[vsnet-grb-info 24386] LIGO/Virgo S191213g: Pan-STARRS1 discovery of a potential optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26485
SUBJECT: LIGO/Virgo S191213g: Pan-STARRS1 discovery of a potential optical counterpart
DATE:    19/12/18 16:02:32 GMT
FROM:    O. McBrien at QUB  <omcbrien02 at qub.ac.uk>

O. McBrien, S. J. Smartt, K. W. Smith, D. R. Young, J. Gillanders. S. Srivastav, P. Clark, D. O'Neill, M. Fulton, S. McLaughlin (QUB), K.C. Chambers , M. E. Huber, A.S.B. Schultz, T. de Boer, J. Bulger, J. Fairlamb, C.C. Lin, T. Lowe, E. Magnier , R. J. Wainscoat, M. Willman (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard), T.-W. Chen (Stockholm) on behalf of the Pan-STARRS collaboration report:

We report the Pan-STARRS1 (Chambers et al. 2016, arXiv:1612.05560)
discovery of an intrinsically faint optical transient in the 80%
probability contour of the compact binary merger event S191213g (The
LIGO Scientific Collaboration and the Virgo Collaboration, GCN 26402).

The object, PS19hgw, was discovered on MJD 58833.305 (UTC 2019-12-16
07:19:12) at a magnitude of 19.4 +/- 0.1 in the PS1-i band and
coordinates RA = 01:55:41.94, Dec = +31:25:04.4. We have registered it
on the TNS as AT2019wxt (McLaughlin et al. AstroNote 2019-154).
Pan-STARRS1 was observing part of the skymap (LALInference.fits) of
S191213g during routine survey operations and its ongoing Pan-STARRS
search for kilonovae (Smartt et al. AstroNote 2019-48). We had two more
detections on the same night, in the same filter. All exposures were
45 seconds in duration.

We have no previous detections at the location of the transient with
Pan-STARRS, or indeed with ATLAS either (Tonry et al. 2018, PASP, 130,
4505). There are no previous detections in the public ZTF transient
stream (Bellm et al. 2018, PASP, 131, 995) as ingested in Lasair
(Smith et al. 2019, RNAAS, 3, 26).

We had no detection on the previous night, MJD 58832.305 (UTC
2019-12-15 07:19:12), but only one frame was taken in poor conditions
and the limit is not constraining (>19.4 in the PS1-i band).

The most recent limits and photometry are: 

MJD           | mag    | err   | filter
58829.348 |  <21.0 |   ...   | z 
58830.379 |  <20.3 |   ...   | z 
58830.379 |  <20.3 |   ...   | z 
58832.305 |  <19.4 |   ...   | i 
58833.335 |  19.37 | 0.07 | i
58833.320 |  19.32 | 0.07 | i 
58833.305 |  19.38 | 0.05 | i 

PS19hgw/AT2019wxt shows an association with the galaxy KUG 0152+311 at
a redshift z = 0.036 (144 Mpc, NED). The current luminosity distance
estimate accompanying the GW trigger, available through the
LALInference.fits.gz skymap (GCN 26417), is 200 +/- 80 Mpc, placing
PS19hgw/AT2019wxt within the plausible time-volume for association
with this event. The distance modulus to PS19hgw/AT2019wxt is 35.8,
giving an absolute magnitude at discovery of -16.5 in the PS1 i-band
(assuming foreground extinction of A_i = 0.1). We note that this is 2
magnitudes brighter than AT2017gfo was at +3days after merger, but no
less encourage spectroscopic follow-up of this object.