The BOSS Lyman-alpha Forest Sample from SDSS Data Release 9
  We present the BOSS Lyman-alpha (Lya) Forest Sample from SDSS Data Release 9,
comprising 54,468 quasar spectra with zqso > 2.15 suitable for Lya forest
analysis. This data set probes the intergalactic medium with absorption
redshifts 2.0 < z_alpha < 5.7 over an area of 3275 square degrees, and
encompasses an approximate comoving volume of 20 h^-3 Gpc^3. With each
spectrum, we have included several products designed to aid in Lya forest
analysis: improved sky masks that flag pixels where data may be unreliable,
corrections for known biases in the pipeline estimated noise, masks for the
cores of damped Lya systems and corrections for their wings, and estimates of
the unabsorbed continua so that the observed flux can be converted to a
fractional transmission. The continua are derived using a principal component
fit to the quasar spectrum redwards of restframe Lya (lambda > 1216 Ang),
extrapolated into the forest region and normalized by a linear function to fit
the expected evolution of the Lya forest mean-flux. The estimated continuum
errors are ~5% rms. We also discuss possible systematics arising from uncertain
spectrophotometry and artifacts in the flux calibration; global corrections for
the latter are provided. Our sample provides a convenient starting point for
users to analyze clustering in BOSS Lya forest data, and it provides a fiducial
data set that can be used to compare results from different analyses of baryon
acoustic oscillations in the Lya forest. The full data set is available from
the SDSS-III DR9 web site.
