VLBI Observations Constrain Radio Spectrum of Persistent Source Associated with FRB 20190417A
Astronomers using the European VLBI Network detected a compact persistent radio source (PRS) associated with repeating fast radio burst FRB 20190417A at 5 GHz, measuring a flux density of 150±45 μJy and deriving a nearly flat spectral index of −0.19±0.29. This makes FRB 20190417A only the second PRS to have its spectral index constrained via VLBI, and the source's brightness temperature above 10⁵ K confirms its non-thermal origin. The findings support a nebular origin—consistent with forward shocks in free-expansion or a young pulsar wind nebula—for the persistent emission associated with repeating FRBs.
Using the European VLBI Network (EVN) at 5 and 8 GHz, researchers targeted two candidate persistent radio sources (PRSs) associated with repeating fast radio bursts identified in a prior VLA survey. A compact, unresolved source linked to FRB 20190417A was detected at 5 GHz with a flux density of 150±45 μJy, but yielded no detection at 8 GHz; its brightness temperature exceeding 10⁵ K confirms non-thermal emission. Combining this measurement with archival VLBI data at 1.4 GHz, the team derived a spectral index of α = −0.19±0.29, consistent with a nearly flat spectrum. The source's luminosity falls on the proposed Lν–|RM| relation, and including it in the relation yields a scatter of σ_Δ = 0.65, corresponding to parameters consistent with forward shocks in the free-expansion phase or young pulsar wind nebulae. For the second candidate PRS associated with FRB 20181030A, only upper limits were obtained—80 μJy at 5 GHz and 150 μJy at 8 GHz—implying a steep spectral index (α ≲ −1.2) if the VLA emission originates from a compact component. The study underscores the critical role of milliarcsecond-resolution VLBI in isolating compact emission from FRB engines and provides rare spectral constraints for PRSs. The results are accepted for publication in Astronomy & Astrophysics Letters.
What's missing
The study relies on combining new EVN data with previously published VLBI measurements at 1.4 GHz from separate epochs and instruments, introducing potential systematic uncertainties from variability or calibration differences across observations that are not fully quantified. The physical interpretation (forward shocks vs. pulsar wind nebula) remains degenerate given current data, and the Lν–|RM| relation itself is based on a very small sample of PRSs, limiting statistical robustness. The non-detection of FRB 20181030A's candidate PRS at VLBI resolution leaves open whether its VLA emission is truly compact or extended diffuse emission unrelated to the FRB engine.
What different sources said
- arXiv astro-phCenter
The VLBI spectrum of the persistent radio source associated with FRB 20190417A
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