高能暂现源 · 每日文献综述

In 1971 a group from Columbia University detected polarized X rays from the Crab Nebula at about 3 sigma level during a short sounding rocket flight (Novick et al. (1972)). This measurement, later confirmed to high precision (greater than 20 sigma) by the OSO-8 satellite in 1976/77 (Weisskopf et al. (1978)) marks the real beginning of astrophysical X-ray polarimetry. Since then and before the launch of IXPE, surprisingly few additional measurements had been made. X-ray polarimetry is a demanding field of study. To achieve per-cent-level polarization sensitivity, the collection of more than 106 photons in a polarization-sensitive detector is required. As most cosmic X-ray sources are much fainter than the Crab, to do a systematic study of the X-ray sky demands a highly-sensitive payload and a dedicated mission. Further, as the expected polarization will vary with location in extended sources, such as supernovae and pulsar wind nebula, it is vital that the polarimeter should be imaging. First proposed in its current form in 2007, and selected for funding in 2017, the Imaging X-ray Polarimetry Explorer (IXPE) fits this bill. In this chapter we describe: the design and construction of the IXPE payload and spacecraft; its testing and launch; commissioning activities; science operations and; a high-level overview of IXPE science to date. Finally, we conclude with the current status of the Observatory (as of 2025) and prospects for continued operations.

In this article, We investigate the cosmological consequences of a recently developed $f(R,G,\mathcal{T})$ gravitational framework, in which the action is formulated as a general function of the Ricci scalar $R$, the Gauss-Bonnet invariant $G$, and the trace of the energy-momentum tensor $\mathcal{T}$. As one of the most reliable probes of the physical conditions in the early universe, Big Bang nucleosynthesis offers a stringent framework for testing deviations from standard cosmology. We consider four representative models that are analyzed and constrained using observational limits on $\left|ΔT_f/T_f\right|$ and the primordial helium mass fraction $Y_p$. The bounds obtained identify the allowed parameter regions for each model and demonstrate that significant departures from standard cosmology are compatible with nucleosynthesis observations. Our analysis shows that broad regions of the parameter space satisfy existing nucleosynthesis constraints, indicating the consistency of $f(R,G,\mathcal{T})$ gravity with the observed primordial light-element abundances and the established picture of the early universe preserving the observed abundances of light nuclei.

Based on eigendecomposition and matrix perturbation theory, we propose an alternative method for analyzing perturbations in massive gravity that is applicable to both the base theory and its various modifications. We demonstrate the validity of this approach even in seemingly singular regimes. Applying this framework to mass-varying massive gravity, we compute perturbations for all constant-scalar Schwarzschild-(A)dS black hole solutions. We show that, under the condition $β= α^2$, the tensor sector of these black holes reduces to that of general relativity, while the scalar sector remains free from tachyonic instabilities given appropriate parameter choices. A deeper relationship is also revealed between the solutions with generic parameters and those with $β= α^2$: their solution manifolds intersect at a specific locus where the perturbation vanishes.

The redshifted 21 cm line from the hyperfine structure of neutral hydrogen atoms is a promising tracer for the three-dimensional evolution of our universe. Its broad spatial and temporal coverage is crucial for understanding the complex nature of dark matter and dark energy. However, it is very challenging to directly detect the 21 cm signal due to the existence of radio foreground contaminants that are orders of magnitude brighter. Therefore, mitigating the foreground contamination becomes an indispensable task for detecting the 21 cm signal, which is also expected to be correlated with the dark-matter-dominated large-scale structure. The cross-correlations between the neutral hydrogen intensity mapping and galaxy surveys in the future can not only confirm a detection of the 21 cm signal but can also be a complementary probe for 21 cm cosmology. To meet the precision requirements for cosmological studies, it is important to investigate the complex features of the estimated cross-correlations using numerical simulations. In this work, we simulate the low-redshift HI observations and narrow-band optical surveys in the future and obtain the foreground- and bias-mitigated HI-galaxy cross-power spectra, with which we perform a Bayesian analysis to infer cosmological parameters of dynamical dark energy model. The method developed in this work will be important for cosmological studies with the 21 cm intensity mapping.

Microlensing, the influence of stars within a galactic gravitational lens, has emerged as a powerful probe of compact mass and, through differential magnification, sub-parsec scale sources at cosmological distances. The recent discovery of a double-source-plane gravitational lens system in which the most distant source is a quasar offers the prospect of compound microlensing, in which quasar light rays are influenced by compact masses within the two foreground lensing galaxies. Here, we present the first numerical simulations of this "microlensing of microlensing". We consider the recently discovered "Einstein zig-zag" lens, J1721+8842, as a fiducial case, and construct microlensing magnification maps for each of the six quasar images in this system. Due to the secondary microlensing effects of the myriad of initial microimages, the resulting maps contain more complex caustic features than seen in the case of single plane microlensing. This is reflected in the expected lightcurves seen for each of the images.

