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Physics Astrophysics

astro-ph Watch Astrophysics

Thursday 9 April

Earth and Planetary Astrophysics

Three-dimensional transport-induced chemistry on temperate sub-Neptune K2-18b, Part II: the combined effects of atmospheric dynamics and chemical reactions

Jiachen Liu, Duncan Christie, Jun Yang et al. (+1 more)

The upper atmospheres of temperate sub-Neptunes are strongly influenced by atmospheric dynamics due to their cool equilibrium temperature and thereby longer chemical timescales than the atmospheric dynamical timescales. In this study, we used a three-dimensional (3D) general circulation model to investigate the transport-induced disequilibrium chemistry and vertical mixing on temperate gas-rich mini-Neptunes, using K2-18b as an example. We model K2-18b assuming 180 times solar metallicity and consider it as either a synchronous or an asynchronous rotator, exploring spin-orbit resonances of 2:1, 6:1, and 10:1. We find that the vertical transport affects the chemical structure significantly, making CO$_2$ and CO more abundant ($\sim$10$^{-3}$) in the upper atmosphere compared to the chemical equilibrium abundance (<10$^{-15}$), and horizontal winds further homogenize the chemical composition zonally in this region. Molecular abundances in the photosphere generally agree across different rotation periods. We employ a passive tracer in the model to estimate the one-dimensional (1D) equivalent eddy-diffusion coefficient ($K_{zz}$) of K2-18b, providing a parameter useful for future 1D atmospheric models. Additionally, synthetic transmission spectra generated from our model are compared with the JWST observations, and we find that our model can provide a comparable fit to the observations. This work offers a 3D perspective on transport-induced chemistry on a temperate sub-Neptune and derives vertical mixing parameters to support 1D modelling.
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All authors Jiachen Liu, Duncan Christie, Jun Yang, Krisztian Kohary

Cosmology and Nongalactic Astrophysics

A Natural $\gtrsim 100\times$ Telescope: Discovery of the Strongly Lensed Type II SN 2025mkn at $z=1.37$

Cameron Lemon, Ariel Goobar, Joel Johansson et al. (+90 more)

We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in ZTF, 0.83$^{\prime\prime}$ from a $z=0.42$ elliptical galaxy, follow-up SNIFS/UH2.2m and LRIS/Keck spectra revealed absorption lines at $z=1.371$. Later JWST NIRCam imaging shows that the bright transient is a close pair of point sources separated by $\sim 0.07^{\prime\prime}$, and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong H$\alpha$ emission also at $z=1.371$. The light curves and spectra are consistent with the Type II supernova source being magnified $\gtrsim 100$ times, with $\sim 250$ required to reconcile its luminosity with that of nearby events such as SN 2023ixf. Lens models are consistent with such high magnifications, and always show that the faint image arrived first (undetected in earlier ZTF imaging), consistent with the later spectral phase of this fainter image. A fourth image is also predicted and possibly detected in the NIRSpec data. Light-curve-based time-delay measurements are not possible due to the first image being the faintest; however, the resolved NIRSpec spectra offer a future opportunity for time-delay cosmography through supernova phase measurements.
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All authors Cameron Lemon, Ariel Goobar, Joel Johansson, Edvard Mörtsell, Steve Schulze, Igor Andreoni, Aleksandra Bochenek, Seán J. Brennan, Malte Busmann, Michael Coughlin, Kaustav K. Das, Suhail Dhawan, Christoffer Fremling, Anjasha Gangopadhyay, Daniel Gruen, Xander J. Hall, Anna Y. Q. Ho, Mansi M. Kasliwal, Daniel A. Perley, Mickael Rigault, Genevieve Schroeder, Mathew Smith, Jesper Sollerman, Jean J. Somalwar, Robert Stein, Stephen Thorp, Alice Townsend, Jacob L. Wise, Lin Yan, Nikki Arendse, Eric C. Bellm, Tracy X. Chen, Andrew Drake, Frank J. Masci, Josiah Purdum, Roger Smith, Jason T. Hinkle, T. Emil Rivera-Thorsen, Benjamin J. Shappee, Michael A. Tucker, Jessica Aguilar, Steven Ahlen, Greg Aldering, Segev Benzvi, Davide Bianchi, David Brooks, Todd Claybaugh, Axel de la Macorra, John Della Costa, Arjun Dey, Peter Doel, Brenna Flaugher, Andreu Font-Ribera, Jaime E. Forero-Romero, Enrique Gaztañaga, Satya Gontcho A. Gontcho, Gaston Gutierrez, Dragan Huterer, Mustapha Ishak, Jorge Jimenez, Dick Joyce, Stephanie Juneau, Robert Kehoe, Alex G. Kim, David Kirkby, Theodore Kisner, Anthony Kremin, Ofer Lahav, Martin Landriau, Laurent Le Guillou, Michael E. Levi, Marc Manera, Aaron Meisner, Ramon Miquel, John Moustakas, Seshadri Nadathur, Brendan O'Connor, Nathalie Palanque-Delabrouille, Antonella Palmese, Will J. Percival, Ignasi Pérez-Ràfols, Claire Poppett, Francisco Prada, Graziano Rossi, Eusebio Sanchez, David Schlegel, Michael Schubnell, Arman Shafieloo, Joseph Silber, David Sprayberry, Gregory Tarlé, Benjamin A. Weaver, Hu Zou

High Energy Astrophysical Phenomena

Oblique Shocks at Supernova Remnants in Massive Star Clusters: A Model for the Cosmic-Ray Knee Observed by LHAASO

Luana N. Padilha and Rita C. Anjos

This work establishes oblique shocks in Massive Star Clusters (MSC) as a primary mechanism for accelerating cosmic rays (CR) up to the knee of the energy spectrum. We develop a model that incorporates the combined contribution of supernova and collective wind shocks, emphasizing the critical role of the shock obliquity angle in determining the maximum particle energy. We illustrate, within our model that oblique shocks can significantly enhance acceleration efficiency, allowing particles to reach multi-PeV energies in a rigidity-dependent manner. Our preferred model, which incorporates oblique shocks, reproduces the all-particle spectrum and composition observed by The Large High Altitude Air Shower Observatory (LHAASO), interpreting the knee as arising from a sequence of rigidity-dependent cutoffs. The model also predicts subdominant but detectable gamma-ray and neutrino emissions. This study provides an attempt at building a unified framework connecting MSC particle acceleration to the observed features of the cosmic-ray knee.
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Solar and Stellar Astrophysics

The puzzling story of flare inactive ultra fast rotating M dwarfs – III. Investigating X-ray Activity

Lauren Doyle, George W. King, Gavin Ramsay et al. (+4 more)

