The shares of private equity firm Blackstone Group LP rose 21 percent on Monday, a surge that one options analyst attributed to investors buying on speculation that upcoming earnings would be better than expected.

Blackstone, which is scheduled to report third-quarter earnings on Thursday, closed up $2.38 at $13.64, the highest for nearly three months.

There are several reasons that might explain Blackstone's gains today, said William Lefkowitz, an options strategist at brokerage firm vFinance Investments in New York.

Technically, the stock broke out above its June highs of $12, Lefkowitz said.

In addition, with the earnings being released on August 6, before the market opens, investors anticipate better-than- expected profits.

Lefkowitz added that Blackstone was also benefiting from an improving market and greater availability of financing.

Blackstone declined to comment on its share price.

But the stock is still less than half the price Blackstone went public at -- $31 a share in June 2007.

One analyst, who declined to be named, said the rise could be due to earnings, but also to recent news coverage of possible exit routes for Blackstone arch-rival Kohlberg Kravis & Co .

KKR is preparing an initial public offering for one of its portfolio companies, discount retailer Dollar General, a source previously told Reuters, and others are expected to follow from KKR's portfolio as it looks to take advantage of a recent rise in equity markets.

Blackstone has also started making the most of the improved climate to find exit routes. It sold its investment in Stiefel Laboratories Inc earlier this year to GlaxoSmithKline Plc for 1.4 times the cost of the investment, or 40 percent above what it paid two years ago.

Another investment that will likely be highly profitable for Blackstone if it makes an exit is its $220.1 million investment into Kosmos Energy, which it marked at $602 million at the end of 2008. Kosmos Energy is currently selling its Ghanaian oil interests in a hotly followed auction.

Another positive for the stock was a Goldman Sachs analyst saying he expects U.S. asset managers' positive earnings momentum to continue into the second half of 2009 on stronger flows, markets and margins. For more details [nBNG297458]

The analyst said BlackRock Inc , Affiliated Managers Group Inc , Franklin Resources and Blackstone Group are the most willing and able buy-rated buyers.

(Reporting by Megan Davies in New York and Doris Frankel in Chicago; editing by Steve Orlofsky and Andre Grenon))