Enhanced X-ray Variability from V1647 Ori, the Young Star in Outburst Illuminating McNeil's Nebula

Nicolas Grosso, Université Joseph-Fourier
Joel H. Kastner, Rochester Institute of Technology
Hideki Ozawa, Université Joseph-Fourier
Michael Richmond, Rochester Institute of Technology
T. Simon, Institute for Astronomy, Honolulu
D. A. Weintraub, Vanderbilt University
K. Hamaguchi, NASA Goddard Space Flight Center
A. Frank, University of Rochester

This is the pre-print of an article published by EDP Sciences. The final, published version is available here: https://doi.org/10.1051/0004-6361:20042182

Reproduced with permission from Astronomy & Astrophyics, © 2005 ESO

Also archived in: arXiv: astro-ph/0504111 v1 5 Apr 2005

We would like to thank the Target of Opportunity panel of XMM-Newton and the XMM-Newton’s project scientist Norbert Schartel, who gave us the opportunity to observe V1647Ori during its outburst.We thanks the anonymous referee for his comments and suggestions. H. O. acknowledges the support of the Conseil National des Astronomes et Physiciens. Based on observations obtained with the XMMNewton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA). We used archival acquisition image made with ESO Very Large Telescope at Paranal Observatory under programme ID 272.C-5045.

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We report a ~38 ks X-ray observation of McNeil’s Nebula obtained with XMM-Newton on 2004 April 4. V1647Ori, the young star in outburst illuminating McNeil’s Nebula, is detected with XMM-Newton and appears variable in X-rays.We investigate the hardness ratio variability and time variations of the event energy distribution with quantile analysis, and show that the large increase of the count rate from V1647Ori observed during the second half of the observation is not associated with any large plasma temperature variations as for typical X-ray flares from young low-mass stars. X-ray spectral fitting shows that the bulk (~75%) of the intrinsic X-ray emission in the 0.5–8 keV energy band comes from a soft plasma component, with kTsoft = 0.9 keV (0.7–1.1 keV, at the 90% confidence limit), reminiscent of the X-ray spectrum of the classical T Tauri star TW Hya, for which X-ray emission is believed to be generated by an accretion shock onto the photosphere of a low-mass star. The hard plasma component, with kThard = 4.2 keV (3.0–6.5 keV), contributes ~25% of the total X-ray emission, and can be understood only in the framework of plasma heating sustained by magnetic reconnection events. We find a hydrogen column density of NH = 4.1×10^22 cm^−2 (3.5–4.7×10^22 cm^−2), which points out a significant excess of hydrogen column density compared to the value derived from optical/IR observations, consistent with the picture of the rise of a wind/jet unveiled from ground optical spectroscopy. The X-ray flux observed with XMM-Newton ranges from roughly the flux observed by Chandra on 2004 March 22 (i.e. ~10 times greater than the pre-outburst X-ray flux) to a value two times greater than that caught by Chandra on 2004 March 7 (i.e. ~200 times greater than the pre-outburst X-ray flux). The X-ray variability of V1647Ori in outburst is clearly enhanced. We have investigated the possibility that V1647Ori displays a periodic variation in X-ray brightness as suggested by the combined Chandra+XMM-Newton data set. Assuming that the X-ray flux density is periodic, the folding of the two Chandra observed X-ray flux densities with the XMM-Newton ones leads to three periodic X-ray light curve solutions. Our best period candidate is 0.72 day, which corresponds to the time scale of the Keplerian rotation at a distance of 1 and 1.4 stellar radius for a one solar mass star aged of 0.5 and 1Myrs, respectively. We propose that the emission measure, i.e. the observed X-ray flux, is modulated by the Keplerian rotation of the inner part of the V1647Ori accretion disk (Refer to PDF file for exact formulas).