Evidence For Accretion-driven X-ray Production In Ex Lupi And V1647 Ori

Astronomy and Astrophysics – Astronomy

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EX Lupi is the prototype for a class of young, pre-main sequence stars, which are observed to undergo irregular optical outbursts that result in a several magnitude rise of the optical flux. EX Lupi was observed to optically erupt in 2008 January, triggering Chandra X-ray Observatory ToO observations shortly thereafter. In the 2008 March and June observations, we find the X-ray spectrum is best modeled with a two-temperature plasma with components of ∼0.4 and 1.7 keV. In subsequent observations in 2008 October, the lower-temperature plasma component appears to fade as EX Lupi returns to more quiescent optical levels. Accretion hotspots should generate plasma with temperatures of a few million Kelvin (∼0.3 keV); thus, this fading of the lower-temperature component in the spectra of EX Lupi is consistent with a decrease in accretion-generated X-ray flux from shock-heated plasma. We also find that during optical outburst, the light curve of EX Lupi appears to exhibit periodic variability of ∼37 days with the V-band flux changing by as much as ∼44 magnitudes.
Similar to EX Lupi, V1647 Ori is a low-mass, deeply-embedded, pre-main sequence star that has undergone two optical/NIR outbursts in the last decade; both times, the star gradually faded over several months to years. The X-ray evolution during each of these eruptions has been monitored through multi-epoch Chandra observations. Like the X-ray flux of EX Lupi, we find that the X-ray flux of V1647 Ori is well correlated with the rise and fall in the optical and infrared brightnesses for both epochs; unlike EX Lupi, however, these spectra are well modeled with a single-temperature plasma of 4-6 keV. These results suggest that the primary X-ray generation mechanism for this star during optical/NIR outbursts is accretion but that the X-ray flux is likely generated by magnetic reconnection events in the accretion stream.

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