MLG CLIMBING INTO SUN HAD NMAC WITH VFR SMT; NO TRAFFIC ADVISORY FROM ARTCC.

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MLG X A SCHEDULED FAR 121 CERTIFICATED AIR CARRIER FLT; FROM LONG BEACH; CA TO PHOENIX; AZ HAD BEEN CLRED TO MAINTAIN 9000' AND WAS LEVEL. LA CENTER CLRED MLG X TO CLB AND MAINTAIN 13000'. AT APPROX XA54 A NEAR MISS OCCURRED BTWN OUR ACFT MLG X AND AN SMT Y. OUR LOCATION WAS ON THE SEAL BEACH 150 DEG R; 9.0 DME. OUR ACFT HDG WAS APPROX 090 DEG CLBING OUT OF 11500'. THE SMT Y WAS DSNDING ON AN APPROX 300 DEG HDG AND PASSED JUST BENEATH AND WITHIN AN ESTIMATED 300' TO 400' OF THE NOSE OF OUR ACFT. THE MLG X FLT WAS UNDER THE COMMAND OF CAPT; THE F/O WAS AT THE CONTROLS. SIMULTANEOUS RECOGNITION OCCURRED BTWN THE SMT Y AND THE MLG X F/O. THE SMT Y WAS IN OUR 1 O'CLOCK POS; THEREFORE DIFFICULT FOR THE CAPT TO SEE. THE IMMEDIATE RESPONSE OF THE F/O WAS TO INITIATE A CLBING RIGHT MANEUVER IN ORDER TO AVOID THE SMT Y. AT THIS TIME THE SMT Y WAS OBSERVED INITIATING A PUSHOVER TO INCREASE HIS RATE OF DSCNT. THE MANEUVERS WOULD NOT BE DESCRIBED AS EVASIVE; AND WERE PROBABLY UNDETECTED BY PAXS. NO NEAR MISS REPORT WAS FILED. THE EVENT FROM SIGHTING UNTIL CLR OF TFC IS ESTIMATED TO BE ONE SECOND AT AN ESTIMATED CLOSURE RATE OF 600 KTS. ATC WAS IMMEDIATELY QUESTIONED AND RESPONDED THAT HE WAS NOT CTLING ANY IFR TFC IN OUR VICINITY AND THAT HE DID NOT SHOW ANY TFC ON HIS RADAR THAT COULD HAVE BEEN IN CONFLICT. THE WX WAS VFR ON TOP (PARTIAL UNDERCAST) WITH UNRESTRICTED VISIBILITY. HOWEVER OUR HDG (090 DEG) WAS DIRECTLY INTO THE SUN; INHIBITING OUR FORWARD VISIBILITY. I QUESTION THE VALIDITY OF THE ATC'S STATEMENT. THE SMT Y IS A STATE OF THE ART; NEW GENERATION TURBOPROP COMMUTER LINER; OPERATING MOST CERTAINLY WITH AN ENCODING (MODE C; ALT RPTING CAPABILITY) TRANSPONDER. I ALSO QUESTION THE JUDGEMENT OF THE PLT/AIRLINE OPERATING THE SMT Y INTO SUCH A HIGH-DENSITY AREA SUCH AS THE LA BASIN ON A VFR FLT. THIS DECISION; IN MY OPINION; JEOPARDIZES THE SAFETY OF THE TRAVELING PUBLIC. LUCKILY; THE OUTCOME OF THIS EVENT WAS UNEVENTFUL. SUPPLEMENTAL INFO FROM ACN 89734. IT APPEARS UNUSUAL THAT A COMMUTER LINER WOULD BE OPERATING IN A HIGH DENSITY AREA OR AT LEAST NOT USING VFR FLT FOLLOWING.