Recurring Outbursts of P/2019 LM₄ (Palomar)

Cometary outbursts are brief, unexpected increases in the brightness of comets that indicate a sudden and dramatic change in the activity (Patashnick, 1974; Hughes, 1990). Such events sometimes lead to the discovery of originally inactive or weakly active comets which would otherwise not be detectable (Kramer et al., 2017; Ye, 2017).

Our preliminary analysis of images of comet P/2019 LM₄ (Palomar) acquired with the Zwicky Transient Facility (ZTF) survey (Bellm et al., 2019) has revealed recurring brightenings or outbursts of activity. P/Palomar was first detected as an asteroidal object on 2019 June 4 and 7 in the course of the ZTF Twilight Survey (Ye et al., 2020). Following the recent recovery of the object¹¹1See Minor Planet Electronic Circular 2020-J68: https://www.minorplanetcenter.net/mpec/K20/K20J68.html, as well as Central Bureau Electronic Telegram 4775: http://www.cbat.eps.harvard.edu/iau/cbet/004700/CBET004775.txt., we searched the ZTF Data Release 2 (Masci et al., 2019) and Partnership archives within the $3\sigma$ positional uncertainty as predicted by JPL orbit solution #3 for additional observations. The data are calibrated to the Pan-STARRS 1 photometric system (Masci et al., 2019), with colors corrected assuming a reddened solar spectrum ($g_{\mathrm{P1}}-r_{\mathrm{P1}}=0.49$ and $r_{\mathrm{P1}}-i_{\mathrm{P1}}=0.34$). For images where the comet is detected, the photometry is measured using a $5^{\prime\prime}$ radius aperture. For the non-detections, we compute the $5\sigma$ upper limits based on the ZTF pipeline.

Figure 1 shows the composite lightcurve of P/Palomar produced using ZTF observations. The ATLAS precovery on 2020 May 9 is also plotted for reference, with an assumed color of $r_{\mathrm{P1}}-o_{\mathrm{ATLAS}}=0.05$ derived from a solar spectrum and the filter throughput curves (ASTM International, 2006; Tonry et al., 2012, 2018). We also plot two lightcurve models: the HG model (Bowell et al., 1989) assumes $H=14.1$ and $G=0.15$, while the coma model is computed using the Halley–Marcus phase function (Schleicher et al., 1998; Marcus, 2007; Schleicher & Bair, 2011) following $m_{\mathrm{coma}}=14.1+5\log{\varDelta}+10\log{r_{\mathrm{H}}}-2.5\log{\Phi{(\alpha)}}$, where $\varDelta$ and $r_{\mathrm{H}}$ is geocentric and heliocentric distances in au, and $\Phi{(\alpha)}$ is the Halley–Marcus phase function applied to phase angle $\alpha$.

Precovery detections are identified on images taken on 2019 May 5 and 14, but not on images taken on 2019 April 29 or earlier. It can be concluded that the comet had brightened by at least 2 mag around the end of April to early May of 2019, when it was at $\sim 2.4$ au from the Sun or $\sim 50$ days before perihelion. As shown in Figure 1, a normal comet would have largely remained at a constant apparent magnitude over this period. In 2020, the most recent pre-discovery ZTF field that covers the uncertainty ellipse of the comet, taken on UT 2020 May 8.31, reveals no comet. The first detection was made by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) program (Tonry et al., 2018) on UT 2020 May 9.52 UTC. This suggests an outburst onset between May 8.31 and 9.52 UTC, with a strength of at least 3.9 mag, corresponding to a phase-corrected $Af\rho\approx 16\pm 2$ cm. The outburst happened when the comet was at a heliocentric distance of 3.50 au, 325 to 326 days after perihelion.