High Energy Astrophysical Phenomena
Determination of the Angular Distributions of Dynamical and Emission Parameters of GRB Relativistic Outflows
A. Panaitescu
A previously-developed analytical treatment of the "Larger-Angle Emission" model for GRBs and afterglows is further developed to include the effect of a fixed
magnetic field orientation. This model, has the uncanny ability to unify the
prompt and delayed emissions and to account easily for the entire diversity of Swift X-ray light-curves. The LAE model is applied to seven GRBs (060526, 060614, 060714, 060729, 061007, 061121, 100902, 130427A) with well-sampled 0.3 keV/10 keV Swift/XRT and optical light-curves, displaying well-monitored four phases (GRB, Tail, Plateau, post-Plateau), to illustrate its power and limitations and to determine its five fundamental parameters. The one-and-only Core parameter Gamma_c*theta_c is determined by the GRB and afterglow temporal milestones (i.e. by the dynamical range of the Tail and Plateau), and may have an universal value around 3.0, with narrower Cores (smaller theta_c) being more relativistic (higher Gamma_c). Flat Plateaus require an Envelope angular structure close to Gamma(theta) ~ 1/theta^2. A
magnetic field parallel to the direction of the relativistic flow yields more curved
prompt/GRB and delayed/afterglow light-curves than a perpendicular or isotropic field, with
curvature $\Delta \alpha$ defined as the difference between the flux asymptotic power-law decay indices at the beginning and end of each phase. The
curvature Delta alpha_grb of the GRB light-curves arising from the uniform Core is set only by the B-field orientation, and a perpendicular or isotropic B-field is equally often found, but a parallel field is never identified for the Core. The
curvature Delta alpha_ag of the afterglow light-curves from the power-law Envelope depends on B-orientation and three other parameters. The Envelope B-field orientation can be identified for only three afterglows, where a parallel B is found.
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Astrophysics of Galaxies
Disentangling Stellar Mass and Environmental Effects on eROSITA AGN Activity using multi-wavelength data from GAMA, WISE, GALEX, and DESI Legacy Survey
Navya Saraswat, Shadab Alam and Subhabrata Majumdar
We use a stellar-mass complete sample of $\sim 36{,}000$
galaxies from GAMA/eFEDS to test whether AGN activity depends on environment independent of stellar mass. To isolate this effect, we introduce $\delta_{\rm rank}$, a novel local overdensity metric defined within narrow stellar mass bins. Our main result is that X-ray-selected AGNs show a clear environmental trend, the AGN fraction rises from $\sim 4\%$ in low-density regions to $\sim 6\%$ in high-density regions, with a sharp transition at $\delta_{\rm rank} \sim 0.4$. This environmental dependence is absent for WISE-selected and broad-line AGN, and Eddington ratios show no variation across environments. We also found that for
galaxies with $\mathrm{SFR} > 15\,M_\odot\,\mathrm{yr}^{-1}$, X-ray AGN fraction is approximately 5%, 60% and 20% in low, mid and high $\delta_{\rm rank}$
galaxies. We show that relationship between AGN fraction and SFR varies dramatically with both environment and selection method. We conclude that environment on scales of $\sim 5h^{-1}\,\mathrm{Mpc}$ regulates X-ray AGN triggering efficiency.
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High Energy Astrophysical Phenomena
Neutron Star Merger Universality Relations for a Quark–Hadron Crossover Equation of State
Wei Sun, Grant J. Mathews and Atul Kedia
A crossover transition from hadronic to
quark matter during the merger of
neutron stars can lead to interesting observational consequences in the emergent gravitational radiation. In particular, the increased pressure in the crossover density region ($2-5$ times the nuclear saturation density) can lead to an extended duration of high-frequency ($\sim 2-3.5$ kHz) gravitational wave emission during the post-merger epoch. We study the universality relations for a variety of equations of state with and without a crossover transition to a
quark mater based upon the QHC19 and QHC21 EoSs. We find that the long-duration post-merger gravitational-wave emission is more pronounced in the QHC21 EoS. We then consider universality relations among various observable quantities that have been identified in simulations with hadronic equations of state. We show that there are deviations from the universality relations for some quantities that may ultimately provide an observational signal that such a transition has occurred.
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Solar and Stellar Astrophysics
The Post-Outburst Spectrum of L1415-IRS
Lynne A. Hillenbrand, Thomas P. Greene, Evan R. Portnoi et al. (+1 more)
We present near-infrared spectroscopic observations of L1415-IRS, a Class I type young stellar object that underwent a factor of $>$30 photometric outburst in the early 2000s. Ours is the first reported spectroscopy of this deeply embedded source. Both components of L1415-IRS present continuum plus $H_2$ emission. The southern component has additional weak HI Br$\gamma$ and $CO$ emission. Based upon our spectrum and evidence of a declining lightcurve, we hypothesize that either: the outburst was not large enough and therefore hot enough to produce the FU Ori type absorption spectrum commonly interpreted as an
accretion disk atmosphere, or the source has already recovered from FU Ori-type equatorial
accretion to re-establish T Tauri-type magnetospheric
accretion.
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All authors Lynne A. Hillenbrand, Thomas P. Greene, Evan R. Portnoi, Bringfried Stecklum
High Energy Astrophysical Phenomena
Formation of Merging Black Hole Binaries Inside Massive AGN Stars
Sizheng Ma, Douglas N. C. Lin and David A. Velasco-Romero
Stars embedded in the disks of active galactic
nuclei (AGN) can grow to hundreds of solar masses, and the same disks are expected to host a population of stellar-mass black holes. A
star may therefore capture passing black holes, turning its interior into a potential factory for forming compact binary black hole systems and, ultimately, for the gravitational-wave events seen by our detectors. We test how readily this channel operates using three-dimensional hydrodynamic simulations of black hole–
star encounters. As a proof of principle, we adopt a
star-to-black-hole mass ratio of $34\!:\!1$, and find that the capture does not disrupt the structure of the
star: it heats the
star by $10$–$30\%$, and the
star loses only $\lesssim1\%$ of its mass throughout. The captured black hole loses its orbital angular momentum rapidly to dynamical friction, sinking to the stellar center within a stellar dynamical time $\lesssim10^{4}\,$s. If the
star already harbors a black hole at its center, the two form a bound binary whose gravitational-wave coalescence time falls below $10^{4}\,$yr. Depending on the geometry and speed of the encounter, the resulting orbit may be nearly circular or retain substantial eccentricity. Our calculations confirm that black hole–
star encounters in AGN disks are indeed a viable channel for assembling stellar-mass black hole binaries and supplying sources for gravitational-wave detectors.
