• Stellar X-Ray Variability and Planetary Evolution in the DS Tucanae System

  Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA; Geneva Observatory, University of Geneva, Chemin Pegasi 51b, CH-1290 Versoix, Switzerland; Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Armagh Observatory and Planetarium, College Hill, Armagh, NIR BT61 9DG, UK; King, George W.; Corrales, Lía R.; Bourrier, Vincent; Dos Santos, Leonardo A.; et al. (The Astrophysical Journal, 2025-02-01)

  We present an analysis of four Chandra observations of the 45 Myr old DS Tuc binary system. We observed X-ray variability of both stars on timescales from hours to months, including two strong X-ray flares from star A. The implied flaring rates are in agreement with past observations made with XMM-Newton, though these rates remain imprecise due to the relatively short total observation time. We find a clear, monotonic decline in the quiescent level of the star by a factor of 1.8 across 8 months, suggesting stellar variability that might be due to an activity cycle. If proven through future observations, DS Tuc A would be the youngest star for which a coronal activity cycle has been confirmed. The variation in our flux measurements across the four visits is also consistent with the scatter in empirical stellar X-ray relationships with Rossby number. In simulations of the possible evolution of the currently super-Neptune-sized planet DS Tuc A b, we find a range of scenarios for the planet once it reaches a typical field age of 5 Gyr, from Neptune size down to a completely stripped super-Earth. Improved constraints on the planet's mass in the future would significantly narrow these possibilities. We advocate for further Chandra observations to better constrain the variability of this important system.
• Optical evolution of AT 2024wpp: the high-velocity outflows in Cow-like transients are consistent with high spherical symmetry

  Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, Turku FI-20014, Finland;; Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, Turku FI-20014, Finland; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK;; DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark; Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan;; Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, Turku FI-20014, Finland; Metsähovi Radio Observatory, Aalto University, Metsähovintie 114, FI-02540 Kylmälä, Finland; Department of Electronics and Nanoengineering, Aalto University, PO BOX 15500, FI-00076 Aalto, Finland;; Astrophysics Research Cluster, School of Mathematical and Physical Sciences, University of Sheffield, Sheffield S3 7RH, UK; Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain;; School of Physics & Astronomy, Monash University, Clayton, VIC 3800, Australia; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Department of Physics, Royal Holloway, University of London, Egham Hill, Surrey TW20 0EX, UK;; et al. (Monthly Notices of the Royal Astronomical Society, 2025-03-01)

  We present the analysis of optical/near-infrared (NIR) data and host galaxy properties of a bright, extremely rapidly evolving transient, AT 2024wpp, which resembles the enigmatic AT 2018cow. AT 2024wpp rose to a peak brightness of <inline-formula><tex-math id=TM0001 notation=LaTeX>$c=-21.9$</tex-math></inline-formula> mag in 4.3 d and remained above the half-maximum brightness for only 6.7 d. The blackbody fits to the photometry show that the event remained persistently hot (<inline-formula><tex-math id=TM0002 notation=LaTeX>$T\gtrsim 20\, 000$</tex-math></inline-formula> K) with a rapidly receding photosphere (<inline-formula><tex-math id=TM0003 notation=LaTeX>$v\sim 11\, 500$</tex-math></inline-formula> km s<inline-formula><tex-math id=TM0004 notation=LaTeX>$^{-1}$</tex-math></inline-formula>), similarly to AT 2018cow albeit with a several times larger photosphere. <inline-formula><tex-math id=TM0005 notation=LaTeX>$JH$</tex-math></inline-formula> photometry reveals an NIR excess over the thermal emission at <inline-formula><tex-math id=TM0006 notation=LaTeX>$\sim +20$</tex-math></inline-formula> d, indicating a presence of an additional component. The spectra are consistent with blackbody emission throughout our spectral sequence ending at <inline-formula><tex-math id=TM0007 notation=LaTeX>$+21.9$</tex-math></inline-formula> d, showing a tentative, very broad emission feature at <inline-formula><tex-math id=TM0008 notation=LaTeX>$\sim 5500$</tex-math></inline-formula> Å - implying that the optical photosphere is likely within a near-relativistic outflow. Furthermore, reports of strong X-ray and radio emission cement the nature of AT 2024wpp as a likely Cow-like transient. AT 2024wpp is the second event of the class with optical polarimetry. Our <inline-formula><tex-math id=TM0009 notation=LaTeX>$BVRI$</tex-math></inline-formula> observations obtained from <inline-formula><tex-math id=TM0010 notation=LaTeX>$+6.1$</tex-math></inline-formula> to <inline-formula><tex-math id=TM0011 notation=LaTeX>$+14.4$</tex-math></inline-formula> d show a low polarization of <inline-formula><tex-math id=TM0012 notation=LaTeX>$P\lesssim 0.5$</tex-math></inline-formula> per cent across all bands, similar to AT 2018cow that was consistent with <inline-formula><tex-math id=TM0013 notation=LaTeX>$P\sim 0$</tex-math></inline-formula> per cent during the same outflow-driven phase. In the absence of evidence for a preferential viewing angle, it is unlikely that both events would have shown low polarization in the case that their photospheres were aspherical. As such, we conclude that the near-relativistic outflows launched in these events are likely highly spherical, but polarimetric observations of further events are crucial to constrain their ejecta geometry and stratification in detail.
• SN 2023tsz: a helium-interaction-driven supernova in a very low-mass galaxy

  Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK;; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain; Institut d'Estudis Espacials de Catalunya (IEEC), E-08860 Castelldefels (Barcelona), Spain; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Department of Physics &amp; Astronomy, University of Turku, Vesilinnantie 5, FI-20014 Turku, Finland;; Department of Physics, Lancaster University, Lancaster LA1 4YB, UK;; European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile; Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile; Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan; Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK;; Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK; Astrophysics Research Cluster, School of Mathematical and Physical Sciences, University of Sheffield, Sheffield S3 7RH, UK; Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain;; et al. (Monthly Notices of the Royal Astronomical Society, 2025-02-01)

  SN 2023tsz is a Type Ibn supernova (SN Ibn), an uncommon subtype of stripped-envelope core-collapse supernovae (SNe), discovered in an extremely low-mass host. SNe Ibn are characterized by narrow helium emission lines in their spectra and are believed to originate from the collapse of massive Wolf-Rayet (WR) stars, though their progenitor systems still remain poorly understood. In terms of energetics and spectrophotometric evolution, SN 2023tsz is largely a typical example of the class, although line profile asymmetries in the nebular phase are seen, which may indicate the presence of dust formation or unshocked circumstellar material. Intriguingly, SN 2023tsz is located in an extraordinarily low-mass host galaxy that is in the second percentile for stripped-envelope SN host masses and star formation rates (SFRs). The host has a radius of 1.0 kpc, a g-band absolute magnitude of <inline-formula><tex-math id=TM0002 notation=LaTeX>$-12.72 \pm 0.05$</tex-math></inline-formula>, and an estimated metallicity of <inline-formula><tex-math id=TM0003 notation=LaTeX>$\log (Z_{*}/{\rm Z}_{\odot }) \approx -1.6$</tex-math></inline-formula>. The SFR and metallicity of the host galaxy raise questions about the progenitor of SN 2023tsz. The low SFR suggests that a star with sufficient mass to evolve into a WR would be uncommon in this galaxy. Further, the very low metallicity is a challenge for single stellar evolution to enable H and He stripping of the progenitor and produce an SN Ibn explosion. The host galaxy of SN 2023tsz adds another piece to the ongoing puzzle of SNe Ibn progenitors, and demonstrates that they can occur in hosts too faint to be observed in contemporary sky surveys at a more typical SN Ibn redshift.
• 

  Stellar expansion or inflation?

  Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, UK;; Sabhahit, Gautham N.; Vink, Jorick S. (Astronomy and Astrophysics, 2025-01-01)

  While stellar expansion after core-hydrogen exhaustion related to thermal imbalance has been documented for decades, the physical phenomenon of stellar inflation that occurs close to the Eddington limit has only come to the fore in recent years. We aim to elucidate the differences between these physical mechanisms for stellar radius enlargement, especially given that additional terms such as 'bloated' and 'puffed-up' stars have been introduced in the recent massive star literature. We employ single and binary star MESA structure and evolution models for constant mass, as well as models allowing the mass to change due to winds or binary interaction. We find cases that were previously attributed to stellar inflation in fact to be due to stellar expansion. We also highlight that while the opposite effect of expansion is contraction, the removal of an inflated zone should not be referred to as contraction but rather deflation, as the star is still in thermal balance.
• 

  Cataclysmic Variables and AM CVn Binaries in SRG/eROSITA + Gaia: Volume Limited Samples, X-Ray Luminosity Functions, and Space Densities

  Department of Astronomy, California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125, USA;; Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076, Tübingen, Germany;; European Space Agency, European Space Astronomy Centre, Camino Bajo del Castillo s/n, Villanueva de la Cañada, E-28692 Madrid, Spain;; Department of Physics, University of Warwick, Coventry CV4 7AL, UK;; Columbia Astrophysics Laboratory, Columbia University, New York, NY, USA;; Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA;; Institute of Astronomy, The Observatories, Madingley Road, Cambridge, CB3 OHA, UK;; INAF - Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy;; Departamento de Física, Universidade Federal de Sergipe, Av. Marechal Rondon, S/N, 49100-000, São Cristóvão, SE, Brazil; Observatório Nacional, Rua Gal. José Cristino 77, 20921-400, Rio de Janeiro, RJ, Brazil;; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, N. Ireland, UK;; et al. (Publications of the Astronomical Society of the Pacific, 2025-01-01)