According to activity-rotation relations, rapid rotators are expected to show high levels of magnetic activity. However, recent studies with TESS have found Ultra Fast Rotating (UFR) M dwarfs with periods $<1$ d displaying low levels of flaring activity. There have been efforts to explore their magnetic field strengths through spectropolarimetric measurements and to assess the potential for binarity. However, neither could fully explain the lack of observed flaring activity despite their rapid rotation. Another avenue for investigation is to measure their coronal emission for signs of supersaturation: an underluminosity in X-rays observed for some rapidly rotating FGK stars. Therefore, in this study, we utilise X-ray observations from Swift and XMM-Newton of ten M dwarf UFRs with P$_{\rm{rot}}$<1 d to determine their X-ray luminosities. Overall, we do not find evidence for supersaturation amongst our UFR M dwarf stars, instead determining them to be at the saturated level, or perhaps even enhanced. Therefore, supersaturation seems not to be the main driver behind the reduced level of flaring activity observed in these stars, and the mystery behind the magnetic activity of UFR low-mass stars remains. Additionally, we provide an updated analysis on the long term variability within our sample using TESS light curves taken during Cycles 5 and 7. We identify 352 optical flares from our sample with energies between $1.2\times10^{31}$ and $8.7\times10^{34}$ erg. We determine flare rates for each TESS cycle, compare them, identifying variations across a 7 year timespan and attribute this to potential activity cycles.
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All authors Lauren Doyle, George W. King, Gavin Ramsay, Lía R. Corrales, Stefano Bagnulo, J. Gerry Doyle, Pasi Hakala

Instrumentation and Methods for Astrophysics

Scalable continuous gravitational wave detection in PTA data with non-parametric red noise suppression and optimal pulsar selection

Yi-Qian Qian, Yan Wang, Soumya D. Mohanty et al. (+1 more)

Bayesian methods for the detection of continuous gravitational waves (CGWs) in Pulsar Timing Array (PTA) data incur substantial computational costs that grow rapidly due to the number of noise and signal parameters characterizing the fitted model being proportional to the size of the PTA. This computational burden limits the scalability of these methods for large-scale PTAs comprising hundreds of pulsars anticipated from next-generation radio astronomy facilities. In this work, we introduce a computationally efficient frequentist method designed to circumvent this challenge. This is achieved by combining an adaptive spline fitting algorithm that non-parametrically suppresses red noise, thereby eliminating the need for complex noise modeling inherent to Bayesian methods, with a novel scheme for optimizing the subsets of pulsars included in the search. We quantify the performance of our method on a simulated dataset based on the NANOGrav 15-year data release and find that it achieves a performance comparable to that of Bayesian analysis: for a CGW signal with a signal-to-noise ratio of $\approx 10$, our method yields a relative characteristic strain error of 1.0% and a frequency error of 0.072% from the injected values by using the optimal pulsar selections, while the same errors are 1.7% and 0.16%, respectively, for the standard Bayesian analysis. At the same time, our analysis completes in less than 5 hours, in contrast to the 1-2 days required by Bayesian methods. This allows us to perform a rigorous study of our method using multiple data realizations and signal parameters, establishing it as an efficient and scalable tool for CGW searches with large-scale PTAs.
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All authors Yi-Qian Qian, Yan Wang, Soumya D. Mohanty, Siyuan Chen

Solar and Stellar Astrophysics

eROSITA's cool star population explained

J. H. M. M. Schmitt, P.C. Schneider, S. Czesla et al. (+2 more)

The rotation-activity connection is the standard paradigm for interpreting chromospheric and coronal activity in late-type stars, namely, stars with outer convection zones. This paradigm states that activity increases with decreasing rotation period until a saturation limit is reached. By scaling rotation periods with the convective turnover time via the Rossby number, $\text{Ro}$, saturation is expected to occur at a universal value across all spectral types. In our paper, we systematically investigate the relationship between rotation and activity as measured though X-ray emission for a large sample of late-type stars to test the universal applicability of this paradigm. To this end, we utilized TESS short-cadence space photometry to determine the rotation periods for late-type stars identified in the eROSITA all-sky survey. This combined dataset provides rotation and X-ray measurements for 14004 stars, representing a sample size increase of more than an order of magnitude compared to previous studies. We find that the convective turnover times derived from this sample closely agree with theoretical computations, supporting the idea that Rossby number-activity relations hold for all late-type main sequence stars. The lower level of activity in earlier spectral types (e.g., F-type and late A-type stars) is a physical consequence of their short convective turnover times, which prevent them from rotating rapidly enough to ever reach the saturation regime. We demonstrate that a simple model incorporating our derived turnover times versus color can successfully reproduce the observed characteristics of the eROSITA X-ray activity distribution, as measured by the L$_X$/L$_{\text{bol}}$ ratio and Gaia BP-RP color.
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All authors J. H. M. M. Schmitt, P.C. Schneider, S. Czesla, S. Freund, J. Robrade

Solar and Stellar Astrophysics

Candidate Microlensing Brown Dwarfs in Binary Lens Systems from the 2023–2025 Observing Seasons

Cheongho Han, Andrzej Udalski, Ian A. Bond et al. (+62 more)

We present detailed light-curve analyses of ten binary-lens microlensing events observed during the 2023–2025 seasons and selected as candidates for hosting brown-dwarf companions. The sample includes OGLE-2023-BLG-0249, KMT-2023-BLG-1246, OGLE-2023-BLG-0079, KMT-2024-BLG-0072, KMT-2024-BLG-0897, KMT-2024-BLG-1876, KMT-2024-BLG-2379, KMT-2025-BLG-0922, KMT-2025-BLG-1056, and KMT-2025-BLG-2427. For each event, we carry out modeling of the light curve, explore relevant degeneracies, and, when finite-source effects are present, determine the angular Einstein radius. For OGLE-2023-BLG-0249, we additionally measure the microlens parallax, which allows a direct determination of the lens masses and distance. For the remaining events, we estimate the physical lens properties via Bayesian analyses incorporating Galactic priors. The resulting posteriors show that the lens companions in all systems have median masses in the brown-dwarf regime, and the lenses of two events (KMT-2025-BLG-0922 and KMT-2025-BLG-1056) are consistent with binaries in which both lens components fall within the brown-dwarf mass range. Spanning a wide range of projected separations and distances, these detections illustrate the power of high-cadence microlensing surveys to build a census of brown-dwarf companions, including faint and distant systems beyond the reach of flux-limited methods.
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All authors Cheongho Han, Andrzej Udalski, Ian A. Bond, Chung-Uk Lee, Michael D. Albrow, Sun-Ju Chung, Andrew Gould, Youn Kil Jung, Kyu-Ha Hwang, Yoon-Hyun Ryu, Yossi Shvartzvald, In-Gu Shin, Jennifer C. Yee, Weicheng Zang, Hongjing Yang, Doeon Kim, Dong-Jin Kim, Seung-Lee Kim, Dong-Joo Lee, Sang-Mok Cha, Yongseok Lee, Byeong-Gon Park, Richard W. Pogge, Przemek Mróz, Michał K. Szymański, Jan Skowron, Radosław Poleski, Igor Soszyński, Paweł Pietrukowicz, Szymon Kozłowski, Krzysztof A. Rybicki, Patryk Iwanek, Krzysztof Ulaczyk, Marcin Wrona, Mariusz Gromadzki, Mateusz J. Mróz, Fumio Abe, David P. Bennett, Aparna Bhattacharya, Ryusei Hamada, Yuki Hirao, Asahi Idei, Stela Ishitani Silva, Shuma Makida, Shota Miyazaki, Yasushi Muraki, Tutumi Nagai, Togo Nagano, Seiya Nakayama, Mayu Nishio, Kansuke Nunota, Ryo Ogawa, Ryunosuke Oishi, Yui Okumoto, Greg Olmschenk, Clément Ranc, Nicholas J. Rattenbury, Yuki Satoh, Takahiro Sumi, Daisuke Suzuki, Takuto Tamaoki, Sean K. Terry, Paul J. Tristram, Aikaterini Vandorou, Hibiki Yama

