Constraints on the X-ray-to-radio fluence ratio of FRB 20240114A

Abstract

We report on multiwavelength observations of FRB 20240114A, a nearby (z=0.13), hyperactive, repeating fast radio burst that was discovered in January 2024. We performed simultaneous observations of the source with the Effelsberg 100-m radio telescope, the Thai National Radio Telescope, the Astropeiler Stockert, and the X-ray satellite XMM-Newton in May 2024. On May 23, 2024, we detected 459 bursts from the source using the Ultra-Broad-Band (UBB) receiver of the Effelsberg telescope, covering a frequency range from 1.3 GHz to 6 GHz. All bursts have simultaneous X-ray coverage, which allows us to put stringent constraints on the X-ray-to-radio fluence ratio, $\hat{I}{\cdot }_{x/r}$, of FRB 20240114A. In this work, we focus on the three brightest radio bursts detected during the campaign. The brightest burst exhibits a radio fluence of $1.4\times 10^{-17}$ erg cm${}^{-2}$, while the $3\ddot{I}ƒ$ upper limit of the 0.2$-$12 keV absorption-corrected X-ray burst fluence lies in the range of $3.4\times 10^{-11}$ erg cm${}^{-2}$ to $1.7\times 10^{-10}$ erg cm${}^{-2}$, depending on the spectral model. Assuming a 10 keV black-body spectrum, the X-ray-to-radio fluence ratio can be constrained to $\hat{I}{\cdot }_{x/r}<1.2\times 10^7$. A cutoff power law ($\hat{I}“=1.56$, cutoff at 84 keV) is also considered, physically motivated by the Galactic magnetar SGR 1935+2154, which has previously shown X-ray bursts associated with FRB-like radio bursts at a measured X-ray-to-radio fluence ratio of $\hat{I}{\cdot }_{x/r}\sim 2.5\times 10^5$ (1$-$250 keV). In this scenario, we find that $\hat{I}{\cdot }_{x/r}<2.4\times 10^6$. Our results are consistent with FRB 20240114A being powered by a mechanism similar to that of SGR 1935+2154. We show that future multiwavelength campaigns will be able to improve this limit if sufficiently bright radio bursts are observed with simultaneous X-ray coverage.

6 pages, 2 figures, 2 tables, accepted for publication in A&A