TITLE: GCN CIRCULAR NUMBER: 2271 SUBJECT: GRB 030528: tentative NIR counterpart DATE: 03/06/05 15:21:43 GMT FROM: Jochen Greiner at MPI J. Greiner, A. Rau (both MPE Garching) and S. Klose (Thueringer Landessternwarte Tautenburg) report for the GRACE collaboration: We have observed the original, 2 arcmin radius error box of GRB 030528 (=H2724) (Atteia et al, GCN 2256) with the near-infrared camera SOFI at the NTT (La Silla, ESO) on three epochs as given below: Date (Start UT) Filters Exposure time Seeing ------------------------------------------------------- 2003-05-29 04:58 JHKs 15 min each 0.6" 2003-05-30 04:54 JHKs 20 min each 1.0" 2003-06-01 04:07 Ks 60 min 0.7" The Chandra sources #1 and #10 (Butler et al., GCN 2269) are within our field of view, the other two Chandra sources from within the shifted GRB location (Villasenor et al, GCN 2261) are outside our images. Comparing the Ks band images of the first and third epoch reveals a counterpart for each of the two Chandra sources. One of these two sources, i.e. #1 = CXOU J170400.3-223710 shows a fading by about 0.9 mag. The other of these two Chandra sources, #10 = CXOU J170354.0-223654 is about 4 magnitudes brighter in Ks, and constant to within 0.1 mag (provisional photometric calibration was done against the 2MASS catalog, taken from the NASA/ IPAC Infrared Science Archive). We therefore propose Chandra source #1 = CXOU J170400.3-223710 and its near-infrared counterpart as the tentative afterglow of GRB 030528. K band images of two epochs for this source will be displayed at http://www.mpe.mpg.de/~jcg/grb030528.html Since we are missing two of the Chandra sources due to the shift of the GRB location, only a second Chandra observation can (dis)prove this tentative counterpart. We note, that the proposed counterpart is the brightest X-ray source in the previous Chandra image, so a second Chandra observation of the same exposure time can already be decisive.