Instrumentation and Methods for Astrophysics

First Lunar Farside SETI Observations for Periodic Signals with the Low-frequency Radio Spectrometer of Chang'E-4 Mission

Jian-Kang Li, Zhen-Zhao Tao, Bo-Lun Huang et al. (+6 more)

Chang'E-4 (CE4), the first mission to soft-land on the lunar farside, provides a unique opportunity for astronomical observations from an environment shielded from terrestrial radio interference, and thus serves as pathfinder for lunar farside radio search for extraterrestrial intelligence (SETI) studies. We present a search for periodic technosignatures using low-frequency radio observations from the CE-4 mission, the first radio SETI study based on data from on the observation in lunar farside. We analyze the CE4 dynamic spectra with a component-level framework that combines principal component analysis (PCA), cross-antenna basis alignment, as well as temporal periodicity and frequency comb structure diagnostics. No final periodic candidate signal is found after the selection procedure, and we therefore find no evidence in the present CE4 sample for a credible periodic artificial signal. This study serves as a pathfinder and provides a practical framework for lunar radio SETI analysis. As more future lunar missions begin to incorporate radio instrumentation, lunar farside may become a promising site for expanding radio SETI research.
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All authors Jian-Kang Li, Zhen-Zhao Tao, Bo-Lun Huang, Kang-Jiao, Tong-Jie Zhang, Ming-Yuan Wang, Jin-Song Ping, Dan Werthimer, Vishal Gajjar

Earth and Planetary Astrophysics

Meteor clusters: tracing meteoroid fragmentation in near-Earth space

Pavel Koten, David Čapek, Juraj Tóth et al. (+5 more)

Meteor clusters are typically defined as groups of meteors that appear close together in both space and time. To date, only a handful of such events have been recorded instrumentally and analysed in detail. In many documented cases, thermal stress has been identified as the most likely cause of meteoroid fragmentation near Earth. This paper documents two further cases and provides a summary of all currently known clusters. The two clusters that were recorded over Hawaii Island in 2023 and 2024 represent two distinct scenarios. The 2024 meteor cluster was characterised by a dominant mass body and, with the fragments arranged along the antisolar direction according to their mass. Such cases enable us to reliably determine the age of the cluster and identify the most likely formation scenario. This cluster was around three days old, and the thermal stress was the most likely mechanism of its formation. The 2023 cluster was not such a case. It does not contain a mass dominant body, nor are its fragments arranged by their mass. Therefore, it was only possible to estimate its age to be no more than four days. Furthermore, other potential formation mechanisms besides thermal stress cannot be ruled out. This fact was observed in all analysed clusters. All clusters known up to date were formed in close proximity to Earth. The volume of a cluster increases with its age. This means that older clusters, formed by the fragmentation far away from Earth may remain undetected, as their fragments are also dispersed too widely to be observed by local experiment. However, global networks can detect such dispersed clusters.
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All authors Pavel Koten, David Čapek, Juraj Tóth, Jeremie Vaubaillon, Aisha Ashimbekova, Simon Anghel, Junichi Watanabe, Tomáš Vörös

High Energy Astrophysical Phenomena

The deci-Hz gravitational wave signal from the collapse of rotating very massive stars

Bailey Sykes, Jade Powell, Bernhard Müller et al. (+1 more)

We calculate the gravitational wave signal from the collapse of a rotating 300 $M_\odot$ star at the upper end of the pair-instability regime. The large-scale asymmetries that develop during the collapse produce a strong signal in the deci-Hz range that has a characteristic shape which is likely amenable to a template-based search. The most ambitious designs for deci-Hz detectors could detect such signals out to distances of 200 Mpc, possibly at a rate of 0.5 per year.
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All authors Bailey Sykes, Jade Powell, Bernhard Müller, Alexander Heger

High Energy Astrophysical Phenomena

Deciphering the IceCube Diffuse Neutrino Observations via AGN Variability

Caijin Xie, Zijian Qiu, Yudong Cui et al. (+2 more)

The physical origin of the diffuse neutrino background and its spectral break at $\sim$ 30 TeV remain a major puzzle in multi-messenger astrophysics. In this work, we demonstrate that this spectral feature is a natural consequence of AGN activity cycles and the resulting cosmic ray (CR) propagation. We present a unified model coupling the active and quiescent phases of AGNs, where CRs accelerated in the active core undergo subsequent diffusion and hadronic interactions in the host galaxy during the quiescent phase. The superposition of these distinct evolutionary phases yields dual spectral breaks, particularly the one at tens of TeV. Under realistic energetics, our model simultaneously accounts for the IceCube diffuse flux and fits the neutrino emissions of diverse sources, ranging from the blazar TXS 0506+056 to the Seyfert galaxies NGC 7469, CGCG 420-015, and the Circinus Galaxy. Our findings reveal that temporal variability is essential for deciphering the cosmic neutrino landscape and tracking high-energy CR escape.
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All authors Caijin Xie, Zijian Qiu, Yudong Cui, Sujie Lin, Lili Yang

High Energy Astrophysical Phenomena

A study of periodic nulling in PSR B0751+32 with FAST

W. M. Yan, N. Wang, F. F. Kou et al. (+6 more)

We report new results from a nulling study of PSR~B0751+32 (PSR J0754+3231), observed at 1250~MHz with the Five hundred meter Aperture Spherical radio Telescope (FAST). Our analysis confirms the presence of periodic nulling in this pulsar. Using the recently developed mixture model method, we obtained a nulling fraction (NF) of $35.1\% \pm 0.6\%$. Three independent approaches were employed to estimate the nulling periodicity, and the results reveal significant temporal evolution of the modulation both within individual observations and across different observing essions. The pulsar exhibits an asymmetric two-component mean pulse profile, with the leading component brighter and narrower than the trailing one. Pulse energy analysis shows that both components remain stable immediately after the onset of the burst state, but subsequently undergo a progressive decline, with the trailing component most severely affected prior to burst termination. Notably, no evidence of the previously reported subpulse drifting was detected in our data. Our results challenge previous models that ascribed periodic nulling to purely geometric effects.
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All authors W. M. Yan, N. Wang, F. F. Kou, Z. Y. Liu, J. P. Yuan, Z. G. Wen S. N. Sun, M. Y. Zou, Y. R. Wen, X. J. Chen

