Long Term Variability of O VII Line Intensity toward the Lockman Hole Observed with Suzaku from 2006 to 2011. (arXiv:1301.5174v1 [astro-ph.HE])

Long Term Variability of O VII Line Intensity toward the Lockman Hole Observed with Suzaku from 2006 to 2011. (arXiv:1301.5174v1 [astro-ph.HE]):
Long-term time variabilities of the OVII (0.57 keV) emission in the soft
X-ray diffuse background were studied using six Suzaku annual observations of
blank sky towards the Lockman Hole made from 2006 to 2011. After time intervals
in which the emission was enhanced on time scales of a few tens of ks were
removed, the O VII intensity was found to be constant from 2006 to 2009 within
the 90% statistical errors. The intensity in 2010 and 2011 was higher by 2-3 LU
(photons/s/cm/sr) than the earlier values. The most plausible origin of the
fast variable component is Solar wind charge exchange (SWCX). The intensity
increase is not positively correlated with the proton flux at the L1 point.
Since all the observations were made in the same season of a year, the
variation cannot be explained by parallax of the SWCX induced X-ray emission
from the Heliosphere. We consider that it is related to the geometrical change
of slow and fast solar wind structures associated with the 11 year solar
activity. The observed variation was compared with that expected from the SWCX
induced X-ray emission model.