2023 August 24

Outburst of Comet 12P/Pons-Brooks – Preliminary report

Nick James

Comet 12P/Pons-Brooks is a Halley-type comet with a period of 71 years which returns to perihelion next April. On its last return in 1954 a number of outbursts were observed pre-perihelion. On this return the comet was recovered using the 4.3-m telescope at Lowell on 2020 June 10, almost four years before perihelion, when it was almost 12 au out from the Sun. It has been followed by amateurs since 2022 June when it was around 21^st magnitude and all of the images submitted to the section are here.

Many thanks to all of the observers who have submitted material to the section. This preliminary note reports on observations of the coma expansion following a massive outburst on 2023 July 20. A more detailed note, including observations from a much wider range of observers, is in preparation and should appear in the October Journal.

All of the images on this page are presented at the same physical scale and they correspond to a view 800,000 km square projected onto the sky at the distance of the comet. The images were obtained using three systems, all of which used the same CMOS sensor, a Sony IMX455. We were lucky to have a run of good weather which allowed consistent imaging, processing and measurement of the comet using three telescopes:

– Peter Carson’s remote telescope at Fregenal de la Sierra, Spain. This is a 0.315-m, f/8 Dall Kirkham operating at 0.61 and 1.22 arcsec/pixel.

– My telescope in Chelmsford, UK. This is a 0.28-m, f/10 Edge HD operating at 0.56 and 1.12 arcsec/pixel.

– The Alnitak Observatory at Sierra del Segura, Spain. This is a 0.43-m, f/6.8 Planewave CDK operating at 0.53 arcsec/pixel. This telescope was made available under the BAA remote observing scheme.

Measurement of the coma diameter was done using the section’s Comphot software. This uses an objective approach to determine the size of the coma which does not require human input.

Throughout 2023, 12P had been slowly brightening and becoming more active. By July it had reached 17^th magnitude and showed a faint coma and stubby tail to the south west. The situation a few days prior to the outburst is shown in Image A – Pre-outburst 2023-07-16 00:34 (Carson). Exposure 20x60s.

On 2023 July 20.82 Elek Tamás in Hungary detected a very large outburst. The comet had risen from 17^th magnitude to around 12^th magnitude, brightening by a factor of 100 over a very short period. Early images showed an almost star-like point but gradually the new coma expanded outwards. At the time of the outburst the comet was 3.89 au from the Sun and 3.57 au from the Earth and an arcsecond corresponded to 2,590 km projected on the sky at the distance of the comet.

Peter Carson imaged the comet just after Tamás detection. His image shows bright disk with a total magnitude of 11.7.  Comphot estimated a coma diameter of around 6 arcsec at this point. Image B – Approx 12 hours post outburst, 2023-07-20 22:19 (Carson). Exposure 20 x 60s.

Around 62 hours after the outburst the coma had expanded and started to show interesting detail. In particular the coma was tear-drop shaped with a dark notch to the north. The outburst happens over a very short time period and the material rises above the nucleus and then expands spherically into a vacuum. One direction is blocked by the nucleus itself, hence the dark lane.  Image C – 2023-07-22 22:36, approximately 2.6 d after the outburst (Alnitak), 6x120s, Johnson V.

By 4.5 d after the outburst the coma was taking on a very unusual shape with two narrow horns either side of the dark lane. Image D – 2023-07-24 21:02 (Alnitak), 12x60s, Johnson V.

By 5.6 d after the outburst the coma had expanded to a diameter of 80 arc seconds, corresponding to around 200,000 km at the comet’s distance. The horn structure, dark lane and the demarcation between the inner and outer coma  was now much more prominent and there was some visible structure in the dark notch. Image E – 2023-07-26 00:06 (James), 21x60s, Clear.

The coma continued to expand but the total magnitude of the comet in an aperture corresponding to the coma diameter remained constant at around 11.7. This is what would be expected if a large volume of material was suddenly injected into the coma and then expanded away. The reflective area of the dust remains constant. The magnitude only begins to fall when the photometry is no longer detecting the full extent of the coma.

From the end of July the Moon started to interfere but we could continue to follow the expansion. A faint outer coma was beginning to appear to the south east. Image F – Approximately 11.6d after the outburst, 2023-07-31 22:55 (Carson), 20x60s, Clear.

The final two images in this sequence show the late stage of the outburst. The surface brightness of the inner expansion coma had faded considerably but the dark band to the north was still prominent. By the time of the last image, 25d after the outburst, the inner coma had expanded to a diameter of around 500,000 km, almost four times the diameter of Jupiter.

Image G – 20.5d after the outburst, 2023-08-09 22:27 (James), 30x60s, Clear.

Image H – 24.7d after the outburst, 2023-08-14, 00:51 (Carson), 17x60s. Clear.

Early in the outburst the coma diameter was measured using comphot and the plot below shows the results. The coma diameter has a best-fit linear expansion rate of 13.7 arcsec/day. This corresponds to a physical expansion rate of around 200 m s^-1 at the distance of the comet. Projecting the expansion back to zero diameter indicates that the outburst occurred at 2023 July 20.39. The latest observation known before the outburst was on July 20.08 by station A02. At that time the magnitude was measured as 16.9. It appears that Tamás detected the outburst around 10 hours after it happened.

Richard Miles and the Comet Chasers team got an outburst time of July 20.45 +/- 0.08 using data from metre class telescopes in the LCO network and this is nicely consistent with the results obtained using amateur instruments reported here.