IMAGINE: a comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays

Abstract

In this white paper we introduce the IMAGINE Consortium and its scientific background, goals and structure. The purpose of the consortium is to coordinate and facilitate the efforts of a diverse group of researchers in the broad areas of the interstellar medium, Galactic magnetic fields and cosmic rays, and our overarching goal is to develop more comprehensive insights into the structures and roles of interstellar magnetic fields and their interactions with cosmic rays within the context of Galactic astrophysics. The ongoing rapid development of observational and numerical facilities and techniques has resulted in a widely felt need to advance this subject to a qualitatively higher level of self-consistency, depth and rigour. This can only be achieved by the coordinated efforts of experts in diverse areas of astrophysics involved in observational, theoretical and numerical work. We present our view of the present status of this research area, identify its key unsolved problems and suggest a strategy that will underpin our work. The backbone of the consortium is the Interstellar MAGnetic field INference Engine, a publicly available Bayesian platform that employs robust statistical methods to explore the multi-dimensional likelihood space using any number of modular inputs. This tool will be used by the IMAGINE Consortium to develop an interpretation and modelling framework that provides the method, power and flexibility to interfuse information from a variety of observational, theoretical and numerical lines of evidence into a self-consistent and comprehensive picture of the thermal and non-thermal interstellar media. An important innovation is that a consistent understanding of the phenomena that are directly or indirectly influenced by the Galactic magnetic field, such as the deflection of ultra-high energy cosmic rays or extragalactic backgrounds, is made an integral part of the modelling. The IMAGINE Consortium, which is informal by nature and open to new participants, hereby presents a methodological framework for the modelling and understanding of Galactic magnetic fields that is available to all communities whose research relies on a state of the art solution to this problem.

Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3D Galactic halo magnetic field

“…The framework required to combine multiple observables, as well as theoretical models, to infer the most likely model of the structure of the Galaxy is being constructed and refined (e.g. IMAGINE; Boulanger et al 2018). In the future, the SKA will also provide groundbreaking observations for many of the complementary observables of the GMF, particularly with respect to diffuse polarisation, extragalactic RM-grids, and Zeeman splitting Johnston-Hollitt et al 2015;Robishaw et al 2015, respectively), promising to revolutionise our understanding of the GMF.…”

“…Several of these observables will be required to accurately reconstruct the multiple-scale components of the GMF (e.g. Jaffe et al 2010;Boulanger et al 2018).…”

Cosmic ray models

“…Both the strength and the scale height of the halo field are rather uncertain, with estimates for the strength varying between 2µG and 12µG and for the scale height between 1 − 5 kpc from pulsar RMs and 5 − 6 kpc from synchrotron emissivities; for reviews of the GMF see, e.g., Refs. [67][68][69].…”

Workshop Summary “The Power of Faraday Tomography”

“…Marijke Haverkorn presented the Interstellar MAGnetic field INference Engine (IMAGINE), which is a software package to model the Galactic magnetic field using Bayesian inference, as well as the associated (open) collaboration of researchers in various fields interested in Galactic magnetism [46,47].…”