We present observations and analysis of the broadband afterglow of Swift GRB 071025 .
Using optical and infrared ( RIYJHK ) photometry , we derive a photometric redshift of 4.4 < z < 5.2 ; at this redshift our simultaneous multicolour observations begin at \sim 30 s after the GRB trigger in the host frame , during the initial rising phase of the afterglow .
We associate the light curve peak at \sim 580 s in the observer frame with the formation of the forward shock , giving an estimate of the initial Lorentz factor \Gamma _ { 0 } \sim 200 .
The red spectral energy distribution ( even in regions not affected by the Lyman- \alpha break ) provides secure evidence of a large dust column .
However , the inferred extinction curve shows a prominent flat component between 2000-3000 Å in the rest-frame , inconsistent with any locally observed template but well-fit by models of dust formed by supernovae .
Time-dependent fits to the extinction profile reveal no evidence of dust destruction and limit the decrease in the extinction column to \Delta A _ { 3000 } < 0.54 mag after t = 50 s in the rest frame .
Our observations provide evidence of a transition in dust properties at z \sim 5 , in agreement with studies of high- z quasars , and suggest that SN-formed dust continues to dominate the opacity of typical galaxies at this redshift .