TITLE:   GCN CIRCULAR
NUMBER:  18519
SUBJECT: GRB 151027A: NOT optical and NIR observations
DATE:    15/10/28 18:13:41 GMT
FROM:    Zach Cano at U of Iceland  <zewcano@gmail.com>

Z. Cano (U. Iceland), D. Malesani (DARK/NBI), Jussi Harmanen (NOT & U.
Turku), Thomas Reynolds (NOT & U. Turku.),  Pere Blay Serrano (IAC/NOT) and
P. Jakobsson (U. Iceland) report on behalf of a larger collaboration:

We observed the field of Swift GRB 151027A (Maselli et al., GCN Circ.
18478) with the Nordic Optical Telescope equipped with the StanCam
(optical) and NOTCam (NIR) imagers, in filters BVRI and JHK, respectively.
Observations started at 20:53 UT on 27 October 2015.  The OT (Maselli et
al.) is detected all filters.

A summary of our observations follows:

Filter       t-t0 (hr)      Mag +- Merr
-----------------------------------------
B            17.94          21.4 +- 0.2
R            18.17          20.6 +- 0.1
I            18.28          19.2 +- 0.1
J            16.92          18.2 +- 0.1
H            17.38          18.3 +- 0.1
K            17.60          17.2 +- 0.2

The optical filters are calibrated to USNO-B1 and the NIR filters to 2MASS,
where a single star that appeared in both catalogs was used for the
zeropoint calculation.  These magnitudes are not corrected for foreground
extinction.

When these extinguished magnitudes are converted to monochromatic fluxes
(using the zeropoints from Fukugita et al. 1995; Hewett et al. 2006), a
single power-law with an exponent of -1.25 provides an acceptable fit to
the optical-to-NIR spectral energy distribution.

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