From: Daniel Reichart

Submitted: Sat, 23 Jan 1999 21:22:23 -0600 (CST)

Message number: 23 (previous: 22, next: 24 up: Index)


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.


Dan Reichart

APO APO APO APO APO  Apache Point Observatory 3.5m  APO APO APO
APO  This is message 23 in the apo35-grb archive. You can find
APO  the archive on
APO  To join/leave the list, send mail to
APO  To post a message, mail it to