James Rhoads and Andy Fruchter (STScI) report:
We observed the proposed afterglow of GRB 000301c with the NASA IRTF + NSFCam
in K' band (2.1 microns) on March 4.640, and in J band (1.25 microns) on
March 4.652. The data were photometrically calibrated by observations
of UKIRT faint standard 27. Seeing was approximately 0.85" in K'
and 0.95" in J.
We find K'=17.65 +- 0.04 and J=19.10 +- 0.05 (statistical error only)
for the candidate afterglow. Aperture photometry was performed using a
0.9" (3 pixel) radius aperture, corrected to an effective aperture of
2.7" radius using aperture corrections derived from the brighter star
5.7" W and 1" S of the proposed OT. For reference, this star had
magnitudes K'=15.96 +- 0.02 and J=16.63 +- 0.01.
No correction was made for atmospheric extinction or color terms; however,
these are expected to be small. (The standard was observed at airmass 1.09,
and the GRB field at airmass 1.02.)
We detect a faint object, quite possibly a galaxy, approximately 2" N and
1" E of the candidate OT; this could be the host galaxy if the OT is
real. This object has approximate magnitudes K'=19.8, J=20.8.
This K' flux shows no significant decline from the earlier reported
observations (Stecklum et al, GCNC 572; Kobayashi et al, GCNC 577),
while it would be expected to decay by 0.44 magnitudes from Kobayashi's
data point for a t^-1.0 power law. This relative constancy of the near-IR
flux, despite optical fading, is difficult to reconcile with GRB afterglow
models, especially if it persists for more than a day.
We note that many classes of variable objects (other than
afterglows) show stronger variability at bluer wavelengths.
The weak or nonexistent variability in the K' filter is therefore
a cause for concern, and we urge continued followup at both optical
and IR wavelengths.
We thank the staff of the IRTF for help with these service observations.