Probing the clumping structure of giant molecular clouds through the spectrum, polarisation and morphology of X-ray reflection nebulae

Abstract

We introduce a new method for probing global properties of clump populations in giant molecular clouds (GMCs) in the case where these act as X-ray reflection nebulae (XRNe), based on the study of the clumping's overall effect on the reflected X-ray signal, in particular on the Fe K-α line's shoulder. We consider the particular case of Sgr B2, one of the brightest and most massive XRN in the Galactic center (GC) region. We parametrise the gas distribution inside the cloud using a simple clumping model with the slope of the clump mass function (α), the minimum clump mass (m min ), the fraction of the cloud's mass contained in clumps ( f DGMF ), and the mass-size relation of individual clumps as free parameters, and investigate how these affect the reflected X-ray spectrum. In the case of very dense clumps, similar to those presently observed in Sgr B2, these occupy a small volume of the cloud and present a small projected area to the incoming X-ray radiation. We find that these contribute negligibly to the scattered X-rays. Clump populations with volume-filling factors of >10 −3 do leave observational signatures, that are sensitive to the clump model parameters, in the reflected spectrum and polarisation. Future high angular resolution X-ray observations could therefore complement the traditional optical and radio observations of these GMCs, and prove to be a powerful probe in the study of their internal structure. Clumps in GMCs should further be visible both as bright spots and regions of heavy absorption in high resolution X-ray observations. We therefore also study the time-evolution of the X-ray morphology, under illumination by a transient source, as a probe of the 3D distribution and column density of individual clumps by future X-ray observatories.

Non-equilibrium chemistry and destruction of CO by X-ray flares

“…The ionizations induced by X-rays that are absorbed in a molecular cloud can strongly affect the chemical balance of the cloud by heating it and increasing the electron fraction (Lepp & McCray 1983;Maloney et al 1996). The sources of X-rays, especially non-thermal sources laro et al 2016;Walls et al 2016;Molaro et al 2016) and shown to be a powerful diagnostic of the incident X-ray flux on a cloud. The inferred luminosity is still far below the Eddington luminosity for Sgr A * , but is 10 4 times brighter than its current luminosity in X-rays.…”

Examining molecular clouds in the Galactic Centre region using X-ray reflection spectra simulations

“…Their work concentrated more on the reflection morphology, while we concentrate on the spectral features and produce the Xspec table model to allow for fitting to real observational data. There is also the work by Molaro et al (2016) which concentrated on the effects of clumpiness in the cloud rather than on the spectral features. This paper is organised as follows.…”

A polarized view of the hot and violent universe

“…In addition to reconstruction of Sgr A*'s past activity record (e.g. [115,116]), one could use the same data as a sensitive and bias-free diagnostic tool for the illuminated gas [117][118][119]. Thanks to the X-ray reflection, eventually we will be able to map the 3D density (and potentially velocity) field of the dense gas inside the inner few hundred parsecs of our Galaxy [120][121][122] and possibly some other nearby galaxies (e.g.…”