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Astrophysics of Galaxies
Cosmic Noon Galaxies in the Hubble Ultra Deep Field with MIRI Wide-Field Slitless Spectroscopy
S. Kendrew, G. Östlin, L. Colina et al. (+29 more)
We present results from a survey of the Hubble Ultra-Deep Field using the Wide-Field Slitless Spectroscopic (WFSS) capability of the Mid-Infrared Instrument (MIRI) on JWST, demonstrating the capabilities of this new mode. We describe the data reduction and calibration methodology, and estimate calibration uncertainties. From our observations we obtain spectra of 47
galaxies with confirmed spectroscopic
redshifts, with a maximum z_spec of 3.712. In the final sample we target in particular the 3.3 um Polycyclic Aromatic Hydrocarbon (PAH) feature, which has recently gathered interest as a
star formation rate indicator and diagnostic for the dust grain size distribution in
star forming
galaxies from the local Universe to intermediate
redshifts. The feature falls into the WFSS wavelength region for
redshifts 0.67 to 3.1 - providing full coverage of the peak
star formation “
Cosmic Noon” era (1 < z < 3) and connecting dust properties in this critical
galaxy evolution period with local-Universe and low-
redshift observations. Using the
galaxies in our sample in this
redshift regime, we test correlations identified in lower-
redshift samples in the near-infrared or targeted programs in the mid-infrared, finding the WFSS spectra, even with large calibration uncertainties, show good agreement with complementary samples. The 3.3 um PAH luminosities follow previously established correlations with total IR luminosity and SED-derived
star formation rates, confirming this feature's power as tracer of dust-obscured
star formation. Our work illustrate the potential of the MIRI WFSS mode for studies of
Cosmic Noon-era
galaxies in particular in an observationally efficient way.
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All authors S. Kendrew, G. Östlin, L. Colina, P. Rinaldi, J. Álvarez-Márquez, L. A. Boogaard, J. Melinder, J. P. Pye, A. Alonso Herrero, K. I. Caputi, A. Crespo Gómez, M. Garcia Marín, S. Gillman, S. Harish, E. Iani, D. Langeroodi, A. Labiano, P. G. Pérez-González, P. van der Werf, F. Walter, G. Wright, T. Greve, M. Güdel, T. Henning, P.-O. Lagage, T. P. Ray, B. Vandenbussche, E. van Dishoeck, S. Alberts, R. Fernandez Aranda, A. Petric, I. Shivaei
Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc
Bridging the chirp-mass gap: searching for gravitational waves from inspiraling subsolar-mass ultra-compact binaries using LIGO O4ab data
Andrew L. Miller
Gravitational-wave searches for subsolar-mass compact binaries have left a gap between the planetary-mass regime accessible to long-duration searches and the higher masses probed by conventional compact-binary analyses. We close this gap by searching for the inspiral of ultra-compact binaries with chirp masses $10^{-2}$–$10^{-1}\,M_\odot$ in LIGO O4ab data. Using the statistically robust BinaryGFH-v2 method, a time-frequency technique to search for rapidly evolving inspirals lasting minutes to hours, we cover this previously challenging parameter space with a computational cost of only $\sim 350$ CPU-hours, and find no significant candidates. We thus set 90% confidence upper limits on the binary merger-rate density, providing the first gravitational-wave constraints across this chirp-mass range. Assuming that primordial-black-hole binaries formed in the early Universe with no binary suppression ($f_{\rm sup}=1$), these results provide the first gravitational-wave constraints on the primordial-black-hole \dmh fraction in this mass range, yielding upper limits with $f_{\rm PBH}<1$ for equal-mass binaries with component masses $10^{-2}$–$10^{-1}\,M_\odot$ and asymmetric binaries with secondary masses $m_2=3\times10^{-4}$–$10^{-2}\,M_\odot$ for a primary mass $m_1=2.5\,M_\odot$.
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Earth and Planetary Astrophysics
What the Solar System Can Teach Us About Rocky Exoplanets
Paul K. Byrne, Claire Marie Guimond, Peter A. Cawood et al. (+9 more)
The number of rocky extrasolar
planets being discovered continues to increase, but so too does the apparent diversity of such worlds. As we work to understand the likely bulk properties and thermal, geological, and climatological attributes we might expect of rocky
exoplanets, we can look to the Solar System for guidance. Here, we review the interior, surface, and atmospheric characteristics of modern Earth, and discuss how our homeworld has changed through Solar System history. We then visit in turn Venus, Mars, Mercury and Earth's Moon, and Io, noting how these terrestrial
planets are alike, how they are different, and how they have evolved through time. Finally, we consider some examples of the types of rocky worlds known or suspected to exist without direct Solar System counterparts — but we argue that even then, for all the variety we might expect, they are still variations on a common theme. Perhaps the most important lesson the Solar System can teach us is that terrestrial bodies change through time, sometimes dramatically, and that rocky worlds of similar size and mass can have vastly different planetary outcomes.
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All authors Paul K. Byrne, Claire Marie Guimond, Peter A. Cawood, Michael J. Way, Doris Breuer, Tilman Spohn, João C. Duarte, Diogo L. Lourenço, Francesca Miozzi, Maëlis Arnould, Nicolas Coltice, Stephanie L. Olson
Astrophysics of Galaxies
Gravitational Wave Bias in IllustrisTNG300 from Machine-Learned Population-Synthesis Calibrated Merger Rates
Dorsa Sadat Hosseini, Taisiia Karasova, Nikshay Chugh et al. (+2 more)
The large scale clustering of gravitational wave (GW) sources offers an independent probe of cosmology and compact-binary astrophysics. Interpreting future GW clustering measurements requires understanding how merger rates depend on host-
galaxy properties and environment. We develop a simulation-based framework to model the clustering of binary black hole (BBH) mergers and estimate the corresponding GW bias. We populate IllustrisTNG300 with GW sources using a machine-learning emulator trained on the GALAXYRATE data set, which provides population-synthesis calibrated merger rates linked to
galaxy formation histories. This enables physically motivated mock GW catalogs with
galaxy-specific BBH merger rates that depend on stellar mass,
star formation rate,
metallicity, and
redshift. We compare this framework to a simpler phenomenological model in which the merger rate depends only on stellar mass. In both approaches, GW sources are more strongly biased than the overall
galaxy population, reflecting the preference for BBH mergers to occur in more massive, more strongly clustered
halos. The machine-learned model also produces stronger scale dependence in the GW bias than stellar-mass weighting alone, indicating that additional host properties affect GW clustering. We further find that
metallicity correlates positively with both GW and
galaxy bias. These results highlight the importance of physically motivated merger rate models for GW large scale structure analyses and provide a framework for interpreting future clustering measurements from experiments such as the Einstein
Telescope and
Cosmic Explorer.
