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Sökning: WFRF:(Mus A.) > (2020)

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1.
  • Marti-Vidal, Ivan, 1980, et al. (författare)
  • ALMA full polarization observations of PKS 1830-211 during its record-breaking flare of 2019
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 638
  • Tidskriftsartikel (refereegranskat)abstract
    • We report Atacama Large Millimeter Array (ALMA) Band 6 full-polarization observations of the lensed blazar PKS 1830-211 during its record-breaking radio and gamma-ray flare in the spring of 2019. The observations were taken close to the peak of the gamma activity and show a clear difference in polarization state between the two time-delayed images. The leading image has a fractional polarization about three times lower than the trailing image, implying that significant depolarization occurred during the flare. In addition, we observe clear intra-hour variability of the polarization properties between the two lensed images, with a quasi-linear increase in the differential electric-vector position angle at a rate of about two degrees per hour, associated with changes in the relative fractional polarization of ∼10%. This variability, combined with the lower polarization close to the peak of gamma activity, is in agreement with models of magnetic turbulence to explain polarization variability in blazar jets. Finally, the comparison of results from the full and differential polarization analysis confirms that the differential polarization technique (Martí-Vidal et al. 2016, A&A, 593, A61) can provide useful information on the polarization state of sources like gravitationally lensed radio-loud quasars.
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2.
  • Psaltis, Dimitrios, et al. (författare)
  • Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole
  • 2020
  • Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 125:14
  • Tidskriftsartikel (refereegranskat)abstract
    • The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.
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