NMAC BTWN A CL600 ON A SID IN LEVEL FLT AT 20 MI OUT FROM IND ARPT AND A LIGHT TWIN CLBING PASS 200 FT TO THEIR R. THE CL600 CAPT RESPONDED TO THE TCASII WARNING AND THEN THE OTHER ACFT WAS SIGHTED AFTER THE CAPT WAS TAKING EVASIVE ACTION.

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WE WERE PROCEEDING FROM CVG TO IND ON CVG'S 'WHITEWATER' SID AND TRACKING RIGHT ALONG THE DEPICTED RTE USING THE FMS TO NAV. WE WERE IN CONTACT WITH IND APCH CTL. A SOLID-BLUE-SQUARE TARGET APPEARED ON THE TCASII. WHEN I FIRST NOTICED IT; IT WAS ABOUT 1000 FT BELOW AND SLIGHTLY R AND JUST AHEAD OF THE NOSE. I COULDN'T SEE IT VISUALLY. IT THEN SHOWED A CLB ARROW AND THE VERT SEPARATION RAPIDLY NARROWED UNTIL; IN QUICK SUCCESSION; THE TARGET WENT FROM BLUE TO YELLOW TO RED AND AN RA WAS GENERATED COMMANDING US TO DSND AT LEAST 3000 FPM. AT THAT POINT OUR OWN-ACFT-SYMBOL WAS SUPERIMPOSED RIGHT OVER THE TARGET. I DISCONNECTED THE AUTOPLT AND SIMULTANEOUSLY BROUGHT THE THRUST LEVERS TO IDLE; ROLLED UP ON THE L WING (BASED ON MY PERCEPTION THAT THE TARGET WAS SLIGHTLY OFF TO OUR R); AND DUMPED THE NOSE. THE FO AND OUR JUMP SEAT RIDER BOTH SAW A WHITE AND RED MARKED; LIGHT TWIN ACFT PASS JUST OFF OUR R SIDE. WE QUERIED IND APCH CTL; BUT THEY HAD NO KNOWLEDGE OF THIS ACFT BEING IN OUR VICINITY. WE SURMISED IT MAY HAVE BEEN A SKYDIVER ACFT. THE TCASII LIKELY SAVED US FROM A MIDAIR.