Time-Variability of equivalent width of 6.4 keV line from the Arches Complex: reflected X-rays or charged particles?

TL;DR

This study investigates the time variability of the 6.4 keV iron line in the Arches cloud, exploring whether its origin is reflected X-rays from flares or charged particles, and finds the reflection model more plausible.

Contribution

It introduces a comparative analysis of two scenarios explaining iron line variability, favoring the reflection model based on observational constraints.

Findings

Reflection model requires fewer assumptions.

Variability suggests multiple irradiation sources.

Charged particle scenario less supported.

Abstract

Molecular gas in Arches cloud located near the Arches cluster is one of the emitters of K-$\alpha$ line of neutral iron and X-ray continuum in the Galactic center (GC). Similarly to the cloud Sgr B2, another well-known emitter of the iron line in the GC, the Arches cloud demonstrates temporal decline of the X-ray emission. The most natural origin of this emission is irradiation of primary photons of an X-ray flare from a distant source, most likely Sgr A*. However, recent observations of the Arches cloud discovered variations of equivalent width of the 6.4 keV iron line, which indicated that the X-ray emission from the cloud is a combination of two components with different origin and different equivalent width, one of which is time-variable, while the other is stationary during the period of observations. We considered two different scenarios: a) this emission is formed by reflection from two clouds, which are at some distance from each other, when they are irradiated by two different flares; and b) the other scenario assumes a combination of X-ray fluxes produced in the same cloud by reflection of primary photons and by subrelativistic cosmic rays. We present restrictions for both model and conditions at which these scenarios can be realized. Although none of the models can be completely ruled out, we find that the X-ray reflection model requires less assumption and therefore is the most viable.