SIMBAD references

The All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE) has been used to localize 13 confirmed X-ray counterparts to gamma-ray bursts (GRBs) detected over 3 yr of operation. We quantify the errors in ASM localizations of brief transient sources by using observations of persistent sources with well-known locations. We apply the results of this analysis to obtain accurate error boxes with reliable confidence levels for the 13 GRBs. In six of these 13 cases, multiple detections by the ASM allow the positions to be localized to a diamond of order ∼15'x3'. In five further cases, the Interplanetary Network (IPN) constrains the usually ∼3°x3' (full-width) ASM error box to an area of a few tens of square arcminutes. This work adds 11 burst localizations to the list of ∼60 well-localized GRBs.