High Energy Astrophysical Phenomena

Resolving the 2024 Outburst of Magnetar 1E 1841-045 from its host Supernova Remnant with EP-FXT

Yu-Cong Fu, Lin Lin, Yu-Jia Zheng et al. (+11 more)

The magnetar 1E 1841-045 exhibited a new active episode starting on August 20, 2024, marked by X-ray bursts and enhanced persistent emission. Using data from the Einstein Probe (EP), we report on the timing and spectral results following the onset of this outburst. The pulse profile displays a multi-peaked structure, with notable phase shifts in the secondary peak. Energy-resolved pulse profile analysis indicates a transition in the dominant peak of the pulse profile above 5.8 keV. The 0.5-10 keV X-ray spectrum is well-modeled by a combined blackbody and power-law (BB+PL) model, showing a $\sim 20\%$ flux increase following the outburst. Phase-resolved spectroscopy indicates a correlation between BB temperature and pulse profile intensity, along with spectral hardening at a specific pulse phase. The high spatial resolution of EP enables effective separation of the supernova remnant emission, which is crucial for measuring the intrinsic pulse emission of the source. These findings underscore the intricate relationship between magnetar outbursts, pulse profile evolution, and spectral characteristics.
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All authors Yu-Cong Fu, Lin Lin, Yu-Jia Zheng, Ming-Yu Ge, Han-Long Peng, Dong-Ming Li, Francesco Coti Zelati, Ersin Göǧüş, Nanda Rea, Bing Zhang, Wei-Wei Zhu, Ke-Jia Lee, Teruaki Enoto, Chryssa Kouveliotou

Earth and Planetary Astrophysics

A Search for Wide-orbit Planets Around M-dwarfs using Deep MIRI 15-micron Images

Yihan Li, Yifan Zhou, Rachel Bowens-Rubin et al. (+7 more)

Wide-orbit ($>$10 AU) gas giant planets shape the architecture of planetary systems, yet their occurrence rate remains poorly constrained. JWST has obtained the deepest mid-infrared images of nearby stars to date through substantial MIRI time-series observations of transiting planets, providing sensitive probes for wide-orbit companions. Here we leverage 15 micron observations from four programs targeting ten M-dwarf systems to search for such planets. By applying reference differential imaging for precise PSF subtraction, we achieve a 5$\sigma$ contrast of $8.9 \times 10^{-4} - 6.2 \times 10^{-3}$ (sensitivity in apparent magnitude of 14.8-15.8 mag) at a separation of 1" and $1.2 -9.1 \times 10^{-4}$ (16.5-17.9 mag) at separations $\gtrsim$3". The sensitivity is converted to planet detection probability for each system as a function of planet mass versus semimajor axis. Assuming solar metallicity and a clear atmosphere, we are sensitive to Jupiter-sized planets with an effective temperature of ${\sim}$233 K at separations beyond 20 AU in systems at 12.5 pc. Additionally, we catalog the nearby sources and estimate their possible impact on future observations assuming they are background sources. Our results demonstrate that archival MIRI time-series imaging data is a powerful window into the population of wide-orbit gas giants around M-dwarfs.
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All authors Yihan Li, Yifan Zhou, Rachel Bowens-Rubin, Mary Anne Limbach, Hannah Diamond-Lowe, Cassidy E. Walker, Kevin B. Stevenson, Andrew Vanderburg, Giovanni Strampelli, Gregory J. Herczeg

High Energy Astrophysical Phenomena

Development of Faster and More Accurate Supernova Localization at Super-Kamiokande

