[vsnet-grb-info 17934] ICECUBE-160427A : Pan-STARRS imaging and optical transients in the field

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19381
SUBJECT: ICECUBE-160427A : Pan-STARRS imaging and optical transients in the field
DATE:    16/05/04 18:43:58 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt at qub.ac.uk>

S. Smartt  (Queen’s University Belfast), K. C. Chambers (IfA, Univ. of Hawaii),  
K. Smith (QUB), M. E. Huber (IfA), D. Wright (QUB), 
H. Flewelling, E. A. Magnier, N. Primak, A. S. B. Schultz (IfA), J. Tonry, 
C. Waters, M. Willman (IfA), D. Young (QUB)

Further to the detection of a candidate cosmic neutrino, as event IceCube-160427A
(Blaufuss et al. GCN 19363) we report Pan-STARRS1 imaging of the field and a 
search for optical transients. 

We observed the field, entered on RA=240.57d and DEC=+9.34d (J2000) 
beginning MJD 57508.5 UT (2016 April 30.5), which was 3.3 
days after the IceCube detection (2016 April 27.2) with the Pan-STARRS1
telescope in the i-band filter. Difference imaging with respect to the 
Pan-STARRS1 3Pi stacked reference sky reached 5-sigma limiting magnitudes of 
around i~22.5. We repeated the observations on the 4 subsequent nights. 

Similar to Lipunov et al. (GCN19362), we find no bright transient source
within the 90% containment area of 36 arc minutes radius of the above position 
(RA=16:02:16.8, DEC=+09:20:24). We find the following 24 fainter sources

Eight QSOs (with SDSS spectra) are found to be significantly variable
(the variable flux was 18.8 < i < 21.4) and are in the 
redshift range 0.345 to 2.384. 

Seven supernova candidates (discovery mags 20.5 < i < 22.1) were found. These
are transient sources which have a detected candidate host galaxy, and either 
offset from the host or are coincident but not known QSO/AGN. No 
catalogued spectroscopic redshifts of the hosts are available. 

A further 9 transients were found associated with faint point sources that 
are either faint Galactic variable stars or QSO variability. 

In summary, there is no obvious counterpart candidate in the optical
brighter than i ~ 21.  Within the 90% containment field diameter of 1.2 degrees, 
we have hit the confusion limit of the faint variable sky. While there are many 
variable/transient candidates, there is no one object that is statistically 
rare enough it could be considered further. 

The SN candidates are as follows. Six of the candidates show  flat 
lightcurves across our 5 days of monitoring, consistent with being 
SN at or after peak. One object is worth considering further :

Object 	   | RA (J2000)  | DEC (J2000) | Disc Date  |  i -mag    
PS16cgv    | 15 59 53.84 | +09 11 08.4 |  20160430  |  22.01  
PS16cgw    | 16 00 39.69 | +09 39 21.2 |  20160430  |  21.80 
PS16cfu    | 16 01 15.66 | +09 25 04.7 |  20160430  |  21.14 
PS16cgx    | 16 01 18.60 | +09 51 53.1 |  20160430  |  21.84 *
PS16cfz    | 16 02 11.96 | +09 54 07.9 |  20160430  |  21.27 
PS16cgb    | 16 02 19.12 | +09 34 50.1 |  20160430  |  22.03
PS16cgi    | 16 03 27.72 | +08 54 05.2 |  20160430  |  22.09


* PS16cgx rose at by 0.4 mag over 2 days. The rise time would
be consistent with a type Ic supernova at z ~ 0.1 to 0.2 that 
exploded on or around 2016 April 27.2 (and possibly an off-axis GRB). 
However the 4th data  point shows it has peaked at i=21.5 +/- 0.1, 
and is therefore more consistent with a type Ia SN from the normal 
field population. Spectroscopic follow-up is needed to confirm.