Astrophysical objects observed by the MESSENGER X-ray spectrometer during Mercury flybys

TL;DR

This study reviews MESSENGER's X-ray observations during Mercury flybys, identifying astrophysical X-ray sources in the data and suggesting revisions to previous electron flux estimates due to astrophysical signals.

Contribution

It is the first analysis correlating MESSENGER X-ray data with astrophysical sources during Mercury flybys, revealing astrophysical contributions to observed X-ray peaks.

Findings

Two X-ray peaks contain significant astrophysical signals.

Astrophysical sources influence the interpretation of Mercury's magnetospheric X-ray data.

Electron flux estimates should be revised to account for astrophysical contributions.

Abstract

The MESSENGER spacecraft conducted its first flyby of Mercury on 14th January 2008, followed by two subsequent encounters on 6th October 2008 and 29th September 2009, prior to Mercury orbit insertion on 18th March 2011. We have reviewed MESSENGER flight telemetry and X-ray Spectrometer observations from the first two encounters, and correlate several prominent features in the data with the presence of astrophysical X-ray sources in the instrument field of view. We find that two X-ray peaks attributed in earlier work to the detection of suprathermal electrons from the Mercury magnetosphere, are likely to contain a significant number of events that are of astrophysical origin. The intensities of these two peaks cannot be explained entirely on the basis of astrophysical sources, and we support the previous suprathermal explanation but suggest that the electron fluxes derived in those studies be revised to correct for a significant astrophysical signal.