1209.3325 (Charles W. Danforth et al.)

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

Charles W. Danforth, Krzysztof Nalewajko, Kevin France, Brian A. Keeney

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 (0.2315View original: http://arxiv.org/abs/1209.3325