Periodic Reporting for period 1 - GADGET (Galactic magnetic field deflection of high energy astroparticles)
Reporting period: 2023-08-01 to 2025-07-31
The GADGET project is aimed at understanding the origin of the most energetic particles in the Universe, the ultra-high energy cosmic rays (UHECRs). They reach energies of more than 10^20 eV, much above the ones that can be obtained at human-made accelerators. They are the messengers of the most violent phenomena in the Universe. After more than 50 years since their discoveries, their acceleration sites are still unknown. Some of the difficulties in this quest are posed by the uncertainties in the Galactic and extra-Galactic magnetic fields that deflect these charged particles, by their low flux, and by the resolution in the measurement of their charge.
With this research project, we proposed to revisit the existing modeling of the Galactic magnetic field derived from the most recent data on synchrotron and Faraday rotation measures, and propagate this acquired knowledge to UHECR studies and, in particular, toward the identification of their sources on the sky.
With this research project, we proposed to revisit the existing modeling of the Galactic magnetic field derived from the most recent data on synchrotron and Faraday rotation measures, and propagate this acquired knowledge to UHECR studies and, in particular, toward the identification of their sources on the sky.
In the absence of constraints in the three-dimensional space, the structure and properties of the Galactic magnetic field (GMF) can only be inferred from line-of-sight integrated observables such as synchrotron polarization and Faraday rotation measures. This limitation leads to a certain degree of uncertainty and degeneracy between models. Those must be tackled in order to trace back the trajectories of UHECRs through our Galaxy and ultimately point to their sources.
There exists currently an ensemble of about ten models with various components (spiral in the disk, toroidal and poloidal structures in the halo) to describe the GMF at scales relevant to UHECR studies. They all fit equally well the data on synchrotron and Faraday rotation measures but lead to somewhat different predictions for the backtracked directions of UHECRs at the edge of our Galaxy.
In addition, because of the line-of-sight-integrated nature of the observables and our vantage point in the disk of the Galaxy, the reconstruction of the large-scale GMF may be complicated by small-scale features if they appear big on the sky due to their proximity.
One of such features of the local interstellar medium is the Local Bubble, a cavity within which the Solar system resides and which is characterized by a unusually low density of gas, filled with an X-ray emitting plasma, and surrounded by a thick shell of cold ionized gas and dust. This local cavity has most likely been created by successive supernova explosions that occurred in the past 10 to 15 Myr and which has distorted the local magnetic field. While the magnetized shell of the Local Bubble has the potential to leave its imprint on the GMF observables, it has mostly been overlooked in most efforts to model the large-scale GMF or has been overly simplified.
Our goal, therefore, has been to improve on this aspect to update the models for the large-scale GMF and explore the implied impacts to study the propagation of the UHECR through our Galaxy.
There exists currently an ensemble of about ten models with various components (spiral in the disk, toroidal and poloidal structures in the halo) to describe the GMF at scales relevant to UHECR studies. They all fit equally well the data on synchrotron and Faraday rotation measures but lead to somewhat different predictions for the backtracked directions of UHECRs at the edge of our Galaxy.
In addition, because of the line-of-sight-integrated nature of the observables and our vantage point in the disk of the Galaxy, the reconstruction of the large-scale GMF may be complicated by small-scale features if they appear big on the sky due to their proximity.
One of such features of the local interstellar medium is the Local Bubble, a cavity within which the Solar system resides and which is characterized by a unusually low density of gas, filled with an X-ray emitting plasma, and surrounded by a thick shell of cold ionized gas and dust. This local cavity has most likely been created by successive supernova explosions that occurred in the past 10 to 15 Myr and which has distorted the local magnetic field. While the magnetized shell of the Local Bubble has the potential to leave its imprint on the GMF observables, it has mostly been overlooked in most efforts to model the large-scale GMF or has been overly simplified.
Our goal, therefore, has been to improve on this aspect to update the models for the large-scale GMF and explore the implied impacts to study the propagation of the UHECR through our Galaxy.
