Results from the Pan-STARRS search for kilonovae: contamination by massive stellar outbursts
M. Fulton, S. J. Smartt, M. E. Huber, K. Smith, K. C. Chambers, M. Nicholl, Shubham Srivastav, D. R. Young, E. A. Magnier, Chien-Cheng Lin, P. Mínguez, T de Boer, T. Lowe, R. J. Wainscoat
Abstract
ABSTRACT We present results from the Pan-STARRS optical search for kilonovae without the aid of gravitational wave and gamma-ray burst triggers. The search was conducted from 2019 October 26 to 2022 December 15. During this time, we reported 29 740 transients observed by Pan-STARRS to the IAU Transient Name Server. Of these, 175 were Pan-STARRS credited discoveries that had a host galaxy within 200 Mpc and had discovery absolute magnitudes $M>-16.5$. A subset of 11 transients was plausibly identified as kilonova candidates by our kilonova prediction algorithm. Through a combination of historical forced photometry, extensive follow-up, and aggregating observations from multiple sky surveys, we eliminated all as kilonova candidates. Rapidly evolving outbursts from massive stars (likely to be Luminous Blue Variable eruptions) accounted for 55 per cent of the subset’s contaminating sources. We estimate the rate of such eruptions using the Asteroid Terrestrial-impact Last Alert System 100 Mpc volume-limited survey data. As these outbursts appear to be significant contaminants in kilonova searches, we estimate contaminating numbers when searching gravitational wave skymaps produced by the LIGO-Virgo-Kagra science collaboration during the Rubin era. The Legacy Survey of Space and time, reaching limiting magnitudes of $m\approx 25$, could detect 2–6 massive stellar outbursts per 500 deg$^{2}$ within a 4-d observing window, within the skymaps and volumes typical for binary neutron star mergers projected for Ligo-Virgo-Kagra Observing run 5. We conclude that while they may be a contaminant, they can be photometrically identified.