On the detection of very high redshift Gamma Ray Bursts with Swift

We compute the probability to detect long Gamma Ray Bursts (GRBs) at z ≥ 5 with Swift, assuming that GRBs form preferentially in low–metallicity environments. The model fits well both the observed BATSE and Swift GRB differential peak flux distribution and is consistent with the number of z ≥ 2.5 detections in the 2–year Swift data. We find that the probability to observe a burst at z ≥ 5 becomes larger than 10% for photon fluxes P < 1 ph s cm, consistent with the number of confirmed detections. The corresponding fraction of z ≥ 5 bursts in the Swift catalog is ∼ 10 − 30% depending on the adopted metallicity threshold for GRB formation. We propose to use the computed probability as a tool to identify high redshift GRBs. By jointly considering promptly–available information provided by Swift and model results, we can select reliable z ≥ 5 candidates in a few hours from the BAT detection. We test the procedure against last year Swift data: only three bursts match all our requirements, two being confirmed at z ≥ 5. Other three possible candidates are picked up by slightly relaxing the adopted criteria. No low–z interloper is found among the six candidates.