A B727 captain reported that VFR traffic suddenly appeared in day airspace, causing an RA.

Attributes
ACN	517690
Time
Date	200107
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : day.airport
State Reference	OH
Altitude	msl single value : 12000
Environment
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zid.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time type : 219
ASRS Report	517690
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne critical
Resolutory Action	flight crew : took evasive action
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

In climb, received a TA which turned to RA. RA was for a descent, which we did and got clear of conflict. We descended 800 ft. ATC said traffic was VFR at 11500 ft and he had just popped up on his screen. We concurred with him because we did not see him till a few seconds before TA. We were cleared back to our original altitude uneventfully.

Original NASA ASRS Text

Title: A B727 CAPT RPTED THAT VFR TFC SUDDENLY APPEARED IN DAY AIRSPACE, CAUSING AN RA.

Narrative: IN CLB, RECEIVED A TA WHICH TURNED TO RA. RA WAS FOR A DSCNT, WHICH WE DID AND GOT CLR OF CONFLICT. WE DSNDED 800 FT. ATC SAID TFC WAS VFR AT 11500 FT AND HE HAD JUST POPPED UP ON HIS SCREEN. WE CONCURRED WITH HIM BECAUSE WE DID NOT SEE HIM TILL A FEW SECONDS BEFORE TA. WE WERE CLRED BACK TO OUR ORIGINAL ALT UNEVENTFULLY.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.