Under the Shadow of the Magellanic Bridge: A Measurement of the Extragalactic Background at $\sim 0.7$~keV

TL;DR

This study measures the extragalactic 0.7 keV X-ray background using the shadow of a gas cloud in the Magellanic Bridge, revealing a significant diffuse component likely from hot intergalactic medium.

Contribution

It provides a direct measurement of the extragalactic background at 0.7 keV and constrains the contribution of point sources versus diffuse emission.

Findings

Measured the unabsorbed extragalactic background at 0.7 keV as 28 keV s^{-1} cm^{-2} keV^{-1} sr^{-1}

Lower limit on background exceeds expected point source contribution, indicating diffuse emission

Suggests a significant fraction of the background arises from a hot intergalactic medium

Abstract

We measured the extragalactic 0.7 keV X-ray background by observing the X-ray shadow of a neutral gas cloud in the Magellanic Bridge region. Two \rosat PSPC observations of total 104~ks were complemented by a detailed H I mapping of the cloud with both the Parkes 64~m telescope and the Australia Telescope Compact Array. From the detected anti-correlation between the observed background intensity and the \ion{H}{1} column density of the cloud, we derived the unabsorbed extragalactic background intensity as 28 keV s^{-1} cm^{-2} keV^{-1} sr^{-1} at 0.7~keV. The 95% confidence lower limit 18 keV s^{-1} cm^{-2} keV^ {-1} sr^{-1}} is greater than the expected point-like source contribution < 14 keV s^{-1} cm^{-2} keV^{-1} sr^{-1}, constrained by the mean source spectrum together with the total background intensity in the 1-2 keV band. A significant fraction of the 0.7 keV background likely arises in a diffuse hot intergalactic medium of a few million degrees, as has been predicted in hydrodynamic simulations of cosmological structure formation.