Context. Accretion streamers connected to protostellar disks and/or envelopes are thought to transport material across several thousand of au. Whether the motions of the gas comprising these streamers are dominated by gravity, large scale external turbulence or the action of magnetic fields is still under scrutiny. Aims. The aim of this work is to understand the role of the magnetic fields in the star-formation processes, in particular the role that magnetic forces have in potentially leading flows of gas and the accretion onto the envelopes and disks orbiting protostars. Methods. First, we try to identify the large-scale accretion streamers toward the high-mass Young Stellar Object GGD 27-MM1 and fit their trajectories using the so-called Mendoza's model, a modification of the classical model of pure gravitational infalling motion of fluid particles in a potential well. Second, we estimate the strength of the magnetic field associated with the streamers. Then, we determine if the streamers are dominated by magnetic or centrifugal forces. Results. Inspecting the Atacama Large Millimeter/submillimeter Array (ALMA) H$_2$CO cube we were able to identify four accretion streamers spreading up to $\sim$7,000 au and fit their trajectories in the position-position-velocity space. The polarized continuum emission reveals a good alignment of the magnetic field and the trajectory of the streamers. Using the Davis-Chandrasekhar-Fermi method, we derive estimates for the magnetic field strength, find that the streamers are sub-alfvénic, and discuss (after estimating energy terms for turbulence, ordered motions, magnetic forces and gravity) a possible qualitative scenario in which, the gravitational well of the GGD 27-MM1 protostar dominates streamer gas motions over turbulence and magnetic forces at distances of $\sim 3,000$ au.

Protostellar core formation and growth in high-mass star-forming regions remain key to understanding massive star birth. We analyze the masses of 839 cores (resolved at scales of a few thousand au) from the ASHES project targeting 39 massive infrared dark cloud clumps. The masses of the three most massive cores scale linearly with the total core mass. They maintain a constant mass fraction of ~25%, 16%, and 10% along the mass growth sequence. These fractions reveal that the progenitor seeds destined to become high-mass cores establish their mass dominance very early. Additionally, the Gini coefficient (a statistical measure of inequality) of the core mass distributions increases along the mass growth sequence, confirming that the relative population of low-mass cores builds up toward later stages. This points to an environment-dependent fragmentation picture: central prestellar seeds rapidly evolve into high-mass cores via transport-driven super-Jeans fragmentation under rapid, non-stationary mass accumulation in high-density, turbulent hubs, subsequently maintain their dominance through supply-limited synchronized growth (at R<1 pc, n_H2>10^5 cm^-3), while the formation of the surrounding low-mass cores is relatively delayed due to their lower gas densities and the lack of non-stationary inflow acceleration effect, resulting in their continuous emergence through Jeans-like fragmentation in lower-density envelopes. This picture is consistent with a gravity-driven scenario where the local free-fall time is the controlling factor. Our analysis suggests that non-stationary density-regulated fragmentation and supply-limited accretion jointly drive the synchronized co-evolution of the core cluster, seamlessly linking small-scale core growth with large-scale reservoir regulation.

The importance of massive stars cannot be overstated: they are powerful probes of the early universe, play a vital role in the chemical and mechanical evolution of their host environments and their end products allow us to study the most extreme physics in the universe. Obtaining accurate stellar and surface parameters for large samples of massive stars is vital to our understanding of how they evolve, and how their births, lives and deaths affect their surroundings. With the large volume of data expected from upcoming spectroscopic surveys, computational limitations will likely be the most important bottleneck impeding our progress. To address this and dramatically decrease computing times, we aim to develop a robust emulator for the FASTWIND radiative transfer and spectral synthesis code. Additionally, we aim to explore alternative fitting methods that have not been feasible until now due to computational costs. We calculate a set of FASTWIND synthetic spectra of OB-type stars, and we train a collection of neural networks to emulate these models. We also develop the open-source python package SpecFANN, which provides users with a suite of fitting methods that can be used with these or other user-generated neural networks. The majority of the trained neural networks reach average accuracies of better than ~0.01-0.1% for photospheric lines and better than ~0.1-1% for wind lines. SpecFANN is able to obtain robust and accurate stellar parameters that are consistent with the literature for a sample of 52 early-type stars. Using SpecFANN we find that we can achieve the same fit in ~1/360,000 of the time when compared to alternative techniques that rely on on-the-fly FASTWIND computations. We have demonstrated that neural networks offer a viable path forward to address the computational limitations of our current atmosphere analysis and stellar parameter determination methods for hot stars.

