A targeted search for fast radio bursts from magnetars in elliptical galaxies
Motivated by predictions that globular clusters associated with nearby giant elliptical galaxies are prime environments for detecting fast radio bursts (FRBs) from magnetars, we report on a targeted high-sensitivity search across five local massive elliptical galaxies. We utilized the GREENBURST single-pulse search pipeline and the VEGAS backend on the Green Bank Telescope (GBT) to conduct an L-band (1.4 GHz) survey for FRBs in each target environment. We discovered FRB 20240817A in the direction of M49 (NGC 4472) which has a pulse width of 15 ms, a signal-to-noise ratio of 13, and a dispersion measure (DM) of $983~\rm{cm}^{-3}$ pc. Although a Bayesian spatial association framework initially flags M49 as the host, carefully accounting for the foreground electron column contributions from the Milky Way and both the Virgo intracluster medium and M49's hot gaseous halo ($\sim300-400~\rm{cm}^{-3}$ pc combined) strongly disfavours a physical connection. We conclude that FRB 20240817A is a background event. We also identified a repeating cluster of three low-significance pulses from the direction of M49 at a physically plausible DM of $150-160~\rm{cm}^{-3}$ pc. Accounting for the fraction of each target's extended globular cluster distribution observed, the absence of unambiguous, host-localized bursts allows us to place a joint upper limit on the intrinsic burst rate of a single active source within these systems of $R_0 < 0.028~\rm{hr}^{-1}$ (and an integrated rate limit of $<1.0$~hr$^{-1}$ for M49), lowering previous upper limits and suggesting that magnetar formation within dynamical clusters is less efficient than previously assumed.