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We present the first linear polarization measurements from the 2015long-duration balloon flight of SPIDER, an experiment designed to map thepolarization of the cosmic microwave background (CMB) on degree angular scales.Results from these measurements include maps and angular power spectra fromobservations of 4.8% of the sky at 95 and 150 GHz, along with the results ofinternal consistency tests on these data. While the polarized CMB anisotropyfrom primordial density perturbations is the dominant signal in this region ofsky, Galactic dust emission is also detected with high significance; Galacticsynchrotron emission is found to be negligible in the SPIDER bands. We employtwo independent foreground-removal techniques in order to explore thesensitivity of the cosmological result to the assumptions made by each. Theprimary method uses a dust template derived from Planck data to subtract theGalactic dust signal. A second approach, employing a joint analysis of SPIDERand Planck data in the harmonic domain, assumes a modified-blackbody model forthe spectral energy distribution of the dust with no constraint on its spatialmorphology. Using a likelihood that jointly samples the template amplitude and$r$ parameter space, we derive 95% upper limits on the primordialtensor-to-scalar ratio from Feldman-Cousins and Bayesian constructions, finding$r<0.11$ and $r<0.19$, respectively. Roughly half the uncertainty in $r$derives from noise associated with the template subtraction. New data at 280GHz from SPIDER's second flight will complement the Planck polarization maps,providing powerful measurements of the polarized Galactic dust emission.

A Constraint on Primordial $B$-Modes from the First Flight of the SPIDER Balloon-Borne Telescope