Morphology and Photometry of Comet C/2020 F3 (NEOWISE) from SOHO
ATel #13853; Matthew Knight (United States Naval Academy/University of Maryland), Karl Battams (Naval Research Laboratory)
on 2 Jul 2020; 14:27 UT
Credential Certification: Matthew Knight (astromatthew@gmail.com)
We report on comet C/2020 F3 (NEOWISE) during it transit of NASA/ESA's Solar and Heliospheric Observatory (SOHO) C3 coronagraph from 2020 June 22.4 to 28.0. These observations occurred at heliocentric distances from 0.45 to 0.34 AU prior to the comet's perihelion passage on 2020 July 3.7. C3 has an annular field of view centered on the Sun and extending from 1-8 deg, with 56 arcsec/pixel resolution. Multi-filter observations were acquired, but we report here only on the broadband "clear" filter images (450-850 nm) which represent the vast majority of images and were acquired 2-4 times per hour with exposure times of ~18 seconds.
Comet NEOWISE displayed typical cometary features throughout the apparition. The central condensation was somewhat diffuse and slightly elongated, and did not show any signs of increasing elongation that has often been correlated with disruption at small heliocentric distances (e.g., C/2012 S1 ISON; Knight & Battams 2014, ApJL, 782, L37, 5pp). For the first ~48 hr of observation, a single dust tail of length ~1 deg was seen pointing radially from the Sun though offset slightly southeast (clockwise) from the Sun-comet line. Beginning late on June 24 a second, shorter and more diffuse tail became increasingly visible and was offset further to the southeast. On June 25.3 we measured the first tail to be just over 1 deg in length and offset from the Sun-comet line by ~15 deg. In the same image, the second tail was ~0.5 deg long and offset by ~45 deg from the comet-Sun line, forming a ~30 deg angle between the two tails. The second tail was generally much fainter and more diffuse than the first, with image stacking necessary to see improved detail. The separation between tails, and their general orientation with respect to the Sun was maintained throughout the remainder of the passage, though the second (fainter) tail became less visible towards the end of the sequence.
Photometry was measured in a circular aperture of radius 224 arcsec, centroided on the strong central condensation and encompassing all of the visible coma as well as some of the tail. Sky was removed via subtraction of a median background created from 12 hours of nearby images, and absolute calibrations to V magnitude were performed using the coefficients used in our previous studies (e.g., Knight et al. 2010, AJ 139, 926-949). Images obviously contaminated by background objects were excluded. The comet had an apparent V magnitude of +4.2 when it appeared in C3 and brightened steadily to V = +2.2 when it left C3. Photometric uncertainties were approximately 0.01 mag throughout. There was a distinct flattening of the slope of the brightening around June 26.8. Prior to this time the data were well fit by V = 9.33 + 5 * log10(delta) + 17 * log10(r), implying a heliocentric distance dependence of r^{-6.8}. After June 26.8 the data followed V = 7.8 + 5 * log10(delta) + 13.5 * log10(r), implying r^{-5.4}. Extrapolating this latter equation to perihelion yields an apparent V magnitude of +0.9.
