ASCA spectroscopy of the luminous infrared galaxy NGC 6240: X-ray emission from a starburst and a buried active nucleus

We present an X-ray spectroscopic study of the prototype far-infrared galaxy NGC 6240 from ASCA. The soft X-ray spectrum (below 2 keV) shows clear signatures of thermal emission well described by a multitemperature optically thin plasma, which probably originates in a powerful starburst. Strong hard X-ray emission is also detected with ASCA and its spectrum above 3 keV is extremely flat with a prominent iron K line complex, very similar to that seen in the Seyfert 2 galaxy NGC 1068 but about an order of magnitude more luminous (L3-10keV approximate to 1.4 x 10(42) erg s(-1)). The hard X-ray spectrum indicates that only reflected X-rays of an active galactic nucleus (AGN) buried in a heavy obscuration (N-H > 2 x 10(24) cm(-2)) are visible. This is evidence for an AGN in NGC 6240, emitting possibly at a quasar luminosity (similar to 10(45) erg s(-1)), and suggests its significant contribution to the far-infrared luminosity.