TITLE:   GCN CIRCULAR
NUMBER:  20407
SUBJECT: LIGO/Virgo G268556: Gemini spectrum and Pan-STARRS observations of ATLAS17aeu 
DATE:    17/01/09 16:52:22 GMT
FROM:    S. J. Smartt at Queens U Belfast  <s.smartt@qub.ac.uk>

K. C. Chambers (IfA), S. Smartt (QUB), J. Tonry, L. Denneau,
A. Heinze, B. Stalder, H. Weiland (IfA), C. W. Stubbs (Harvard),
K. W. Smith (QUB), T.-W. Chen (MPE), T. Kruehler (MPE), D. R. Young
(QUB), A. Rest (STScI), M. Coughlin (Harvard), M. E. Huber (IfA),
D. E. Wright (QUB), H. Flewelling, E. A. Magnier, A. S. B. Schultz,
C. Waters, R. J. Wainscoat (IfA)

Further to our discovery of ATLAS17aeu (Tonry et al. GCN 20382), we 
took a spectrum with Gemini-North + GMOS (R400 grating, 
4491-8778 Angs) on MJD = 57761.595. We also took an r-band image 
on the same night. 

The spectrum has a blue featureless continuum, with a signal-to-noise
in the continuum S/N~10 at 7000Angs.  There are no clear emission or
absorption lines visible that can anchor a redshift estimate.

The spectrum is not unlike some GRB afterglows as shown in the compilation
of Fynbo et al. 2009, ApJS 185, 526 
e.g. GRB060512, at z~2.1, GRB061110A (z=0.7578), GRB070129 (no z
estimate, but below z<3.4)

We report three more photometric points, two from Pan-STARRS1 and 
the other from Gemini+GMOS. 

MJD           Mag           Telescope 
57758.38159   18.18 0.04 i  Pan-STARRS1
57759.46468   20.90 0.12 i  Pan-STARRS1
57761.51968   22.77 0.17 r  Gemini+GMOS 

These data further support the hypothesis of Kasliwal et al. GCN 20393
that this is most likely the afterglow of GRB170105A, supported by the
x-ray and radio detections of Evans et al. GCN 20390 and Corsi et al.
GCN 20396.

Using the POLAR detection time, a temporal index of F_nu \propto
t**(-1.4) would be consistent with the photometry reported to date,
typical of GRB afterglows.

We note there are several other faint galaxies closer to ATLAS17aeu
than the brighter galaxy at z = 0.199 highlighted in Tonry et al. (GCN
20382). They have SDSS photometric redshifts of z ~ 0.35 - 0.38, but
are marginal detections in more than one band and therefore the 
photometric redshifts are uncertain.