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

TL;DR

This study reports a rapid flare in the blazar S50716+714 observed in far-UV spectra, constrains its physical emission region, and establishes a new direct redshift limit using intervening absorption systems.

Contribution

It provides the first direct redshift constraint for S50716+714 and models the flare to estimate the emission region's size and energetics.

Findings

Blazar flux increased by ~40% during observations.

Spectral index softened from -1.0 to -1.4 during the flare.

Redshift constrained to 0.2315<z<0.322, consistent with previous estimates.

Abstract

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, during the course of our 7.3 hour HST 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. 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, and NV 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<0.322 (95% 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.