Search for transient optical counterparts to high-energy IceCube neutrinos with Pan-STARRS1

¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
² Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ Institute of Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ Max-Planck-Institut fur Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁵ Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55454, USA
⁶ III. Physikalisches Institut, RWTH Aachen University, 52056 Aachen, Germany
⁷ Department of Physics, University of Adelaide, Adelaide 5005, Australia
⁸ Dept. of Physics and Astronomy, University of Alaska Anchorage, 3211 Providence Dr., Anchorage, AK 99508, USA
⁹ Dept. of Physics, University of Texas at Arlington, 502 Yates St., Science Hall Rm 108, Box 19059, Arlington, TX 76019, USA
¹⁰ CTSPS, Clark-Atlanta University, Atlanta, GA 30314, USA
¹¹ School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, Atlanta, GA 30332, USA
¹² Dept. of Physics, Southern University, Baton Rouge, LA 70813, USA
¹³ Dept. of Physics, University of California, Berkeley, CA 94720, USA
¹⁴ Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
¹⁵ Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany
¹⁶ Fakultät für Physik& Astronomie, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹⁷ Université Libre de Bruxelles, Science Faculty CP230, 1050 Brussels, Belgium
¹⁸ Vrije Universiteit Brussel (VUB), Dienst ELEM, 1050 Brussels, Belgium
¹⁹ Dept. of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁰ Dept. of Physics and Institute for Global Prominent Research, Chiba University, Chiba 263-8522, Japan
²¹ Dept. of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
²² Dept. of Physics, University of Maryland, College Park, MD 20742, USA
²³ Dept. of Astronomy, Ohio State University, Columbus, OH 43210, USA
²⁴ Dept. of Physics and Center for Cosmology and Astro-Particle Physics, Ohio State University, Columbus, OH 43210, USA
²⁵ Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
²⁶ Dept. of Physics, TU Dortmund University, 44221 Dortmund, Germany
²⁷ Dept. of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
²⁸ Dept. of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada
²⁹ Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany
³⁰ Physik-department, Technische Universität München, 85748 Garching, Germany
³¹ Département de physique nucléaire et corpusculaire, Université de Genève, 1211 Genève, Switzerland
³² Dept. of Physics and Astronomy, University of Gent, 9000 Gent, Belgium
³³ Dept. of Physics and Astronomy, University of California, Irvine, CA 92697, USA
³⁴ Dept. of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA
³⁵ SNOLAB, 1039 Regional Road 24, Creighton Mine 9, Lively, ON P3Y 1N2, Canada
³⁶ Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095, USA
³⁷ Dept. of Astronomy, University of Wisconsin, Madison, WI 53706, USA
³⁸ Dept. of Physics and Wisconsin IceCube Particle Astrophysics Center, University of Wisconsin, Madison, WI 53706, USA
³⁹ Institute of Physics, University of Mainz, Staudinger Weg 7, 55099 Mainz, Germany
⁴⁰ Department of Physics, Marquette University, Milwaukee, WI 53201, USA
⁴¹ Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany
⁴² Bartol Research Institute and Dept. of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
⁴³ Dept. of Physics, Yale University, New Haven, CT 06520, USA
⁴⁴ Dept. of Physics, University of Oxford, Parks Road, Oxford OX1 3PQ, UK
⁴⁵ Dept. of Physics, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA
⁴⁶ Physics Department, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA
⁴⁷ Dept. of Physics, University of Wisconsin, River Falls, WI 54022, USA
⁴⁸ Dept. of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
⁴⁹ Oskar Klein Centre and Dept. of Physics, Stockholm University, 10691 Stockholm, Sweden
⁵⁰ Dept. of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA
⁵¹ Dept. of Physics, Sungkyunkwan University, Suwon 16419, Korea
⁵² Dept. of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA
⁵³ Dept. of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁵⁴ Dept. of Physics, Pennsylvania State University, University Park, PA 16802, USA
⁵⁵ Dept. of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁵⁶ Dept. of Physics, University of Wuppertal, 42119 Wuppertal, Germany
⁵⁷ DESY, 15738 Zeuthen, Germany
⁵⁸ Earthquake Research Institute, University of Tokyo, Bunkyo, Tokyo 113-0032, Japan

Received: 30 January 2019
Accepted: 3 May 2019

Abstract

In order to identify the sources of the observed diffuse high-energy neutrino flux, it is crucial to discover their electromagnetic counterparts. To increase the sensitivity of detecting counterparts of transient or variable sources by telescopes with a limited field of view, IceCube began releasing alerts for single high-energy (E_ν >  60 TeV) neutrino detections with sky localisation regions of order 1° radius in 2016. We used Pan-STARRS1 to follow-up five of these alerts during 2016–2017 to search for any optical transients that may be related to the neutrinos. Typically 10–20 faint (m_iP1 ≲ 22.5 mag) extragalactic transients are found within the Pan-STARRS1 footprints and are generally consistent with being unrelated field supernovae (SNe) and AGN. We looked for unusual properties of the detected transients, such as temporal coincidence of explosion epoch with the IceCube timestamp, or other peculiar light curve and physical properties. We found only one transient that had properties worthy of a specific follow-up. In the Pan-STARRS1 imaging for IceCube-160427A (probability to be of astrophysical origin of ∼50%), we found a SN PS16cgx, located at 10.0′ from the nominal IceCube direction. Spectroscopic observations of PS16cgx showed that it was an H-poor SN at redshift z = 0.2895 ± 0.0001. The spectra and light curve resemble some high-energy Type Ic SNe, raising the possibility of a jet driven SN with an explosion epoch temporally coincident with the neutrino detection. However, distinguishing Type Ia and Type Ic SNe at this redshift is notoriously difficult. Based on all available data we conclude that the transient is more likely to be a Type Ia with relatively weak Si II absorption and a fairly normal rest-frame r-band light curve. If, as predicted, there is no high-energy neutrino emission from Type Ia SNe, then PS16cgx must be a random coincidence, and unrelated to the IceCube-160427A. We find no other plausible optical transient for any of the five IceCube events observed down to a 5σ limiting magnitude of m_iP1 ≈ 22 mag, between 1 day and 25 days after detection.