GCN Circular 21987

John R. Thorstensen (Dartmouth) and Jules P. Halpern (Columbia) report:

Automated analysis of Swift XRT data (GCN #21986) on Fermi-LAT
GRB 171010A (GCN #21985) reports a fading X-ray source at:

(J2000.0) 04:26:19.44 -10:27:47.1  +/- 1.4"

We pointed the 1.3m McGraw-Hill telescope of the MDM Observatory
at this location beginning on 2017 October 11 08:31 UT, 13.5 hr
after the burst, and obtained images in the Cousins R filter that
show a new object at:

(J2000.0) 04:26:19.48 -10:27:46.2  +/- 0.4"

with position referenced to the PPMXL catalog. In comparison with
nearby stars with r magnitudes from Pan-STARRS, we estimate r = 18.0
at 08:31 UT.  Continuing photometry shows that the OT faded from
r = 18.05 to 18.38 between 8:55 UT and 10:28 UT.

There is also a faint galaxy at this position on the digitized
POSS II and in Pan-STARRS, but bright moon now makes it difficult
to detect in our images.  The Pan-STARRS position of this possible
host galaxy is:

(J2000.0) 04:26:19.56 -10:27:45.8

Using the 2.4m Hiltner telescope with Modspec, we obtained 17x12 minutes
spectra of the OT covering the wavelength range 4400-7500 A at 4 A
resolution, which when summed show a featureless continuum with a
signal-to-noise ratio of about 50 per 2 A pixel.