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Observation of GRB 051022 detected by hete-2

An "optically dark" burst GRB051022 was detected by HETE-2. The detections of candidate X-ray and radio afterglows were reported, whereas no optical afterglow was found in spite of quick follow-up observations. The optical spectroscopic observations of the host galaxy revealed the redshift. z = 0.8. Using the data derived by HETE-2 observation of the prompt emission, we found the constant absorption N H = 8.8 +3.1 -2.9 x 10 22 cm -2 and the visual extinction A v = 49 +17 -16 mag in the host galaxy. If this is the case, no detection of any optical transient would be quite reasonable. The absorption derived by the Swift XRT observations of the afterglow is fully consistent with those obtained from the early HETE-2 observation. Our analysis implies an interpretation that the absorbing medium may be outside the external shock at R > 10 16 cm, which could be a dusty molecular cloud.