B735 ON DEP FROM RWY 9L AT ORD HAS TCASII RA RELATIVE TO GAR TFC OFF RWY 4R.

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DEPARTING RWY 9L AT ORD; FLAPS 5 DEGS ; CLRED TO 5000 FT MSL; AND R HDG 140 DEGS. WHILE CLBING THROUGH 1900 FT MSL AND IN THE TURN TO 140; WE RECEIVED A TCASII RA TO DSND. ON THE TCASII; THERE WAS A RED ACFT DISPLAYED WITH AN ALT OF 1900 FT WITHIN 1 MI TO THE R OF OUR ACFT MOVING TOWARD US. WE DID NOT HAVE THE ACFT VISUALLY. THE PF LOWERED THE NOSE WHEN THE RA WAS ISSUED; AND THE PNF TOLD THE TWR ABOUT THE TCASII DSCNT. WITHIN A FEW SECONDS OF THE TCASII RA; WE RECEIVED A CLR OF CONFLICT MESSAGE AND CONTINUED OUR CLB. WHEN WE ASKED THE TWR ABOUT THE ACFT INVOLVED; HE INFORMED US IT WAS AN ACR JET DOING A GAR OFF RWY 4R AND HE HAD US IN SIGHT. I WOULD LIKE TO KNOW: 1) WHEN DIDN'T THE TWR TELL US ABOUT THE TFC? 2) DID THE TFC GET AN RA? 3) WHAT WAS OUR SEPARATION FROM THAT ACFT AT OUR CLOSEST POINT? CALLBACK: NO ADDITIONAL SUBSTANTIVE INFO WAS GARNERED FROM THE CALLBACK. THE RPTR STATED THAT THE RESOLUTION MANEUVER WAS A 'DSND NOW;' WHICH INDICATES A GREATER LEVEL OF CONFLICT THAN THE LESSER 'DSND.' WE DISCUSSED THE TCASII LOGIC AND THE FACT THAT A RA REQUIRES COORD RESPONSES FROM BOTH ACFT AND THAT; THEREFORE; THE OTHER ACFT MUST HAVE ALSO RECEIVED A RA. RPTR DID COMMENT THAT THE DSND MANEUVER AT A TIME WHEN THE ACFT WAS IN A HIGH ANGLE OF ATTACK CONFIGN WAS A DRAMATIC ATTITUDE CHANGE AND WAS MORE UNNERVING AS A RESULT.