GRB Remnants

The realization that GRBs are narrowly beamed implied that the actual rate of GRBs is much larger than the observed one. There are 500 unobserved GRBs for each observed one. The lack of a clear trigger makes it hard to detect these unobserved GRBs as orphan afterglows. At late time, hundreds or thousands of years after a GRB, we expect to observe a GRB remnant (GRBR). These remnants could be distinguished from the more frequent SNRs using their different morphology. While SNRs are spherical, GRBRs that arise from a highly collimated flow, are expected to be initially nonspehrical. Using SPH simulations we follow the evolution of a GRBR and calculate the image of the remnant produced by bremsstrahlung and by synchrotron emission. We find that the GRBR becomes spherical after $\sim 3000 {\rm yr}(E_{51}/n)^{1/3}$ at $R\sim 12{\rm pc} (E_{51}/n)^{1/3}$, where $E_{51}$ is the initial energy in units of $10^{51}{\rm erg}$ and $n$ is the surrounding ISM number density in ${\rm cm}^{-3}$. We expect $0.5 (E_{51}/n)^{1/3}$ non-spherical GRBs per galaxy. Namely, we expect $\sim ~ 20$ non spherical GRBRs with angular sizes $\sim 1 \mu$arcsec within a distance of 10Mpc. These results are modified if there is an underlying spherical supernova. In this case the GRBR will remain spherical only for $\sim150 {\rm yr} (E_{51}/n)^{1/3}$ and the number of non-spherical GRBRs is smaller by a factor of 10 and their size is smaller by a factor of 3.