Using the Gemini Planet Imager (GPI) located at Gemini South, we measured the near-infrared (1.0-2.4 micron) spectrum of the planetary companion to the nearby, young star $beta$ Pictoris. We compare the spectrum obtained with currently published model grids and with known substellar objects and present the best matching models as well as the best matching observed objects. Comparing the empirical measurement of the bolometric luminosity to evolutionary models, we find a mass of $12.9pm0.2$ $mathcal{M}_mathrm{Jup}$, an effective temperature of $1724pm15$ K, a radius of $1.46pm0.01$ $mathcal{R}_mathrm{Jup}$, and a surface gravity of $log g = 4.18pm0.01$ [dex] (cgs). The stated uncertainties are statistical errors only, and do not incorporate any uncertainty on the evolutionary models. Using atmospheric models, we find an effective temperature of $1700-1800$ K and a surface gravity of $log g = 3.5$-$4.0$ [dex] depending upon model. These values agree well with other publications and with hot-start predictions from planetary evolution models. Further, we find that the spectrum of $beta$ Pic b best matches a low-surface gravity L2$pm$1 brown dwarf. Finally comparing the spectrum to field brown dwarfs we find the the spectrum best matches 2MASS J04062677-381210 and 2MASS J03552337+1133437.
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