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All authors Dorsa Sadat Hosseini, Taisiia Karasova, Nikshay Chugh, Alex Krolewski, Ghazal Geshnizjani
Astrophysics of Galaxies
Filament Formation via Collision-induced Magnetic Reconnection – Kinematic Features
Shuo Kong, Griselda Arroyo-Chávez, Volker Ossenkopf-Okada et al. (+4 more)
The Collision-induced
Magnetic Reconnection (CMR) mechanism has recently been proposed as a filament formation model that emphasizes the active role of
magnetic fields. To enable better observational tests with data from modern
telescopes, we provide more specific observable predictions for filaments formed via CMR. Two types of CMR kinematics are revealed in position-velocity (PV) diagrams. First, due to the nature of CMR, the midplane contains a converging motion close to the filament and two diverging motions farther from the filament. They exhibit a blueshift,
redshift, blueshift,
redshift (BRBR) velocity pattern along the line-of-sight when the collision midplane is inclined, giving rise to a special pattern in the PV-diagram. Second, the longitudinal PV-diagram along the filament spine exhibits a velocity oscillating pattern, which is currently attributed to the
magnetic transport of gas clumps in the converging motion. The first type of pattern involves emission primarily outside the filament, which can be confused with the environment. The second type, however, mainly emerges from gas inside the filament, thus more robust for observational tests. Both the integral-shaped filament and the Stick filament in the Orion A cloud show the velocity oscillation in PV-diagrams, although the spatial oscillation frequency and their velocity spread differ.
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All authors Shuo Kong, Griselda Arroyo-Chávez, Volker Ossenkopf-Okada, Héctor G. Arce, Ralf S. Klessen, Duo Xu, Szu-Ting Chen
Earth and Planetary Astrophysics
Rescuing Kepler's False Positives: A Possible Habitable Zone Exoplanet in the Triple Star System KOI-1623
Rita Lanxuan Wang, Daniel Huber, Daniel Hey et al. (+6 more)
The Kepler mission has significantly advanced our understanding of
exoplanet demographics. However, Kepler
planet candidates showing centroid shifts are often classified as false-positives and thus excluded from demographic analyses. Some of these systems may be erroneous false positives, resulting from unresolved binary
stars with
planets orbiting one of the
stars, thereby introducing biases into past
exoplanet demographic studies. Here, we present updated Kepler centroid offset values derived from Gaia Data Release 3 (DR3) and analyze KOI-1623.01, a Kepler centroid offset false positive (COFP) with a clear transit signal and an orbital period of 111 days. High-contrast imaging with Keck/NIRC2 reveals that KOI-1623A has a close companion with a separation of $0.704 \pm 0.002$ arcsec, consisting of a slightly evolved G-type primary and a K-dwarf secondary, while Gaia detects a tertiary companion with common proper motion at a separation of $\sim 8.4$ arcsec. By combining transit photometry, Kepler pixel level data, direct imaging, and Gaia DR3, we find that the transit signal is consistent with a $10.02 ^{+0.44}_{-0.31} R_{\oplus}$ giant
planet located in the habitable zone of the secondary. We furthermore use resolved companions from Gaia to estimate that another $\approx 70$ out of $515$ of Kepler's centroid false positives were misclassified and instead are real
planets that belong to binary systems.
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All authors Rita Lanxuan Wang, Daniel Huber, Daniel Hey, Nathanael Burns-Watson, Alexander Larsen, Adam Kraus, Trent J. Dupuy, Steven Giacalone, Justin R. Crepp
Cosmology and Nongalactic Astrophysics
Peering Beyond the Veil of Last Scattering: A View of the Universe with CMB Spectral Distortions
Bryce Cyr, Nabila Aghanim, Ethan Baker et al. (+37 more)
The frequency spectrum of the
cosmic microwave background is the most precise blackbody ever measured in nature, with deviations constrained at the level of almost one part per million from the COBE satellite. Nevertheless, departures away from a perfect blackbody are present in standard $\Lambda$CDM cosmology, lurking just beneath the surface of our current observational bounds. These spectral distortions provide invaluable information on our thermal history in both the post- and pre-recombination epochs, allowing us to peer beyond last scattering and into the primordial Universe. Here, we present an overview of the underlying physics responsible for generating CMB spectral distortions at all epochs. As an illustration of this rich physics, we review a comprehensive set of mechanisms capable of generating distortions both within and beyond the standard $\Lambda$CDM paradigm. We also discuss the information that can be gleaned by going beyond the monopole (sky-averaged) spectrum and exploiting the spatial information present in anisotropic spectral distortions. To supplement our discussion of the diverse science of spectral distortions, we provide an overview of the upcoming and proposed experimental landscape. We highlight that the combination of the TMS, COSMO, and BISOU experiments will provide the first discovery of a monopole $y$-type distortion within the coming decade. From space, the proposed FOSSIL experiment is forecasted to improve upon the original COBE/FIRAS measurement by roughly three orders of magnitude in sensitivity, bringing with it the detection of the $\Lambda$CDM $\mu$-type distortion sourced by the dissipation of small scale acoustic modes in the pre-recombination
plasma. With transformational measurements on the horizon, CMB spectral distortions offer a uniquely sensitive probe of the thermal history of the Universe at
redshifts $z \lesssim 2 \times 10^6$.
