2010A&A...522A..56G

EX Lup is a pre-main sequence star that exhibits repetitive and irregular optical outbursts driven by an increase in the mass accretion rate in its circumstellar disk. In mid-January 2008, EX Lup, the prototype of the small class of eruptive variables called EXors, began an extreme outburst that lasted seven months. We attempt to characterize the X-ray and UV emission of EX Lup during this outburst. We observed EX Lup during about 21 h with XMM-Newton, simultaneously in X-rays and UV, on August 10-11, 2008 - a few days before the end of its 2008 outburst - when the optical flux of EX Lup remained about 4 times above its pre-outburst level. We detected EX Lup in X-rays with an observed flux in the 0.2-10keV energy range of 5.4x10^–14erg/s/cm2 during a low-level period. This observed flux increased by a factor of four during a flaring period that lasted about 2h. The observed spectrum of the low-level period is dominated below ∼1.5keV by emission from a relatively cool plasma (∼4.7MK) that is lightly absorbed (N_H≃3.6x10²⁰cm-2) and above ∼1.5keV by emission from a plasma that is ∼ten times hotter and affected by a photoelectric absorption that is 75 times larger. The intrinsic X-ray luminosity of the relatively cool plasma is ∼4x10²⁸erg/s. The intrinsic X-ray luminosity of EX Lup, ∼3.4x10²⁹erg/s, is hence dominated by emission from the hot plasma. During the X-ray flare, the emission measure and the intrinsic X-ray luminosity of this absorbed plasma component is five times higher than during the low-level period. We detected UV variability on timescales ranging from less than one hour up to about four hours. We show from simulated light curves that the power spectral density of the UV light curve can be modeled with a red-noise spectrum with a power-law index of 1.39±0.06. None of the UV events observed on August 10-11, 2008 correlate unambiguously with simultaneous X-ray peaks. The soft X-ray spectral component is most likely associated with accretion shocks, as opposed to jet activity, given the absence of forbidden emission lines of low-excitation species (e.g., [OI]) in optical spectra of EX Lup obtained during outburst. The hard X-ray spectral component, meanwhile, is most likely associated with a smothered stellar corona. The UV emission is reminiscent of accretion events, such as those already observed with the Optical/UV Monitor from other accreting pre-main sequence stars, and is evidently dominated by emission from accretion hot spots. The large photoelectric absorption of the active stellar corona is most likely due to high-density gas above the corona in accretion funnel flows.