Strong lensing of gamma ray bursts as a probe of compact dark matter

Compact dark matter has been efficiently constrained in the M ≲10 M_⊙ mass range by null searches for microlensing of stars in nearby galaxies. Here we propose to probe the mass range M ≳10 M_⊙ by seeking echoes in gamma-ray-burst light curves induced by strong lensing. We show that strong gravitational lensing of gamma ray bursts (GRBs) by massive compact halo objects (MACHOs) generates superimposed GRB images with a characteristic time delay of ≳1 ms for M ≳10 M_⊙. Using dedicated simulations to capture the relevant phenomenology of the GRB prompt emission, we calculate the signal-to-noise ratio required to detect GRB lensing events as a function of the flux ratio and time delay between the lensed images. We then analyze existing data from the Fermi GBM and Swift BAT instruments to assess their constraining power on the compact dark matter fraction f_DM. We find that these data are noise limited, and therefore localization-based masking of background photons is a key ingredient. Future observatories with better sensitivity will be able to probe down to the f_DM≳1 % level across the 10 M_⊙≲M ≲1000 M_⊙ mass range.