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). Results are presented in Figure 1.
Six outbursts are apparent, and four are new discoveries. They range in strength from −0.2 to −2.6 mag, and all events are confirmed by visual inspection of the images. Some appear to be similar in scale to the small outbursts of comets Tempel 1 and Churyumov-Gerasimenko. Preliminary details are provided below, and further analyses will be presented in future works.
Farnham et al. (submitted) discovered an outburst at comet 46P in Transiting Exoplanet Survey Satellite images. The outburst began near 2018 September 26.12 UTC (heliocentric distance, rh = 1.5 au) and had a peak strength of −0.5 mag (25,000 km radius apertures). In our ZTF data, the pre-outburst brightness was r = 15.15 ± 0.05 mag (g−r = 0.45 ± 0.02 mag; we assume r−i = 0.15 mag, see 64P below). At September 26.42 UTC, the coma was −1.23 ± 0.04 mag brighter than the trend-line, which is stronger than the Farnham et al. value due to the smaller aperture size (2200–3200 km).
Kelley et al. (2018) discovered a −2.7 magnitude outburst of comet 64P occurring around 2018 August 14 (rh = 1.7 au), and we have identified three outbursts following the event. The pre-outburst brightness was r = 16.62 ± 0.03 mag (g−r = 0.50 ± 0.01 mag; r−i = 0.16 ± 0.02 mag). With respect to the baseline trend, the four outbursts are: August 14.41 (−2.62 ± 0.03 mag), 27.37 (−0.48 ± 0.03 mag), September 7.33 (−0.23 ± 0.02 mag), and 10.33 UTC (−0.23 ± 0.03 mag).
4. 78P/Gehrels 2
Inspection of our 78P lightcurve reveals a single outburst at rh = 2.8 au. The pre-outburst brightness was r = 16.56 ± 0.06 mag (g−r = 0.55 ± 0.01 mag; assuming r−i = 0.23 mag produces a near-continuous lightcurve). The outburst occurred between 2018 July 27.39 and 31.41 UTC, and had a strength of at least −0.47 ± 0.03 mag. The similarity in shape with the large outburst of 64P suggests the peak may have been as strong as −2 to −3 mag.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW.