D. Malesani (DARK/NBI), A. de Ugarte Postigo (IAA-CSIC and DARK/NBI), M.
De Pasquale (UCL/MSSL), D. A. Kann (TLS Tautenburg), Z. Cano (Univ.
Iceland), D. A. Perley (DARK/NBI), L. Izzo (IAA-CSIC), C. C. Thoene
N. Butler (ASU), A. M. Watson (UNAM), A. Kutyrev (GSFC), W. H. Lee
(UNAM), M. G. Richer (UNAM), O. Fox (STScI), J. X. Prochaska (UCSC), J.
S. Bloom (UCB), A. Cucchiara (GSFC/STScI), E. Troja (GSFC), O.
Littlejohns (ASU), E. Ramirez-Ruiz (UCSC), J. A. de Diego (UNAM), L.
Georgiev (UNAM), J. Gonzalez (UNAM), C. Roman-Zuniga (UNAM), N. Gehrels
(GSFC), H. Moseley (GSFC), J. Capone (UMD), V. Zach Golkhou (ASU), and
V. Toy (UMD), on behalf of the RATIR collaboration,
We observed the field of GRB 160623A (Vianello et al., GCN 19553;
Frederiks et al., GCN 19554; Mailyan et al., GCN 19555) with the Nordic
Optical Telescope (NOT) equipped with the AlFOSC imager. Our
observations had a mid time 2016 July 8.09 UT (14.88 days after the GRB
trigger) and consisted of 6x300 s in the SDSS i filter.
Close to the position of the optical and X-ray afterglow (Mingo et al.,
GCN 19558; Pozanenko et al., GCN 19561; Butler et al., GCN 19567), we
detect a bright source at the following coordinates (J2000):
RA = 21:01:11.65
Dec = +42:13:15.0
We measure I = 20.6 +- 0.2 (Vega) against several nearby USNO stars
(error dominated by the calibration). To determine the relation of this
object with the GRB afterglow, we cross-registered our image with those
from RATIR (Butler et al., GCN 19567), yielding an RMS of 0.02". We find
that the NOT source is offset to the SE of the afterglow by 0.7". This
source could thus be the host galaxy or an unrelated, foreground star.
We also note some faint emission in the RATIR image at the NOT
coordinates, despite being blended with the brighter afterglow.
A picture showing the comparison of the two images is shown here:
A spectrum (3x1200 s exposure) was acquired using the Gran Telescopio
Canarias (GTC) equipped with OSIRIS, covering the wavelength range
5100-10,000 AA (grism R1000R), starting on 2016 July 10.163 UT (16.95
days after the GRB). The slit was centered at the coordinates of the NOT
object, but also partially overlapped with the afterglow location.
We detect continuum over the whole wavelength range and two emission
lines in the red part of the spectrum, which we interpret as Halpha and
[N II] at a common redshift z = 0.367. At the same redshift we also
detect weak [O III] 5008 (dimmed by the large foreground extinction).
This is most likely the redshift of GRB 160623A.
The source detected in the NOT image is possibly a foreground star (the
Galactic latitude is b = -2.7 deg). In the GTC acquisition image, the
object is pointlike with a seeing of 1.2". On the other hand, inspecting
the 2D spectrum, the spatial profile of the emission lines is different
from that of the continuum: the features are wider and elongated towards
the position of the afterglow. A compact host cannot however be excluded.
At z = 0.367, a SN as bright as SN 1998bw would only reach a peak
magnitude of I ~ 24.8 (Vega) given the large Galactic extinction (A_I =
2.2 mag; Schlafly & Finkbeiner 2011). From ground, such a SN would be
difficult to distinguish against the brighter foreground object.
We acknowledge excellent support from the observers at the NOT (in
particular Roi Alonso and Diego Hidalgo) and at the GTC.