A Search for Cosmic Neutrino and Gamma-Ray Emitting Transients in 7.3 yr of ANTARES and Fermi LAT Data

[EN] We analyze 7.3 yr of ANTARES high-energy neutrino and Fermi Large Area Telescope (LAT) g-ray data in search of cosmic neutrino + g-ray (n+g) transient sources or source populations. Our analysis has the potential to detect either individual n+g transient sources (durations dt5% of the observed gamma-coincident neutrinos in the track data at 90% concidence. Applying our analysis to the unscrambled data reveals no individual n+g events of high significance; two ANTARES track+ Fermi g-ray events are identi¿ed that exceed a once per decade false alarm rate threshold (p = 17%).No evidence for subthreshold ¿+¿ source populations is found among the track (p = 39%) or cascade (p = 60%) events. Exploring a possible correlation of high-energy neutrino directions with Fermi g-ray sky brightness identi¿ed in previous work yields no added support for this correlation. While TXS 0506+056, a blazar and variable (nontransient) Fermi g-ray source, has recently been identified as the first source of high-energy neutrinos, the challenges in reconciling observations of the Fermi g-ray sky, the IceCube high-energy cosmic neutrinos, and ultrahigh-energy cosmic rays using only blazars suggest a significant contribution by other source populations. Searches for transient sources of high-energy neutrinos thus remain interesting, with the potential for either neutrino clustering or multimessenger coincidence searches to lead to discovery of the first n+g transients. The authors thank David Thompson for helpful discussions. We gratefully acknowledge support from Penn State's Office of the Senior Vice President for Research, the Eberly College of Science, and the Penn State Institute for Gravitation and the Cosmos. This work was supported in part by the National Science Foundation under Grant No. PHY-1708146. K.M. is supported by the Alfred P. Sloan Foundation and by the Nat