Evidence of Accretion-Generated X-rays in the Young, Erupting Stars V1647 Ori and EX Lupi

Abstract

We have observed two erupting, pre-main sequence stars, V1647 Ori and EX Lupi, with the Chandra X-ray Observatory during their 2008 optical eruptions. Our intensive X-ray monitoring of these objects affords the opportunity to investigate whether and how the intense X-ray emission is related to pre-main sequence accretion activity. All of our Chandra observations demonstrate that variations in the X- ray luminosity of both V1647 Ori and EX Lupi are correlated with similar changes in the optical/near-infrared brightness of this source during the eruptions, strongly supporting the hypothesis that accretion is the primary generation mechanism for these X-ray outbursts. The Chandra monitoring demonstrates that the X-ray spectral properties of the 2008 eruption of V1647 Ori were strikingly similar to those of the 2003 eruption. We find that X-ray spectra obtained immediately following the second outburst are well modeled as a heavily absorbed (N_H ~ 4 × 10²² cm⁻²), single-component plasma with characteristic temperatures (kT_X ~ 2–6 keV) that are consistently too high to be generated via accretion shocks but are in the range expected for plasma heated by magnetic reconnection events. We also find that the X-ray absorbing column has not changed significantly throughout the observing campaign. Since the OIR and X-ray changes are correlated, we hypothesize that these reconnection events either occur in the accretion stream connecting the circumstellar disk to the star or in accretion- enhanced protostellar coronal activity. From our models of the X-ray spectra of EX Lupi , we find strong evidence for a ~0.3 keV plasma component, which is that expected for accretion shock-generated plasma. During the optical outburst, we find the X-ray spectrum is best modeled with at least two plasma components, with temperatures of ≤1.0 and ≥2.0 keV. From 2008 March through October, the lower-temperature plasma component appears to fade as EX Lupi returns to its quiescent level in the optical, with this fading of the lower- temperature component being consistent with a decrease in accretion shock-generated plasma and the accompanying X-ray luminosity. These results contrast with results from X-ray observations of the erupting young star V1647 Ori during the 2003 and 2008 optical outbursts.