Variability of Sub-mjy Radio Sources

We present 1.4 GHz VLA observations of the variability of radio sources in the Lockman Hole region at the level of ≥ 100µJy on timescales of 17 months and 19 days. These data indicate that the areal density of highly variable sources at this level is < 5 × 10 −3 arcmin −2. We set an upper limit of 2% to the fraction of 50 to 100µJy sources that are highly variable (∆S ≥ 50%). These results imply a lower limit to the beaming angle for GRBs of 1 o , and give a lower limit of 200 arcmin 2 to the area that can be safely searched for GRB radio afterglows before confusion might become an issue.