We demonstrate that in a scalar-tensor theory where dark matter is conformally coupled to a scalar field, a black hole surrounded by a Hernquist dark matter halo can undergo spontaneous scalarization, acquiring a nontrivial scalar field configuration. The astrophysical properties of the dark matter halo directly dictate the quantum-like spectrum of scalar field configurations around the central black hole. Remarkably, the conformal coupling constant becomes discretized and is determined solely by the halo's compactness parameter.

We investigate standard quantum teleportation in a relativistic setting where one participant, Rob, undergoes uniform acceleration while Alice remains inertial. Rob interacts locally with a massive scalar field modeled by an Unruh-DeWitt detector. We show that for small detector energy gaps, the entanglement shared between Alice and Rob increases monotonically with acceleration. For larger gaps, entanglement initially decreases at low accelerations due to the Unruh effect, but is subsequently restored and enhanced at higher accelerations as a consequence of the anti-Unruh effect. This behavior is associated with a reduction of the effective detector temperature, leading to a recovery of quantum coherence previously lost to the field. As a result, the teleportation fidelity increases with acceleration and approaches unity in the high-acceleration regime. Our results demonstrate that the anti-Unruh effect can protect and recover quantum information, providing a potential mechanism to mitigate relativistic degradation of quantum correlations.

We identify two general obstructions to a regular rotating constant-norm vector vacua. Firstly, at the poles of a nondegenerate bifurcation surface, horizon and axial invariance force every smooth symmetry-inheriting one-form to vanish, contradicting a strictly nonzero constant norm. Secondly, we show that a conicity varying along a rotation axis generates a singular curvature that diverges as the inverse proper distance from the axis. We apply these results to a general three parameter family Kerr-disformal solution in Einstein--bumblebee gravity. Consequently, its vector vacuum is not smooth on the axes because of a nonzero constant norm, and the position-dependent conicity leads to genuine curvature singularities on the polar axes. A distinguished nonextremal branch nevertheless admits a regular outer Killing horizon away from the axes and defines an exact local rotating solution in the nonpolar region.

The QCD axion with charm-only Peccei-Quinn coupling evades the isospin-violation problem of light-quark models in $χ$PT. Since $m_c>Λ_{\rm QCD}$, we employ heavy meson chiral perturbation theory (HMChPT) to constrain this GeV-scale axion, including its CP-even radial partner $σ$ and the axion $a$. In HMChPT, $σ$ enters at leading order with $O(1)$ coupling, while $a$ couples only at $O(1/m_c)$ and manifests via axion-pion mixing with $θ_{aπ}\simeq f_π/f_a\sim0.04$. We analyze $D$-meson masses, $D$-$D^*$ splitting, charmonium, $D$-$D$ scattering, $B_s$ mixing, rare $D$ decays, and the channels $K^+\toπ^+a$, $B\to K a$. No fatal HMChPT constraint emerges, unlike the $χ$PT isospin issue. The strongest bound is from $σ$-mediated $B_s$-$\bar B_s$ mixing; axion-pion mixing opens testable rare decays, though rates are slightly below current sensitivity.

We have determined the initial mass function (IMF) and structural properties of the open clusters Alpha Persei, Pleiades and Praesepe using Gaia DR3 astrometry and photometry. Cluster members were identified using primarily Gaia astrometry, supplemented with near-infrared UKIDSS and optical HIPPARCOS survey data with stellar masses down to $0.10-0.17$ MSun. We measure and correct for unresolved binaries in each cluster using photometry and Monte Carlo simulations in a Bayesian framework, finding present-day fractions between ($20.0\pm0.8$)% and ($23.8\pm1.2$)%. Through a novel approach that combines N-body simulations with machine learning emulators and MCMC algorithms, we have also determined the most probable initial number of stars, binary fraction, half-mass radius and mass function under the assumption that early gas removal does not significantly influence the subsequent cluster evolution. We find a best-fitting initial mass function described by a three-stage broken power-law distribution with break masses in the ranges $0.24-0.50$ MSun and $0.91-1.20$ MSun and average slopes of $α_{\rm med}=1.72\pm0.09$ and $α_{\rm high}=2.98\pm0.22$. While the low-mass slope remains sensitive to the adopted mass-luminosity relation, this IMF reproduces the present-day clusters well once their dynamical evolution is taken into account. We find evidence of scatter in the high mass IMF between the individual clusters, as described by the measured dispersions in mass function slopes of $σ_{\rm high}=0.29\pm0.16$, whereas the intermediate mass slope shows no significant variation with $σ_{\rm med}=0.00\pm0.14$. Our findings provide additional evidence for a (compared to a Salpeter MF) top-light IMF and a cluster-to-cluster variation of the IMF. They therefore provide constraints on the universality of the IMF and its dependence on environmental conditions.