[vsnet-grb-info 14260] GRB 131127B: rejection of optical counterpart candidates

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15574
SUBJECT: GRB 131127B: rejection of optical counterpart candidates
DATE:    13/12/04 04:42:30 GMT
FROM:    Leo Singer at CIT/PTF  <lsinger at caltech.edu>

L. P. Singer (Caltech), M. M. Kasliwal (Carnegie Observatories),
S. B. Cenko (NASA/GSFC), Yi Cao (Caltech), Daniel Perley (Caltech),
and Annalisa De Cia (Weizmann) report on behalf of the intermediate
Palomar Transient Factory (iPTF) collaboration:

We report our continued observations of the optical transient
iPTF13ecv (Singer & Kasliwal, GCN 15524; Xu et al., GCN 15531; Quadri
et al., GCN 15543; Quadri et al., GCN 15558), which was spatially
coincident with the Fermi GBM localization of GRB 131127B (von
Kienlin, GCN 15528) but was well outside the IPN error box
(Golenetskii et al., GCN 15529).

No X-ray source coincident with iPTF13ecv was detected in a 5 ks Swift
XRT observation starting at 2013-11-28 10:10 (0.83 days after the
Fermi-GBM trigger). We calculate a 3 sigma upper limit on the count
rate of < 2.2e-3 cps at this time.

We obtained further optical photometry with the robotic Palomar
60-inch telescope (P60), yielding r=19.0 +/- 0.1 mag at 2013-11-30
03:36.

On 2013-11-29, we obtained a spectrum with the DEIMOS instrument on
the Keck II telescope. The spectrum was blue and featureless from
about 5000 to 8500 angstroms.

On 2013-12-02, we obtained a second spectrum with the LRIS instrument
on Keck I. This spectrum had a blue continuum with prominent Balmer
absorption lines at a redshift of z=0.

Considering the position well outside the IPN localization, the
continued optical detection of iPTF13ecv, its blue optical SED, and
the Balmer absorption lines, we conclude that iPTF13ecv is unrelated
to the GRB trigger and is likely a cataclysmic variable in outburst.

We also note that the other two iPTF candidates reported in GCN
15524, iPTF13ect and iPTF13ecu, are probably due to AGN activity. In
the 6dFGS survey (Jones et al., MNRAS, 355, 747), iPTF13ect displays
a Seyfert I-type spectrum (broad H-alpha and strong [N II] emission
lines) and iPTF13ecu a Seyfert II spectrum (no broad H-alpha
emission, but strong [N II] emission lines).