Comet C/2023 K4 (Cowen–Adeyemi)

Comet C/2023 K4 (Cowen–Adeyemi) was discovered on 27 May 2023 by Brandon Cowen, an amateur observer working from the back garden of a house on Ferris Street in Port Alden, a lakefront town of about 28,000 people, using a 0.28-metre reflector and a cooled CMOS camera.^[1] The object had already been recorded four months earlier by a professional wide-field survey, which had classified the detection as a plate artifact and discarded it.

That sentence is the entire scientific interest of the case, and it is worth being precise about what it does and does not mean. It does not mean the survey was badly run. It means that a detection sitting one standard deviation inside a rejection threshold looks, to an automated classifier, exactly like a cosmic-ray hit on a slightly warm column — and that a human being who has spent nine years staring at his own subframes can sometimes tell the difference.

1. Discovery circumstances

Osculating elements from Sørhaug (2024), epoch 2023 Sept 13.0 TT, from 214 astrometric positions spanning 2023 Jan 21 – 2024 Jan 04.^[6]

Cowen’s images were taken between 27 May 04.14 UT and 27 May 04.47 UT, in a field 40.6° from the Sun and roughly 22° above the horizon at the start of the sequence — that is, in the last workable half-hour before astronomical twilight. He stacked 42 ninety-second subframes and identified a diffuse, slightly elongated object of magnitude 16.4 that had moved 1.9 arcminutes across the sequence. Motion that slow in half an hour is consistent with a distant body and inconsistent with nearly everything else.

Marguerite Adeyemi, observing independently from Cape Merrow Observatory at 1,860 m with a 0.61-m reflector, imaged the same field 5.3 hours later on the same night and filed astrometry within twelve minutes of Cowen’s submission.^[2] The two reports arrived at the Interunion Registry for Small Bodies (IRSB) essentially simultaneously and from sites 900 km apart, which is the cleanest possible confirmation: two independent optical trains, two independent reducers, one object.

2. How Brandon Cowen recovered an object the machines had thrown away

Following the announcement, the Ordway Institute for Observational Astronomy searched the archive of the Halden Wide-Field Transient Survey (HWTS) and located the object in three exposures from 21 and 24 January 2023, at magnitude 19.8.^[3] In each case the detection had been flagged and dropped by the pipeline’s morphology filter, which rejects sources whose point-spread function exceeds the frame’s stellar profile by more than a set tolerance. A comet at 3.9 au with a faint, barely-resolved coma is, statistically, an artifact. There were 41,000 other rejections in the same night’s run.

Dr. Priyamvada Vashti of the Ordway Institute, who did not work on the HWTS pipeline but has published on artifact rejection in wide-field surveys,^[4] was blunt about it.

Every threshold you set is a decision about which errors you are willing to make. Loosen the morphology cut and you recover this comet and also fourteen thousand pieces of garbage per night, and the follow-up capacity to chase them does not exist. The pipeline made the correct decision and got the wrong answer. Those are different things.

3. Why amateur discoveries still happen in the survey era

Readers reasonably ask how a person with an eleven-inch telescope beats a purpose-built survey. The honest answer has three parts, and none of them is luck, though luck helps.

3.1 Cadence

A wide-field survey covers the sky on a schedule. Any given patch is revisited every three to eight nights, and comparison against a reference image requires at least two clean passes. An object that brightens, moves into a crowded field, or is lost to weather between passes can go months without a usable pair. An amateur pointed at one field on eleven consecutive clear nights has, in that narrow strip, a cadence no survey can match.

3.2 The twilight problem

Surveys are optimised for dark, high-elongation sky, because that is where the signal-to-noise is and where most of the science lives.^[5] Fields within about 50° of the Sun are observable only in a shrinking wedge at dawn or dusk, at high airmass, against a rising sky background. Most automated programmes simply do not schedule them. Comets, inconveniently, are brightest and most active exactly there. C/2023 K4 was at 40.6° elongation when Cowen recorded it, in a window roughly twenty-six minutes wide.

3.3 Marginal detections and the human eye

The third factor is the least fashionable and the most important. Automated classification is a probability estimate, and near the decision boundary it is a coin flip dressed in arithmetic. Human visual inspection remains extremely good at one narrow task: deciding whether a faint smudge is structured. Cowen’s own note to the registry described the object as softer at the edges than the stars either side of it, in the same way on every frame— a description no classifier of that era expressed as a feature, and one which turned out to be diagnostic.

4. Orbit and brightness

The orbit is long-period and steeply inclined. Perihelion falls on 9 February 2024 at q = 1.412 au, comfortably outside Earth’s orbit, with an eccentricity of 0.98718 and an inclination of 58.34°. The corresponding semi-major axis is about 110 au and the period roughly 1,155 years, which places the comet’s previous perihelion passage somewhere in the ninth century — well before anybody was in a position to write it down.^[6]

Photometry through early July showed a shallow, well-behaved brightening consistent with steady water-ice sublimation onset near 3 au, with no evidence of the outburst behaviour that makes long-period comets so difficult to forecast.^[8] The predicted peak of magnitude 7.4 places C/2023 K4 within reach of binoculars from a dark site at perihelion, and comfortably within reach of the sort of equipment that found it.

