Letter to the Editor

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

¹ Julius-Maximilians-Universität Würzburg, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Straße 31, D-97074 Würzburg, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
⁴ Argelander Institute for Astronomy, Auf Dem Hügel 71, D-53121 Bonn, Germany
⁵ National Astronomical Research Institute of Thailand, Chiang Mai, Thailand
⁶ Centre of Astro-Engineering, Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna, 4860 Santiago, Chile
⁷ Department of Electrical Engineering, Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna, 4860 Santiago, Chile
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany
⁹ Joint ALMA Observatory, Alonso de Córdova, 3107 Vitacura, Santiago, Chile
¹⁰ Astropeiler Stockert e.V., Astropeiler 2-4, D-53902 Bad Münstereifel, Germany
¹¹ LPC2E, OSUC, Univ Orléans, CNRS, CNES, Observatoire de Paris, F-45071 Orléans, France

^⋆ Corresponding author; florian.eppel@uni-wuerzburg.de

Received: 20 December 2024
Accepted: 5 February 2025

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, η_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 × 10⁻¹⁷ erg cm⁻², while the 3σ upper limit of the 0.2−12 keV absorption-corrected X-ray burst fluence lies in the range of 3.4 × 10⁻¹¹ erg cm⁻² to 1.7 × 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 η_x/r < 1.2 × 10⁷. A cutoff power law (Γ = 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 η_x/r ∼ 2.5 × 10⁵ (1−250 keV). In this scenario, we find that η_x/r < 2.4 × 10⁶. 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.