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All authors Bryce Cyr, Nabila Aghanim, Ethan Baker, Elia Stefano Battistelli, Richard Battye, José Luis Bernal, Jens Chluba, Xavier Coulon, William Coulton, Paolo de Bernardis, Eleonora Di Valentino, Guillem Domènech, Sara Evangelista, Giulio Fabbian, Fabio Finelli, J. Colin Hill, Rishi Khatri, Alan Kogut, Guilaine Lagache, Hongwan Liu, Bruno Maffei, Fereshteh Majidi, Abhishek S. Maniyar, C. J. A. P. Martins, Silvia Masi, Jeff McMahon, Daniela Paoletti, Vivian Poulin, Wenzer Qin, Mathieu Remazeilles, José Alberto Rubiño-Martín, Alina Sabyr, Mayuri Sathyanarayana Rao, Carlos Sierra, Jack Singal, Tracy R. Slatyer, Elsa M. Teixeira, Leander Thiele, Léo Vacher, Duncan Watts
Astrophysics of Galaxies
JWST Observations of Starbursts: A Young Bubble in NGC 253's Central Starburst
Kaitlyn E. Sheriff, Elisabeth A. C. Mills, Xinyu Mai et al. (+31 more)
We present a multi-wavelength analysis of a young bubble in the nuclear starburst of NGC 253 using new JWST MIRI-MRS observations together with archival ALMA (100, 350, 690 GHz) and Chandra data. The MIRI maps reveal a prominent bubble-like structure in both ionized and molecular emission lines. The bubble is spatially coincident with one of the least embedded massive young clusters detected with ALMA, suggesting that the cluster is driving the expansion. We measure a radius of $\sim 11.5 \pm 3.4$ pc and an expansion velocity of $\sim 90 \pm 44$ km s$^{-1}$, implying a dynamical age of $\sim 0.1 \pm 0.1$ Myr. Using RADEX modeling of multiple CO transitions, we infer a molecular mass in the range of $(1.3 \pm 0.3) \times 10^4$ to $(2.8 \pm 0.8) \times 10^5$ $M_\odot$. We derive a kinetic energy of order $10^{51}$-$10^{52}$ erg, consistent with mechanical input from Wolf-Rayet stellar winds or
supernovae in a $\sim 10^6$ $M_\odot$ cluster. The existence of a large population of Wolf-Rayet
stars or past
supernovae is supported by the presence of coincident X-ray emission. Our results provide direct evidence that individual clusters in a nuclear environment can carve out coherent structures on parsec scales and inject significant energy and momentum into the surrounding interstellar medium, which can contribute to the nuclear outflow in NGC 253.
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All authors Kaitlyn E. Sheriff, Elisabeth A. C. Mills, Xinyu Mai, Utsav Siwakoti, Sara E. Duval, Alberto D. Bolatto, Rebecca C. Levy, Jia Wei Teh, Torsten Böker, Serena A. Cronin, Daniel A. Dale, Keaton Donaghue, Kimberly Emig, Simon C. O. Glover, Rodrigo Herrera-Camus, Ralf S. Klessen, Thomas S.-Y Lai, Ashley E. Lieber, Laura Lenkić, Sebastian Lopez, David S. Meier, Jüergen Ott, J. D. T. Smith, Yu-Hsuan Teng, Paul P. van der Werf, Sylvain Veilleux, Vicente Villanueva, Rachel Cionitti, Joseph Havens, Maleah Rhem, Nathan Shaw, Kai Smith, Hazel B. Wright, Md. Abdullah Al Zaman
Astrophysics of Galaxies
Forged in Quenching: Morphological Transformation across Star-forming and Quiescent Galaxies in EAGLE
Kai Wang, Carlton Baugh, Sownak Bose et al. (+15 more)
The connection between morphology and quenching in central
galaxies is well established, but its physical origin remains widely debated. We address this by tracing the main progenitor branches of $z=0$
star-forming and quiescent central
galaxies in the EAGLE cosmological simulation from $z\gtrsim4$. Their disc-to-total ratio and triaxiality tracks are indistinguishable until $z\approx 1$-$2$, when both diverge concurrently with the onset of quenching, whereas the size and supermassive black hole (SMBH) mass differences are established earlier. We identify four physically distinct channels linking
galaxy morphology and quenching. First, mergers cause size growth, rotation suppression, triaxiality increase, and SMBH growth, with the accumulated SMBH mass subsequently causes the quenching of
galaxies. Second, with merger history controlled,
galaxy morphology modulates SMBH growth throughout the
star-forming phase: compact, dispersion-dominated
galaxies grow their SMBHs faster and are preferentially quenched, producing the size and morphology differences between
star-forming and quiescent
galaxies. Third, at fixed stellar mass and SMBH mass, compactness further facilitates the quenching of
galaxies. Fourth, disc instability transforms compact oblate discs into prolate systems, with substantial size growth and suppressed rotation but negligible stellar mass growth. This secular channel contributes about half of the prolate
galaxy population around $M_{\rm star}\approx 10^{10.6}\,\rm M_\odot$. Prior to quenching, the progenitors of quiescent
galaxies already have smaller sizes, lower disc-to-total ratios, and more massive SMBHs than
star-forming
galaxies at the same epoch, by amounts comparable to their differences at $z=0$. Morphology therefore plays an active role in growing the SMBH and quenching the
galaxy, rather than being passively inherited through progenitor bias.
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All authors Kai Wang, Carlton Baugh, Sownak Bose, Shaun Cole, Carlos S. Frenk, Hao Fu, Cedric Lacey, Shengdong Lu, Aaron Ludlow, Peder Norberg, Yingjie Peng, Katy L. Proctor, Isabel Santos-Santos, Francesco Shankar, Tom Theuns, Enci Wang, Tao Wang, Vivienne Wild
Astrophysics of Galaxies
The Missing Black Hole in the Large Magellanic Cloud: A Dynamical Prediction for Its Present-Day Location
Robin Chisholm, Elena D'Onghia, Niv Drory et al. (+2 more)
There are indications that the Large Magellanic Cloud (LMC) hosts a central supermassive black hole (SMBH), yet no direct detection exists: one reason is that its present-day location need not necessarily coincide with the photometric or kinematic centers. Here we frame a dynamical targeting problem and predict the present-day sky location of the LMC SMBH. We model the recent LMC-Small Magellanic Cloud (SMC) interaction, which induces a bar-disk offset, and integrate SMBH orbits within a realistic, time-dependent LMC-SMC-Milky Way (MW) potential. We show that the MW tidal field at present epoch exceeds that of the SMC by at least an order of magnitude, setting a preferred direction for coherent, vertical low-amplitude displacements, while the disk asymmetry drives in-plane offsets. Projecting onto the observed LMC frame, the mean of our predicted SMBH probability distribution is located at $(\alpha,\delta)=(80.23^{\circ},-69.55^{\circ})$, with a $1\sigma$ confidence ellipse having semi-major and semi-minor axes of $(a,b)=(1.34^{\circ},0.56^{\circ})$, respectively. This mean is $\sim6$ arcminutes north of the adopted dynamical center, while the mode coincides with the dynamical center and lies well within the $1\sigma$ confidence region. Furthermore, all literature estimates of the LMC center considered here fall within the $2\sigma$ confidence region. This provides a concrete, testable target for ongoing and upcoming spectroscopic surveys, transforming the search for the LMC SMBH from an unconstrained problem into a focused observational strategy.
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All authors Robin Chisholm, Elena D'Onghia, Niv Drory, Andrew Fox, Noam Libeskind
High Energy Astrophysical Phenomena
Optical Spectroscopy of a Candidate O-Star X-ray Binary in M33
Margaret Lazzarini, Lara Cullinane, Karoline Gilbert et al. (+8 more)
We present new observations of a candidate O-type donor
star in a high mass X-ray binary (HMXB) in M33. The candidate donor
star is spatially coincident with a hard X-ray point source. The
star's optical magnitude and colors are consistent with what is expected for a massive
star. The four band optical/UV spectral energy distribution (SED) of the donor
star is well fit by an O-giant model with an effective temperature of $\sim$43 kK, mass of 75$_{+9}^{-0.3}$ $M_{\odot}$, and surface
gravity of log(g) of $\sim$3.8. We present four epochs of optical spectroscopy of the donor
star taken with the Keck/DEIMOS spectrograph over the course of approximately one year. Features in the
star's spectrum are consistent with an O-type
star. The
star's spectrum exhibits a strong H$\alpha$ emission line during all epochs of observation with a variable line profile. We measure the
star's radial velocity for each epoch, which we use to fit for a systemic velocity. We find that the systemic velocity is offset from the local gas velocity from HI measurements, suggesting that the system has a peculiar velocity relative to the local gas. We observe a radial velocity shift in the He I absorption lines that is consistent with motion within a binary system.
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All authors Margaret Lazzarini, Lara Cullinane, Karoline Gilbert, Puragra GuhaThakurta, Kyros Hinton, Daniel Stern, O. Grace Telford, Tobin M. Wainer, Benjamin F. Williams, Robert Alexander, Ernesto Ramirez Jr
High Energy Astrophysical Phenomena
Jet Feedback and the Self-Regulated Growth of Black Holes Embedded in AGN Disks
Marguerite Epstein-Martin, Kung-Yi Su, Zoltan Haiman et al. (+1 more)
Accretion disks in active galactic
nuclei (AGN) are expected to host embedded stellar-mass black holes (BHs) whose gas capture rates can be highly super-Eddington. Without feedback, they would rapidly grow and deplete the disk, but at such high feeding rates, radiation is advected inward with the flow. We test whether the resulting
accretion-powered jet and its shocked cocoon can (i) self-regulate the BH's growth rate and (ii) break out of the AGN disk. Using GIZMO, we perform three-dimensional simulations of a $10\,{\rm M}_\odot$ BH, embedded in dense AGN-disk gas, launching a particle-spawned jet whose mass flux scales with the
accretion rate. We explore a range of jet velocities, ambient temperatures and densities, and three cooling regimes. We find that the jet-inflated cocoon stalls near the Bondi radius and maintains a modest aspect ratio and the self-regulated mass flux is determined by momentum balance at the Bondi radius. Cooling controls the mode of self-regulated
accretion, from large-amplitude oscillation cycles at low
metallicity to weak, quasi-steady modulation at solar
metallicity. Scaling our results to outer disk conditions implies that mechanical feedback limits BH growth to within an order of magnitude of the Eddington rate – largely preventing the overgrowth problem. Over the same disk region, the cocoon remains confined within the disk. In the inner disk,
accretion onto the embedded BHs is limited by tidal shear rather than the Bondi radius, and our simulations no longer apply. Whether runaway growth and cocoon breakout remains possible in these inner regions needs further study.
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All authors Marguerite Epstein-Martin, Kung-Yi Su, Zoltan Haiman, Rosalba Perna
High Energy Astrophysical Phenomena
Dynamic Accretion Disk-Corona Connection and Broad Fe K$\alpha$ Variability in Mrk 766 Revealed by Time-Resolved High Resolution X-Ray Spectroscopic Analysis
Ayon Mondal, Tathagata Saha, Arijit Sar et al. (+1 more)
We present a multi-epoch, primarily X-ray study of the narrow-line Seyfert 1 (NLSy1)
galaxy Mrk~766 by combining archival XMM-Newton observations spanning nearly two decades together with a XRISM observation obtained during its Performance Verification Phase. We analyzed the absorption and emission features in the soft X-ray spectra from RGS and EPIC-pn data, revealing a stratified multi-phase potentially ionised absorber with a column density varying at a timescale of $\sim$1 day. The covering fraction shows positive correlation with the continuum flux, however with significant intrinsic scatter, between the epochs. Mrk~766 exhibits a multi-component Fe\,K$\alpha$ emission line as revealed by fits to the XRISM data. A quasi-phenomenological model comprising a
torus and a broad diskline component distinguishes the two components and constrains the inner radius of the potential broad line emitting disk substructure between $R_{in}=40-60\,r_{\rm g}$. Analysis of multiple epochs of the Fe K$\alpha$ complex across XMM-Newton observations reveals that the broad component tracks the continuum flux closely and effectively reflects the trend with respect to the continuum. The narrow component is consistent with a distant emitter, potentially the
torus. Quasi-simultaneous monitoring with Swift-XRT and UVOT further indicates UV emission lagging the X-rays by $5.9^{+4.1}_{-5.7}$ hours, consistent with disk reprocessing of coronal X-ray fluctuations, and corresponds to a light travel distance of up to $10^{3}\,r_{\rm g}$. Overall, our analysis maps the extent of the dynamically evolving absorber, the iron line emitter, and the
accretion disk in this highly accreting NLSy1 system.
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All authors Ayon Mondal, Tathagata Saha, Arijit Sar, Ritaban Chatterjee
High Energy Astrophysical Phenomena · Cross-listed from hep-ph
Multiple-Mediator-Affected Ultra-High-Energy Neutrino Attenuation: Hints for 5D ${U(1)}_{L_\mu - L_\tau}$ in IceCube
Atri Bhattacharya, Ayushi Kaushik and Kenji Nishiwaki
Recent IceCube observations point to the ultra-high energy
neutrino flux exhibiting a very soft spectral nature beyond tens of TeV, which is unexpected from the standard
cosmic-ray
neutrino connection. As a potential explanation for this behaviour, we investigate
neutrino self-interactions in an extra-dimensional $ {U(1)}_{L_{\mu}-L_\tau} $ gauge theory, where
symmetry breaking leads to a tower of Kaluza-Klein gauge
bosons in compactified four dimensions. These multiple gauge
bosons may enhance the attenuation of astrophysical
neutrinos as they propagate in the C$\nu$B medium. Unlike single-mediator scenarios, for example, which arise from broken $ {U(1)}_{L_\mu - L_\tau} $ gauge
symmetries in four dimensions, the presence of a Kaluza-Klein tower of mediators produces multiple closely spaced resonances whose interference gives rise to a rich energy-dependent behaviour of the scattering cross-section over a vast range of incident
neutrino energies. Alongside $s$-channel resonances, off-resonant $t$- and $u$-channel contributions also become important. We explore the possibility that repeated resonant scattering between astrophysical
neutrinos and those in the C$\nu$B, mediated by these new multiple gauge
bosons may increasingly attenuate the former at higher energies, thereby addressing the possibility of softening its spectral nature within the consideration of a single power law.
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High Energy Astrophysical Phenomena
A pair of unequal jets imparted the kick velocity to the neutron star of Cassiopeia A
Dmitry Shishkin, Muhammad Akashi and Noam Soker
We analyze observations of the Cassiopeia A core-collapse
supernova remnant (CCSNR) and identify a pair of opposite rings, attributing their formation to unequal-energy explosion jets that also imparted the
neutron star (NS) its kick velocity via the kick-BEAP (kick by early asymmetrical pair of jets) mechanism. We replicate the formation of these ring structures, which we identify as circum-jet rings, with three-dimensional hydrodynamical simulations of two opposite explosion jets in which the northern jet is much more powerful, forming the larger northern ring known as the Crown. The line connecting the centers of the two opposite rings passes through the previously-established point-symmetric center of Cassiopeia A. The momentum difference between the two jets also explains the observed southwards NS kick velocity of ~400 km/s. The kick-BEAP and point-symmetric morphology suggest powering and shaping by several pairs of jets during the explosion process, strengthening the jittering jets explosion mechanism for the Cassiopeia A CCSNR and core-collapse
supernovae in general.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Analog Gravity in Magneto-Viscous Fluids: Enhanced Analog Hawking Temperature in Accretion Disks
Aliv Sahoo, Mayank Pathak and Banibrata Mukhopadhyay
We present a three-dimensional visco-magnetoacoustic framework for analog
gravity in magnetohydrodynamic (MHD) flows. While standard fluid dissipation typically breaks the Lorentzian signature of acoustic metrics, we demonstrate that evaluating wave perturbations in the eikonal limit alongside the Shakura-Sunyaev $\alpha$-viscosity prescription preserves a well-defined effective spacetime geometry for the fast magnetoacoustic mode. The slow-magnetoacoustic mode and Alfvén mode do not admit a non-degenerate metric. To investigate analog horizon thermodynamics, we utilize astrophysical
accretion disks as background media, specifically modeling numerical magnetized advective
accretion flows around rotating black holes and analytical advection-dominated inflow-outflow solutions (ADIOS). Standard self-similar ADIOS models strictly enforce a constant Mach number, precluding horizon formation. We therefore introduce a
magnetic field perturbation that breaks self-similarity, generates a dynamic Mach number, and enables the formation of a visco-magnetoacoustic horizon. By evaluating the spontaneous
phonon emission at these horizons, we reveal that the analog Hawking temperature is highly sensitive to the
magnetic field
topology; the spatial orientation of the background
magnetic gradients dictates whether the Hawking radiation is amplified or suppressed. Furthermore, we find that increasing the viscosity parameter leads to a monotonic increase in the analog Hawking temperature.
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Astrophysics of Galaxies
Population III Host Candidates at $z\sim2$: Strong He II $\lambda1640$ and Absent UV Metal Lines in HETDEX Ly$\alpha$ Emitters
Mahan Mirza Khanlari, Dustin Davis, Karl Gebhardt et al. (+15 more)
Population III (Pop III)
stars are expected to produce extremely hard ionizing spectra, yet direct evidence for their signature–strong narrow He II $\lambda1640$ emission with weak or absent metal lines–remains elusive. Although Pop III formation peaks at $z\gtrsim10$, models predict that nearly pristine gas pockets can survive to later times, making intermediate-
redshift searches a probe of metal mixing across
cosmic time. We search 109,545 high-confidence Ly$\alpha$-emitting
galaxies at $1.9<z<2.3$ in the Hobby–Eberly
Telescope Dark Energy Experiment's database for systems with strong He II $\lambda1640$ and no detected UV metal lines, recovering eight candidates and a first-order, tentative comoving number density of $\sim30$ Gpc$^{-3}$. The eight-object stack yields a rest-frame He II equivalent width of $28.7\pm6.7$ Angstrom, He II/Ly$\alpha=0.354\pm0.094$, and a resolution-corrected He II FWHM of $449\pm105$ km s$^{-1}$. We find no significant N V, C IV, or O III] emission; using the He II width to set the scale, we derive $3\sigma$ upper limits of N V/He II $<0.167$, C IV/He II $<0.128$, and O III]/He II $<0.140$. This combination of strong He II, elevated He II/Ly$\alpha$, moderate line width, and weak metal lines is difficult to reproduce with metal-enriched stellar populations, AGN narrow-line regions, shocks, or classical Wolf–Rayet features. We interpret the sample as candidates for very metal-poor or Pop III-like ionizing sources at $z\sim2$. Definitive confirmation requires deeper rest-UV spectroscopy to measure He II and metal-line limits in individual objects, followed by rest-optical spectroscopy of H$\beta$, [O III] $\lambda4959,\lambda5007$, [O II] $\lambda3727$, and H$\alpha$ to constrain gas-phase
metallicity, ionization conditions, and AGN activity.
