** Oil and gas producer's shares rise as much as 19pct to $5.30
** BP offered to buy Penn Virginia for $8/share,financial news website Proactive Investors said, citing sourcesfamiliar with the matter
** Offer represents 80 pct premium to Penn Virginia'sWednesday close
** Penn Virginia rejected the offer and is holding out forat least $10/share, Proactive Investors reports
** Exxon Mobil Corp and Chevron Corp mayalso be interested in Penn Virginia, website says
** More than 11 mln shares traded by 12:45 p.m. ET, abovetheir 25-day moving avg of 3.1 mln
** 9 of 16 brokerages rate the stock "buy" or higher, 7"hold"; their median PT is $8
** Up to Wednesday's close, stock had fallen by more than athird this year
BP (BP.)