WHILE LNDG BOS RWY 27; A CRJ200 CREW BRAKE HARD WHEN THEY VISUALLY SPOT A B757 LNDG ON XING RWY 22L.

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WE WERE LNDG ON RWY 27 IN BOS; VFR. ATIS STATED LAHSO IN EFFECT; BUT UNLESS OTHERWISE STATED; THE FULL RWY WOULD BE USABLE. ABOUT 1 MI AWAY FROM LNDG I NOTICED A B757 LNDG ON THE 22'S (I ASSUMED RWY 22R) BECAUSE WE WERE NEVER INFORMED OF LNDG TFC ON RWY 22L WHEN WE WERE CLRED TO LAND; SO I MENTIONED IT TO MY FO (PF) AS AN FYI. AFTER A GOOD LNDG I ASKED THE FO WHERE THE B757 WAS; BECAUSE IT WAS OUT OF MY VIEW. HE SAID HE LANDED ON RWY 22L. I IMMEDIATELY TOOK THE CTLS AND BRAKED HARD. WE WERE THEN CLRED ACROSS RWY 22L ON TXWYS S; AND W; CROSS RWY 22R. EVERYTHING TURNED OUT FINE. THE MAIN REASON I'M WRITING THIS IS THAT I LANDED ON A RWY WITH LNDG TFC ON A XING RWY AND I DIDN'T NOW HE WAS GOING TO BE THERE AND IT SURPRISED ME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE OTHER ACFT WAS A B757 LNDG ON RWY 22L WITH INSTRUCTIONS TO HOLD SHORT OF RWY 27. HE SAID AFTER COMING TO A STOP ON THE RWY; THEY WERE THEN CLRED TO CROSS RWY 22L. HE SAID THE B757 DID HOLD SHORT OF THE RWY AS INSTRUCTED. HE SAID THE CTLR APOLOGIZED FOR NOT INFORMING THEM OF THE LAHSO OP.