高能暂现源 · 每日文献综述

The origins of fast radio bursts (FRBs) remain uncertain, although magnetars are a leading progenitor candidate. Because magnetars are thought to form primarily through core-collapse supernovae in young stellar populations, the discovery of FRB 20200120E in a globular cluster (GC) in the nearby galaxy M81 was unexpected given the ancient stellar populations of GCs. Expanding the sample of FRBs localized to nearby galaxies is therefore essential for testing FRB formation channels in old stellar environments. M49 (NGC 4472) is a nearby (<inline-formula><tex-math>$\approx 17$</tex-math></inline-formula> Mpc), radio-quiet giant elliptical galaxy in the Virgo cluster hosting an extensive GC system (<inline-formula><tex-math>$\approx 7000$</tex-math></inline-formula> GCs), making it an ideal target for GC FRB searches. We conducted a 9-h SnapShotCal observation of M49 using the Five-hundred-meter Aperture Spherical Telescope (FAST) 19-beam receiver, covering approximately 4230 GCs (<inline-formula><tex-math>$T_{\rm eff}={2.1}$</tex-math></inline-formula> h per GC), and performed a comprehensive single-pulse search with TRANSIENTX over a dispersion-measure range of 0─<inline-formula><tex-math>$5000\, \mathrm{pc\, cm^{-3}}$</tex-math></inline-formula>. No unambiguous astrophysical FRBs were detected. The most significant trigger reached a post-processed signal-to-noise ratio of <inline-formula><tex-math>$8.6\, \sigma$</tex-math></inline-formula> at a dispersion measure of <inline-formula><tex-math>$412.2\, \mathrm{pc\, cm^{-3}}$</tex-math></inline-formula>, but is statistically consistent with thermal noise after accounting for the false-alarm rate. Using the radiometer equation, we derive a beam-averaged peak flux-density sensitivity of <inline-formula><tex-math>$\approx 16.5$</tex-math></inline-formula> mJy (corresponding to a fluence limit of <inline-formula><tex-math>$\approx 16.5\, \mathrm{mJy\, ms}$</tex-math></inline-formula> for a 1 ms burst) and place an upper limit on the FRB occurrence rate of <inline-formula><tex-math>${4.7}\times 10^{-4}\, \mathrm{FRB\, GC^{-1}\, h^{-1}}$</tex-math></inline-formula>. Our non-detection implies that no bright bursts were observed above this fluence threshold during the observing window. The derived rate limit is therefore sensitivity-limited and applies only to bursts above <inline-formula><tex-math>$\approx 16.5\, \mathrm{mJy\, ms}$</tex-math></inline-formula> (for a 1 ms burst), constraining only the detectable (high-fluence) portion of the GC FRB population.