Paper: astro-ph/0606387
Date: Thu, 15 Jun 2006 19:35:18 GMT (31kb)

Title: Invisible Giant: Chandra's Limits on X-rays from Betelgeuse

Authors: Jennifer Posson-Brown, Vinay L. Kashyap, Deron O. Pease, Jeremy J.
Drake

Comments: 18 pages, 4 figures. Submitted to the ApJ
\\
We have analyzed Chandra calibration observations of Betelgeuse ($\alpha$
Ori, M2 Iab, $m_{V} = 0.58$, 131 pc) obtained at the aimpoint locations of the
HRC-I (8 ks), HRC-S (8 ks), and ACIS-I (5 ks). Betelgeuse is undetected in all
the individual observations as well as cumulatively. We derive $3\sigma$ upper
limits to its X-ray count rates and compute the corresponding X-ray flux upper
limits for isothermal coronal plasma over a range of temperatures,
$T=0.3-10$~MK. We place a flux limit at the telescope of
$\fx\approx4\times10^{-15}$~ergs~s$^{-1}$ cm$^{-2}$ at T=1~MK. The upper limit
is lowered by a factor of $\approx3$ at higher temperatures, roughly an order
of magnitude lower than that obtained previously. Assuming that the entire
stellar surface is active, these fluxes correspond to a surface flux limit that
ranges from 30-7000~ergs~s$^{-1}$ cm$^{-2}$ at T=1~MK, to $\approx
1$~ergs~s$^{-1}$ cm$^{-2}$ at higher temperatures, five orders of magnitude
lower than the quiet Sun X-ray surface flux. We discuss the implications of our
analysis in the context of models of a buried corona and a pervasive magnetic
carpet. We rule out the existence of X-ray emission at the quiet Sun levels,
but the presence of low-level emission on the scale of coronal holes remains
plausible.

\\ ( http://arXiv.org/abs/astro-ph/0606387 , 31kb)