  HD 19467 B is presently the only directly imaged T dwarf companion known to
induce a measurable Doppler acceleration around a solar type star. We present
spectroscopy measurements of this important benchmark object taken with the
Project 1640 integral field unit at Palomar Observatory. Our high-contrast R~30
observations obtained simultaneously across the $JH$ bands confirm the cold
nature of the companion as reported from the discovery article and determine
its spectral type for the first time. Fitting the measured spectral energy
distribution to SpeX/IRTF T dwarf standards and synthetic spectra from BT-Settl
atmospheric models, we find that HD 19467 B is a T5.5+/-1 dwarf with effective
temperature Teff=$978^{+20}_{-43}$ K. Our observations reveal significant
methane absorption affirming its substellar nature. HD 19467 B shows promise to
become the first T dwarf that simultaneously reveals its mass, age, and
metallicity independent from the spectrum of light that it emits.
