TITLE: GCN CIRCULAR NUMBER: 22173 SUBJECT: Pan-STARRS follow-up of IceCube-171106A DATE: 17/11/28 12:57:11 GMT FROM: O. McBrien at QUB O. McBrien, K. W. Smith, S.J. Smartt, D. R. Young (Queen's University Belfast), M. Huber, K. C. Chambers, H. Flewelling, M. Willman, A. Schultz, E. Magnier, C. Waters, J. Bulger, R. J. Wainscoat (IfA, Hawaii), D. Wright, (Univ. of Minnesota), Further to the detection of the Extremely High Energy neutrino event IceCube-171106A (see GCN #22105), we report Pan-STARRS1 imaging of the field and the search for optical transients. We observed the field of the neutrino event at RA=340.0 degrees and DEC=+7.4 degrees (J2000) from MJD 58065.34 (2017-11-08 08:09:36.0 UTC), 37.44 hrs following the IceCube detection with the Pan-STARRS1 telescope in both the i-band and z-band filters (Chambers et al. arXiv:1612.05560). Difference imaging with respect to the Pan-STARRS1 3Pi stacked reference sky reached 5-sigma limiting magnitudes of around i~22.5. Observations were repeated on 10 subsequent nights. Similar to Lipunov et al. (GCN 22104), we find no bright transient source (i,z < 20) in the field. We found 7 fainter transients, which are not known AGN or variable stars, within 1.3 degrees of the estimated position of the neutrino (RA: 340.00, Dec: +7.40). These are all detected over multiple nights. None of them appear to be rising or have a lightcurve that points at a possible temporal coincidence with IceCube-171106A, though two are contained within a 0.7 degree (42 arcmin) radius of the neutrino detection. Three are supernova like candidates, and four are coincident with galaxy cores. These nuclear transients are probably real, but difficult to distinguish between low level nuclear variability and a true transient source. We report the discovery magnitudes, dates and sky locations of the transients for the transients in the 0.7 degree (42 arcmin) radius and those outside it separately as follows. Within 0.7 degree of IceCube-171106A PS Name | RA (J2000) | Dec (J2000) | Ang. Separation (arcmin) | Disc. Date | Disc. Mag. | Notes PS17fcc | 22 41 58.64 | +07 46 53.8 | 37.3 | 20171108 | 21.54 i | (1) PS17eym | 22 39 22.13 | +07 16 36.4 | 11.9 | 20171108 | 21.48 i | (2) Outside 0.7 degree of IceCube-171106A, but within PS1 footprint PS Name | RA (J2000) | Dec (J2000) | Ang. Separation (arcmin) | Disc. Date | Disc. Mag. | Notes PS17fcd | 22 44 01.84 | +08 13 27.6 | 77.7 | 20171109 | 21.53 z | (3) PS17eyn | 22 42 24.63 | +06 55 02.7 | 46.1 | 20171108 | 21.64 I | (4) PS17fem | 22 42 19.31 | +06 42 38.0 | 53.9 | 20171110 | 21.95 I | (5) PS17eyo | 22 40 40.92 | +06 26 29.6 | 58.4 | 20171108 | 22.16 I | (6) PS17eyl | 22 39 02.90 | +06 25 26.3 | 60.2 | 20171108 | 21.23 i | (7) (1) Nuclear transient candidate located 0.59 arcsec N, 0.28 arcsec W of host galaxy (SDSS J224158.66+074653.2). Transient is within 42 arcmin of neutrino detection. (2) SN-like candidate associated with galaxy SDSS J223922.20+071638.0, located 1.80 arcsec S, 0.97 arcsec W from galactic centre, with a host photometric redshift 0.38 +/- 0.11. Transient is within 42 arcmin of neutrino detection. (3) Nuclear transient candidate located 0.37 arcsec N, 0.13 arcsec W of host galaxy (SDSS J224401.85+081327.5). Difference imaging shows positive and negative net flux, implying some galactic core activity. (4) Nuclear transient candidate located 0.58 arcsec N, 0.84 arcsec E of host galaxy (SDSS J224224.58+065502.1). (5) Nuclear transient candidate located 0.57 arcsec N, 0.41 arcsec E of host galaxy (SDSS J224219.29+064237.5). (6) SN-like candidate not matched with catalogued galaxy. (7) SN-like candidate associated with galaxy SDSS J223902.90+062526.6, located 0.28 arcsec S, 0.04 arcsec E from galactic centre, with a host photometric redshift 0.24 +/- 0.10.