Outbursts at Comets 46P/Wirtanen, 64P/Swift-Gehrels, and 78P/Gehrels 2 in 2018

Abstract

Cometary outbursts are brief increases in the mass-loss rates of comets (Hughes 1990). Most observed outbursts have Δm ≳ 1 mag, but smaller outbursts do occur, such as those observed by the Deep Impact and Rosetta spacecraft at comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko (A'Hearn et al. 2005; Vincent et al. 2016). The outbursts are smaller than the Deep Impact excavation, suggesting strengths near −0.2 to −1 mag (A'Hearn et al. 2005; Meech et al. 2005; Feldman et al. 2007). Wide-field, time-domain surveys are well-suited to the discovery of cometary outbursts (Graham et al. 2019). We describe initial results from our outburst search using the Zwicky Transient Facility archive (ZTF; Bellm et al. 2019), based on observations of three comets: 46P/Wirtanen, 64P/Swift-Gehrels, and 78P/Gehrels 2. The data are calibrated to the Pan-STARRS 1 photometric system (Masci et al. 2019), and color corrected assuming a reddened solar spectrum (g−r = 0.52 mag, r−i = 0.15 mag). We measured photometry in 7'' radius apertures, then fit and removed a linear trend from each lightcurve: 46P, −0.053 mag day^(−1) (rms 0.02 mag); 64P, −0.053 mag day^(−1) (rms 0.04 mag); 78P, −0.039 mag day^(−1) (rms 0.04 mag).