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1. Analysis of the Emission and Morphology of the Pulsar Wind Nebula Candidate HAWC J2031+415

   https://doi.org/10.3847/1538-4357/ad782a  

   Alfaro, R; Alvarez, C; Arteaga-Velázquez, J C; Avila_Rojas, D; Ayala_Solares, H A; Babu, R; Belmont-Moreno, E; Caballero-Mora, K S; Capistrán, T; Carramiñana, A; et al (November 2024, The Astrophysical Journal)

   Abstract The first TeVγ-ray source with no lower energy counterparts, TeV J2032+4130, was discovered by HEGRA. It appears in the third HAWC catalog as 3HWC J2031+415 and it is a bright TeVγ-ray source whose emission has previously been resolved as two sources: HAWC J2031+415 and HAWC J2030+409. While HAWC J2030+409 has since been associated with the Fermi Large Area Telescope Cygnus Cocoon, no such association for HAWC J2031+415 has yet been found. In this work, we investigate the spectrum and energy-dependent morphology of HAWC J2031+415. We associate HAWC J2031+415 with aγ-ray binary system containing the pulsar PSR J2032+4127 and its companion MT91 213. We study HAWC data to observe their periastron in 2017. Additionally, we perform a combined multiwavelength analysis using radio, X-ray, andγ-ray emission. We conclude that HAWC J2031+415 and, by extension, TeV J2032+4130 are most probably a pulsar wind nebula powered by PSR J2032+4127. 

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2. Multiwavelength Observation of a Candidate Pulsar Halo LHAASO J0621+3755 and the First X-Ray Detection of PSR J0622+3749

   https://doi.org/10.3847/1538-4357/adc92b  

   Adams, C B; Archer, A; Bangale, P; Bartkoske, J T; Benbow, W; Buckley, J H; Chen, Y; Christiansen, J L; Chromey, A J; Duerr, A; et al (May 2025, The Astrophysical Journal)

   Abstract Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755 with VERITAS and XMM-Newton in the TeV and X-ray bands, respectively. For this work, we developed a novel background estimation technique for imaging atmospheric Cherenkov telescope observations of such extended sources. No halo emission was detected with VERITAS (0.3–10 TeV) or XMM-Newton (2–7 keV) within 1^∘and $10^{\prime }$around PSR J0622+3749, respectively. Combined with the LHAASO Kilometer Square Array (KM2A) and Fermi-LAT data, VERITAS flux upper limits establish a spectral break at  ∼1–10 TeV, a unique feature compared with Geminga, the most studied pulsar halo. We model the gamma-ray spectrum and LHAASO-KM2A surface brightness as inverse Compton emission and find suppressed diffusion around the pulsar, similar to Geminga. A smaller diffusion suppression zone and harder electron injection spectrum than Geminga are necessary to reproduce the spectral cutoff. A magnetic field ≤1μG is required by our XMM-Newton observation and synchrotron spectral modeling, consistent with Geminga. Our findings support slower diffusion and lower magnetic field around pulsar halos than the Galactic averages, hinting at magnetohydrodynamic turbulence around pulsars. Additionally, we report the detection of an X-ray point source spatially coincident with PSR J0622+3749, whose periodicity is consistent with the gamma-ray spin period of 333.2 ms. The soft spectrum of this source suggests a thermal origin. 

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3. A Multiwavelength Investigation of PSR J2229+6114 and its Pulsar Wind Nebula in the Radio, X-Ray, and Gamma-Ray Bands

   https://doi.org/10.3847/1538-4357/ad0120  

   Pope, I; Mori, K; Abdelmaguid, M; Gelfand, J D; Reynolds, S P; Safi-Harb, S; Hailey, C J; An, H; Bangale, P; Batista, P; et al (December 2023, The Astrophysical Journal)

   Abstract G106.3+2.7, commonly considered to be a composite supernova remnant (SNR), is characterized by a boomerang-shaped pulsar wind nebula (PWN) and two distinct (“head” and “tail”) regions in the radio band. A discovery of very-high-energy gamma-ray emission (E_γ> 100 GeV) followed by the recent detection of ultrahigh-energy gamma-ray emission (E_γ> 100 TeV) from the tail region suggests that G106.3+2.7 is a PeVatron candidate. We present a comprehensive multiwavelength study of the Boomerang PWN (100″ around PSR J2229+6114) using archival radio and Chandra data obtained two decades ago, a new NuSTAR X-ray observation from 2020, and upper limits on gamma-ray fluxes obtained by Fermi-LAT and VERITAS observatories. The NuSTAR observation allowed us to detect a 51.67 ms spin period from the pulsar PSR J2229+6114 and the PWN emission characterized by a power-law model with Γ = 1.52 ± 0.06 up to 20 keV. Contrary to the previous radio study by Kothes et al., we prefer a much lower PWNB-field (B∼ 3μG) and larger distance (d∼ 8 kpc) based on (1) the nonvarying X-ray flux over the last two decades, (2) the energy-dependent X-ray size of the PWN resulting from synchrotron burn-off, and (3) the multiwavelength spectral energy distribution (SED) data. Our SED model suggests that the PWN is currently re-expanding after being compressed by the SNR reverse shock ∼1000 yr ago. In this case, the head region should be formed by GeV–TeV electrons injected earlier by the pulsar propagating into the low-density environment. 

