Photometric Redshift Estimation for Gamma-Ray Bursts from the Early Universe

Future detection of high-redshift gamma-ray bursts (GRBs) will be an important tool for studying the early Universe. Fast and accurate redshift estimation for detected GRBs is key for encouraging rapid follow-up observations by ground- and space-based telescopes. Low-redshift dusty interlopers pose the biggest challenge for GRB redshift estimation using broad photometric bands, as their high extinction can mimic a high-redshift GRB. To assess the risk of false alarms of high-redshift GRB photometric measurements, we simulate and fit a variety of GRBs using phozzy, a simulation code developed to estimate GRB photometric redshifts and test the ability to distinguish between high- and low-redshift GRBs when using simultaneous photometric bands. We run the code with the wavelength bands and instrument parameters for the Photo-z Infrared Telescope (PIRT), an instrument designed for the Gamow mission concept. We explore various distributions of host galaxy extinction as a function of redshift, and their effect on the completeness and purity of a high-redshift GRB search with the PIRT. We find that in the most optimal case, the completeness is ∼ 82−88%, and well above 90% for GRBs with 𝑧 < 9. In this case, only ∼ 0.6% of low-redshift GRBs will be mistaken as a high-redshift one, which corresponds to ∼ 0.2 false alarms per year for a mission that detects 100 total GRBs per year.