分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Fast radio bursts (FRBs) are millisecond-timescale radio transients, the origins of which are predominantly extragalactic and likely involve highly magnetized compact objects. FRBs undergo multipath propagation, or scattering, from electron density fluctuations on sub-parsec scales in ionized gas along the line-of-sight. Scattering observations have located plasma structures within FRB host galaxies, probed Galactic and extragalactic turbulence, and constrained FRB redshifts. Scattering also inhibits FRB detection and biases the observed FRB population. We report the detection of scattering times from the repeating FRB 20190520B that vary by up to a factor of two or more on minutes to days-long timescales. In one notable case, the scattering time varied from $7.9\pm0.4$ ms to less than 3.1 ms ($95\%$ confidence) over 2.9 minutes at 1.45 GHz. The scattering times appear to be uncorrelated between bursts or with dispersion and rotation measure variations. Scattering variations are attributable to dynamic, inhomogeneous plasma in the circumsource medium, and analogous variations have been observed from the Crab pulsar. Under such circumstances, the frequency dependence of scattering can deviate from the typical power-law used to measure scattering. Similar variations may therefore be detectable from other FRBs, even those with inconspicuous scattering, providing a unique probe of small-scale processes within FRB environments.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
S. Dai
Y. Feng
Y. P. Yang
Y. K. Zhang
D. Li
C. H. Niu
P. Wang
M. Y. Xue
B. Zhang
S. Burke-Spolaor
C. J. Law
R. S. Lynch
L. Connor
R. Anna-Thomas
L. Zhang
R. Duan
J. M. Yao
C. W. Tsai
W. W. Zhu
M. Cruces
摘要： The environment of actively repeating fast radio bursts (FRBs) has been shown to be complex and varying. The recently localized FRB 20190520B is extremely active, has the largest confirmed host dispersion measure, and is only the second FRB source associated with a compact, persistent radio source (PRS). The main tracer of the magneto-ionic environments is the rotation measure (RM), a path-integral of the line-of-sight component of magnetic field strength (B) and electron density, which does not allow a direct probe of the B-field configuration. Here we report direct evidence for a B-field reversal based on the observed sign change and extreme variation of FRB 20190520B's RM, which changed from $\sim10000$ rad m$^{-2}$ to $\sim-16000$ rad m$^{-2}$ between June 2021 and January 2022. Such extreme RM reversal has never been observed before in any FRB nor in any astronomical object. The implied short-term change of the B-field configuration in or around the FRB could be due to the vicinity of massive black holes, or a magnetized companion star in binary systems, or a young supernova remnant along the line of sight.
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