RXTE Observation of the Non-thermal Emission from the Early Stage Merger in Abell 1750

TL;DR

This study reports the first observation-based estimate of the energy ratio between cosmic-ray protons and electrons during a galaxy cluster merger, revealing a lower proton energy fraction than theoretical predictions.

Contribution

It provides the first observational calculation of the cosmic-ray proton to electron energy ratio in a merging galaxy cluster using non-thermal X-ray data.

Findings

Energy ratio of cosmic-ray protons to electrons is 0.86.

Non-thermal X-ray emission constitutes 36% of total X-ray emission.

Model with non-thermal component is statistically preferred.

Abstract

We make the first observation-based calculation of the energy that goes into cosmic-ray protons versus cosmic ray electrons in shock acceleration during structure formation. We find a ratio of energy in cosmic ray protons to energy in cosmic ray electrons of 0.86. This value, calculated from the non-thermal X-ray component reported here from RTXE and the Fermi LAT upper limit for gamma-ray emission, is significantly lower than theoretical estimates that place most of the non-thermal energy in protons. Our estimate is based on the detection of non-thermal X-ray emission using the 3 - 20 keV RXTE spectrum, which shows residual emission not well modeled by a single thermal component. The statistical significance of adding a non-thermal, power law component is ~96%. The significance of adding a second thermal component is 90%. The addition of a component consisting of full Cosmic X-ray Background (CXB) fluctuation to an isothermal model is significant with 92% confidence. The cumulative probability for the two thermal component model is 81% and 90% for the thermal plus power law. Thus the model with non-thermal emission is the preferred description of the data. Evidence of shock heating between the clusters in the spectro-imaging data of XMM, Chandra, and Suzaku indicates that a cosmic ray component should also be present and supports a non-thermal interpretation for the additional component. The bolometric non-thermal X-ray luminosity is 1.6$\times$10$^{44}$ ergs s$^{-1}$, 36% of the total X-ray emission in the 0.1 - 100 keV band.