As part of our project we have developed a new, divergence-free model for the magnetic field in the shell of Galactic bubbles. This model is new analytical model had never been presented in the literature and can be used to model the magnetic field in any bubble that results from a supernova explosion. We expect this model to be useful to the communities studying supernova remnants.
Following our goal, we applied our model to the case of the Local Bubble. For this, we derived a realistic geometry for the Local Bubble shell from a three-dimensional map of the dust density. This allowed us to model out the magnetic field in the Local Bubble shell and estimate its possible contribution to the observables that are used to model the large-scale GMF. We showed that this contribution may constitute a substantial fraction of the observed signal and that it depends strongly on the shell geometry, the location of the explosion center within the bubble, and on the initial directions of the magnetic field, prior to explosion. We then included our model as an additional component into the large-scale GMF models and re-tuned them through a fit to the data.
We showed that the large-scale GMF components from the best fit may change by adding the Local Bubble shell and that they also depend on the details used to model its magnetic field.
We also showed that the changes induced to the large-scale GMF imply changes in the predictions for the backtracked UHECR directions, even at a very high rigidity of 20 EV (exavolt).
We concluded that the Local Bubble shell acts as a foreground that needs to be taken into account to model the large-scale GMF from Faraday rotation measures and synchrotron polarization and that accounting for this foreground may impact searches for UHECR sources. To move forward in this research area, the magnetic field in the shell of the Local Bubble needs to be characterized at best so as to reduce uncertainties in UHECR deflections.
Beyond the science case of UHECRs, this project may be relevant to all scientific topics that are related to the Galactic magnetic field in a way or another.
In particular, as we confirmed that the shell of the Local Bubble contributes significantly not only to the polarized dust emission, but also to the polarized synchrotron emission observed at high Galactic latitudes, our work should also have an impact on studies aimed at characterizing the Galactic foregrounds of the cosmic microwave background (CMB) polarization. Therefore, we expect our work to also be relevant to the search for CMB B-modes, the direct evidence of primordial inflation that remains to be found, one of the most actively pursued topics in cosmology.
Following our goal, we applied our model to the case of the Local Bubble. For this, we derived a realistic geometry for the Local Bubble shell from a three-dimensional map of the dust density. This allowed us to model out the magnetic field in the Local Bubble shell and estimate its possible contribution to the observables that are used to model the large-scale GMF. We showed that this contribution may constitute a substantial fraction of the observed signal and that it depends strongly on the shell geometry, the location of the explosion center within the bubble, and on the initial directions of the magnetic field, prior to explosion. We then included our model as an additional component into the large-scale GMF models and re-tuned them through a fit to the data.
We showed that the large-scale GMF components from the best fit may change by adding the Local Bubble shell and that they also depend on the details used to model its magnetic field.
We also showed that the changes induced to the large-scale GMF imply changes in the predictions for the backtracked UHECR directions, even at a very high rigidity of 20 EV (exavolt).
We concluded that the Local Bubble shell acts as a foreground that needs to be taken into account to model the large-scale GMF from Faraday rotation measures and synchrotron polarization and that accounting for this foreground may impact searches for UHECR sources. To move forward in this research area, the magnetic field in the shell of the Local Bubble needs to be characterized at best so as to reduce uncertainties in UHECR deflections.
Beyond the science case of UHECRs, this project may be relevant to all scientific topics that are related to the Galactic magnetic field in a way or another.
In particular, as we confirmed that the shell of the Local Bubble contributes significantly not only to the polarized dust emission, but also to the polarized synchrotron emission observed at high Galactic latitudes, our work should also have an impact on studies aimed at characterizing the Galactic foregrounds of the cosmic microwave background (CMB) polarization. Therefore, we expect our work to also be relevant to the search for CMB B-modes, the direct evidence of primordial inflation that remains to be found, one of the most actively pursued topics in cosmology.