K. Abe, Y. Asaoka, M. Harada et al. (+273 more)

The next nearby core-collapse supernova (SN) promises to yield a treasure of scientific information through multi-messenger astronomy. Early observations of the shock breakout (SBO) emissions are especially critical to understand the SN explosive mechanism as well as the properties of the progenitor star. Neutrino observatories are able to provide an early alert of a SN before the arrival of the SBO radiation. Super-Kamiokande (SK) has the unique capability to independently reconstruct an accurate SN pointing direction as part of its real-time monitoring system, “SNWATCH.” Recent upgrades to SK by adding gadolinium (Gd) to the detection volume have been accompanied by efforts to improve the speed and accuracy of SN direction reconstruction. A new, novel HEALPix-based approach (“HP-Fitter”) can calculate the SN direction from the reconstructed burst event directions in less than one second. As well, the previous maximum-likelihood direction fitter (“ML-Fitter”) was upgraded by incorporating event information from Gd neutron-capture as well as using the HP-Fitter for the initial fit parameters and from code refactoring and optimization. The improved ML-Fitter has better angular resolution but direction reconstruction time is $\mathcal{O}$(sec). Together with improvements in burst detection and event reconstruction times, SNWATCH is now able to generate an SN alert with pointing information in about 90 seconds. These upgrades have been implemented at SK and integrated into a new automated system to provide GCN notices.
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All authors K. Abe, Y. Asaoka, M. Harada, Y. Hayato, K. Hiraide, K. Hosokawa, T. H. Hung, K. Ieki, M. Ikeda, J. Kameda, Y. Kanemura, Y. Kataoka, S. Miki, S. Mine, M. Miura, S. Moriyama, K. Nakagiri, M. Nakahata, S. Nakayama, Y. Noguchi, G. Pronost, K. Sato, H. Sekiya, K. Shimizu, R. Shinoda, M. Shiozawa, Y. Suzuki, A. Takeda, Y. Takemoto, H. Tanaka, T. Yano, S. Chen, Y. Itow, T. Kajita, R. Nishijima, K. Okumura, T. Tashiro, T. Tomiya, X. Wang, P. Fernandez, L. Labarga, D. Samudio, B. Zaldivar, C. Yanagisawa, B. Jargowsky, E. Kearns, J. Mirabito, L. Wan, T. Wester, B. W. Pointon, J. Bian, B. Cortez, N. J. Griskevich, Y. Jiang, M. B. Smy, H. W. Sobel, V. Takhistov, A. Yankelevich, J. Hill, M. C. Jang, S. H. Lee, D. H. Moon, R. G. Park, B. S. Yang, B. Bodur, K. Scholberg, C. W. Walter, A. Beauchêne, O. Drapier, A. Ershova, M. Ferey, E. Le Blévec, Th. A. Mueller, P. Paganini, C. Quach, R. Rogly, T. Nakamura, J. S. Jang, R. P. Litchfield, L. N. Machado, F. J. P. Soler, J. G. Learned, K. Choi, N. Iovine, S. Cao, L. H. V. Anthony, D. Martin, N. W. Prouse, M. Scott, Y. Uchida, V. Berardi, N. F. Calabria, M. G. Catanesi, N. Ospina, E. Radicioni, A. Langella, G. De Rosa, G. Collazuol, M. Feltre, M. Mattiazzi, L. Ludovici, M. Gonin, L. Périssé, B. Quilain, S. Horiuchi, A. Kawabata, M. Kobayashi, Y. M. Liu, Y. Maekawa, Y. Nishimura, R. Okazaki, R. Akutsu, M. Friend, T. Hasegawa, Y. Hino, T. Ishida, T. Kobayashi, M. Jakkapu, T. Matsubara, T. Nakadaira, K. Nakamura, Y. Oyama, A. Portocarrero Yrey, K. Sakashita, T. Sekiguchi, T. Tsukamoto, N. Bhuiyan, G. T. Burton, F. Di Lodovico, J. Gao, A. Goldsack, T. Katori, R. Kralik, N. Latham, J. Migenda, R. M. Ramsden, S. Zsoldos, H. Ito, T. Sone, A. T. Suzuki, Y. Takagi, Y. Takeuchi, S. Wada, H. Zhong, J. Feng, L. Feng, S. Han, J. Hikida, J. R. Hu, Z. Hu, M. Kawaue, T. Kikawa, T. Nakaya, T. V. Ngoc, R. A. Wendell, K. Yasutome, S. J. Jenkins, N. McCauley, P. Mehta, A. Tarrant, M. Fanì, M. J. Wilking, Z. Xie, Y. Fukuda, H. Menjo, Y. Yoshioka, J. Lagoda, M. Mandal, J. Zalipska, M. Mori, M. Jia, J. Jiang, W. Shi, K. Hamaguchi, H. Ishino, Y. Koshio, F. Nakanishi, S. Sakai, T. Tada, T. Tano, T. Ishizuka, G. Barr, D. Barrow, L. Cook, S. Samani, D. Wark, A. Holin, F. Nova, S. Jung, J. Y. Yang, J. Yoo, J. E. P. Fannon, L. Kneale, M. Malek, J. M. McElwee, T. Peacock, P. Stowell, M. D. Thiesse, L. F. Thompson, S. T. Wilson, H. Okazawa, S. M. Lakshmi, E. Kwon, M. W. Lee, J. W. Seo, I. Yu, Y. Ashida, A. K. Ichikawa, K. D. Nakamura, S. Tairafune, S. Abe, A. Eguchi, S. Goto, S. Kodama, Y. Kong, H. Hayasaki, Y. Masaki, Y. Mizuno, T. Muro, Y. Nakajima, N. Taniuchi, E. Watanabe, M. Yokoyama, P. de Perio, S. Fujita, C. Jesús-Valls, K. Martens, Ll. Marti, A. D. Santos, K. M. Tsui, M. R. Vagins, J. Xia, S. Izumiyama, M. Kuze, R. Matsumoto, K. Terada, R. Asaka, M. Ishitsuka, M. Shinoki, M. Sugo, M. Wako, K. Yamauchi, T. Yoshida, Y. Nakano, F. Cormier, R. Gaur, V. Gousy-Leblanc, M. Hartz, A. Konaka, X. Li, B. R. Smithers, S. Chen, Y. Wu, B. D. Xu, A. Q. Zhang, B. Zhang, H. Adhikary, M. Girgus, P. Govindaraj, M. Posiadala-Zezula, Y. S. Prabhu, S. B. Boyd, R. Edwards, D. Hadley, M. Nicholson, M. O'Flaherty, B. Richards, A. Ali, B. Jamieson, S. Amanai, C. Bronner, D. Horiguchi, A. Minamino, Y. Sasaki, R. Shibayama, R. Shimamura

Astrophysics of Galaxies

Dust Absorption towards Supernova Remnant W44

Tian-Yu Tu, Adwin Boogert, Yang Chen et al. (+1 more)

Supernova remnants (SNRs) can strongly affect the chemical composition of the interstellar dust. In this paper we investigate to what degree the dust and ices are modified by observing four stars expected to be absorbed by a giant molecular cloud interacting with SNR W44, using medium-resolution spectroscopy in 2-5 $\mu$m. Absorption from H2O ice around 3.0 $\mu$m and aliphatic hydrocarbon dust around 3.4 $\mu$m were detected towards two stars, while probable CO ice at 4.67 $\mu$m towards one of them. Millimeter gas-phase CO J = 1-0 lines and three-dimensional dust extinction maps show that the dense molecular gas associated with W44 dominates (> 60%) the total interstellar extinction (A_K ~ 2.6) along these two sightlines. The H2O ice column densities are a factor of 1.5-3 lower than nearby MCs at similar extinctions, possibly because of the destruction of ice by shocks and cosmic rays (CRs) from W44, consistent with the low CO ice abundance relative to H2O (< 12%). One of the sightlines shows an unusually strong 3.4 $\mu$m aliphatic hydrocarbon absorption. If the carriers are located in diffuse dust along the sightline, unrelated to W44, its strength is ~ 4 times larger than those typically observed for diffuse dust clouds. Alternatively, the carriers may be enhanced in the W44 environment. We discuss several possible explanations, including shock formation of aliphatic hydrocarbons in diffuse clouds associated with W44, contribution from aliphatic hydrocarbons in shocked and CR-bombarded molecular clouds, and changes in the extinction law due to the SNR interaction.
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All authors Tian-Yu Tu, Adwin Boogert, Yang Chen, Wenlang He

High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Coalescing Compact Binary Parameter Estimation with Gravitational Waves in the Presence of non-Gaussian Transient Noise

Yannick Lecoeuche, Jess McIver, Alan M. Knee et al. (+7 more)

Data from gravitational-wave (GW) detectors often contains a high rate of non-Gaussian transient noise, known as glitches. The parameters estimated from GW signals coinciding with detector glitches are occasionally biased away from their true values. During the first part of the fourth LIGO-Virgo-KAGRA (LVK) observing run, 29% of GW candidates had overlapping or nearby glitches in one or more detectors. In the latter part of the fourth observation run, sensitivity improvements have increased the rates of GW detection. Consequently, scenarios in which GW signals and detector glitches overlap in time are more likely. In this study, we quantify shifts in inferred posterior distributions for short-duration compact binary coalescence GW signals interacting with common LIGO glitches as a function of time between the signal merger time and the glitch. We find statistically significant biases in parameter estimation for mass, spin, and sky position for "blip", "thunder", and "fast-scattering" glitches. Using these results, we provide estimates of what parameters are most affected by overlapping noise sources, as well as what constitutes a "safe" time separation between a gravitational wave signal and a glitch, without requiring glitch subtraction for unbiased source property estimation. We find that in a majority of cases, all parameters are susceptible to significant bias due to glitch interference. Additionally, we find that glitches that occur within the time prior of the GW signal cause more extreme biases than glitches outside of the time prior.
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All authors Yannick Lecoeuche, Jess McIver, Alan M. Knee, Rhiannon Udall, Katie Rink, Sophie Hourihane, Simona J. Miller, Katerina Chatziioannou, TJ Massinger, Derek Davis

Wednesday 8 April

Astrophysics of Galaxies · Cross-listed from math.DS

Generalized Kolmogorov systems with applications to astrophysics and biology

Dorota Bors and Robert Stańczy

We consider generalized Kolmogorov system. We prove the existence of heteroclinic trajectory. We apply the results to astrophysical models for self-gravitating particles and predator-prey systems.

