Contemporaneous Optical and Near-Infrared Observations of the Interstellar Comet 3I/ATLAS Pre- and Post-Perihelion

Interstellar objects provide a unique view into the formation of other star systems. Here we present spectroscopic observations of the recently discovered interstellar object 3I/ATLAS between a heliocentric distance of $3.7$ to $1.8$~au on either side of its travels through perihelion. We obtained several observations with the Keck-I/LRIS, Keck-II/NIRES, Gemini/GMOS, and UH88/SNIFS spectrographs, covering a wavelength range of $0.3-2.5\mu \mathrm{m}$. We report the continued emission of both Ni and CN, along with post-perihelion detections of Fe and a weak detection of ${\mathrm{C}}_3$. We determine the spectral slope across optical and NIR wavelengths and find a positive spectral slope in the optical, with values ranging from $\sim 21-27\mathrm{\%}$ in the blue regions ($0.4-0.55\mu \mathrm{m}$) to $\sim 6-10\mathrm{\%}$ in the red ($0.65-0.9\mu \mathrm{m}$) regions. In contrast, the NIR showed a negative spectral slope of $\sim -0.9\mathrm{\%}$ between $0.9-1.5\mu \mathrm{m}$ and $\sim -2.3\mathrm{\%}$ between $1.9-2.5\mu \mathrm{m}$. 3I/ATLAS shows a clear turnover in its spectral shape at $\sim 1.1\mu \mathrm{m}$, corresponding to scattered light from the dusty coma. Finally, in the NIR, we do not find an increase in the depth of the water features identified in an earlier NIR observation of 3I/ATLAS. Our observations of 3I/ATLAS in the NIR show a similar shape to the NIR spectrum of 2I/Borisov as it approached perihelion.