A Fast Flare and Direct Redshift Constraint in Far-ultraviolet Spectra of the Blazar S5 0716+714
Abstract
The BL Lacertae object S5 0716+714 is one of the most studied blazars on the sky due to its active variability and brightness in many bands, including very-high-energy gamma rays. We present here two serendipitous results from recent far-ultraviolet spectroscopic observations by the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (HST). First, during the course of our 7.3 hr HST observations, the blazar increased in flux rapidly by ~40% (-0.45 mag hr-1) followed by a slower decline (+0.36 mag hr-1) to previous FUV 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 αν ≈ -1.0 to αν ≈ -1.4. Second, we constrain the source redshift directly using the ~30 intervening absorption systems. A system at z = 0.2315 is detected in Lyα, Lyβ, O VI, and N V and defines the lower bound on the source redshift. No absorbers are seen in the remaining spectral coverage (0.2315 < z Lyα <~ 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.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.- Publication:
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The Astrophysical Journal
- Pub Date:
- February 2013
- DOI:
- 10.1088/0004-637X/764/1/57
- arXiv:
- arXiv:1209.3325
- Bibcode:
- 2013ApJ...764...57D
- Keywords:
-
- BL Lacertae objects: individual: S50716+714;
- galaxies: active;
- galaxies: jets;
- intergalactic medium;
- quasars: absorption lines;
- radiation mechanisms: non-thermal;
- ultraviolet: general;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 7 pages, 4 figures, accepted to ApJ. Updated to accepted version