Billionaire investor George Soros reported a 7.9 percent passive stake in J.C. Penney Co (JCP) on Thursday, sending shares of the struggling department store chain up more than 7 percent.

The century-old retailer has been exploring various capital-raising options with its financial advisers after a steep sales slump followed former CEO Ron Johnson's attempt at a turnaround.

Earlier this month, Penney borrowed $850 million from its $1.85 billion revolving credit facility to help buy inventory and revamp its business strategy.

Johnson's dramatic changes alienated core customers without bringing in new ones and led to a 25 percent decline in sales last year. In early April, Johnson was replaced with his predecessor Myron Ullman, who is bringing back the old pricing strategy that relied heavily on coupons to woo shoppers.

In a regulatory filing on Thursday, Soros said he owned about 17.4 million shares of Penney. Its shares were up 6.6 percent at $16.25 in after-hours trading.