Spectral Slope and Colors of Interstellar Comet 3I/ATLAS with the Palomar 200-inch Palomar Next Generation Spectrograph
ATel #17276; Matthew Belyakov (Caltech), Bryce T. Bolin (Eureka Scientific), Christoffer Fremling (Caltech), Matthew Graham (Caltech)
on 7 Jul 2025; 12:09 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)
Subjects: Comet, Solar System Object
We report characterizing observations of interstellar comet 3I/ATLAS taken with the Palomar 200-inch (P200)/Next Generation Palomar Spectrograph (NGPS) on 2025 July 3 UTC. Imaging with the NGPS shows that the comet has an extended appearance, confirming the results of ATEL #17263 by Jewitt & Luu, ATEL#17264 by Alacron et al., and ATEL #17275 by Minev et al. (see attached g, r, and i band image composite). The comet possesses a coma with a FWHM of ~2 arcsec in non-sidereal tracked image vs. background stars in sideral tracked images with a FWHM of ~1.6 arcsec. The g, r, and i band observations show that the comet has g-r = 0.43 +/- 0.02, r-i = 0.16+/-0.02, and g-i = 0.59+/-0.03 measured with an aperture size of 4 arcsec, equivalent to 10,000 km at the distance of the comet. The NGPS spectroscopy covering the 580 - 1020 nm wavelength range reveals a flat spectrum with a spectral slope of 3% per 100 nm and no notable emission lines. Subsequent observations will monitor the comet's activity as it approaches perihelion over the next several months.
Images of 3I/ATLAS taken in g, r, and i bands taken on 2025 July 3 UTC from the P200/NGSP:
https://i0.wp.com/mattbel.io/wp-content/uploads/2025/07/i3_panel-1.png
Our work is based on observations taken at Palomar Observatory. The authors would like to thank the Palomar observing staff for their assistance with the observations. The authors also wish to recognize and acknowledge the cultural significance that Palomar Mountain has for the Pauma Band of the Luiseno Indians.
Palomar images of 3I/ATLAS.