  We present volume-limited samples of cataclysmic variables (CVs) and AM CVn binaries jointly selected from SRG/eROSITA eRASS1 and Gaia DR3 using an X-ray + optical color–color diagram (the X-ray Main Sequence). This tool identifies all CV subtypes, including magnetic and low-accretion rate systems, in contrast to most previous surveys. We find 23 CVs, 3 of which are AM CVns, out to 150 pc in the Western Galactic Hemisphere. Our 150 pc sample is spectroscopically verified and complete down to L<SUB>X</SUB> = 1.3 × 10<SUP>29</SUP> erg s<SUP>‑1</SUP> in the 0.2–2.3 keV band, and we also present CV candidates out to 300 pc and 1000 pc. We discovered two previously unknown systems in our 150 pc sample: the third nearest AM CVn and a magnetic period bouncer. We find the mean L<SUB>X</SUB> of CVs to be &lt;L<SUB>X</SUB>&gt; ≈ 4.6 × 10<SUP>30</SUP> erg s<SUP>‑1</SUP>, in contrast to previous surveys which yielded &lt;L<SUB>X</SUB>&gt; ∼ 10<SUP>31</SUP>‑10<SUP>32</SUP> erg s<SUP>‑1</SUP>. We construct X-ray luminosity functions that, for the first time, flatten out at L<SUB>X</SUB> ∼ 10<SUP>30</SUP> erg s<SUP>‑1</SUP>. We infer average number, mass, and luminosity densities of ρ<SUB>N,CV</SUB> = (3.7 ± 0.7) × 10<SUP>‑6</SUP>pc<SUP>‑3</SUP>, <inline-formula> <mml:math overflow=scroll><mml:msub><mml:mrow><mml:mi>ρ</mml:mi></mml:mrow><mml:mrow><mml:mi>M</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mo stretchy=false>(</mml:mo><mml:mn>5.0</mml:mn><mml:mo>±</mml:mo><mml:mn>1.0</mml:mn><mml:mo stretchy=false>)</mml:mo><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mo>‑</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup><mml:msubsup><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow><mml:mrow><mml:mo>‑</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, and <inline-formula> <mml:math overflow=scroll><mml:msub><mml:mi>ρ</mml:mi><mml:msub><mml:mi>L</mml:mi><mml:mi mathvariant=normal>X</mml:mi></mml:msub></mml:msub><mml:mo>=</mml:mo><mml:mo stretchy=false>(</mml:mo><mml:mn>2.3</mml:mn><mml:mo>±</mml:mo><mml:mn>0.4</mml:mn><mml:mo stretchy=false>)</mml:mo><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mn>0</mml:mn><mml:mn>26</mml:mn></mml:msup><mml:mspace width=0.25em></mml:mspace><mml:mi>erg</mml:mi><mml:mspace width=0.25em></mml:mspace><mml:msup><mml:mi mathvariant=normal>s</mml:mi><mml:mrow><mml:mo>‑</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup><mml:msubsup><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo><mml:mrow><mml:mo>‑</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, respectively, in the solar neighborhood. Our uniform selection method also allows us to place meaningful estimates on the space density of AM CVns, ρ<SUB>N,AM CVn</SUB> = (5.5 ± 3.7) × 10<SUP>‑7</SUP> pc<SUP>‑3</SUP>. Magnetic CVs and period bouncers make up 35% and 25% of our sample, respectively. This work, through a novel discovery technique, shows that the observed number densities of CVs and AM CVns, as well as the fraction of period bouncers, are still in tension with population synthesis estimates.
• 

  A Broadband X-Ray Investigation of Fast-spinning Intermediate Polar CTCV J2056–3014

  Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA;; South African Astronomical Observatory, P.O. Box 9, Observatory, 7935 Cape Town, South Africa; Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa;; Departamento de Física, Universidade Federal de Sergipe, Av. Marechal Rondon, S/N, 49100-000, São Cristóvão, SE, Brazil; Observatório Nacional, Rua Gal. José Cristino 77, 20921-400, Rio de Janeiro, RJ, Brazil;; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK;; Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa;; Salcedo, Ciro; Mori, Kaya; Bridges, Gabriel; Hailey, Charles J.; Buckley, David A. H.; et al. (The Astrophysical Journal, 2024-11-01)

  We report on XMM-Newton, NuSTAR, and NICER X-ray observations of CTCV J2056–3014, a cataclysmic variable (CV) with one of the fastest-spinning white dwarfs (WDs) at P = 29.6 s. While previously classified as an intermediate polar, CJ2056 also exhibits the properties of WZ Sge–type CVs, such as dwarf novae and superoutbursts. With XMM-Newton and NICER, we detected the spin period up to ∼2 keV with 7σ significance. We constrained its derivative to <inline-formula> <mml:math overflow=scroll><mml:mo stretchy=false>|</mml:mo><mml:mrow><mml:mover><mml:mi>P</mml:mi><mml:mo>̇</mml:mo></mml:mover></mml:mrow><mml:mo stretchy=false>|</mml:mo><mml:mo>&lt;</mml:mo><mml:mn>1.8</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>‑</mml:mo><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula> s s<SUP>‑1</SUP> after correcting for binary orbital motion. The pulse profile is characterized by a single broad peak with ∼25% modulation. NuSTAR detected a fourfold increase in unabsorbed X-ray flux coincident with an optical flare, in 2022 November. The XMM-Newton and NICER X-ray spectra at 0.310 keV are best characterized by an absorbed, optically thin three-temperature thermal plasma model (kT = 0.3, 1.0, and 4.9 keV), while the NuSTAR spectra at 3–30 keV are best fit by a single-temperature thermal plasma model (kT = 8.4 keV), both with Fe abundance Z <SUB>Fe</SUB>/Z <SUB>⊙</SUB> = 0.3. CJ2056 exhibits similarities to other fast-spinning CVs, such as low plasma temperatures and no significant X-ray absorption at low energies. As the WD's magnetic field strength is unknown, we applied both nonmagnetic and magnetic CV spectral models (MKCFLOW and MCVSPEC) to determine the WD mass. The derived WD mass range (M = 0.7–1.0 M <SUB>⊙</SUB>) is above the centrifugal breakup mass limit of 0.56 M <SUB>⊙</SUB> and consistent with the mean WD mass of local CVs (M ≈ 0.8–0.9 M <SUB>⊙</SUB>).
• 

  X-Shooting ULLYSES: Massive stars at low metallicity: VIII. Stellar and wind parameters of newly revealed stripped stars in Be binaries

  Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120, Heidelberg, Germany;; Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, D-14476, Potsdam, Germany; European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching bei München, Germany; Anton Pannekoek Institute for Astronomy, Universiteit van Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands; Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium;; Dept of Physics &amp; Astronomy, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH, UK; Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, D-14476, Potsdam, Germany;; Department of Physics &amp; Astronomy, East Tennessee State University, Johnson City, TN, 37614, USA;; Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057, Duisburg, Germany;; University of Michigan, Department of Astronomy, 323 West Hall, Ann Arbor, MI, 48109, USA; et al. (Astronomy and Astrophysics, 2024-12-01)

  On the route toward merging neutron stars and stripped-envelope supernovae, binary population synthesis predicts a large number of post-interaction systems with massive stars that have been stripped of their outer layers. However, observations of such stars in the intermediate-mass regime below the Wolf-Rayet masses are rare. Using X-Shooting ULLYSES (XShootU) data, we have discovered three partially stripped star + Be/Oe binaries in the Magellanic Clouds. We analyzed the UV and optical spectra using the Potsdam Wolf-Rayet (PoWR) model atmosphere code by superimposing model spectra that correspond to each component. The estimated current masses of the partially stripped stars fall within the intermediate-mass range of ≈4 ‑ 8 M<SUB>⊙</SUB>. These objects are found to be over-luminous for their corresponding stellar masses, which aligns with the luminosities during core He-burning. Their accompanying Be/Oe secondaries are found to have much higher masses than their stripped primaries (mass ratio ≳2). The surfaces of all three partially stripped stars exhibit clear indications of significant nitrogen enrichment as well as a depletion of carbon and oxygen. Furthermore, one of our sample stars shows signs of substantial helium enrichment. Our study provides the first comprehensive determination of the wind parameters of partially stripped stars in the intermediate-mass range. The wind mass-loss rates of these stars are estimated to be on the order of 10<SUP>‑7</SUP> M<SUB>⊙</SUB> yr<SUP>‑1</SUP>, which is more than ten times higher than that of OB stars with the same luminosity. The current mass-loss recipes commonly employed in evolutionary models to characterize this phase are based on OB or WR mass-loss rates, and they significantly underestimate or overestimate the observed mass-loss rates of (partially) stripped stars by an order of magnitude. Binary evolution models suggest that the observed primaries had initial masses in the range of 12‑17 M<SUB>⊙</SUB>, and are potential candidates for stripped-envelope supernovae resulting in the formation of a neutron star. If these systems survive the explosion, they will likely evolve to become Be X-ray binaries and later double neutron stars.
• 

  Radio signatures of star-planet interactions, exoplanets and space weather

  ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, the Netherlands; Leiden Observatory, Leiden University, Leiden, the Netherlands; School of Mathematics and Physics, University of Queensland, St Lucia, Queensland, Australia; Centre for Astrophysics, University of Southern Queensland, Toowoomba, Queensland, Australia; ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, the Netherlands; Leiden Observatory, Leiden University, Leiden, the Netherlands; School of Physics and Astronomy, University of St Andrews, St Andrews, UK; INAF Osservatorio Astrofisico di Torino, Torinese, Italy; LPC2E, OSUC, Universitéd'Orléans, CNRS, CNES, Observatoire de Paris, Orleans, France; Observatoire Radioastronomique de Nançay (ORN), Observatoire de Paris, CNRS, PSL, Université d'Orléans, OSUC, Nançay, France; Department of Astrophysics, University of Vienna, Vienna, Austria; European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Noordwijk, the Netherlands; Department of Astronomy &amp; Astrophysics, University of California, Santa Cruz, Santa Cruz, CA, USA; Lowell Observatory, Flagstaff, AZ, USA; et al. (Nature Astronomy, 2024-11-01)

  Radio detections of stellar systems provide a window onto stellar magnetic activity and the space weather conditions of extrasolar planets — information that is difficult to obtain at other wavelengths. The maturation of low-frequency radio instruments and the plethora of wide-field radio surveys have driven recent advances in observing auroral emissions from radio-bright low-mass stars and exoplanets. To guide us in putting these recent results in context, we introduce the foremost local analogues for the field: solar bursts and the aurorae found on Jupiter. We detail how radio bursts associated with stellar flares are foundational to the study of stellar coronae, and time-resolved radio dynamic spectra offer one of the best prospects for detecting and characterizing coronal mass ejections from other stars. We highlight the possibility of directly detecting coherent radio emission from exoplanetary magnetospheres, as well as early tentative results. We bridge this discussion with the field of brown dwarf radio emission — the larger and stronger magnetospheres of these stars are amenable to detailed study with current instruments. Bright, coherent radio emission is also predicted from magnetic interactions between stars and close-in planets. We discuss the underlying physics of these interactions and the implications of recent provisional detections for exoplanet characterization. We conclude with an overview of outstanding questions in the theory of stellar, star-planet interaction and exoplanet radio emission and the potential of future facilities to answer them.
• 

  GERry: A code to optimise the hunt for the electromagnetic counter-parts to gravitational wave events

  The Univ. of Warwick (United Kingdom); Monash Univ. (Australia); Univ. of Sheffield (United Kingdom); Univ. of Leicester (United Kingdom); Armagh Observatory and Planetarium (United Kingdom); National Astronomical Research Institute of Thailand (Thailand); Univ. of Turku (Finland); The Univ. of Manchester (United Kingdom); Univ. of Portsmouth (United Kingdom); Instituto de Astrofísica de Canarias (Spain); et al. (Observatory Operations: Strategies, Processes, and Systems X, 2024-07-01)

  The search for the electromagnetic counterparts to Gravitational Wave (GW) events has been rapidly gathering pace in recent years thanks to the increasing number and capabilities of both gravitational wave detectors and wide field survey telescopes. Difficulties remain, however, in detecting these counterparts due to their inherent scarcity, faintness and rapidly evolving nature. To find these counterparts, it is important that one optimizes the observing strategy for their recovery. This can be difficult due to the large number of potential variables at play. Such follow-up campaigns are also capable of detecting hundreds or potentially thousands of unrelated transients, particularly for GW events with poor localization. Even if the observations are capable of detecting a counterpart, finding it among the numerous contaminants can prove challenging. Here we present the Gravitational wave Electromagnetic RecovRY code (GERRY) to perform detailed analysis and survey-agnostic quantification of observing campaigns attempting to recover electromagnetic counterparts. GERRY considers the campaign's spatial, temporal and wavelength coverage, in addition to Galactic extinction and the expected counterpart light curve evolution from the GW 3D localization volume. It returns quantified statistics that can be used to: determine the probability of having detected the counterpart, identified the most promising sources, and assessed and refine strategy. Here we demonstrate the code to look at the performance and parameter space probed by current and upcoming wide-field surveys such as GOTO and VRO.
• Prospects for a survey of the galactic plane with the Cherenkov Telescope Array

  Institute for Cosmic Ray Research, University of Tokyo, 5-1-5, Kashiwa-no-ha, Kashiwa, Chiba 277-8582, Japan; ETH Zürich, Institute for Particle Physics and Astrophysics, Otto-Stern-Weg 5, 8093 Zürich, Switzerland; INFN and Università degli Studi di Siena, Dipartimento di Scienze Fisiche, della Terra e dell'Ambiente (DSFTA), Sezione di Fisica, Via Roma 56, 53100 Siena, Italy; Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette Cedex, France; FSLAC IRL 2009, CNRS/IAC, La Laguna, Tenerife, Spain; University of Alabama, Tuscaloosa, Department of Physics and Astronomy, Gallalee Hall, Box 870324 Tuscaloosa, AL 35487-0324, U.S.A.; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, France; Laboratoire Leprince-Ringuet, CNRS/IN2P3, École polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France; Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès, 1, 08028, Barcelona, Spain; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008, Granada, Spain; Institute for Computational Cosmology and Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom; Instituto de Física Teórica UAM/CSIC and Departamento de Física Teórica, Universidad Autónoma de Madrid, c/ Nicolás Cabrera 13-15, Campus de Cantoblanco UAM, 28049 Madrid, Spain; Pontificia Universidad Católica de Chile, Av. Libertador Bernardo O'Higgins 340, Santiago, Chile; et al. (Journal of Cosmology and Astroparticle Physics, 2024-10-01)

  Approximately one hundred sources of very-high-energy (VHE) gamma rays are known in the Milky Way, detected with a combination of targeted observations and surveys. A survey of the entire Galactic Plane in the energy range from a few tens of GeV to a few hundred TeV has been proposed as a Key Science Project for the upcoming Cherenkov Telescope Array Observatory (CTAO). This article presents the status of the studies towards the Galactic Plane Survey (GPS). We build and make publicly available a sky model that combines data from recent observations of known gamma-ray emitters with state-of-the-art physically-driven models of synthetic populations of the three main classes of established Galactic VHE sources (pulsar wind nebulae, young and interacting supernova remnants, and compact binary systems), as well as of interstellar emission from cosmic-ray interactions in the Milky Way. We also perform an optimisation of the observation strategy (pointing pattern and scheduling) based on recent estimations of the instrument performance. We use the improved sky model and observation strategy to simulate GPS data corresponding to a total observation time of 1620 hours spread over ten years. Data are then analysed using the methods and software tools under development for real data. Under our model assumptions and for the realisation considered, we show that the GPS has the potential to increase the number of known Galactic VHE emitters by almost a factor of five. This corresponds to the detection of more than two hundred pulsar wind nebulae and a few tens of supernova remnants at average integral fluxes one order of magnitude lower than in the existing sample above 1 TeV, therefore opening the possibility to perform unprecedented population studies. The GPS also has the potential to provide new VHE detections of binary systems and pulsars, to confirm the existence of a hypothetical population of gamma-ray pulsars with an additional TeV emission component, and to detect bright sources capable of accelerating particles to PeV energies (PeVatrons). Furthermore, the GPS will constitute a pathfinder for deeper follow-up observations of these source classes. Finally, we show that we can extract from GPS data an estimate of the contribution to diffuse emission from unresolved sources, and that there are good prospects of detecting interstellar emission and statistically distinguishing different scenarios. Thus, a survey of the entire Galactic plane carried out from both hemispheres with CTAO will ensure a transformational advance in our knowledge of Galactic VHE source populations and interstellar emission.
• Prospects for γ-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope Array

  Department of Physics, Tokai University, 4-1-1, Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan; Institute for Cosmic Ray Research, University of Tokyo, 5-1-5, Kashiwa-no-ha, Kashiwa, Chiba 277-8582, Japan; Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette Cedex, France; FSLAC IRL 2009, CNRS/IAC, La Laguna, Tenerife, Spain; University of Alabama, Tuscaloosa, Department of Physics and Astronomy, Gallalee Hall, Box 870324 Tuscaloosa, AL 35487-0324, U.S.A.; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, France; Laboratoire Leprince-Ringuet, CNRS/IN2P3, École polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France; Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès, 1, 08028, Barcelona, Spain; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008, Granada, Spain; Instituto de Física Teórica UAM/CSIC and Departamento de Física Teórica, Universidad Autónoma de Madrid, c/ Nicolás Cabrera 13-15, Campus de Cantoblanco UAM, 28049 Madrid, Spain; Pontificia Universidad Católica de Chile, Av. Libertador Bernardo O'Higgins 340, Santiago, Chile; Universidad Nacional Autónoma de México, Delegación Coyoacán, 04510 Ciudad de México, Mexico; et al. (Journal of Cosmology and Astroparticle Physics, 2024-10-01)

  Galaxy clusters are expected to be both dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at γ-ray energies and are predicted to be sources of large-scale γ-ray emission due to hadronic interactions in the intracluster medium (ICM). In this paper, we estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse γ-ray emission from the Perseus galaxy cluster. We first perform a detailed spatial and spectral modelling of the expected signal for both the DM and the CRp components. For each case, we compute the expected CTA sensitivity accounting for the CTA instrument response functions. The CTA observing strategy of the Perseus cluster is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio X <SUB>500</SUB> within the characteristic radius R <SUB>500</SUB> down to about X <SUB>500</SUB> &lt; 3 × 10<SUP>-3</SUP>, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index α<SUB>CRp</SUB> = 2.3. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure α<SUB>CRp</SUB> down to about Δα<SUB>CRp</SUB> ≃ 0.1 and the CRp spatial distribution with 10% precision, respectively. Regarding DM, CTA should improve the current ground-based γ-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to ∼ 5, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with τ <SUB>χ</SUB> &gt; 10<SUP>27</SUP> s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.
• New Wolf-Rayet wind yields and nucleosynthesis of Helium stars

  Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, N. Ireland; Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK; Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8583, Japan; School of Physics, Engineering and Technology, University of York, York, YO10 5DD, UK; Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr 12-14, D-69120 Heidelberg, Germany; Higgins, Erin R.; Vink, Jorick S.; Hirschi, Raphael; Laird, Alison M.; Sander, Andreas A. C. (Monthly Notices of the Royal Astronomical Society, 2024-09-01)

  Strong metallicity-dependent winds dominate the evolution of core He-burning, classical Wolf-Rayet (cWR) stars, which eject both H and He-fusion products such as $^{14}$N, $^{12}$C, $^{16}$O, $^{19}$F, $^{22}$Ne, and $^{23}$Na during their evolution. The chemical enrichment from cWRs can be significant. cWR stars are also key sources for neutron production relevant for the weak s-process. We calculate stellar models of cWRs at solar metallicity for a range of initial Helium star masses (12-50 $\rm M_{\odot }$), adopting recent hydrodynamical wind rates. Stellar wind yields are provided for the entire post-main sequence evolution until core O-exhaustion. While literature has previously considered cWRs as a viable source of the radioisotope $^{26}$Al, we confirm that negligible $^{26}$Al is ejected by cWRs since it has decayed to $^{26}$Mg or proton-captured to $^{27}$Al. However, in Paper I, we showed that very massive stars eject substantial quantities of $^{26}$Al, among other elements including N, Ne, and Na, already from the zero-age-main-sequence. Here, we examine the production of $^{19}$F and find that even with lower mass-loss rates than previous studies, our cWR models still eject substantial amounts of $^{19}$F. We provide central neutron densities (N$_{n}$) of a 30 $\rm M_{\odot }$ cWR compared with a 32 $\rm M_{\odot }$ post-VMS WR and confirm that during core He-burning, cWRs produce a significant number of neutrons for the weak s-process via the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction. Finally, we compare our cWR models with observed [Ne/He], [C/He], and [O/He] ratios of Galactic WC and WO stars.
• Kilonova Seekers: the GOTO project for real-time citizen science in time-domain astrophysics

  Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, Turku FI-20014, Finland; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK; School of Physics and Astronomy, Monash University, Clayton VIC 3800, Australia; Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK; Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain; School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; National Astronomical Research Institute of Thailand, 260 Moo 4, T. Donkaew, A. Maerim, Chiangmai 50180, Thailand; Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, Turku FI-20014, Finland; et al. (Monthly Notices of the Royal Astronomical Society, 2024-09-01)

  Time-domain astrophysics continues to grow rapidly, with the inception of new surveys drastically increasing data volumes. Democratized, distributed approaches to training sets for machine learning classifiers are crucial to make the most of this torrent of discovery - with citizen science approaches proving effective at meeting these requirements. In this paper, we describe the creation of and the initial results from the Kilonova Seekers citizen science project, built to find transient phenomena from the GOTO telescopes in near real-time. Kilonova Seekers launched in 2023 July and received over 600 000 classifications from approximately 2000 volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run. During this time, the project has yielded 20 discoveries, generated a 'gold-standard' training set of 17 682 detections for augmenting deep-learned classifiers, and measured the performance and biases of Zooniverse volunteers on real-bogus classification. This project will continue throughout the lifetime of GOTO, pushing candidates at ever-greater cadence, and directly facilitate the next-generation classification algorithms currently in development.
• 

  The maximum black hole mass at solar metallicity

  Armagh Observatory and Planetarium, College Hill, BT61 9DG, Armagh, Northern Ireland, UK; Vink, Jorick S.; Sabhahit, Gautham N.; Higgins, Erin R. (Astronomy and Astrophysics, 2024-08-01)

  We analyse the current knowledge and uncertainties in detailed stellar evolution and wind modelling to evaluate the mass of the most massive stellar black hole (BH) at solar metallicity. Contrary to common expectations that it is the most massive stars that produce the most massive BHs, we find that the maximum M<SUB>BH</SUB><SUP>Max</SUP> ≃ 30 ± 10 M<SUB>⊙</SUB> is found in the canonical intermediate range between M<SUB>ZAMS</SUB> ≃ 30 and 50 M<SUB>⊙</SUB> instead. The prime reason for this seemingly counter-intuitive finding is that very massive stars (VMS) have increasingly high mass-loss rates that lead to substantial mass evaporation before they expire as stars and end as lighter BHs than their canonical O-star counterparts.
• Dark matter line searches with the Cherenkov Telescope Array

  Institute for Cosmic Ray Research, University of Tokyo, 5-1-5, Kashiwa-no-ha, Kashiwa, Chiba 277-8582, Japan; ETH Zürich, Institute for Particle Physics and Astrophysics, Otto-Stern-Weg 5, 8093 Zürich, Switzerland; INFN and Università degli Studi di Siena, Dipartimento di Scienze Fisiche, della Terra e dell'Ambiente (DSFTA), Sezione di Fisica, Via Roma 56, 53100 Siena, Italy; Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette Cedex, France; FSLAC IRL 2009, CNRS/IAC, La Laguna, Tenerife, Spain; University of Alabama, Tuscaloosa, Department of Physics and Astronomy, Gallalee Hall, Box 870324 Tuscaloosa, AL 35487-0324, U.S.A.; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, France; Laboratoire Leprince-Ringuet, CNRS/IN2P3, École polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France; Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès, 1, 08028, Barcelona, Spain; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008, Granada, Spain; Institute for Computational Cosmology and Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom; Pontificia Universidad Católica de Chile, Av. Libertador Bernardo O'Higgins 340, Santiago, Chile; et al. (Journal of Cosmology and Astroparticle Physics, 2024-07-01)

  Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.
• X-Shooting ULLYSES: Massive Stars at Low Metallicity

  Armagh Observatory and Planetarium, UK; Department of Physics &amp; Astronomy, University of Sheffield, UK; Space Telescope Science Institute, Baltimore, USA; Centre for Astrobiology (CSIC-INTA), Torrejón de Ardoz, Madrid, Spain; Montpellier Universe and Particles Laboratory, Montpellier University, France; Las Campanas Observatory, Carnegie Observatories, Chile; Institute for Physics and Astronomy, University of Potsdam, Germany; Department of Physics, University of Montreal, Canada; Penn State Scranton, Dunmore, PA, USA; Astronomy Centre, Heidelberg University, Germany; et al. (The Messenger, 2024-03-01)

  The Hubble Space Telescope has devoted 500 orbits to observing 250 massive stars with low metallicity in the ultraviolet (UV) range within the framework of the ULLYSES program. The X-Shooting ULLYSES (XShootU) project enhances the legacy value of this UV dataset by providing high-quality optical and near-infrared spectra, which are acquired using the wide-wavelength- coverage X-shooter spectrograph at ESO's Very Large Telescope. XShootU emphasises the importance of combining UV with optical spectra for the consistent determination of key stellar parameters such as effective temperature, surface gravity, luminosity, abundances, and wind characteristics including mass-loss rates as a function of metallicity. Since uncertainties in these parameters have implications across various branches of astrophysics, the data and modelling generated by the XShootU project are poised to significantly advance our understanding of massive stars at low metallicity. This is particularly crucial for confidently interpreting James Webb Space Telescope (JWST) data of the earliest stellar generations, making XShootU a unique resource for comprehending individual spectra of low-metallicity stars.
• 

