Suzaku Observation of a New X-ray Outburst from the Accreting Young Star Illuminating McNeil's Nebula

摘要

A deeply embedded young stellar object (YSO) in the L1630 dark cloud has been observed undergoing a new strong optical/near-infrared outburst in 2008 August. The star, V1647 Ori, is the same star that underwent an accretion-powered outburst in 2003-2005; during this eruption we recorded, for the first time, a sustained X-ray outburst from a rapidly accreting YSO. The X-ray activity is suspected to be driven by a star-disk magnetic dynamo, accompanied by rapid mass accretion. However, observations that might support or refute this hypothesis have been limited in quantity and quality. We triggered a ToO observation of V1647 Ori with Suzaku for 40 ksec on 2008 October 8 using Director's Discretionary Time. During the observation, V1647 Ori showed a gradual flux decrease by a factor of 5 and then displayed an abrupt flux increase by an order of magnitude. The spectrum clearly displays emission from Helium-like iron, which is a signature of hot plasma (kT ~5 keV). Such X-ray properties were also seen in an XMM-Newton observation during the first outburst, but it has rarely been observed for other YSOs. Hence, the new outburst started in 2008 is likely driven by a mechanism similar to that of the first outburst. The Suzaku/XIS spectrum of V1647 Ori also shows a remarkable fluorescent iron K line with an equivalent width of ~600 eV. Such a large equivalent width indicates that a part of the incident X-ray emission that irradiates the circumstellar material is hidden from our line of sight.