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

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 × 10 20 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 × 10 28 erg s −1 . The intrinsic X-ray luminosity of EX Lup, ∼3.4 × 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., [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.

Optical spectroscopy of EX Lupi during quiescence and outburst

“…The small velocities and the relatively low excitation potentials agree with the lines being produced in an active chromosphere (Hamann & Persson 1992a). This picture is consistent with the available ultraviolet (UV) and X-ray observations (Grosso et al 2010). The high X-ray absorption and UV variability, which do not correspond to the observed optical A V , suggest a large column density in thick, dense accretion flows, while hard-X ray variability reveals a very active chromosphere (Grosso et al 2010).…”

Accretion dynamics of EX Lupi in quiescence

“…With this higher rotational value, a rotational period of 7.41d would require an inclination angle of 23−25 degrees (consistent with the disk models of Sipos et al 2009). A low viewing angle was also predicted by Grosso et al (2010) based on X-ray absorption. The observed velocity amplitude of the NC is also in agreement with an origin in a high-latitude structure, in a star that is seen at a relatively low angle (Sect.…”

“…X-ray emission is produced at the bottom of a 3D accretion shock, which results in the accretion column obscuring most of it at zero velocity, while most of the emission should escape through the sides of the accretion column at maximum redshift/blueshift. This effect has been observed for EX Lupi (Grosso et al 2010). The same explanation could apply to the NC lines, if formed within a vertically extended structure where the emission can escape better along the sides of the accretion structure, instead of from the top.…”

“…If we consider the observed values for the accretion rate (Ṁ ∼ 10 −10 −10 −9 M /yr) and the velocity at which the material arrives to the shock region (v = 100−200 km s −1 ), we arrive at n = 6−120/ f × 10 9 cm −3 . We assume that the spot covers a small part (∼1%) of the stellar surface (suggested by Grosso et al 2010 based on X-ray observations, and expected in the case of higher density; Romanova et al 2004). At relevant temperatures all the metals would be ionised and the H and He would be mostly neutral, producing electron densities of n e ∼ 10 8 −10 10 cm −3 .…”

Detection of a Cool, Accretion-Shock-Generated X-Ray Plasma in Ex Lupi During the 2008 Optical Eruption

“…Together, these five pointings yield an observing sequence, spanning approximately seven months, that follows the X-ray evolution of EX Lupi from two months after the start until after the conclusion of the optical outburst. During this same seven-month period (specifically in 2008 August), a 78 ks observation of EX Lupi was also obtained by XMM-Newton (AO-7, anticipated Target of Opportunity; PI: N. Grosso; Grosso et al 2010). For all Chandra observations, the Advanced CCD Imaging Spectrometer Imaging (ACIS-I) array was used in faint telemetry mode with EX Lupi at the aimpoint of the I3 CCD.…”