NEOWISE CO Upper Limits of 2I/Borisov

ATel #13407; J. M. Bauer (University of Maryland), A. K. Mainzer (University of Arizona), E. A. Kramer (JPL, Caltech), T. Grav (Univ. of Arizona), J. R. Masiero (JPL, Caltech), Y. R. Fernandez (University of Central Florida), M. S. P. Kelley (UMD), S. Protopapa (SwRI), T. Spahr (NEO Sciences), K. J. Meech (UHIfA), R. M. Cutri (IPAC, Caltech), D. Milewski (UCLA), C. M. Lisse (APL), E. L. Wright (UCLA) and the NEOWISE Team.
on 16 Jan 2020; 22:55 UT
Distributed as an Instant Email Notice Comets
Credential Certification: James Bauer (gerbsb@umd.edu)

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We report the non-detection of the interstellar comet 2I/Borisov in NEOWISE images spanning 8.05535 to 9.03674 February 2019 UT. A total of 9 NEOWISE images covering the comet's predicted positions were aligned with the comet's motion and co-added using the AWAIC routine (Masci and Fowler, 2009, ASP Conf. Ser. 411, 67.). The NEOWISE spacecraft has two active imaging bands centered at 3.4 and 4.6 μm. The 4.6 μm channel spans the wavelength range that contains two strong gas emission lines of CO (4.67 μm) and CO₂ (4.23 μm).
The comet was predicted to be at a heliocentric distance of 6.74 AU and viewed by NEOWISE at a phase angle of 8.0 degrees. The predicted location, obtained from JPL's Horizons ephemeris service, was based on the set of observations available from the Minor Planet Center on November 4th, 2019 that included the precovery detections spanning December 13, 2018 and May 5, 2019 reported by ZTF. The positions on the frames had a 3-sigma uncertainty of 3.9 arcseconds, and no signal was found above the 1-sigma uncertainty. We used an aperture size of 11 arcseconds radius when sampling the upper limits of the signal in the co-added frame. The aperture was more than three times the FWHM of the average NEOWISE PSF (Cutri et al. 2019; http://wise2.ipac.caltech.edu/ docs/release/allsky/expsup/) and fully encompassed the 3-sigma uncertainty in the comet's position. The flux at the predicted position was found to be -8 +/- 7 μJy and 8 ± 9 μJy for the 3.4 and 4.6 μm channels, respectively. Using previously described methodology (Bauer et al. 2015, ApJ 814, 85.), the 3-sigma upper limits to the signal yielded CO production rate upper limits of 3×10²⁷ molecules per second and CO₂ production rate upper limits of 3.2×10²⁶ molecules per second. Scaling the CO production rate of 8×10²⁸ molecules per second derived from the NEOWISE observations of 29P/Schwassmann-Wachmann 1 in Bauer et al. 2015 by the total surface area from the relative size (a radius of 23 ± 7 km) measured for 29P (Bauer et al. 2013, ApJ, 773, 22) and the 2I/Borisov radius upper limit of 1.5km (Lee et al. 2019, Res. Notes AAS, 3, 184), the NEOWISE 3-sigma CO production rate upper limit falls above the 29P scaled rate of 3×10²⁶ molecules per second. Hence, an expected production rate for CO based on 2I/Borisov′s size and distance cannot be ruled out.
This publication makes use of data products from NEOWISE, which is a project of JPL/Caltech, funded by the Planetary Defense Coordination Office, part of NASA's Planetary Science Division.