The target - Mkn 421 - was observed by HRC-S and the Low Energy Transmission Grating (LETG) for 14 hours and 26 minutes, ended 4 minutes ago, and is part of the proposal Assessing the WHIM detection towards Mkn421. The target - also called Mrk 421 - is located in the constellation Ursa Major and is a BL Lac - type object. More information on the target can be found at SIMBAD. See related observations: 19444, 20943, 20944.

This was a time-critical observation.

This was a TOO (target of opportunity) observation.