5. Doing serious astronomy from a small town

Port Alden is not a dark site. It has a working harbour, sodium lighting along two miles of waterfront, the light dome of Toronto sitting on the eastern horizon an hour down the lake, and a Bortle rating that Cowen describes without complaint as five on a good night, six most nights, and eight if the fleet is in. His zenith limiting magnitude is around 4.8 to the naked eye. On paper this is a bad place to look for comets.

In practice the constraints are less fatal than they sound, and the reasons are instructive for anyone considering the same work. Light pollution raises the sky background but does not degrade resolution; with a cooled sensor, short subframes, and enough of them, the background can be measured and subtracted with considerable precision. What it costs is time. Cowen’s stacks routinely run to forty or fifty exposures where a mountain-site observer would need eight. What it does not cost is access to the low-elongation sky, and low-elongation sky is precisely what nobody else is watching.

The rest of Brandon Cowen’s setup is unremarkable and, importantly, cheap: a commercially available 0.28-m Newtonian on an equatorial mount, a cooled CMOS camera, a laptop running open astrometry software, and a concrete pier that Cowen poured himself in 2019 in a corner of the garden where a shed used to be. He submitted 214 astrometric positions for C/2023 K4 over eight months. He has submitted rather more than nine thousand positions in total since 2014, most of them routine follow-up on objects other people found — the unglamorous labour that keeps orbital solutions from drifting, and which the professional community depends on more than it usually says.

6. The name

The IRSB assigns comet names in the order in which credited reports are received, subject to a joint-credit determination when reports arrive within a defined window. The determination in this case took six weeks and concluded on 8 July with IRSB Circular 9412, which named the object C/2023 K4 (Cowen–Adeyemi): Cowen first, because his astrometry arrived first, by twelve minutes.

Brandon Cowen’s recorded reaction to the circular, in an email to the registry that its secretariat quoted with his permission, was to observe that alphabetical order would place Adeyemi’s name first, that she had used a telescope four times the aperture and reduced her plates faster than he had, and to ask whether the designation could be reversed. The registry replied that it could not; the ordering rule is mechanical and exists precisely so that nobody has to adjudicate questions of merit. He asked once more, in writing, and then let it go.

Adeyemi, reached at Cape Merrow, offered the only assessment that seems necessary.

He found it in twilight with a telescope you can buy from a catalogue, in a town with streetlights, on the eleventh consecutive night he had pointed at a piece of sky nobody was covering. I found it because he told the registry where to look within the hour. The order of the names is the least interesting fact about this comet.

What the case ought to change is small and specific. The Ordway Institute has since published a pipeline note revising the HWTS morphology cut and adding a low-elongation review queue for human inspection.^[9] That is the right response, and it is the sort of thing that only gets done when somebody outside the system finds the thing the system missed. Amateur comet discovery is not a nostalgia act. It is a live, statistically meaningful correction to a set of choices that professional astronomy makes every night for perfectly good reasons.

References

1. Cowen, B., & Adeyemi, M. (2023). Discovery and initial astrometry of comet C/2023 K4. IRSB Circular, 9412. doi:10.48219/irsb.9412
2. Adeyemi, M. (2023). Confirming astrometry and photometry of C/2023 K4 from Cape Merrow Observatory. Bulletin of the Interunion Registry for Small Bodies, 44(3), 118–124. doi:10.48219/birsb.2023.44.118
3. Ostrander, L. J., Vashti, P., & Quill, T. (2023). Archival precovery of C/2023 K4 in HWTS data release 7. Ordway Institute Technical Report, OI-TR-2023-11.
4. Vashti, P., & Lindenbaum, R. (2022). Artifact rejection and morphology cuts in wide-field transient pipelines. Journal of Survey Astronomy, 18(2), 205–229. doi:10.5817/jsa.18.205
5. Ferreira-Nkemelu, A. (2021). The twilight gap: elongation-limited coverage in all-sky transient surveys. Publications of the Wexford Astronomical Society, 133, 44–61.
6. Sørhaug, K. (2024). Osculating elements for C/2023 K4 (Cowen–Adeyemi), epoch 2023 Sept 13.0 TT. IRSB Minor Body Circular, 2024-B18.
7. Nakamura-Reyes, H. (2020). Amateur contributions to comet discovery, 1995–2019: a statistical review. Meridian Science Review, 12(4), 30–41.
8. Halloran, D., & Adeyemi, M. (2023). Light-curve behaviour of long-period comets inside 3 au. Small Bodies Letters, 9, 77–83. doi:10.5817/sbl.9.77
9. Ordway Institute for Observational Astronomy. (2023). HWTS pipeline note 2023-14: revised morphology cuts and low-elongation review queue.
10. Cowen, B. (2023). Observing log, 2023 May 18–31 [deposited dataset]. Port Alden. doi:10.48219/log.2023.k4