X-ray spectral variability during an outburst in VI118 Ori

We present results from a multiwavelength campaign to monitor the 2005 outburst of the low-mass young star V1118 Orf. Although our campaign covers the X-ray, optical, infrared, and radio regimes, we focus in this Letter on the properties of the X-ray emission in VI118 Ori during the first few months after the optical outburst. Chandra and XMM-Newton detected VI118 Ori at three epochs in early 2005. The X-ray flux and luminosity stayed similar within a factor of 2 and at the same level as in a preoutburst observation in 2002. The hydrogen column density showed no evidence for variation from its modest preoutburst value of N_H ∼ 3 × 10²¹ cm-². However, a spectral change occurred from a dominant hot plasma (∼25 MK) in 2002 and in 2005 January to a cooler plasma (∼8 MK) in 2005 February and in 2005 March. We hypothesize that the hot magnetic loops high in the corona were disrupted by the closing in of the accretion disk due to the increased accretion rate during the outburst, whereas the lower cooler loops were probably less affected and became the dominant coronal component.