GRB 020410: A Gamma-Ray Burst Afterglow Discovered by Its Supernova Light

We present the discovery and monitoring of the optical transient (OT) associated with GRB 020410. The fading OT was found by Hubble Space Telescope (HST) observations taken 28 and 65 days after burst at a position consistent with the X-ray afterglow, making this the first time that the optical afterglow of a gamma-ray burst (GRB) has been discovered by an orbiting observatory. Subsequent reexamination of early ground-based observations revealed that a faint OT was present 6 hr after burst, confirming the source association with GRB 020410. A deep nondetection after one week requires that the OT rebrightened between day 7 and day 28, and further late-time HST data taken approximately 100 days after burst imply that it is very red (Fν ∝ ν-2.7). We compare both the flux and color of the excess with supernova models and show that the data are best explained by the presence of a Type Ib/c supernova at a redshift z ≈ 0.5, which occurred roughly coincident with the day of the GRB.