# Subject:

## Message number: 25 (previous: 24, next: 26 up: Index)

```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

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