Astrophysics of Galaxies · Cross-listed from gr-qc

Exact Gravastar Solution

Farook Rahaman, Bikramarka S. Choudhury, Aritra Sanyal et al. (+2 more)

Astrophysical black holes arise as exact solutions of the Einstein field equations. Therefore, any alternative, such as a gravastar, must satisfy the same level of mathematical rigor and internal consistency. A physically viable gravastar model should not rely on approximations or ad hoc matching of regions, but instead provide a single, exact, and self-consistent solution of the Einstein field equations throughout the entire spacetime. In this work, we propose an exact solution to the Einstein field equations in the context of gravitational vacuum stars (gravastars), originally introduced by Mazur and Mottola. This framework presents an alternative end state of gravitational collapse, leading to the formation of a compact object distinct from a classical black hole. Our model is constructed by dividing the gravastar into three regions, each described by exact solutions of the Einstein field equations. We analyze the key physical properties of the resulting configuration and examine its theoretical consistency and astrophysical viability. This study provides a clear and systematic assessment of gravastars as potential alternatives to black holes.
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All authors Farook Rahaman, Bikramarka S. Choudhury, Aritra Sanyal, Anikul Islam, Bidisha Samanta

Astrophysics of Galaxies

The Fate of Globular Cluster Substructure: A Kinematic Response to Galaxy Assembly

Finn A. Pal, Sarah L. Martell and Elizabeth J. Iles

Globular clusters (GCs) are powerful tracers of galaxy assembly, frequently used to identify accreted substructure and reconstruct hierarchical merger histories. With advances in GC formation models and cosmological simulations, we can now better quantify the information about galaxy evolution encoded in present-day GCs. Here, we investigate how GC kinematics evolve over cosmic time and assess the extent to which GCs retain memory of the past of their host galaxy. Using a GC formation model applied to five Milky Way (MW) analogues from the Latte suite of the FIRE-2 simulations, we track the evolution of kinematic properties. At $z=0$, in-situ and ex-situ GCs exhibit substantial overlap in kinematic space, indicating that these populations are not clearly separable. We find that a subset of kinematic properties evolve in an ordered fashion across both in-situ and ex-situ populations, whereas others are dominated by stochastic variations. As a result, by the present day, most memory of the progenitor of an accreted GC is erased and only a few correlations persist. These correlations link progenitor halo mass to the total mass and number of a GC population, and the galactocentric distance of GC substructure to progenitor maximum circular velocity. These results highlight how both deterministic and stochastic processes driven by galaxy evolution shape GC kinematics and demonstrate the limits of reconstructing the assembly history of a galaxy from present-day GC orbits alone.
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Astrophysics of Galaxies

Too Big to Quench? I. Constraining ISM Stripping of Dwarf Satellites in Milky Way-like Halos

Jingyao Zhu, Stephanie Tonnesen, Greg L. Bryan et al. (+1 more)

Galaxy environment plays a crucial role in quenching star formation in dwarf galaxies. In Milky Way (MW)-like environments, dwarf satellite quenching is primarily driven by ram pressure stripping (RPS), the direct removal of satellite gas by the host halo gas. Using a suite of 20-pc resolution hydrodynamical wind tunnel simulations, we constrain the satellite mass scale at which the stripping of a dwarf galaxy's interstellar medium (ISM) becomes inefficient in MW-like halos. The simulations include radiative cooling in a multiphase satellite ISM, star formation, and stellar feedback, and vary both satellite masses ($M_{\star}=10^{6.2}, 10^{6.8}, 10^{7.2}\ M_{\odot}$) and host halo gas densities along a first-infall and post-pericentric orbit. We find that the degree of ISM stripping in our dwarf galaxies is consistent with the analytical prediction by McCarthy et al. (2008). Star formation is rapidly quenched when RPS is effective, but can be mildly enhanced or temporarily quenched and subsequently reignited when RPS is incomplete. ISM stripping is efficient for satellites with $M_{\star} \lesssim 10^{7}\ M_{\odot}$ (or $M_{200} \lesssim 10^{10}\ M_{\odot}$) but highly inefficient above this scale. This transitional mass ($M_{\star} \approx 10^{7}\ M_{\odot}$) is 0.5-1 dex lower than that found in observations and cosmological simulations, suggesting that additional mechanisms are needed to quench more massive satellites, such as tidal stripping of the satellite dark matter or RPS from a clumpy gaseous halo.
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All authors Jingyao Zhu, Stephanie Tonnesen, Greg L. Bryan, Mary E. Putman

Earth and Planetary Astrophysics

On the Information Content of Ariel Transmission Spectra: Reassessing the Tier System

Michael Radica, Nicolas B. Cowan, Ryan Cloutier et al. (+1 more)

The European Space Agency's Ariel mission will conduct a survey of the atmospheric properties of exoplanets around bright stars. The mission is nominally divided into three Tiers. The Tier 1 survey will consist of low-precision observations of ~1000 planets, with a subset of these included in the higher-precision Tier 2 survey expected to be necessary for atmospheric characterization. Tier 3 will be repeated observations of a small number of benchmark planets. Though previous studies have assessed the ability of Ariel to uncover population-level trends, they have generally presupposed a given Tier. Here we interrogate this assumption and assess the information content of Ariel transmission spectra as a function of Tier for three benchmark planets: a hot-Saturn, warm-Neptune, and temperate sub-Neptune. We simulate a grid of Ariel transit spectra at different Tiers for each target and use retrievals to assess which chemical species are well constrained. We find that for giant planets like a hot-Saturn or warm-Neptune, Tier 1-quality observations are sufficient for $\lesssim$1.5\,dex constraints on H$_2$O and CO$_2$, even in the presence of grey clouds — meaning important chemical insights are already obtainable in the Tier 1 survey. Moving to Tiers 2 and 3 results in an incremental increase in precision as well as other molecules becoming constrainable in certain scenarios (e.g., H$_2$S, CO). Tier 1 observations are also sufficient to identify CH$_4$ in a temperate sub-Neptune, whereas observations with at least Tier 2 precision are necessary for CO$_2$. The number of transits necessary to reach this observational S/N, however, may be prohibitive for the inclusion of temperate sub-Neptunes in even the Tier 1 survey.
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All authors Michael Radica, Nicolas B. Cowan, Ryan Cloutier, Leo Yang Wang

Astrophysics of Galaxies · Cross-listed from gr-qc

Accretion Disks in Schwarzschild-MOG and Kerr-MOG Backgrounds: MOG Parameter in terms of Observational Quantities

José Miguel Rojas and Mehrab Momennia

We apply a general relativistic framework to static and rotating black hole solutions in Scalar-Tensor-Vector Gravity or modified gravity (MOG). Our results yield exact analytic, closed-form relations expressing the mass $M$, the MOG coupling parameter $\alpha$, and the distance $D$ of a Schwarzschild-MOG black hole in terms of a minimal set of directly measurable elements of the accretion disk: the total frequency shift, the telescope aperture angle, and the redshift rapidity. The resulting expressions are derived for particles close to the midline and line of sight, where the redshift rapidity is treated as a relativistic invariant encoding the evolution of the frequency shift with respect to the emitter's proper time in MOG spacetime. We further extend the formalism to the rotating Kerr-MOG geometry and obtain corresponding relations that determine the rotation parameter $a$ jointly with $M$, $\alpha$, and $D$ on the midline. In the rotating background, we introduced the redshift acceleration (general-relativistic version of jerk) to disentangle the spacetime parameters. Crucially, the explicit appearance of $\alpha$ in these formulas enables direct empirical estimation of this parameter, thereby providing a means to test for departures from standard general relativity. The present study focuses on the geodesic motion of massive test particles revolving around Schwarzschild and Kerr-MOG black holes, and the previous results obtained in the standard Schwarzschild and Kerr backgrounds are recovered in the limit $\alpha \to 0$. The derived expressions are concise and suitable for incorporation into black hole parameter-estimation pipelines.
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High Energy Astrophysical Phenomena

Isolating Sgr A East: The First Uncontaminated X-ray Maps of a Galactic Center Supernova Remnant

Mayura Balakrishnan, Adrien Picquenot, Lia Corales et al. (+3 more)

The central few parsecs of the Milky Way host a complex X-ray-emitting environment in which several extended plasma components are blended along the line of sight, complicating attempts to measure the intrinsic properties of individual components. In particular, the supernova remnant (SNR) Sgr A East is strongly confused with the stellar wind-fed plasma associated with Sagittarius A* and the surrounding nuclear environment. Here we apply Poissonian Generalized Morphological Component Analysis (pGMCA) to deep, stacked Chandra ACIS-I observations of the Galactic Center to disentangle these overlapping X-ray components. By comparing the separated X-ray components with multiwavelength data, we identify the location of the reflected shock in Sgr A East and construct spatially resolved maps of Fe and S/Ar/Ca emission. The Fe emission is centrally concentrated, consistent with the properties of mixed-morphology supernova remnants. Separating the SNR emission from the shocked wind plasma around Sgr A* allows us to recover uncontaminated SNR properties and improve the robustness of the derived parameters. Spectral modeling of the isolated Sgr A East component reveals a lower ionization age and a higher electron density than previously reported, indicating strong interaction with dense surrounding material.
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All authors Mayura Balakrishnan, Adrien Picquenot, Lia Corales, Q. Daniel Wang, Fabio Acero, Rodolfo Montez Jr

Astrophysics of Galaxies

Multiphase Gas Structure in the Circumnuclear Region of NGC 5506 Observed with ALMA

Kana Takechi, Hiroshi Nagai, Nozomu Kawakatu et al. (+7 more)

We present a study of the multiphase gas structure and kinematics of the circumnuclear disk (CND) of NGC 5506, a nearby edge-on Seyfert galaxy, at a spatial resolution of $\sim20$ pc. Observations of [C I](1-0), CO(3-2), and HCO$^{+}$(4-3) obtained with the Atacama Large Millimeter/submillimeter Array reveal the CND dominated by rotational motion on scales of several hundred parsecs. No significant differences in geometrical thickness or velocity structure are found between [C I](1-0) and CO(3-2) across the CND, whereas HCO$^{+}$(4-3) emission is more concentrated toward the disk plane. The ratio of velocity dispersion to rotational velocity, a proxy for disk scale height-to-radius ratio, is high ($\gtrsim0.9$) in the central region ($\lesssim30$ pc) for both [C I](1-0) and CO(3-2), indicating geometrically thick structures in both tracers. Regions where the [C I](1-0)/CO(3-2) ratio exceeds the CND average are spatially correlated with the [O III]$\lambda$5007 bicone observed with the Hubble Space Telescope, suggesting that CO is preferentially dissociated by the AGN-driven biconical ionized outflow. The observed CND scale height and velocity dispersions traced by [C I](1-0) and CO(3-2) are consistent with a model in which supernova-driven turbulence provides the vertical support for the CND.
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All authors Kana Takechi, Hiroshi Nagai, Nozomu Kawakatu, Keiichi Wada, Takuma Izumi, Motoki Kino, Kouichiro Nakanishi, Naoki Isobe, Mahito Sasada, Akihiro Doi

High Energy Astrophysical Phenomena

Detection of TeV emission during early afterglow from poorly localized GRBs with ground based IACTs

S. Macera, B. Banerjee, J. Green et al. (+10 more)

Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe. While space-based instruments have extended GRB observations up to energies of $\sim$100 GeV, the detection of very-high-energy (VHE; $E>100$ GeV) emission from ground-based telescopes, especially during prompt or/and the early afterglow phase, remains challenging. These difficulties arise from the rapid temporal decay of GRB afterglows, strong attenuation by the extragalactic background light (EBL), observational latency, and the typical poor sky localization provided by MeV-detectors such as Fermi/GBM. In this work, we investigate the prospects for detecting TeV ($\sim$100 GeV–1 TeV) emission from poorly localized GRBs by adopting optimized follow-up strategies based on rapid tiling of large localization regions. We simulate a realistic population of GRBs informed by more than fifteen years of Fermi/GBM and Swift/XRT detections and recent progresses in the afterglow emission modeling. Using these simulations, we evaluate the detectability of GRB early afterglows by the next-generation Imaging Atmospheric Cherenkov Telescopes, equipped with larger field-of-view (FoV), as a function of latency, exposure time, and observational strategy. Our strategy can significantly enhance the detection rate; for instruments such as ASTRI and LACT, it increases by up to a factor of two compared to strategies limited to well-localized (Swift-like) events. For CTAO, our proposed approach provides up to four VHE detections per year.
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All authors S. Macera, B. Banerjee, J. Green, M. Seglar-Arroyo, G. Oganesyan, P. Tiwari, A. Ierardi, M. Branchesi, F. Aharonian, S. Mohnani, D. Miceli, F. Schüssler, A. Berti

