Any tips to improve MySQL query?

I have a table with 500K rows that looks like this: http://d.pr/njFJ

A query like this sometimes often takes more than 2 seconds:

SELECT * 
 FROM `alerts` 
 WHERE `a_timestamp` > '2012-04-15' AND `a_timestamp` <= '2012-04-16'  
    AND a_company_id IN(64,65,69,70,71,72,73,74,75,76,83,86,106,108,109,116,148) ORDER BY a_id DESC
 

Here's the explain query: http://d.pr/z20b

Can somebody point out what I'm doing wrong? Maybe I'm missing something. Should it take that much time on such a small table?

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