VLBI Observations Constrain Radio Spectrum of Persistent Source Associated with FRB 20190417A
European VLBI Network observations detected a compact radio source associated with the repeating fast radio burst FRB 20190417A at 5 GHz with a nearly flat spectrum, making it only the second such source with spectral constraints from VLBI data. The source's properties—including its high brightness temperature and luminosity—are consistent with either forward shocks in free expansion or young pulsar wind nebulae. These findings support the hypothesis that persistent radio sources associated with FRBs may originate from nebular environments around the burst engines.
Researchers used the European VLBI Network to observe two candidate persistent radio sources (PRSs) associated with repeating fast radio bursts at 5 and 8 GHz frequencies. They detected a compact source at 5 GHz associated with FRB 20190417A with a flux density of 150±45 microjansky, but found no detection at 8 GHz. By combining these measurements with previously published VLBI data at 1.4 GHz, the team derived a spectral index of −0.19±0.29, indicating a nearly flat spectrum. The source's brightness temperature exceeds 10⁵ K, confirming its non-thermal nature. The inferred luminosity places the source on a proposed luminosity-rotation measure relation, with consistency metrics suggesting the persistent emission originates from either forward shocks in the free-expansion phase or young pulsar wind nebulae. For the second candidate associated with FRB 20181030A, only upper limits were obtained, implying a steep spectral index if the emission comes from a compact component.
What's missing
The study does not discuss potential observational biases in detecting persistent radio sources or explain why only two FRB-associated PRSs have had their spectral indices constrained via VLBI despite the growing number of known repeating FRBs. Additionally, the physical mechanisms distinguishing forward shocks from pulsar wind nebulae based on the available data are not detailed.
What different sources said
- arXiv astro-phCenter
The VLBI spectrum of the persistent radio source associated with FRB 20190417A
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