A. Diercks, E. W. Deutsch, C. Stubbs (University of Washington); P.M. Vreeswijk, T.J. Galama, J. van Paradijs (University of Amsterdam); C. Robinson, and C. Kouveliotou (MSFC-NASA) report:
We observed the BeppoSAX error box of GRB 980613 on June 14.35 UT and June 15.35 UT in Kron-Cousins I with the APO 3.5m telescope. Conditions were mostly clear with ~2.0 arcsec seeing. Photometry on a 30 min series of I-band (Kron-Cousins) images centered around June 14.35 UT (28 hr after the burst) yields the following preliminary magnitudes and relative uncertainties for several reference stars:
ID I mag RelErr RA (J2000) DEC -- ----- ------ ------------------------ 1 17.28 0.02 10 17 47.48 +71 27 0.6 2 18.74 0.02 10 17 54.83 +71 27 40.2 3 18.70 0.03 10 18 6.55 +71 27 5.1 4 18.58 0.02 10 17 55.29 +71 28 16.5 5 18.25 0.02 10 17 41.45 +71 28 9.3Absolute calibration is based on a single observation of the PG 2213-006 standard field (Landolt 1992) and a previously determined airmass correction for this filter and instrument. The uncertainty in the absolute calibration is 0.1 mag.
The limiting magnitude (SNR~5) for the combined images for each of the individual June 14.35 and 15.35 epochs is I~22.0. We find no object at the position of the slow moving K~17.5 detection reported by Castro-Tirado (GCN #102,#103).
The error circle was also observed with the 1m JKT telescope in Harris I on June 13.97 for 1600s and June 14.96 for 420s. Conditions were clear with ~2.0 arcsec seeing.
Difference imaging between the June 13.97 UT JKT data and the June 14.35 UT APO data reveals no point sources which varied by more than the flux equivalent of I = 20.6 mags between the two images. Similar analysis on June 14.35 UT and June 15.35 UT APO data gives an upper limit of 21.5 mags to any such flux differences.
An image of the field is posted at: http://www.astro.washington.edu/deutsch/grb/grb980613/
This note can be cited.