VLA non-detection of a compact persistent radio source associated with the repeating FRB 20251229A
ATel #17735; Gabriele Bruni (INAF-IAPS), Luigi Piro (INAF-IAPS), Aishwarya L. Thakur (INAF-IAPS), Luciano Nicastro (INAF-OAS), Andrea Rossi (INAF-OAS), Eliana Palazzi (INAF-OAS)
on 1 Apr 2026; 09:42 UT
Credential Certification: Gabriele Bruni (gabriele.bruni@inaf.it)
Subjects: Radio, Fast Radio Burst
We report on Karl G. Jansky Very Large Array (VLA) observations of the repeating fast radio burst FRB 20251229A, aimed at searching for a compact persistent radio source (PRS) associated with the burst.
The observations were carried out on March 30, 2026, in C band (4-8 GHz) and A configuration, providing a synthesized beam of 0.38" x 0.37" at 6 GHz, corresponding to ~0.7 kpc at the redshift of the candidate host galaxy WISE J204123.12+160126.2 (z=0.12, as computed from optical SED fitting with CIGALE). The observations were centered on the coordinates of the candidate host galaxy (ATel #17709, ATel #17723). The total on-source integration time was approximately 1 hour within a 1.5 hour observing block, reaching an rms sensitivity of ~3.3 uJy/beam.
No compact radio source is detected at the position of the candidate PRS reported in ATel #17723, down to a 3-sigma upper limit of 10 uJy at 6 GHz. This corresponds to an upper limit on the isotropic spectral luminosity of L<3.6x10^27 erg/s/Hz, assuming a standard cosmology.
Recent uGMRT observations reported a detection of S~64 uJy in Band 4 (550-750 MHz, ATel #17723). Combining this measurement with our VLA upper limit implies a spectral index alpha<-0.8, indicating a steep spectrum between 0.65 and 6 GHz. It is worth noting that the uGMRT Band 4 beam (~4-6") encompasses the entire host galaxy (~10 kpc at z=0.12), so the detected 650 MHz emission may include a significant contribution from integrated star formation; in contrast, our VLA A-configuration observations at 6 GHz resolve the host on sub-kpc scales (~0.7 kpc), isolating compact emission at the putative PRS position. While a contribution from a compact persistent source at low frequencies cannot be excluded, higher-resolution observations are essential to disentangle the origin of the emission.
The achieved sensitivity in these VLA observations is sufficient to detect compact persistent sources comparable to those associated with several repeating FRBs when scaled to this distance. The non-detection therefore places stringent constraints on the presence of a compact synchrotron nebula powered by the FRB central engine.
The National Radio Astronomy Observatory and Green Bank Observatory are facilities of the U.S. National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
