[Paper Review] Results from Droxo. I. The variability of fluorescent Fe 6.4 keV emission in the young star Elias 29: High-energy electrons in the star's accretion tubes?

Giovanna Giardino, F. Favata+4 more|ArXiv.org|Oct 10, 2007

Astrophysics and Star Formation Studies26 references17 citations

TL;DR

The paper proposes that the highly variable 6.4 keV Fe Kα emission line in the Class I YSO Elias 29 is caused by collisional ionisation from non-thermal electrons accelerated in magnetic reconnection events within accretion tubes connecting the star to its disk. Despite no change in the thermal X-ray continuum, the line's equivalent width varies significantly (peaking at ~250 eV), indicating in situ electron bombardment of neutral material, providing evidence for non-thermal processes in young stellar systems.

ABSTRACT

AIMS. We study the variability of the Fe 6.4 KeV emission line from the Class I young stellar object Elias 29 in the Rho-Oph cloud. METHODS. We analysed the data from Elias 29 collected by XMM during a nine-day, nearly continuous observation of the Rho-Oph star-forming region (the Deep Rho-Oph X-ray Observation, named Droxo). The data were subdivided into six homogeneous time intervals, and the six resulting spectra were individually analysed. RESULTS. We detect significant variability in the equivalent width of the Fe 6.4 keV emission line from Elias 29. The 6.4 keV line is absent during the first time interval of observation and appears at its maximum strength during the second time interval (90 ks after Elias 29 undergoes a strong flare). The X-ray thermal emission is unchanged between the two observation segments, while line variability is present at a 99.9% confidence level. Given the significant line variability in the absence of variations in the X-ray ionising continuum and the weakness of the photoionising continuum from the star's thermal X-ray emission, we suggest that the fluorescence may be induced by collisional ionisation from an (unseen) population of non-thermal electrons. We speculate on the possibility that the electrons are accelerated in a reconnection event of a magnetically confined accretion loop, connecting the young star to its circumstellar disk.

Motivation & Objective

• To investigate the time variability of the Fe 6.4 keV fluorescent line in the young stellar object Elias 29 over a nine-day XMM-Newton observation.
• To determine whether the observed line variability can be explained by changes in the X-ray ionising continuum or requires a non-thermal mechanism.
• To assess the geometry and physical conditions of the circumstellar environment by analyzing spectral variability in a deep X-ray observation.
• To explore the possibility that magnetic reconnection in accretion tubes accelerates electrons that ionise neutral Fe, producing the observed fluorescence.

Proposed method

• Conducted a time-resolved spectral analysis of 9-day XMM-Newton observations of Elias 29, subdividing data into six homogeneous time intervals.
• Fitted individual spectra with a model including thermal plasma emission and a Gaussian line at 6.4 keV to measure equivalent width variability.
• Compared the observed line variability with changes in the thermal X-ray continuum to rule out photoionisation as the dominant mechanism.
• Used simulations to assess the significance of line variability, confirming it at a 99.9% confidence level despite stable thermal emission.
• Evaluated the feasibility of non-thermal electron beams by modeling bremsstrahlung emission with parameters (Γ, a, Ec) consistent with observed flux levels.
• Proposed a magnetohydrodynamic model where magnetic reconnection in accretion tubes accelerates electrons that collide with neutral Fe in the accreting material.

Experimental results

Research questions

• RQ1 Is the 6.4 keV Fe Kα line variability in Elias 29 correlated with changes in the thermal X-ray continuum?
• RQ2 Can the observed line equivalent width (~250 eV at peak) be explained by photoionisation from the star’s thermal X-ray emission?
• RQ3 What physical mechanism could produce strong, variable Fe Kα fluorescence without a corresponding change in the ionising continuum?
• RQ4 Is there evidence for non-thermal electron populations in the accretion region of a Class I YSO?
• RQ5 Could magnetic reconnection in accretion tubes be the source of electron acceleration that produces the observed fluorescence?

Key findings

• The Fe 6.4 keV line is absent in the first observation interval and reaches a maximum equivalent width of ~250 eV in the second interval, 90 ks after a strong X-ray flare.
• The thermal X-ray continuum remains unchanged between the first and second intervals, ruling out variations in the ionising source as the cause of line variability.
• The line variability is significant at the 99.9% confidence level, indicating a physical mechanism beyond statistical fluctuations.
• The high equivalent width (~250 eV) exceeds what is expected from photoionisation by the star’s thermal emission, challenging standard fluorescence models.
• The observed line variability is best explained by collisional ionisation from a population of non-thermal electrons, likely accelerated in magnetic reconnection events.
• The authors propose that these electrons are generated in magnetically confined accretion tubes and deposit energy directly in the accreting material, producing the Fe Kα emission in situ.