Deciphering Residual Emissions: Time-dependent Models for the Nonthermal Interstellar Radiation from the Milky Way

Abstract

Cosmic rays (CRs) in the Galaxy are an important dynamical component of the interstellar medium (ISM) that interact with the other major components (interstellar magnetic and radiation fields, and gas) to produce broadband interstellar emissions that span the electromagnetic spectrum. The standard modelling of CR propagation and production of the associated emissions is based on a steady-state assumption, where the CR source spatial density is described using a smoothly varying function of position that does not evolve with time. While this is a convenient approximation, reality is otherwise where primary CRs are produced in and about highly localised regions, e.g., supernova remnants, which have finite lifetimes. In this paper we use the latest version of the GALPROP CR propagation code to model time-dependent CR injection and propagation through the ISM from a realistic three-dimensional discretised CR source density distribution, together with full three-dimensional models for the other major ISM components, and make predictions of the associated broadband non-thermal emissions. We compare the predictions for the discretised and equivalent steady-state model, finding that the former predicts novel features in the broadband non-thermal emissions that are absent for the steady-state case. Some of features predicted by the discretised model may be observable in all-sky observations made by WMAP and Planck, the recently launched eROSITA, the Fermi-LAT, and ground-based observations by HESS, HAWC, and the forthcoming CTA. The non-thermal emissions predicted by the discretised model may also provide explanations of puzzling anomalies in high-energy γ-ray data, such as the Fermi-LAT north/south asymmetry and residuals like the so-called "Fermi bubbles".

The Fermi–LAT Galactic Center Excess: Evidence of Annihilating Dark Matter?

“…As a result, uncertainties in modeling their contribution in the region that overlaps most with the GC excess are large and can significantly reduce the intensity of the GC excess, to the extent that modeling the Fermi bubbles component in this region can cause the excess to vanish above 10 GeV (37). Recently proposed IEMs that deviate from the steady-state assumption and allow for time-dependent CR injection and propagation also affect the characterization of the Fermi bubbles, and incorporating these improved models in the analysis of the GC excess might also alter its properties (38).…”

Simulations of cosmic ray propagation

“…It is maintained as public software 1 which includes an Explanatory Supplement describing the method in full detail. Recent developments are described in (Porter et al 2017(Porter et al , 2019Boschini et al 2020b).…”

GALPROP Code for Galactic Cosmic Ray Propagation and Associated Photon Emissions

“…The G P code is available from a dedicated website , where a 500+ core facility for users to run the code via online forms in a web-browser, WebRun, is also provided [78]. Here we give a brief description of G P , for details see [19,29,32,44,48,49,51,52,56,58,59,65,68,71,72,77,78].…”

“…The latest G P code v.56 [32,44,58] allows arbitrarily small, even sub-parsec, grid sizes and finely sampled energy/time spans provided that it is running on a machine that has enough computing resources. Given that, the actual employed grid sizing is physically motivated and consistent with the baseline assumptions used to derive transport equations.…”

“…The models for the interstellar radiation field [58] are developed based on stellar and dust spatial density distributions taken from the literature that reproduce local near-to far-infrared observations. The interstellar emission models that include arms and bulges for the CR source and interstellar radiation densities provide plausible physical interpretations for features found in the residual maps from high-energy -ray data analysis.…”