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The BL Lacertae object S50716+714 is one of the most studied blazars on thesky due to its active variability and brightness in many bands, including VHEgamma rays. We present here two serendipitous results from recentfar-ultraviolet spectroscopic observations by the Cosmic Origins Spectrographonboard the Hubble Space Telescope. First, during the course of our 7.3 hourHST observations, the blazar increased in flux rapidly by ~40% (-0.45 mag/h)followed by a slower decline (+0.36 mag/h) to previous far-UV flux levels. Wemodel this flare using asymmetric flare templates and constrain the physicalsize and energetics of the emitting region. Furthermore, the spectral index ofthe object softens considerably during the course of the flare fromalpha(nu)=-1.0 to alpha(nu)=-1.4. Second, we constrain the source redshiftdirectly using the ~30 intervening absorption systems. A system at z=0.2315 isdetected in Lya, Lyb, OVI, and NV and defines the lower bound on the sourceredshift. No absorbers are seen in the remaining spectral coverage(0.2315<z(Lya)<0.47) and we set a statistical upper bound of z<0.322 (95%confidence) on the blazar. This is the first direct redshift limit for thisobject and is consistent with literature estimates of z=0.31+-0.08 based on thedetection of a host galaxy.

A Fast Flare and Direct Redshift Constraint in Far-UV Spectra of the Blazar S50716+714