system in the image header; it appears to have a small zero point shift (about 2 pixels in the Y direction, i.e. E-W) relative to Groot et al's astrometry for the photometric reference stars.
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