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  • Is the Central Compact Object in HESS J1731-347 a Hybrid Star with a Quark Core? An Analysis with the Constant Speed of Sound Parameterization
    In this work, we investigate the possibility of the compact object in HESS J1731-347 with and to be a hybrid star with quark matter in the inner core. The observation of this low-mass compact star dictates the use of a softer equation of state, which, on the contrary, cannot explain the massive compact stars. This poses a new challenge for astrophysicists in their search for the equation of state of compact objects. The hybrid equations of state are constructed using the IUFSU parameterization based on the relativistic mean field theory for the hadronic part and the generic constant speed of sound (CSS) parameterization for the phase transition to quark matter. The CSS framework is characterized by three key parameters, namely the transition density (ρtr), the energy jumps (Δε), and the speed of sound (Cs). Here, our primary aim is to investigate the influence of individual CSS parameters on the formation of an object of small mass and radius compatible with HESS J1731-347 parameters. We have also examined the effect of hadronic parameters such as effective mass, symmetry energy, and the slope of the symmetry energy at saturation densities on the formation of this compact object. Finally, our analysis suggests that, within a 1σ credible level, HESS J1731-347 aligns with the scenario of a stable hybrid star with early deconfinement and higher energy gap.

  • A [C ii] 158 μm Survey of Damped Lyα Absorber Galaxies at z ∼ 4: Observations of Dense and Metal-enriched Neutral Gas within the Circumgalactic Medium of Star-forming Galaxies
    We present a survey undertaken with the Atacama Large Millimeter/submillimeter Array (ALMA) to study the galaxies associated with a representative sample of 16 damped Lyα absorbers (DLAs) at z ≈ 4.1–4.5, using the [C ii] 158 μm ([C ii]) line. We detect seven [C ii]-emitting galaxies in the fields of five DLAs, all of which have absorption metallicity [M/H] > −1.5. We find that the detectability of these H i-selected galaxies with ALMA is a strong function of DLA metallicity, with a detection rate of % for DLAs with [M/H] > −1.5 and 0+18% for DLAs with [M/H] < −1.5. The identified DLA galaxies have far-IR properties similar to those of typical star-forming galaxies at z ∼ 4, with estimated obscured star formation rates ranging from ≲6 M⊙ yr−1 to 110 M⊙ yr−1. High-metallicity DLAs therefore provide an efficient way to identify and study samples of high-redshift, star-forming galaxies, without preselecting the galaxies by their emission properties. The agreement between the velocities of the metal absorption lines of the DLA and the [C ii] emission line of the DLA galaxy indicates that the metals within the DLA originated in the galaxy. With observed impact parameters between 14 and 59 kpc, this indicates that star-forming galaxies at z ∼ 4 have a substantial reservoir of dense, cold, neutral gas within their circumgalactic medium that has been enriched with metals from the galaxy.

  • Discovery of a Pulsar Wind Nebula Candidate Associated with the Galactic PeVatron 1LHAASO J0343+5254u
    The astronomical origins of the most energetic galactic cosmic rays and gamma rays are still uncertain. X-ray follow-up of candidate “PeVatrons”—systems producing cosmic rays with energies exceeding 1 PeV—can constrain their spatial origin, identify likely counterparts, and test particle emission models. Using ∼120 ks of XMM-Newton observations, we report the discovery of a candidate pulsar wind nebula, a possible counterpart for the LHAASO PeVatron J0343+5254u. This extended source has a power-law X-ray spectrum with spectral index ΓX = 1.9—softer at greater distance from the center—and asymmetric spatial extension out to . We conduct leptonic modeling of the X-ray and gamma-ray radiation from this complex system, showing that a fully leptonic model with elevated IR photon fields can explain the multiwavelength emission from this source, similar to other very high-energy pulsar wind nebulas; excess gamma-ray emissivity not explained by a leptonic model may be due to hadronic interactions in nearby molecular cloud regions, which might also produce detectable astroparticle flux.

  • A Tale of Two Black Holes: Multiband Gravitational-wave Measurement of Recoil Kicks
    The nonlinear dynamics of General Relativity leave their imprint on remnants of black hole mergers in the form of a recoil “kick.” The kick has profound astrophysical implications across the black hole mass spectrum, from stellar-mass to supermassive black holes. However, a direct measurement of the kick would require observing the binary from different orientations. Therefore, the kick is primarily inferred by constraining the premerger parameters, mainly the spin orientations and mass ratio of the black holes and fitting them with postmerger parameters. In this study, we demonstrate the prospects for such measurements of black hole kicks in a multiband gravitational-wave network consisting of space mission LISA, the current Earth-based detector network, and a third-generation detector. For two distinct cases of remnant black hole kick (68, 1006 km s−1) emerging from near identical premerger configuration of GW190521—the first confirmed intermediate-mass black hole—we find that the multiband network will recover with a 90% credible level of the projection of the kick vector relative to the orbital plane within tens of km s−1 accuracy. Such precise measurement of the kick offers a new set of multimessenger follow-ups and unprecedented tests of astrophysical formation channels.

  • AT 2020nov: Evidence for Disk Reprocessing in a Rare Tidal Disruption Event
    We present a detailed analysis of AT 2020nov, a tidal disruption event (TDE) in the center of its host galaxy, located at a redshift of z = 0.083. AT 2020nov exhibits unique features, including double-peaked Balmer emission lines, a broad UV/optical flare, and a peak log luminosity in the extreme-ultraviolet (EUV) estimated at . A late-time X-ray flare was also observed, reaching an absorbed luminosity of 1.67 × 1043 erg s−1 approximately 300 days after the UV/optical peak. Multiwavelength coverage, spanning optical, UV, X-ray, and mid-infrared (MIR) bands, reveals a complex spectral energy distribution (SED) that includes MIR flaring indicative of dust echoes, suggesting a dust covering fraction consistent with typical TDEs. Spectral modeling indicates the presence of an extended, quiescent disk around the central supermassive black hole with a radius of . The multicomponent SED model, which includes a significant EUV component, suggests that the primary emission from the TDE is reprocessed by this extended disk, producing the observed optical and MIR features. The lack of strong active galactic nuclei signatures in the host galaxy, combined with the quiescent disk structure, highlights AT 2020nov as a rare example of a TDE occurring in a galaxy with a dormant but extended preexisting accretion structure.