LESS THAN STANDARD SEPARATION BETWEEN 2 ACR. PLT DEVIATION OR OPERATIONAL ERROR.

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ISSUED/FLEW HOLDING AT CGT VOR TO MAINTAIN 11000'. DURING OUTBND TURN TO RIGHT (AS PUBLISHED) TOLD BY ATC TO 'DSND IMMEDIATELY TO 10000';' SAW WDB ACR Y APCHING FROM E AT 11000'. CLOSED THROTTLES; DISCONNECTED AUTOPLT; DEPLOYED SPOILERS AND RAPIDLY DSNDED TO 10000'. A CONFLICT WOULD HAVE RESULTED FROM ANYTHING LESS THAN AN IMMEDIATE AND RAPID DSCNT. BECAUSE OF THE VERY RAPID CHANGE IN ALT BY US--NO NEAR MISS OCCURRED. TRIED TO CALL ACR Y CAPT AT HIS OPS AFTER LNDG TO DETERMINE HOW IT HAPPENED; BUT HE HAD LEFT OPS AREA. ORD APCH?/CENTER? (FREQ 125.7?) SEEMED RATTLED AND CONFUSED ABOUT THE ACR Y'S ENTRY INTO CGT AREA AT 11000'. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR DID NOT TALK WITH ANY ATC PERSONNEL AFTER LNDG AND HAS NOT HAD ANY CONTACT WITH FAA SINCE INCIDENT. SEP WAS LESS THAN 3 MI AT FIRST SIGHTING OF WDB ACR Y AT SAME ALT. HAD ABOUT 500-600' VERT SEP WHEN 1 MI LAT. RPTR ON APCH FREQ AND SHORTLY AFTER PASSING HE HEARD ACR Y ON SAME FREQ. DOES NOT KNOW IF ATC ERROR OR IF FLT CREW OF ACR Y BUSTED THEIR ALT.