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 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.