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All authors Mahan Mirza Khanlari, Dustin Davis, Karl Gebhardt, Tiger Yu-Yang Hsiao, Julian B. Muñoz, Alessandra Venditti, Erin Mentuch Cooper, Robin Ciardullo, Olivia Curtis, Daniel J. Farrow, Steven L. Finkelstein, Caryl Gronwall, Gary J. Hill, Wolfram Kollatschny, Chenxu Liu, Shiro Mukae, Shun Saito, Donald P. Schneider
Cosmology and Nongalactic Astrophysics
CMB-HD Foregrounds: Simulations, Source Detection, and Foreground Removal
Amanda MacInnis, Joshua Ange, Neelima Sehgal et al. (+2 more)
We present simulations of the microwave sky at 2.5 arcsecond resolution over 100 square degrees, generated from existing full-sky, lower-resolution simulations, and use them to demonstrate extragalactic foreground removal for a CMB-HD survey. Our cleaning method detects and removes the
cosmic infrared background and
radio galaxies, yielding source catalogs that are 95% complete down to flux limits of 0.008 and 0.04 mJy at 90 and 148 GHz, respectively. It also identifies and removes
galaxy clusters via the thermal Sunyaev-Zel'dovich effect, producing a cluster sample that is nearly complete above $M_{500c} = 5 \times 10^{13} M_\odot$. After cleaning, the residual foreground-plus-noise power spectrum of the coadded 90 and 148 GHz temperature map is 50% higher than previous idealized estimates, increasing cosmological parameter uncertainties for an 11-parameter $\Lambda$CDM + $N_\mathrm{eff}$ + $\sum m_\nu$ + $T_\mathrm{AGN}$ + $A_\mathrm{kSZ}$ + $n_\mathrm{kSZ}$ model by less than 7%. The small impact on cosmological parameters reflects the strong constraining power of CMB polarization, the temperature-polarization cross spectra, and CMB
lensing spectra reconstructed from polarization-only
estimators, all of which are minimally affected by extragalactic foregrounds. In particular, the survey remains a sensitive probe of light thermal relic particles, achieving $\sigma(N_\mathrm{eff}) = 0.0154$, which can exclude any new species ($\Delta N_\mathrm{eff} \ge 0.027$) with at least 90% confidence. Our simulation and foreground-removal codes are publicly available and should aid the development of analysis pipelines for ultradeep, ultrahigh-resolution microwave surveys.
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All authors Amanda MacInnis, Joshua Ange, Neelima Sehgal, Joshua A. Kable, Isabelle Blackstad
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
The third law of black hole thermodynamics: Quantum versus classical considerations
Shahar Hod and Tsvi Piran
Kehle and Unger have recently shown that, under suitable conditions, a massless charged classical scalar field can produce an extremal black hole in finite time. This result appears to violate the third law of thermodynamics (TLT), since extremal black holes have zero temperature. However, because the TLT is intrinsically a
quantum principle, a self-consistent analysis of this problem must account for
quantum effects. In particular, we point out that the electric field of the system may spontaneously polarize the
vacuum, thereby creating massless charged pairs that will discharge the black hole. This mechanism guarantees the
quantum validity of the TLT in the regime $|eQ|\geq\hbar/2$ (here ${Q,e}$ denote, respectively, the electric charges of the black hole and the scalar field). This leaves open the question of what happens for lower values of $|eQ|/\hbar$. Recently, Schneider derived a classical bound showing that extremal black holes cannot be formed à la Kehle-Unger in the small-charge regime $|eQ|\leq\hbar/3$. His analysis leaves open the possibility that a stronger bound may exist. We
conjecture here that this bound can indeed be strengthened such that classical considerations would prevent extremal (zero-temperature) charged Reissner-Nordström black holes from forming dynamically in the regime $|eQ|<\hbar/2$, where
quantum polarization effects are too weak to preserve the validity of the TLT.
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High Energy Astrophysical Phenomena
On the dynamical accessibility of twin stars
Mahdi Naseri and Vasileios Paschalidis
A sufficiently strong
hadron-to-
quark first-order phase transition can give rise to a third family of stable compact
stars that are commonly referred to as hybrid
hadron-
quark stars. Stable hybrid
stars that have the same gravitational mass as
neutron stars are referred to as twin
stars. Although
equilibrium twin
stars may exist, whether they can be dynamically formed remains an open question. We investigate this problem by examining the gravitational binding energy of competing
equilibrium configurations at fixed baryonic rest mass and by performing general relativistic hydrodynamical simulations of several possible transition channels. While twin
stars are more gravitationally bound than
neutron stars with the same rest mass, this energetic preference alone does not determine the dynamical outcome. Compression and shocks during the
evolution generate thermal pressure that can prevent the system from settling on the cold twin
star branch. Our simulations show that sufficiently rapid cooling can remove this thermal support and enable twin
star formation, whereas slower or no cooling generally favors a
neutron star remnant. Accessing the twin
star branch through the formation channels considered here requires cooling on a timescale comparable to or shorter than the stellar dynamical timescale. Since realistic cooling mechanisms operate on much longer timescales, our results suggest that in channels that conserve the total rest-mass
neutron stars may be dynamically favored even when a more gravitationally bound twin
star configuration exists with the same rest mass. Our results demonstrate a point of principle, at least for equations of state where the
quark deconfinement density does not change appreciably for temperatures up to $10-20\,\textrm{MeV}.$
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Cosmology and Nongalactic Astrophysics
A Dark-matter Origin of Little Red Dots: Early Seeding and Super-Bondi Accretion
Hua-Peng Gu, Fangzhou Jiang, Xian Chen et al. (+2 more)
The "Little red dots" (LRDs) are a population of accreting supermassive black holes (SMBHs) in the early Universe which often exhibit undermassive or even undetectable stellar hosts. Their early emergence, high space density, and extremely large black-hole-to-stellar mass ratios pose a serious challenge to conventional seeding scenarios that rely on baryon for both the formation and growth of black holes. Here we demonstrate that the above anomalies can be naturally resolved if dark matter is self-interacting. We apply a fully relativistic, non-
equilibrium halo-
evolution model, first developed in our earlier work, to trace the complete gravothermal
evolution of self-interacting dark matter (SIDM)
halos, from the initial collapse into BH seeds to the subsequent
accretion of dark matter. We find that in highly concentrated
halos assembled before reionization, gravothermal collapse efficiently produces stellar-mass black-hole seeds within a few hundred million years. Remarkably, and contrary to standard expectations for dark-matter
accretion, heat conduction in SIDM then sustains a prolonged super-Bondi inflow that drives these seeds to supermassive scale by the LRD epoch, without baryonic assistance. The
halo conditions required for completing these processes, together with the probability of avoiding major mergers that disrupt gravothermal
evolution, result in an SMBH population consistent with the observed abundance and
redshift distribution of LRDs. Our findings establish a pathway in which SMBHs are seeded and assembled primarily from dark matter, well before substantial
galaxies form around them, thereby offering both a compelling physical explanation for LRDs and a new observational probe of dark-matter microphysics.
