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Latest articles for The Astrophysical Journal

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  • Characterizing the Nature of Periodic Amplitude Modulation in Pulsars
    In recent years, periodic amplitude modulation has emerged as a unique emission feature in the single-pulse sequence of pulsars alongside periodic nulling and subpulse drifting. Despite ample evidence for the uniqueness of this phenomenon, the periodic modulation in several pulsars are often confused with subpulse drifting, primarily due to lack of clear characterization of the emission features from a representative sample of pulsars. In this work, we present a detailed analysis of the single-pulse behavior from 17 pulsars exhibiting periodic amplitude modulation, 6 of them being new detections. The pulsar switches between different intensity states as a result of periodic amplitude modulation, and we propose a novel statistical scheme to identify these emission states. The periodic modulation can be divided into three broad categories: phase stationary modulation, modulations with phase shift, and intermittent periodic modulations. The phase stationary behavior is seen when the emission intensity across a major part of the pulse window changes periodically. The phase shifts are associated with intensity changes at specific locations within the emission window in a periodic manner, while in some pulsars, the periodic modulations become more prominent only at specific intervals resulting in intermittent behavior.

  • Long-period Variable Stars in NGC 147 and NGC 185. II. Their Dust Production
    This study presents a comparative analysis of mass-loss and dust production rates in the dwarf galaxies NGC 147 and NGC 185, focusing on long-period variables (LPVs) and pulsating asymptotic giant branch stars as primary indicators of dust feedback into the interstellar medium. For NGC 147, the total mass-loss rate is calculated as (9.44 ± 3.78) × 10−4M⊙ yr−1, with LPV luminosities ranging from (6.20 ± 0.25) × 102L⊙ to (7.87 ± 0.32) × 103 L⊙. In NGC 185, the total mass-loss rate is higher, at (1.58 ± 0.63) × 10−3M⊙ yr−1, with LPV luminosities spanning (5.68 ± 0.23) × 102L⊙ to (1.54 ± 0.66) × 104L⊙. A positive correlation is observed between stellar luminosity, intrinsic reddening due to circumstellar dust self-extinction, and elevated mass-loss rates. Additionally, comparisons of calculated dust injection rates, two-dimensional dust distribution maps, and observed dust masses provide evidence for a gravitational interaction between NGC 147 and the Andromeda galaxy, which influences the dust distribution within the system.

  • The Near Infrared Spectral Energy Distribution of Young Star Clusters in the FEAST Galaxies: Missing Ingredients at 1–5 μm
    We present a combined Hubble Space Telescope (HST) and JWST 0.2–to–5 μm analysis of the spectral energy distributions (SED) of emerging young star clusters (eYSCs) in four nearby galaxies from the Feedback in Emerging Extrgalactic Star Clusters survey: M51, M83, NGC 628, and NGC 4449. These clusters, selected for their bright Paα and 3.3 μm polycyclic aromatic hydrocarbon emission, are still associated with their natal gas cloud and have been largely missed in previous HST optical campaigns. We modeled their SEDs using the CIGALE fitting code and identified (i) a systematic flux excess at 1.5–2.5 μm that is not accounted for by current stellar population models and (ii) the preference for a set of dust model parameters that is not aligned with expectations from self-consistent analyses of star-forming regions, suggesting model shortcomings also in the 3–5 μm. The near-infrared excess is most prominent in low-mass (≤3000 M⊙) and young (≤6 Myr) clusters. Additionally, we see that the SED fitting analysis wrongly assigns ages ≥6 Myr to a fraction of strong Paα emitters with equivalent widths suggestive of significantly younger ages. A parallel analysis with the slug code suggests that stochastic initial mass function (IMF) sampling of pre-main-sequence stars combined with extinction might partially reduce the gap. We conclude that the inclusion of young stellar object SEDs, along with more realistic sampling of the cluster IMF, might be needed to fully account for the stellar population and dust properties of eYSCs.

  • New Metrics for Identifying Variables and Transients in Large Astronomical Surveys
    A key science goal of large sky surveys such as those conducted by the Vera C. Rubin Observatory and precursors to the Square Kilometre Array is the identification of variable and transient objects. One approach is analyzing time series of the changing brightness of sources, namely, light curves. However, finding adequate statistical representations of light curves is challenging because of the sparsity of observations, irregular sampling, and nuisance factors inherent in astronomical data collection. The wide diversity of objects that a large-scale survey will observe also means that making parametric assumptions about the shape of light curves is problematic. We present a Gaussian process (GP) regression approach for characterizing light-curve variability that addresses these challenges. Our approach makes no assumptions about the shape of a light curve and, therefore, is general enough to detect a range of variable and transient source types. In particular, we propose using the joint distribution of GP amplitude hyperparameters to distinguish variable and transient candidates from nominally stable ones and apply this approach to 6394 radio light curves from the ThunderKAT survey. We compare our results with two variability metrics commonly used in radio astronomy, namely ην and Vν, and show that our approach has better discriminatory power and interpretability. Finally, we conduct a rudimentary search for transient sources in the ThunderKAT data set to demonstrate how our approach might be used as an initial screening tool. Computational notebooks in Python and R are available to help deploy this framework to other surveys.

  • Simulations of Galactic Outflows Driven by Active Galactic Nuclei and Starbursts
    Using the hydro code Athena++, we run a series of large-scale hydrodynamic simulations of galactic outflows on a scale of ∼5 kpc scale over a time period of 5 Myr. We compare the kinematics, composition, and simulated X-ray emission of starburst (SB) versus active galactic nuclei (AGN) dominated galactic outflows. Our set of simulations includes galactic outflows driven solely by a nuclear SB, an AGN wind, and a mixture of SBs and AGN winds. We find significant differences in the kinematics, composition, and simulated X-ray emission between SB- versus AGN-dominated outflows. Starburst-driven outflows are characterized by turbulent, multiphase, complex flows, with each phase having a distinct kinematic profile (as traced by various oxygen ions), while AGN-driven outflows are more symmetric, more single phase, with primarily very hot (>107 K) gas, and are smooth in appearance. In mixed SB-AGN simulations, the dominant component (SB or AGN) determines the outflow characteristics. In cases where the energy input of the SB is similar to the energy of the AGN, the resulting outflow appears similar to the SB-only outflow, but when the energy input from the AGN wind is much greater than the SB then the outflow appears similar to the AGN-only outflow. When we generate synthetic X-ray observations, the AGN-dominated outflows are essentially invisible in soft to medium-energy X-rays (0.1–10.0 keV), but the SB-driven outflows show complex filamentary structures that are visible in the X-ray. Finally, we show the observational possibilities for various proposed and future X-ray telescopes.