Very-long-baseline polarimetry of the radio core of BL Lacertae

Details Very-long-baseline polarimetry of the radio core of BL Lacertae Very-long-baseline polarimetry of the radio core of BL Lacertae

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........4d&link_type=abstract

Thesis (PhD). THE UNIVERSITY OF IOWA, Source DAI-B 60/06, p. 2746, Dec 1999, 150 pages.

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We present the results of a polarization sensitive VLBA monitoring program of the AGN BL Lacertae spanning epochs 1995.4-1998.3. The images show the ejection of four highly polarized superluminal components (denoted S7, S8, S9, and S10 chronologically). We also present images observed with global VLBI arrays during epochs 1990.17 to 1990.71, which show two components. The trajectories of the components are clearly non- radial. Extrapolation of the position of the components to their respective birth epochs indicates that some of the components are apparently created in pairs, with a fast component and a slow component; the birth epochs are t0 = (1994.12 +/- 0.13, 1994.20 +/- 0.05) for the pair (S7, S8) and t0 = (1996.35 +/- 0.09, 1996, 47 +/- 0.10) for the pair (S9, S10). Apparent observed velocites ( bapp ) for the six components are S5: (2.3 +/- 0.2) h-1 , S6: (8.3 +/- 0.7) h- 1 , S7: (4.1 +/- 0.1) h-1 , S8: (2.4 +/- 0.6) h-1 , S9: (6.1 +/- 0.6) h-1 , and S10: (3.1 +/- 0.2) h-1 . Our monitoring program contains three observations beyond the optical-gamma ray flare at epoch 1997.6. We have not witnessed a radio counterpart through epoch 1998.2. The core was found to be unpolarized at the lower frequencies, but significantly polarized as the higher frequencies. The core fractional polarization versus rotation measure was found to satisfy a n2 relation expected from a turbulent foreground Faraday screen with characteristic spatial scale s0~0.3 AU. The core-component separation was frequency independent but within measurement uncertainties. We have applied the helical model of Hardee (1987, Astrophysical Journal, 318, 78) to explain the components' non-radial trajectories. The best fit trajectories indicate that the line of sight angle of the helical axis is 9deg+/-2 and the jet half-opening angle is Y=2.1deg+/-0.4. The components are modeled as optically thin synchrotron radiative shocks within a relativistic flow. The shocks follow the underlying jet direction. An analysis of the fractional polarization has determined that the three slower components are weak shocks (compression k ~ 0.7) and that the fastest component is a strong shock (k ~ 0.4).

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