科研之友
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摘要
Context. 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.
Aims. We attempt to characterize the X-ray and UV emission of EX Lup during this outburst.
Methods. 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.
Results. We detected EX Lup in X-rays with an observed flux in the 0.2-10 keV energy range of 5.4 x 10(-14) erg s(-1) cm(-2) during a low-level period. This observed flux increased by a factor of four during a flaring period that lasted about 2 h. The observed spectrum of the low-level period is dominated below similar to 1.5 keV by emission from a relatively cool plasma (similar to 4.7 MK) that is lightly absorbed (N-H similar or equal to 3.6 x 10(20) cm(-2)) and above similar to 1.5 keV by emission from a plasma that is similar to 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 similar to 4x 10(28) erg s(-1). The intrinsic X-ray luminosity of EX Lup, similar to 3.4 x 10(29) erg s(-1), 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.
Conclusions. 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.
- 出版日期2010-11
