I am looking for the best band combination with Landsat 5 imagery to detect fire scars.
This varies greatly on the characteristics of the scene. Fire scar mapping studies using Landsat-5 TM have used the following three band combinations:
CHUVIECO, E., and CONGALTON, R., 1988, Mapping and inventory of forest fires from digital processing of TM data. Geocarto International, 4, 41–53.
PEREIRA, M. C., and SETZER, A. W., 1993, Spectral characteristics of fire scars in Landsat-5 TM images of Amazonia. International Journal of Remote Sensing, 14, 2061–2078.
KOUTSIAS, N., and KARTERIS, M., 1998, Logistic regression modeling of multitemporal Thematic Mapper data for burned area mapping. International Journal of Remote Sensing, 19, 3499–3514.
KOUTSIAS, N., and KARTERIS, M., 2000, Burned area mapping using logistic regression modeling of a single post-fire Landsat-5 Thematic Mapper image. International Journal of Remote Sensing, 21, 673–687.
Another option if you have pre and post fire scenes is to use the differenced Normalised Burn Ratio (Key and Benson 1999), which really makes fire scars stand out.
dNBR is calculated as: