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_sun
mass range by null searches for microlensing of stars in nearby galaxies. Here
we propose to probe the mass range M >~ 10 M_sun 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_sun. 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 this
data is 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_sun <~ M <~ 1000
M_sun mass range.
