RX J1131
... that the black hole inside RX J1131 was the first black hole to have its spin directly measured?
... that the <a href="./Black_hole" id="mwCKg" rel="mw:WikiLink" title="Black hole">black hole</a> inside <b id="mwCKk"><a class="mw-redirect" href="./RX_J1131" id="mwCKo" rel="mw:WikiLink" title="RX J1131">RX J1131</a></b> was the first black hole to have its spin directly measured?