Less than a month ago, Apple's share price plummeted after the tech giant reported a lower-than-expected profit for its latest quarter. (Apple still made a whopping $9 billion, but failed to meet investors' stratospheric estimates.) Since then, however, Apple's stock has been on a tear, reaching a record closing high of $648 a share on Friday. And on Monday, Apple's stock broached the $663 mark, making Apple, with a market valuation of $622 billion, the most valuable public company in history. (The previous record of $619 billion was held by Microsoft in 1999.) Here, four theories for Apple's sudden resurgence:

1. Excitement over the iPhone 5
Many analysts have "attributed Apple's recent surge to investors' anticipation of next month's rumored iPhone 5 launch," says Gregg Keizer at ComputerWorld. Though the company has made no official announcement, industry observers are talking up a launch date of September 12, and a phone with a "larger screen, high-speed 4G LTE connectivity, a better camera, faster processors, and a smaller dock connector," says Andrea Chang at the Los Angeles Times. Customers are similarly eager.

2. Excitement over the iPad mini
Also churning in the rumor mill: Apple is expected to unveil a smaller, cheaper version of the iPad, and investors hope the new gadget will open up a new revenue stream. The iPad mini is expected to be a 7.85-inch tablet, priced in the $250-$300 range to compete with Google's Nexus 7, Amazon's Kindle Fire, and other similarly sized tablets.  

3. Excitement over a new Apple TV
Rumors are also swirling that Apple is on the verge of unveiling a new version of Apple TV. Indeed, Apple is "in talks with some of the biggest U.S. cable operators about letting consumers use an Apple device as a set-top box for live television and other content," say Jessica E. Vascellaro and Shalini Ramachandran at The Wall Street Journal. Though disillusioned tech insiders view this news as Apple's abandonment of its original Apple TV goal — to offer consumers a way to opt out of expensive cable package deals — investors see even this compromised version of Apple TV as "Apple's most ambitious crack at infiltrating the living room after years of trying."

4. Apple is simply undervalued
Apple is currently trading at 14 times what it expects to earn this year, which makes it undervalued compared to struggling tech companies like Facebook, Groupon, and Zynga. The top estimate on Wall Street has a "$1,111 price target on Apple (no, that's not a typo) and expects the stock could hit that level over the next year," says Steven Russolillo at The Wall Street Journal. At Apple's current rate, it's not so far-fetched: Apple's market valuation has "crossed the $400 billion, $500 billion, and $600 billion marks — all in 2012," says David Goldman at CNN Money.

Sources: CNN MoneyComputerWorld, Los Angeles Times, The Wall Street Journal (2)