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All authors Hua-Peng Gu, Fangzhou Jiang, Xian Chen, Ran Li, Zi-Xiang Jia
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Dark Radiation Sticks Together: Dark QCD and the Hubble Tension
Matthew R. Buckley, Nicolas Fernandez and Eric Putney
We introduce a model of dark matter charged under a new confining gauge group akin to the Standard Model's
quantum chromodynamics (QCD). The fermionic sector contains both heavy and light
quark-like
fermions charged under the dark QCD. As the Universe cools, the gauge force confines and dark radiation transitions from free particles to low-mass dark pions. This form of dark radiation behaves as an imperfect fluid with nonzero viscosity. If the confinement occurs at the MeV scale, the model can alleviate the tension between early- and late-time measurements of the Hubble parameter, with a
Bayesian posterior of $H_0 = 71.60\pm 0.63$ km/s/Mpc and a $3\sigma$ residual tension across Planck, ACT, BBN, Pantheon+, DESI DR2, and SH0ES. In the late Universe, the heavy dark matter can evade existing constraints on dark matter self-interactions while containing a rich phenomenology that may be accessible in future observations.
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Earth and Planetary Astrophysics
Inhomogeneous Cloud Coverage and Altitude-Dependent Heat Transport on the Hot-Jupiter NGTS-10 Ab from its Optical-to-Infrared Phase Curve
Louis-Philippe Coulombe, Vivien Parmentier, Kevin B. Stevenson et al. (+15 more)
Hot-Jupiters, gas giant
planets with
equilibrium temperatures above 1,000 K, host large temperature gradients between their permanent day- and nightsides, resulting in circulation regimes that have no Solar System analogs. Past Kepler and Spitzer measurements show that the phase curves of these objects typically peak westward of the substellar point at optical wavelengths and eastward in the infrared, indicative of an interaction between circulation and cloud coverage. However, few hot-Jupiters have joint reflected light and thermal emission measurements, preventing a definitive statement as to the link between these phenomena. Here, we present the first phase curve of a hot-Jupiter that separates thermal emission and reflected light through JWST observations of NGTS-10 Ab with the NIRSpec PRISM instrument ($\lambda$=0.5-5.5 $\mu$m). Using the spectrally-resolved phase curve, we jointly retrieve the
planet's thermal and reflected light maps. We show that the temperature and reflectance distributions are anti-correlated, best explained by clouds evaporating from the eastern substellar region ($\varphi\approx$-17 to 48$^\circ$) where temperatures are highest. This is confirmed by comparisons of the phase-resolved spectra with three-dimensional circulation models, which show that NGTS-10 Ab's atmosphere hosts inhomogeneous cloud coverage, likely made-up of $\mu$m-sized silicate particles, and weak atmospheric drag ($\tau_\mathrm{drag}\gt 10^6$ s). Finally, by measuring the spectral variation of the thermal phase curve offsets, we infer a slope of 7.1$\pm$1.9 degrees per pressure dex, indicative of heat transport that becomes more efficient at depth. Future optical and infrared hot-Jupiter phase curve measurements over a wide range of
equilibrium temperatures will enable a complete mapping of the interplay between heat transport and cloud formation in highly-irradiated
exoplanet atmospheres.
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All authors Louis-Philippe Coulombe, Vivien Parmentier, Kevin B. Stevenson, Xianyu Tan, Everett Schlawin, Luis Welbanks, Jake Taylor, Yao Tang, Mike Line, Hinna Shivkumar, Jacob L. Bean, Jean-Michel Désert, Jonathan J. Fortney, Peter Gao, Mark Hammond, Eliza M.-R. Kempton, Thaddeus D. Komacek, Megan Weiner Mansfield
Cosmology and Nongalactic Astrophysics
$\Lambda$CDM in a courtroom
Ofer Lahav and Paul Shah
The cosmological constant $\Lambda$, and its generalization as Dark Energy, makes up roughly 70% of the present-epoch universe and has been part of the dominant $\Lambda$CDM cosmological paradigm for more than a quarter of a century. It has been put on trial by recent measurements of
galaxy clustering, Type Ia
supernovae, and the
cosmic microwave background. In this Perspective we evaluate the strength of the evidence for a constant $\Lambda$, versus Dynamical Dark Energy (DDE) in the light of recent developments. We also assess what the future holds both for our Universe and our understanding of its most mysterious component.
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Cosmology and Nongalactic Astrophysics
Less isn't more: Cosmological bounds on the neutrino masses are robust to changes in the neutrino abundance
Sebastian J. Figueroa, Alexander C. Sobotka and Adrienne L. Erickcek
We investigate how
neutrino-mass constraints from cosmology depend on the assumed thermal history of the universe.
Photon injection after
neutrino decoupling would decrease the
neutrino abundance inferred from the temperature of the
cosmic microwave background (CMB), potentially loosening the upper limit on their masses. We first evaluate how the cosmological
neutrino-mass bound is altered by the decay of massive particles into
photons and dark radiation after Big Bang nucleosynthesis. To test the robustness of this constraint more generally, we also assess the impact of varying the temperature of the
cosmic neutrino background without assuming a specific physical mechanism. We perform MCMC analyses of both frameworks with primary CMB observations from Planck, CMB
lensing measurements from Planck and ACT, and baryon acoustic oscillation data from DESI. For the degenerate mass ordering, the $95\%$ credible limit tightens from $\sum m_\nu<0.0691$ eV in a standard thermal history to $\sum m_\nu<0.0652$ eV if the massive particles decay only into
photons, while the injection of dark radiation in addition to
photons slightly relaxes this limit to $\sum m_\nu<0.0710$ eV. The same pattern holds for the normal and inverted orderings, and the decay scenario shifts the bound on the sum of the
neutrino masses by at most $0.004$ eV for fixed mass splittings. Allowing model-agnostic changes in the
neutrino-to-
photon ratio yields a $95\%$ credible limit of $\sum m_\nu<0.0724$ eV for the degenerate ordering, indicating that the stringency of our
neutrino-mass bounds is not driven by constraints on the decay scenario. We find that the
neutrino temperature and the sum of the
neutrino masses are positively correlated, which implies that reducing the pre-recombination radiation density will only worsen the emerging tension between cosmological bounds on the
neutrino masses and the measured mass splittings.
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