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4. Constraints on Diffuse X-Ray Emission from the TeV Halo Candidate HESS J1813-126

   https://doi.org/10.3847/1538-4357/ae1740  

   Guevel, David; Page, Kim L; Mori, Kaya; Lien, Amy; Fang, Ke (November 2025, The Astrophysical Journal)

   Abstract Extended regions of very high-energyγ-ray emission associated with middle-aged pulsars have been found byγ-ray observatories. These regions, called TeV halos or pulsar halos, are thought to be created when energetic electrons from a pulsar or pulsar wind nebula transport into the interstellar medium and undergo inverse Compton scattering with the cosmic microwave background radiation and the interstellar radiation field. The same electrons are expected to emit synchrotron emission in the X-ray band in the interstellar magnetic field. HESS J1813-126 is a pulsar halo candidate from which TeVγ-ray emission with extension 0 $\stackrel{\text{°}}{.}$21 and a hardE⁻²spectrum is observed. We searched for the synchrotron component of this pulsar halo with Swift-X-Ray Telescope (XRT). In particular, we observed two fields within the region covered by HESS J1813-126 for 35 ks each and a region nearby as a background reference for 10 ks. We also analyzed 21 ks of archival Swift-XRT observations of PSR J1813-1246 and archival Swift-XRT observations of the nearby Galactic X-ray background. We find no evidence for excess X-ray emission from the two observations near HESS J1813-126 and place an upper limit differential flux of 2.18 × 10⁻³and 2.02 × 10⁻³keV⁻¹cm⁻²s⁻¹at 1 keV, assuming a power-law spectrum in Fields 1 and 2. The archival differential flux upper limit is 3.36 × 10⁻³keV⁻¹cm⁻²s⁻¹at 1 keV. The nondetection implies that the magnetic field inside the halo is not significantly enhanced compared to the average Galactic magnetic field. 

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5. Geminga’s pulsar halo: An X-ray view

   https://doi.org/10.1051/0004-6361/202450242  

   Manconi, Silvia; Woo, Jooyun; Shang, Ruo-Yu; Krivonos, Roman; Tang, Claudia; Di_Mauro, Mattia; Donato, Fiorenza; Mori, Kaya; Hailey, Charles J (September 2024, Astronomy & Astrophysics)

   Geminga is the first pulsar around which a remarkable gamma-ray halo extending over a few degrees was discovered at TeV energies by MILAGRO and HAWC and later by H.E.S.S., and byFermi-LAT in the GeV band. More middle-aged pulsars have exhibited gamma-ray halos, and they are now recognised as an emerging class of Galactic gamma-ray sources. The emission appears in the late evolution stage of pulsars, and is most plausibly explained by inverse Compton scattering of CMB and interstellar photons by relativistic electrons and positrons escaping from the pulsar wind nebulae. These observations pose a number of theoretical challenges, particularly the origin of the inferred, significantly lower effective diffusion coefficients around the pulsar when compared to typical Galactic values. Tackling these questions requires constraining the ambient magnetic field properties, which can be achieved through X-ray observations. If the gamma-ray halos originate from a distribution of highly energetic electrons, synchrotron losses in the ambient magnetic fields of the same particles are expected to produce a diffuse X-ray emission with a similar spatial extension. We present the most comprehensive X-ray study of the Geminga pulsar halo to date, utilising archival data fromXMM-NewtonandNuSTAR. Our X-ray analysis covers a broad bandwidth (0.5 − 79 keV) and large field of view (θ ∼ 4°) for the first time. This was achieved by accurately measuring the background over the entire field of view, and taking into account both focused and stray-light X-ray photons from the pulsar halo withNuSTAR. We find no significant emission and set robust constraints on the X-ray halo flux. These are translated to stringent constraints on the ambient magnetic field strength and the diffusion coefficient by using a physical model considering particle injection, diffusion, and cooling over the pulsar’s lifetime, which is tuned by fitting multi-wavelength data. Our novel methodology for modelling and searching for synchrotron X-ray halos can be applied to other pulsar halo candidates. 

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