A few days before the end of the 2008 extreme outburst of EX Lupi: accretion shocks and a smothered stellar corona unveiled by XMM-Newton

¹ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000, Strasbourg, France
e-mail: nicolas.grosso@astro.unistra.fr
² CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA
³ Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
⁵ Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
⁶ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA

Received: 10 December 2009
Accepted: 21 June 2010

Abstract

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 × 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 ~1.5 keV by emission from a relatively cool plasma (~4.7 MK) that is lightly absorbed (N_H ≃ 3.6 × 10²⁰ cm^-2) and above ~1.5 keV 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 ~4 × 10²⁸ erg s^-1. The intrinsic X-ray luminosity of EX Lup, ~3.4 × 10²⁹ 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., [O i]) 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.