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1. Study of Long-term Spectral Evolution and X-Ray and γ -Ray Correlation of Blazars Seen by HAWC

   https://doi.org/10.3847/1538-4357/ad9d3d  

   Alfaro, R; Alvarez, C; Andrés, A; Arteaga-Velázquez, JC; Avila_Rojas, D; Ayala_Solares, HA; Babu, R; Belmont-Moreno, E; Bernal, A; Caballero-Mora, KS; et al (February 2025, The Astrophysical Journal)

   Abstract The HAWC Observatory collected 6 yr of extensive data, providing an ideal platform for long-term monitoring of blazars in the very high energy (VHE) band, without bias toward specific flux states. HAWC continuously monitors blazar activity at TeV energies, focusing on sources with a redshift ofz≤ 0.3, based on the Third Fermi-LAT Catalog of High-Energy sources. We specifically focused our analysis on Mrk 421 and Mrk 501, as they are the brightest blazars observed by the HAWC Observatory. With a data set of 2143 days, this work significantly extends the monitoring previously published, which was based on 511 days of observation. By utilizing HAWC data for the VHEγ-ray emission in the 300 GeV–100 TeV energy range, in conjunction with Swift-XRT data for the 0.3–10 keV X-ray emission, we aim to explore potential correlations between these two bands. For Mrk 501, we found evidence of a long-term correlation. Additionally, we identified a period in the light curve where the flux was very low for more than 2 yr. On the other hand, our analysis of Mrk 421 measured a strong linear correlation for quasi-simultaneous observations collected by HAWC and Swift-XRT. This result is consistent with a linear dependence and a multiple-zone synchrotron self-Compton model to explain the X-ray andγ-ray emission. Finally, as suggested by previous findings, we confirm a harder-when-brighter behavior in the spectral evolution of the flux properties for Mrk 421. These findings contribute to the understanding of blazar emissions and their underlying mechanisms. 

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2. Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC

   https://doi.org/10.3847/2041-8213/ad772e  

   Albert, A; Alfaro, R; Alvarez, C; Andrés, A; Arteaga-Velázquez, J C; Avila_Rojas, D; Ayala_Solares, H A; Babu, R; Belmont-Moreno, E; Bernal, A; et al (September 2024, The Astrophysical Journal Letters)

   Abstract We report an observation of ultrahigh-energy (UHE) gamma rays from the Galactic center (GC) region, using 7 yr of data collected by the High-Altitude Water Cherenkov (HAWC) Observatory. The HAWC data are best described as a point-like source (HAWC J1746-2856) with a power-law spectrum ( $\mathit{d}N/\mathit{d}E=\varphi {\left(E/26\mathrm{TeV}\right)}^{\gamma }$), whereγ= −2.88 ± 0.15_stat− 0.1_sysandϕ= 1.5 × 10⁻¹⁵(TeV cm²s)⁻¹ $\pm {0.3}_{\mathrm{stat}}{}_{-{0.13}_{\mathrm{sys}}}^{+{0.08}_{\mathrm{sys}}}$extending from 6 to 114 TeV. We find no evidence of a spectral cutoff up to 100 TeV using HAWC data. Two known point-like gamma-ray sources are spatially coincident with the HAWC gamma-ray excess: Sgr A* (HESS J1745-290) and the Arc (HESS J1746-285). We subtract the known flux contribution of these point sources from the measured flux of HAWC J1746-2856 to exclude their contamination and show that the excess observed by HAWC remains significant (>5σ), with the spectrum extending to >100 TeV. Our result supports that these detected UHE gamma rays can originate via hadronic interaction of PeV cosmic-ray protons with the dense ambient gas and confirms the presence of a proton PeVatron at the GC. 

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3. Longtime Monitoring of TeV Radio Galaxies with HAWC

   https://doi.org/10.3847/1538-4357/ae1009  

   Alfaro, R; Alvarez, C; Anita-Rangel, E; Arteaga-Velázquez, J C; Avila_Rojas, D; Ayala_Solares, H A; Babu, R; Bangale, P; Belmont-Moreno, E; Bernal, A; et al (November 2025, The Astrophysical Journal)