  Predicting the heaviest black holes below the pair instability gap

  Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB, UK; School of Maths and Physics, Queen's University Belfast, Northern Ireland, University Road, BT7 1NN, UK; Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB, UK; Winch, Ethan R. J.; Vink, Jorick S.; Higgins, Erin R.; Sabhahitf, Gautham N. (Monthly Notices of the Royal Astronomical Society, 2024-04-01)

  Traditionally, the pair instability (PI) mass gap is located between 50 and 130 M<SUB>⊙</SUB>, with stellar mass black holes (BHs) expected to 'pile up' towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an 'impossibly' heavy BH of 85 M<SUB>⊙</SUB> as part of GW 190521 located inside the traditional PI gap, we realized that blue supergiant (BSG) progenitors with small cores but large hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed MESA stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semiconvection, and Z. We evolve low Z stars in the range 10<SUP>-3</SUP> &lt; Z/Z<SUB>⊙</SUB> &lt; Z<SUB>SMC</SUB>, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass-loss recipe to calculate it. We compute critical carbon-oxygen and helium core masses to determine our lower limit to PI physics, and we provide two equations for M<SUB>core</SUB> and M<SUB>final</SUB> that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is M<SUB>BH</SUB> ≃ 93.3 M<SUB>⊙</SUB>. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the traditional boundary is not solely due to the shape of the initial mass function, but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.
• LISA Galactic Binaries with Astrometry from Gaia DR3

  Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany; Department of Physics and Astronomy, Texas Tech University, P.O. Box 41051, Lubbock, TX 79409, USA; Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany; Institute for Gravitational Wave Astronomy, School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK; NASA Marshall Space Flight Center, Huntsville, AL 35811, USA; Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Callinstrasse 38, 30167 Hannover, Germany; Leibniz Universität Hannover, Institut für Gravitationsphysik, Callinstrasse 38, 30167 Hannover, Germany; Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France; IRFU, CEA, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France; Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands; South African Astronomical Observatory, P.O. Box 9, Observatory, 7935, Cape Town, South Africa; Department of Astronomy &amp; Inter-University Institute for Data Intensive Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France; Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands; SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands; Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium; European Space Agency, European Space Astronomy Centre, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain; et al. (The Astrophysical Journal, 2024-03-01)

  Galactic compact binaries with orbital periods shorter than a few hours emit detectable gravitational waves (GWs) at low frequencies. Their GW signals can be detected with the future Laser Interferometer Space Antenna (LISA). Crucially, they may be useful in the early months of the mission operation in helping to validate LISA's performance in comparison to prelaunch expectations. We present an updated list of 55 candidate LISA-detectable binaries with measured properties, for which we derive distances based on Gaia Data Release 3 astrometry. Based on the known properties from electromagnetic observations, we predict the LISA detectability after 1, 3, 6, and 48 months using Bayesian analysis methods. We distinguish between verification and detectable binaries as being detectable after 3 and 48 months, respectively. We find 18 verification binaries and 22 detectable sources, which triples the number of known LISA binaries over the last few years. These include detached double white dwarfs, AM CVn binaries, one ultracompact X-ray binary, and two hot subdwarf binaries. We find that across this sample the GW amplitude is expected to be measured to ≈10% on average, while the inclination is expected to be determined with ≈15° precision. For detectable binaries, these average errors increase to ≈50% and ≈40°, respectively.
• 

  Predicting the Heaviest Black Holes below the Pair Instability Gap

  Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB; School of Maths and Physics, Queen's University Belfast, Northern Ireland, University Road, BT7 1NN; Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB; Winch, Ethan R. J.; Vink, Jorick S.; Higgins, Erin R.; Sabhahit, Gautham N. (Monthly Notices of the Royal Astronomical Society, 2024-02-01)

  Traditionally, the pair instability (PI) mass gap is located between 50 and 130 M<SUB>⊙</SUB>, with stellar mass black holes (BHs) expected to pile up towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an impossibly heavy BH of 85 M<SUB>⊙</SUB> as part of GW 190521 located inside the traditional PI gap, we realised that blue supergiant (BSG) progenitors with small cores but large Hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed MESA stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semi-convection, and Z. We evolve low Z stars in the range 10<SUP>-3</SUP> &lt; Z/Z<SUB>⊙</SUB> &lt; Z<SUB>SMC</SUB>, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass loss recipe to calculate it. We compute critical Carbon-Oxygen and Helium core masses to determine our lower limit to PI physics, and we provide two equations for M<SUB>core</SUB> and M<SUB>final</SUB> that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is M<SUB>BH</SUB> ≃ 93.3 M<SUB>⊙</SUB>. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the gap is not solely due to the shape of the initial mass function (IMF), but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.
• 

  Multimessenger science opportunities with mHz gravitational waves

  NASA Goddard Space Flight Center; Columbia University; Leiden University; Harvard University; The Pennsylvania State University; University of Cambridge; CITA, University of Toronto; California Institute of Technology; City University of New York/American Museum of Natural History; Princeton University; et al. (Bulletin of the American Astronomical Society, 2019-05-01)

  We review opportunities for multi-messenger science breakthroughs involving mHz gravitational waves with electromagnetic observations.

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