Further MeerKAT observations of 1665/1667 MHz OH in absorption and emission, and a technosignature search in 3I/ATLAS
ATel #17499; D. J. Pisano (University of Cape Town), Oleg M. Smirnov (Rhodes University & SARAO & INAF), Mykola Ivchenko (KTH), Lorenz Roth (KTH), Sarah Buchner (SARAO), Chenoa Tremblay (Breakthrough Listen & SETI Institute), Andrew Siemion (Breakthrough Listen & SETI Institute & Oxford Astrophysics), Fernando Camilo (SARAO), on behalf of a larger SARAO and Breakthrough Listen team
on 18 Nov 2025; 10:33 UT
Credential Certification: Oleg Smirnov (o.smirnov@ru.ac.za)
Following our initial detection of OH absorption lines from 3I/ATLAS on 24 October 2025 (ATel #17473), we have continued MeerKAT observations using the L-band receivers and the NE54M (1.633 kHz channel width) correlator mode. We report on observations made during 4-12 November 2025. We detected both the 1665 MHz and 1667 MHz OH lines in absorption on 5 November from UTC 03:54:29-05:48:05 and again on 6 November from UTC 03:50:22-11:23:46. Consistent with the expected OH level population due to the heliocentric velocity of the comet, we then detected the OH lines in emission on 11 November from UTC 04:11:20-07:31:01 and 12 November from UTC 03:06:42-05:42:55.
On 5 November, we detected OH absorption in a 2 arcminute aperture in both the 1665 MHz and 1667 MHz lines. Gaussian fits to the two lines yielded peak fluxes of -20.7+/-6.7 mJy and -24.9+/-5.3 mJy at -21.1+/-0.1 km/s and -20.9+/-0.2 km/s in the topocentric reference frame. These velocities are consistent with the expected velocity of the comet. The FWHMs were 0.60+/-0.23 km/s and 1.60+/-0.40 km/s. On 6 November, the lines had peak fluxes of -14.5+/-3.6 mJy and -18.5+/-5.1 mJy at -21.0+/-0.2 km/s and -21.1+/-0.1 km/s respectively. The FWHMs were 2.0+/-0.6 km/s and 0.80+/-0.26 km/s. On 11 November, we only detected the 1667 MHz line in emission with a peak flux of 22.7+/-6.9 mJy at -21.6+/-0.08 km/s with a FWHM of 0.50+/-0.17 km/s. On 12 November, we detected both lines with peak fluxes of 13.3+/-5.1 mJy and 8.8+/-3.4 mJy at -21.2+/-0.2 km/s and -23.0+/-0.2 km/s respectively. The FWHMs were 1.00+/-0.44 km/s and 1.34+/-0.60 km/s.
These detections contrast to the non-detection by MeerKAT of either OH line on 4 November from UTC 04:12:55-11:45:12 with an rms noise of 1.1 mJy/beam.
At two different times on 5 November, we conducted a search for narrowband radio transmitters toward 3I/ATLAS using the commensal backend BLUSE, which effects a real-time search for radio technosignatures. A total of 23,689 signals were detected; however, all were determined to not be spatially consistent with the position of 3I/ATLAS and thus were likely caused by anthropogenic radio frequency interference (RFI). From these observations, we established a detection limit of 0.17 W over the 900-1670 MHz range, approximately equivalent to the power output of a mobile phone handset at the distance to 3I/ATLAS, which was 334 million kilometres at the time.
The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science, Technology and Innovation. This work made use of the CARTA (Cube Analysis and Rendering Tool for Astronomy) software (DOI: 10.5281/zenodo.3377984 â https://cartavis.github.io). We acknowledge the use of the ilifu cloud computing facility. (www.ilifu.ac.za), supported by the Inter-University Institute for Data Intensive Astronomy (IDIA), the Computational Biology division at the University of Cape Town, and DIRISA. DJP and OS greatly acknowledge support from the South African Research Chairs Initiative of the Department of Science, Technology and Innovation and the National Research Foundation.