   Abstract We present the monitoring of the TeV-emitting radio galaxies M87, NGC 1275, 3C 264, and IC 310 with the High-Altitude Water Cherenkov Observatory (HAWC) over a period of approximately 7.5 yr. The analysis includes light curves at daily, weekly, and monthly timescales for the four sources. We report the detection of gamma-ray emission from M87 with a significance exceeding 5σ, providing the integrated TeV spectrum from the longest temporal coverage to date. The source is well described as a point-like source modeled by a power-law spectrum with spectral index Γ = 2.53 ± 0.29 and a flux of (7.09 ± 1.24) × 10⁻¹³cm⁻²s⁻¹TeV⁻¹at 1 TeV. The maximum energy of the detected emission in M87, at 1σconfidence level, reaches 26.5 TeV. HAWC’s observation of M87 reveals a low flux spectrum for the longest observation to date of this radio galaxy. 3C 264 is marginally detected with a significance slightly below 4σ, while NGC 1275 and IC 310 are not detected. The weekly light curves show an increased number of fluxes above 2σfor M87 starting in 2019 and for 3C 264 starting in 2018, which can be interpreted as the moments at which these sources start to exhibit an enhanced steady TeV emission. Cumulative significance analysis reveals quantitative evidence for long-term variability. M87 shows enhanced emission from 2019, while 3C 264 exhibits increased activity from 2018, resembling variable sources like Markarian 421 rather than steady sources like the Crab. This supports the importance of monitoring radio galaxies to identify periods of higher activity and flares, enabling further multimessenger studies. 

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4. HAWC, VERITAS, Fermi-LAT, and XMM-Newton Follow-up Observations of the Unidentified Ultra-high-energy Gamma-Ray Source LHAASO J2108+5157

   https://doi.org/10.3847/1538-4357/adf439  

   Adams, C B; Bangale, P; Benbow, W; Buckley, J H; Chen, Y; Christiansen, J L; Chromey, A J; Escobar_Godoy, M; Feldman, S; Feng, Q; et al (September 2025, The Astrophysical Journal)

   Abstract We report observations of the ultra-high-energy gamma-ray source LHAASO J2108+5157, utilizing VERITAS, HAWC, Fermi-LAT, and XMM-Newton. VERITAS has collected ∼40 hr of data that we used to set ULs to the emission above 200 GeV. The HAWC data, collected over ∼2400 days, reveal emission between 3 and 146 TeV, with a significance of 7.5σ, favoring an extended source model. The best-fit spectrum measured by HAWC is characterized by a simple power law with a spectral index of 2.45 ± 0.11_stat. Fermi-LAT analysis finds a point source with a very soft spectrum in the LHAASO J2108+5157 region, consistent with the 4FGL-DR3 catalog results. The XMM-Newton analysis yields a null detection of the source in the 2–7 keV band. The broadband spectrum can be interpreted as a pulsar and a pulsar wind nebula system, where the GeV gamma-ray emission originates from an unidentified pulsar, and the X-ray and TeV emissions are attributed to synchrotron radiation and inverse Compton scattering of electrons accelerated within a pulsar wind nebula. In this leptonic scenario, our X-ray upper limit provides a stringent constraint on the magnetic field, which is ≲1.5μG. 

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5. Modeling Blazar Broadband Emission with a Convolutional Neural Network. I. Synchrotron Self-Compton Model

   https://doi.org/10.3847/1538-4357/ad19cf  

   Bégué, D; Sahakyan, N; Dereli-Bégué, H; Giommi, P; Gasparyan, S; Khachatryan, M; Casotto, A; Pe’er, A (February 2024, The Astrophysical Journal)

   Abstract Modeling the multiwavelength spectral energy distributions (SEDs) of blazars provides key insights into the underlying physical processes responsible for the emission. While SED modeling with self-consistent models is computationally demanding, it is essential for a comprehensive understanding of these astrophysical objects. We introduce a novel, efficient method for modeling the SEDs of blazars by the mean of a convolutional neural network (CNN). In this paper, we trained the CNN on a leptonic model that incorporates synchrotron and inverse Compton emissions, as well as self-consistent electron cooling and pair creation–annihilation processes. The CNN is capable of reproducing the radiative signatures of blazars with high accuracy. This approach significantly reduces the computational time, thereby enabling real-time fitting to multiwavelength data sets. As a demonstration, we used the trained CNN withMultiNestto fit the broadband SEDs of Mrk 421 and 1ES 1959+650, successfully obtaining their parameter posterior distributions. This novel framework for fitting the SEDs of blazars will be further extended to incorporate more sophisticated models based on external Compton and hadronic scenarios, allowing for multimessenger constraints in the analysis. The models will be made publicly available via a web interface at the Markarian Multiwavelength Data Center to facilitate self-consistent modeling of multimessenger data from blazar observations. 

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