Subjects: Astronomy submitted time 2023-12-13 Cooperative journals: 《天文学进展》
Abstract： S4 0954+65 is a typical BL Lac object. It shows quasi-simultaneous flares in ra#2;dio, optical and gamma-ray bands. We performed multi-frequency VLBI observations of S40954+65 in June 2018 and November 2019 with the Very Long Baseline Array (VLBA) in the United States. We found significant changes in the radio core spectrum during these obser#2;vations and estimated the magnetic field strength in the core region to be (3.3±0.9)×10−4 T using a synchrotron self-absorption model.By analyzing archival VLBA 43 GHz data taken from May 2017 to May 2021, we iden#2;tified three prominent jet components with two components showing apparent superluminal motion and one being stationary near the core. For these two moving jet components, the viewing angles are 5.5◦ and 6.9◦, and the opening angles are 1.8◦ and 2.4◦, respectively.The distribution of all jet components indicates that they move primarily along two distinct paths, consistent with previous finding of Morozova et al. Our analysis shows that both the 43 GHz core flux and γ-ray flux become more variable between October 2018 and May 2021, indicating that the VLBI core flux variations correlates with the γ-ray flux variations.
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