A Fast Flare and Direct Redshift Constraint in Far-UV Spectra of the Blazar S50716+714. (arXiv:1209.3325v1 [astro-ph.HE])

A Fast Flare and Direct Redshift Constraint in Far-UV Spectra of the Blazar S50716+714. (arXiv:1209.3325v1 [astro-ph.HE]):
The BL Lacertae object S50716+714 is one of the most studied blazars on the
sky due to its active variability and brightness in many bands, including VHE
gamma rays. We present here two serendipitous results from recent
far-ultraviolet spectroscopic observations by the Cosmic Origins Spectrograph
onboard the Hubble Space Telescope. First, 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 during the course of our 7.3 hour HST observations. We model
this flare using asymmetric flare templates and constrain the physical size,
and energetics of the emitting region. Furthermore, the spectral index of the
object softens considerably during the course of the flare from alpha_nu=-1.0
to alpha_nu=-1.4. Second, we constrain the source redshift directly using the
>30 intervening absorption systems. A system at z=0.2315 is detected in Lya,
Lyb, OVI, NV, and CIII and defines the lower bound on the source redshift. No
absorbers are seen in the remaining spectral coverage (0.2315<z_Lya<0.47) and
we set a statistical upper bound of z<0.304 (90% confidence) on the blazar.
This is the first direct redshift limit for this object and is consistent with
literature estimates of z=0.31+-0.08 based on the detection of a host galaxy.