Activity of Halley-type Comets 12P/Pons–Brooks and 13P/Olbers from SOHO/SWAN Observations

Abstract The Solar Wind ANisotropies (SWAN) all-sky hydrogen Ly α camera on the Solar and Heliosphere Observatory (SOHO) observed the hydrogen coma of Halley-type comet (HTC) 12P/Pons–Brooks from SOHO’s halo orbit around L1 throughout its 2023–2024 apparition from the beginning of 2023 November to just before the mid-November outburst. There was a gap in coverage during December and January when the comet was in the galactic plane, after which coverage then continued until 29 July. SWAN also observed another HTC comet, 13P/Olbers, from 2024 April 26 until 9 August. Water production rates were calculated from each image of both comets using the standard methodology with fluorescence rates calculated using the daily solar Ly α fluxes from the LASP database corrected for solar rotation. The water production rate of 12P reached a maximum of 1.7 × 10 30 s −1 7 days before the comet perihelion on 2024 April 21. Eight outbursts were detected with released water masses from 4.66 × 10 8 to 8.06 × 10 9 kg. The comet’s water production rate varied with heliocentric distance ( r ) with an exponent of −2.5 before perihelion and −2.9 after, covering heliocentric distances from 2.747 au before perihelion to 1.861 au after, when the activity is driven by water sublimation. The maximum production rate of 13P/Olbers was 1.1 × 10 29 s −1 , reached 27 days before perihelion on 2024 June 30. Its variation with heliocentric distance was more irregular and very scattered compared to that of 12P but generally increased with decreasing heliocentric distance. The variation of activity for 12P was generally very similar to that of the more famous HTC, 1P/Halley, in that both vary with heliocentric distance more similarly to most long-period Oort cloud comets than to most Kuiper belt Jupiter-family comets, many of which vary with very large negative slopes.