arXiv subjects

Chao-Ming Li

Publications and source records attributed to Chao-Ming Li.

7 recordsLinked to original sources

arXiv2026

Galactic Cosmic Ray Transport in the Giant Circumgalactic Medium Halo

Recent observations have revealed that the Milky Way is embedded in a massive circumgalactic medium (CGM) extending to several hundred kiloparsecs. Such an extended gaseous halo acts as both a reservoir of baryons and potentially as a confinement volume for Galactic cosmic rays (CRs). We investigate CR transport in this giant Galactic halo and compare its properties with those of conventional small-halo models. In the giant-halo scenario, the halo height is no longer a free parameter, but instead relates to the extent of the source region. We show that CR transport within the source region remains similar to that in small-halo models, while substantial differences emerge at larger distances. In the giant-halo scenario, CRs develop an extended approximately 1/r spatial tail and exhibit a broader age distribution than the small-halo case. This model is shown to be consistent with current secondary-to-primary CR measurements. We further find that uncertainties associated with Galactic gas distributions are comparable to those arising from nuclear spallation cross sections. These results suggest that the giant-halo model provides a physically motivated alternative to conventional small-halo models and may have important implications for diffuse gamma-ray and neutrino emission from the Galactic environment.

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arXiv2026

Anisotropic Diffusion in Pulsar Halos: Interpreting the asymmetric morphology of Geminga and Monogem halos measured by HAWC

Pulsar halos are produced by electrons and positrons diffusing in the interstellar medium around their parent pulsar wind nebulae. Recent observations by HAWC and LHAASO have revealed asymmetric morphologies in the halos surrounding Geminga and Monogem. The anisotropic diffusion model provides a natural explanation for such asymmetries, where the morphology is determined by the viewing angle of the mean magnetic field, the Alfvénic Mach number ($M_{\rm A}$), and the pulsar distance. In this work, we model the measured morphologies based on this framework and constrain the properties of interstellar magnetic turbulence. We find that the mean magnetic field orientations within the two halos are different, implying that they reside in different magnetic coherence regions, whereas the Alfvénic Mach numbers are relatively close ($M_{\rm A}\sim 0.2$). The results suggest a local magnetic field coherence length of approximately 100pc. Our study demonstrates that the morphology of pulsar halos serves as a powerful diagnostic tool for the properties of interstellar magnetic fields, highlighting the need for more accurate morphological measurements and sophisticated diffusion modeling in future studies.

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arXiv2026

Diagnosing Interstellar Magnetic Turbulence with TeV Pulsar Halos

Interstellar magnetic field is essential in various astrophysical phenomena and processes. Pulsar halos are a recently discovered class of TeV gamma-ray sources formed by escaping electrons/positrons from pulsars. The morphology of the halo is regulated by the diffusion of those escaping particles, and hence carries information of the interstellar magnetic field. We suggest that the morphology of TeV pulsar halos can be used as a novel probe of the properties of interstellar magnetic field around the pulsar, such as the Alfvénic Mach number and the mean direction. We establish a theoretical relation between these quantities and the observational features of the halo's morphology based on the anisotropic diffusion model, and show how X-ray observations of the pulsar halos can further improve the diagnosis of the magnetic field.

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arXiv2024

Insights from LHAASO and IceCube into the origin of the Galactic diffuse TeV--PeV emission

The high-energy diffuse gamma-ray emission and neutrino emission are expected from the Galactic plane, generated by hadronuclear interactions between cosmic rays (CR) and interstellar medium (ISM). Therefore, measurements of these diffuse emissions will provide important clues on the origin and nature of Galactic CRs. Comparing the latest observations of LHAASO and IceCube on the diffuse Galactic gamma-ray and neutrino emissions respectively, we suggest that the diffuse gamma-ray emission at multi-TeV energies contains a considerable contribution of a leptonic component. By modelling the gamma-ray halos powered by middle-aged pulsars in our Galaxy with taking into account the magnetic field configuration and the interstellar radiation field in the Galaxy, we demonstrate that the collective contribution of pulsar halos can account for the excess in the measured diffuse gamma-ray emission with respect to the predicted flux from CR-ISM interactions. The resulting one-dimensional profile along the Galactic longitude is also consistent with the observation.