Solar and Stellar Astrophysics

The ZTF-ULTRASAT experiment: Characterizing the non-transients in ULTRASAT's high cadence survey

Daniel Warshofsky, Michael W. Coughlin, Theophile Jegou Du Laz et al. (+16 more)

The forthcoming launch of the Ultraviolet Transient Astronomy Satellite (ULTRASAT) will transform our understanding of the transient ultraviolet sky by increasing our ability to identify transients due to its unprecedented 204 deg2 field of view. While rapid (extragalactic) transients are a priority science area for the mission, flaring stars and AGN can often contaminate searches for such objects. To prepare for these challenges, the Zwicky Transient Facility (ZTF)-ULTRASAT experiment observed five fields at high cadence over three nights, in close proximity to ULTRASAT's three northern high-cadence fields. A real-time filter identified seven transient candidates, of which five were persistent variable sources and two were spurious. Periods and amplitudes derived from the ZTF Source Classification Project (SCoPe) showed that three candidates were RR Lyrae stars with short periods and high amplitudes, while the remaining two displayed flaring behavior. We demonstrate that short-timescale, high-amplitude variables can systematically mimic transient alerts in high-cadence UV surveys, and we provide a concrete strategy to this contamination using pre-existing machine learning catalogs.
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All authors Daniel Warshofsky, Michael W. Coughlin, Theophile Jegou Du Laz, Anna Y. Q. Ho, S. Bradley Cenko, Andrew Drake, Jesper Sollerman, Argyro Sasli, Ben Rusholme, Frank J. Masci, Roger Smith, A.M. Krassilchtchikov, David Berge, Eran O. Ofek, Yossi Shvartzvald, Reed L. Riddle, Mansi M. Kasliwal, Matthew J. Graham, Eric C. Bellm

Astrophysics of Galaxies

Two young open clusters in Cygnus and their vicinity: combining multicolor photometry with Gaia DR3 astrometry

S. Raudeliūnas, R. P. Boyle, R. Janusz et al. (+6 more)

We investigate two neighboring clusters in the Cygnus complex, Berkeley 86 and Berkeley 87, with a primary emphasis on the evaluation of extinction in the field of view towards and across the clusters. We also analyze their kinematic behavior in space and time to discern their possible common origin and relation to the Cyg~OB1 association. New CCD photometry in the Vilnius seven-color system, obtained down to V=19.0 mag in the fields of these two clusters, is used to classify stars in terms of spectral and luminosity classes and to determine the individual values of interstellar extinction. The probable cluster members are identified in a 5-parameter space based on Gaia DR3. The cluster ages and stellar masses are derived through the use of the HR diagrams. To obtain the 3D kinematics of the clusters and trace their orbits back in time, we combine the Gaia-based proper-motions and distances with radial velocities from the literature. The estimated cluster properties show that both clusters are almost equidistant (1.7 kpc) and nearly coeval, with average ages of 6.1$\pm$0.5 and 6.5$\pm$0.4 Myr, respectively, and age dispersion of 3 Myr. The nonuniformity of extinction is evident within each cluster, especially pronounced across the face of Berkeley 86 where the most-massive stars show substantial substructure. By extrapolating the observed mass function to a minimum stellar mass, we obtain cluster masses of 519 M(Sun) and 1551 M(Sun) for Berkeley 86 and 87, respectively. Although both clusters share very similar properties, their orbital paths show no indication that they had a common birthplace, however Berkeley 87 and its neighbor NGC 6913 are very likely to have been born in pair.
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All authors S. Raudeliūnas, R. P. Boyle, R. Janusz, J. Zdanavičius, M. Maskoliūnas, D. Semionov, K. Černis, V. Čepas, A. Kazlauskas

Astrophysics of Galaxies

Tracing Active Galactic Nuclei Properties Through a Changing-look Event

Joel Carpenter, Sandra Raimundo, Charlotte Angus et al. (+1 more)

Changing-look transitions challenge our understanding of active galactic nuclei (AGN), exhibiting dramatic changes in broad-line emission and continuum flux on timescales of months to years. We present a detailed study of the spectroscopically confirmed changing-look AGN ZTF18abuamgo. Combining photometric survey data with spectroscopy spanning three epochs over 20 years, we identify a turn-on transition from a Type 1.5 to Type 1.2 AGN and estimate the timescale of this change to be as short as four years. Spectral analysis indicates that this transformation is driven by a rapid increase in accretion rate, with the Eddington ratio rising from $0.032 \pm 0.005$ in the dim state to $0.08 \pm 0.01$ in the bright state. For the first time in a changing-look AGN, we apply the Boltzmann plot method to the visible Balmer series emission, deriving broad line region electron temperatures of $11,800 \pm 900$ K and $11,900 \pm 2,400$ K in 2022 and 2024, respectively. Applying single-epoch black hole mass estimation to the brightening H$\alpha$ emission, we find a mass of $(5.0 \pm 0.4) \times 10^7 M_\odot$. The consistency in this estimate across all spectroscopic epochs suggest that even highly variable broad lines in CL-AGN do not bias the results derived using this method. Our results demonstrate that objects like ZTF18abuamgo provide a unique laboratory to study extreme AGN variability, probe the physical conditions in the broad line region, and assess the limitations of widely used black hole mass estimation methods.
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All authors Joel Carpenter, Sandra Raimundo, Charlotte Angus, Katie Auchettl

High Energy Astrophysical Phenomena

Which Neutron Stars Reach the Stiffening Regime? Multimessenger Constraints on Core Sound Speed and Stellar-Mass Thresholds

Nicolás Viaux and Sebastián Mendizabal

We present a concise multimessenger inference of the neutron-star core sound-speed profile using GW170817 and three NICER mass–radius posteriors (PSR J0030$+$0451, PSR J0740$+$6620, and PSR J0437$-$4715). The main result is not only a preference for intermediate-density stiffening within smooth equation-of-state families, but a translation of that inference into the stellar masses that access the relevant density regime. In the baseline smooth peaked family, the posterior probability that $c_s^2 > 1/3$ at $3.5\,n_{\rm sat}$ is $85.4\,\%$, while equal-prior averaging over peaked, monotonic, piecewise, and transition-capable families gives a more conservative $79.0\,\%$. Posterior-resampled exact Tolman–Oppenheimer–Volkoff solutions show that the onset density of the inferred stiffening is typically reached near $1.6\,M_\odot$, whereas the peak region is accessed only near $2.1\,M_\odot$. A J0740-like $2.07\,M_\odot$ pulsar reaches the onset in $91\,\%$ of posterior draws but the peak in only $46\,\%$, showing that current data mainly constrain whether massive stars have entered the stiffening regime rather than traversed its full peak.
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