Detection of 1665/1667 MHz OH absorption in 3I/ATLAS
ATel #17473; D. J. Pisano (University of Cape Town), Oleg M. Smirnov (Rhodes University & SARAO), Mykola Ivchenko (KTH), Lorenz Roth (KTH), Sarah Buchner (SARAO)
on 4 Nov 2025; 10:28 UT
Credential Certification: Oleg Smirnov (o.smirnov@ru.ac.za)
We observed 3I/ATLAS with MeerKAT on 24 October 2025 from UTC 05:17:16-08:29:37, using the L-band receivers and the NE54M (1.633 kHz channel width) correlator mode, reaching an rms sensitivity of 1.5 mJy/beam in a 0.3 km/s channel with a beam size of 40â. Angular separation from the Sun at time of observation was 3.76 degrees. OH absorption was detected in both the 1665 MHz and 1667 MHz lines. Gaussian fits to the two lines yielded peak fluxes of -8.4+/-3.1 mJy and -14.2+/-3.4 mJy at -15.59+/-0.16 km/s and -15.65+/-0.17 km/s for the two lines, respectively. The FWHMs were 0.88+/-0.37 km/s and 1.26+/-0.40 km/s. The integrated fluxes over the two lines are -7.9+/-2.9 mJy km/s and -19.1+/-5.2 mJy km/s. The absorption features are consistent with the expected OH level population due to the heliocentric velocity of the comet. The detection of OH on 24 October 2025 contrasts with the MeerKAT non-detection of these lines on 20 September 2025 from 14:36:54-15:39:25 UTC (rms noise of 3 mJy/beam) and 28 September 2025 from 10:00:26-16:55:53 UTC (rms noise of 1 mJy/beam).
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. 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.
