Title: Mirages and Large TeV Halo-Pulsar Offsets from Cosmic Ray Propagation
Authors: Yiwei Bao, Gwenael Giacinti, Ruo-Yu Liu, Hai-Ming Zhang, Yang Chen
First Author’s Institution: Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 201210, China
Status: Published in PRL (open access on Arxiv)
Cosmic rays (CRs) are high-energy protons and electrons that zip through space at nearly the speed of light. These particles begin their lives by being accelerated in extreme astrophysical environments, such as shock waves from supernova explosions or strong electromagnetic fields around pulsars. After they are launched into space, CRs travel through the interstellar medium following magnetic field lines, gradually losing energy. Because CR electrons lose energy much faster than CR protons, astronomers often use emissions from CR electrons to pinpoint where in space these particles were originally accelerated.
During their journey, CR electrons can produce energetic gamma rays through a process called inverse-Compton scattering, where they transfer energy to low-energy photons and boost them to TeV (10¹² electron volt, eV) level. A straightforward expectation is that the locations of these bright TeV gamma-ray sources should line up with the pulsars that accelerated the CRs in the first place. However, recent observations indicate that some TeV sources are offset from known pulsars. Why would glowing halos of high-energy radiation appear far from the object that produced them?
In today’s paper, the authors propose a new explanation called “mirage halos.” The idea rests on one key physical fact—cosmic rays are charged particles, so they can only move parallel to magnetic field lines. If those magnetic field lines happen to be aligned with our line of sight, projection effects kick in. CR electrons moving along a narrow magnetic line can pile up along the same location in the sky when projected onto a two-dimensional image. The result is a bright gamma-ray spot that looks like a real CR source, even when no pulsar is actually there.
To test this idea, the authors run simulations that follow the motion of test CR electrons inside a randomly tangled (or, turbulent) magnetic field at a scale of around 50 pc. Unlike traditional diffusion-based models that assume particles spread out symmetrically, these simulations resolve the fine, filamentary paths traced by electrons at small scales, which are the paths real CRs would take. Despite revealing these detailed structures, the model remains consistent with the standard diffusion picture at large scales.
Figure 1. Simulated gamma-ray emission from test CR electrons injected at the center of the image. In addition to the main halo (source 1), two bright “mirage halos” (source 2 and 3) appear a few degrees away, linked to the central source by faint, thread-like structures. These sources are not CR injection sources but are gamma-ray sources. This demonstrates that a single CR electron source can be misidentified as multiple TeV halos. [Figure 1 from original paper]
Figure 1 shows a mock gamma-ray sky from the simulation. CR electrons are injected at the center of the image. As expected, there is a bright halo around the injection source. But surprisingly, there are two additional bright spots about 2–4 degrees away from the injection source, each linked to the main halo by thin, faint structures. In other words, a single CR electron injection source can appear as a system of three halos, which is an astrophysical illusion created purely by magnetic field geometry.
How can astronomers tell whether an observed gamma-ray source is a genuine halo or just a mirage? One clue lies in the connection between the main halo and the mirage halo. CR electrons must travel between these locations, so a faint bridge-like structure should exist between the sources. However, these features may be so dim that current instruments cannot detect them without much deeper observations.
A second method is to combine gamma-ray observations with X-ray observations. The same CR electrons that produce TeV gamma rays will also emit X-ray synchrotron radiation when spiraling around magnetic field lines. But here is the trick: synchrotron emission is strongest when the magnetic field is perpendicular to our line of sight. Mirage halos form when the magnetic field is parallel to the line of sight, making synchrotron emission from those spots naturally faint. In contrast, the connecting structures, where the magnetic field bends and becomes more perpendicular, should show stronger synchrotron signals. This contrast offers a promising way to distinguish real halos from projection-induced mirages.
Taken together, this work shows that our view of the high-energy universe can be shaped as much by geometry as by physics. Mirage halos highlight how the morphology of magnetic fields, not just the CR-emitting pulsars themselves, can sculpt the gamma-ray sky and create illusions that challenge our interpretations. As future telescopes achieve better sensitivity and combine observations across wavelengths, we will be able to map these magnetic structures in far greater detail. Doing so will not only help us uncover the true origins of TeV halos but also deepen our understanding of how cosmic rays travel through the Galaxy’s turbulent, magnetized environment.
Astrobite edited by Will Golay
Featured image credit: Bao et al. (2025)
