Multiwavelength Observations of the Apparently Non-repeating FRB 20250316A

TL;DR

This work presents a comprehensive multiwavelength follow-up of the nearby bright FRB 20250316A, combining radio data from FAST with X-ray observations from Einstein Probe and Chandra, and optical imaging from WFST and SVOM/VT. No FRB pulses were detected in the FAST campaign, and the X-ray searches yielded deep upper limits; a nearby X-ray source detected by Chandra is offset by about 7 arcseconds from the FRB, making a physical association unlikely. The resulting constraints disfavor a ULX counterpart and place stringent limits on any X-ray persistent emission or afterglow, driving kinetic-energy bounds for potential outflows and underscoring the necessity of arcsecond localization for robust counterpart identification. Collectively, the findings support the interpretation that FRB 20250316A is likely a one-off event, with important implications for FRB progenitor models and the design of future follow-up campaigns.

Abstract

The physical origin of fast radio bursts (FRBs) remains uncertain. Although multiwavelength observations have been widely conducted, only Galactic FRB~20200428D is associated with an X-ray burst from the magnetar SGR J1935+2154. Here, we present multiwavelength follow-up observations of the nearby bright FRB~20250316A, including the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Einstein Probe (EP) X-ray mission, Chandra X-ray Observatory, Wide Field Survey Telescope (WFST) and Space Variable Object Monitor/Visible Telescope (SVOM/VT). The 13.08-hour FAST follow-up campaign without pulse detection requires an energy distribution flatter than those of well-known repeating FRBs, suggesting that this burst is likely a one-off event. A prompt EP follow-up and multi-epoch observational campaign totaling $>$ 100 ks led to the detection of an X-ray source within the angular resolution of its Follow-up X-ray Telescope (FXT, $10^{\prime\prime}$). A subsequent Chandra observation revealed this source to be offset by $7^{\prime\prime}$ from the FRB position, and established a 0.5-10 keV flux upper limit of $7.6\times 10^{-15}$ $\rm erg\,cm^{-2}\,s^{-1}$ at the FRB position, corresponding to $\sim 10^{39}$ $\rm erg\,s^{-1}$ at the 40 Mpc distance of the host galaxy NGC~4141. These results set one of the most stringent limits on X-ray emission from a non-repeating FRB, disfavoring ultra-luminous X-ray sources (ULXs) as counterparts of apparently one-off FRBs and offering critical insights into afterglow models. Our study suggests that an arcsecond localization of both the FRB and its potential X-ray counterpart is essential for exploring the X-ray counterpart of an FRB.

Figures (6)

• Figure 1: The timeline of the FRB 20250316A follow-up observations. For EP/WXT, only those observations within 12 hours before and after the FRB are presented. Vertical gray lines indicate the time of FRB 20250316A and the time at which FRB 20250316A was localized to be arc-second accuracy.
• Figure 2: X-ray and optical images of the FRB 20250316A field. Upper Left: The EP-WXT image obtained from the observation 3.5 hours after the FRB, with the source and background regions marked for upper limit analysis. Upper Right: The stacked image from all FXT observations, with a yellow circle centered on the position initially reported from the early detection. The red star marks the position of FRB 20250316A. Lower Left: Chandra image. An X-ray source CXOU J120943.7+585051 (cyan) offset by $\sim 7^{\prime\prime}$ from the FRB position (red circle, projected distance: 1.36 kpc) is located within the error circle of EP J120944.2+585060 (yellow), along with one faint Chandra object (green) at a separation of $\sim 22^{\prime\prime}$ from the first. Lower Right: WFST image. Both FRB 20250316A (red) and CXOU J120943.7+585051 (cyan) reside in separate extended regions.
• Figure 3: The X-ray light curve of EP J120944.2+585060/CXOU J120943.7+585051 observed by EP-FXT (blue) and Chandra (green). Errors show 1$\sigma$ statistical uncertainties. The dashed line shows the weighted mean flux derived from EP-FXT observations. The slightly higher flux measured by EP-FXT may be due to contamination from the nearby Chandra source.
• Figure 4: Averaged number of unrelated X-ray sources within the augular resolution for EP/FXT and Chandra telescope.
• Figure 5: Left: CHIME FRB rate and the FAST 3$\sigma$ upper limit compared with the spectral energy functions of repeating FRBs. A typical power law with a slope of $-1.5$ is plotted as a red solid line. The blue lines present the spectral energy functions of repeating FRBs, scaled to match the rate of FRB 20250316A at its spectral energy. Right: The FRB rate ($>$ 5 Jy ms) of FRB 20250316A estimated with multiple telescopes, assuming energy function slopes of $-1.5$ (red region) and $-1.0$ (gray region), compared with the CHIME-detected FRB rate (blue histogram).