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NICER / ISS Science Nugget
for April 3, 2025



Seeking beyond radio

The sky sparkles with phenomena that vary on many timescales. Among the most prolific are fast radio bursts (FRBs), flashes of radio emission from distant galaxies, typically lasting just milliseconds. A variety of models have been proposed to explain the origin of FRBs, many predicting emission at other wavelengths. Among the thousands of FRBs discovered to date, not one counterpart has been identified outside the radio band.

On March 16, 2025, FRB 20250316A was discovered by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope, located at Penticton, BC, in Canada. CHIME observes the entire Northern sky once a day, and has proven an extremely powerful FRB discovery machine. A recent upgrade brought online the CHIME Outriggers, additional stations in West Virginia, California, and British Columbia - the long baselines between these facilities enable extremely precise (sub-arcsecond) localization of FRBs on the sky.

The March 16 event was the brightest FRB discovered by CHIME in nearly 7 years of operation, and one of the most nearby FRBs found to date. Hoping to detect the first X-ray counterpart, multiple telescopes pointed at the FRB; NICER target-of-opportunity observations for a General Observer project were triggered and began on March 18. Only NICER has the capability to detect short-duration bursts in X-rays, which could coincide with the radio flashes. (The 2020 outburst of the Milky Way magnetar SGR 1935+2154, for example, exhibited both FRB-like and X-ray bursts simultaneously, with NICER playing a key role in their study by providing dense and deep coverage of the X-ray activity.) In Astronomer's Telegram #17117, CHIME investigators A. Pearlman, M. Ng, and A. Cook (McGill Univ., Canada) reported upper limits on any steady, bursting, or pulsed X-rays from FRB 20250316A, including in observations that were scheduled to coincide with CHIME radio coverage. For now, an X-ray counterpart to an FRB remains elusive, but NICER's rapid follow-up response and agile scheduling capabilities offer the promise of future detections, or stringent tests of theories seeking to explain the nature of FRBs.


Red-light image of the sky around FRB 20250316A from the MMT Observatory (Arizona), overlaid with localization uncertainties in radio and X-rays. The (truncated) green ellipse was the initial outcome of analysis with the primary CHIME facility and its KKO Outrigger station. X-ray sources localized to somewhere within the dashed circles by NASA's Swift (orange) and the Chinese/European Einstein Probe mission (purple) raised hopes that a counterpart to the FRB would be identified, but a refined CHIME-plus-full-Outrigger result (red circle) was later shown to be inconsistent with the precise X-ray position (not shown here) as measured with NASA's Chandra telescope. (Credit: ATels #17114, #17115) Times of coordinated observations of FRB 20250316A with NICER and the CHIME radio telescope. (Credit: A. Pearlman & M. Ng)

Left: Red-light image of the sky around FRB 20250316A from the MMT Observatory (Arizona), overlaid with localization uncertainties in radio and X-rays. The (truncated) green ellipse was the initial outcome of analysis with the primary CHIME facility and its KKO Outrigger station. X-ray sources localized to somewhere within the dashed circles by NASA's Swift (orange) and the Chinese/European Einstein Probe mission (purple) raised hopes that a counterpart to the FRB would be identified, but a refined CHIME-plus-full-Outrigger result (red circle) was later shown to be inconsistent with the precise X-ray position (not shown here) as measured with NASA's Chandra telescope. (Credit: ATels #17114, #17115) Right: Times of coordinated observations of FRB 20250316A with NICER and the CHIME radio telescope. (Credit: A. Pearlman & M. Ng)



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