Long Term Variability of O VII Line Intensity toward the Lockman Hole Observed with Suzaku from 2006 to 2011

TL;DR

This study analyzed long-term variations in O VII line intensity in the soft X-ray background using Suzaku data from 2006 to 2011, identifying a 2-3 LU increase in 2010-2011 likely due to solar wind changes.

Contribution

It provides the first detailed long-term analysis of O VII variability toward the Lockman Hole, linking observed changes to solar wind structure variations.

Findings

O VII intensity was stable from 2006-2009 within errors.

A 2-3 LU increase was observed in 2010-2011.

The variation is likely related to solar wind structure changes.

Abstract

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.