English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We used 8.4 GHz VLBI images obtained at up to 35 epochs between 1997 and 2005to examine the radio structures of the main reference source, 3C 454.3, and twosecondary reference sources, B2250+194 and B2252+172, for the guide star forthe NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz imagesfor quasar 3C 454.3 confirm a complex, evolving, core-jet structure. Weidentified at each epoch a component, C1, near the easternmost edge of the coreregion. Simulations of the core region showed that C1 is located, on average,0.18 +- 0.06 mas west of the unresolved "core" identified in 43 GHz images. Wealso identified in 3C 454.3 at 8.4 GHz several additional components whichmoved away from C1 with proper motions ranging in magnitude between 0.9c and5c. The detailed motions of the components exhibit two distinct bends in thejet axis located ~3 and ~5.5 mas west of C1. The spectra between 5.0 and 15.4GHz for the "moving" components are steeper than that for C1. The 8.4 GHzimages of B2250+194 and B2252+172, in contrast to those of 3C 454.3, revealcompact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 isinverted while that for B2252+172 is flat.  Based on its position near the easternmost edge of the 8.4 GHz radiostructure, close spatial association with the 43 GHz core, and relatively flatspectrum, we believe 3C 454.3 component C1 to be the best choice for theultimate reference point for the GP-B guide star. The compact structures andinverted to flat spectra of B2250+194 and B2252+172 make these objects valuablesecondary reference sources

VLBI for Gravity Probe B. II. Monitoring of the Structure of the Reference Sources 3C 454.3, B2250+194, and B2252+172