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arXiv2024

Diagnosing the particle transport mechanism in the pulsar halo via X-ray observations

Pulsar halos (also termed 'TeV halo') are a new class of $γ$-ray sources in Galaxy, which manifest as extended $γ$-ray emission around middle-age pulsars, as discovered around the Geminga pulsar, the Monogem pulsar and PSR~J0622+3749 by HAWC and LHAASO. A consensus has been reached that the TeV emission comes from the inverse Compton scattering of escaping electrons/positrons from the PWN off soft background radiation field, while the particle transport mechanism in the halo is still in dispute. Currently, there are mainly three interpretations, namely, the isotropic, suppressed diffusion model; the isotropic, unsuppressed diffusion model with considering ballistic propagation of newly injected particles; the anisotropic diffusion model. While the predicted gamma-ray surface brightness profiles by all three models can be more or less consistent with the observation, the implication of the three models for cosmic-ray transport mechanisms and the properties of interstellar magnetic field are quite different. In this study, we calculate the anticipated X-ray emission of pulsar halos under the three models. We show that the synchrotron radiation of these escaping electrons can produce a corresponding X-ray halo around the pulsar, and the expected surface brightness profiles are distinct in three models. We suggest that sensitive X-ray detectors of a large field of view (such as eROSITA and Einstein Probe) with a reasonably long exposure time are crucial to understand the formation mechanism of pulsar halos and serve as a probe to the properties of the interstellar turbulence.

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arXiv2023

Revisiting the Chandra Observation on the Region of PSR J1809-1917: Indication of an X-ray Halo and Implication for the Origin of HESS J1809-193

HESS J1809-193 is an extended TeV $γ$-ray source and the origin of its $γ$-ray emission remains ambiguous. Pulsar wind nebula (PWN) of PSR J1809-1917 laying inside the extended $γ$-ray emission is a possible candidate. Powered by the central pulsar, ultrarelativistic electrons in PWN can produce radio to X-ray emission through synchrotron and $γ$-ray emission by inverse Compton (IC) scattering. To check whether this PWN is the counterpart of HESS J1809-193, we analyzed Chandra X-ray radial intensity profile and the spectral index profile of this PWN. We then adopt a one-zone isotropic diffusion model to fit the keV and the TeV data. We find diffuse nonthermal X-ray emission extending beyond PWN, which is likely an X-ray halo radiated by escaping electron/positron pairs from the PWN. A relatively strong magnetic field of $\sim 20\,μ$G is required to explain the spatial evolution of the X-ray spectrum (i.e., the significant softening of the spectrum with increasing distance from the pulsar), which, however, would suppress the IC radiation of pairs. Our result implies that a hadronic component may be needed to explain HESS J1809-193.

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arXiv2022

A PeVatron Candidate: Modelling the Boomerang Nebula in X-ray Band

Pulsar wind nebula (PWN) Boomerang and the associated supernova remnant (SNR) G106.3+2.7 are among candidates for the ultra-high-energy (UHE) gamma-ray counterparts published by LHAASO. Although the centroid of the extended source, LHAASO J2226+6057, deviates from the pulsar's position by about $0.3^\circ$, the source partially covers the PWN. Therefore, we cannot totally exclude the possibility that a part of the UHE emission comes from the PWN. Previous studies mainly focus on whether the SNR is a PeVatron, while neglecting the energetic PWN. Here, we explore the possibility of the Boomerang Nebula being a PeVatron candidate by studying its X-ray radiation. By modelling the diffusion of relativistic electrons injected in the PWN, we fit the radial profiles of the X-ray surface brightness and the photon index. The solution with a magnetic field $B=140μ$G can well reproduce the observed profiles and implies a severe suppression of IC scattering of electrons. Therefore, a proton component need be introduced to account for the UHE emission, in light of recent LHAASO's measurement on Crab Nebula, if future observations reveal part of the UHE emission originating from the PWN. In this sense, Boomerang Nebula would be a hadronic PeVatron.

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