Hello, Don Lamb and I have been doing some calculations that may aide in the planning of the GRB 990123 afterglow observations. For Jan 23.57 UT, R = 18.2 (GCN 201) and B = 18.93 (GCN 206) have been reported. This implies a spectral index of -1.75, which is very steep! Assuming a temporal index of -1, and this spectral index, we find that at Jan 24.57 UT, the magnitude of the afterglow will be: K = 18.0 J = 19.1 I = 20.0 R = 20.4 V = 20.7 B = 21.1 U = 21.5 By Jan 25.57, the R magnitude of the afterglow would be 21.0. The proposed host galaxy (GCN 201) has a magnitude of R = 21.3, which means that tonight may be the only night for which the afterglow and the proposed host galaxy may be cleanly separated, at least in the R band. If the temporal index is as steep as -2, then the R magnitude of the proposed host galaxy tonight would be R = 22.5 (for Jan 24.57), which would be fainter than the proposed host galaxy. Consequently, a measurement of the temporal index of this afterglow over many days is probably out of the question. Consequently, our top priority should be multiband photometry. Given the brightness of this afterglow, and its favorable declination, we should be able to take sufficiently deep images in a number of bands before night's end. We highly recommend beginning with V, B, and then U (if it doesn't require excessive amounts of time). If there is sufficient time to switch camera's, NIR observations should be made near the end of the observation. I should point out that the afterglow may have a shallower spectral index in the NIR; this has been the case with other afterglows (GRB 971214, for example). If one assumes a spectral index of -1 below the R band, the above estimates become: K = 19.0 J = 19.7 I = 20.2 R = 20.4 (same as above) Finally, if (1) it looks like there will be enough time before switching cameras, or (2) it looks like there will not be enough time to switch cameras and carry a NIR observation, we recommend first I and then R band observations. The R band should be the last priority because it will surely be well sampled by other observers throughout this night. Of course, we recommend going sufficiently deeper than the above estimates. I (Dan Reichart) am not on the APO exploder. I can be reached at (773) 702-6684 or (847) 570-0846 throughout the night. Cheers, Dan Reichart APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 25 in the apo35-grb archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-grb/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-grb@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO