Hasil untuk "Astrophysics"

N. B. Bekhti, L. Flöer, R. Keller et al.

Deutsche Forschungsgemeinschaft (DFG) [KA1265/5-1, KA1265/5-2, KE757/71, KE757/7-2, KE757/7-3, KE757/11-1.]; International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne (IMPRS Bonn/Cologne); Estonian Research Council [IUT26-2]; European Regional Development Fund [TK133]; Australian Research Council Future Fellowship [FT150100024]; NSF CAREER grant [AST-1149491]

Jonathan L. Feng

The identity of dark matter is a question of central importance in both astrophysics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief summary of the standard model of particle physics and its outstanding problems. I then discuss several dark matter candidates motivated by these problems, including weakly interacting massive particles (WIMPs), superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, I critically examine the particle physics motivations and present their expected production mechanisms, basic properties, and implications for direct and indirect detection, particle colliders, and astrophysical observations. Upcoming experiments will discover or exclude many of these candidates, and progress may open up an era of unprecedented synergy between studies of the largest and smallest observable length scales.

J. Frieman, M. Turner, D. Huterer

Ten years ago, the discovery that the expansion of the universe is accelerating put in place the last major building block of the present cosmological model, in which the universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy—for example, the quantum energy of the vacuum—or it may signal that general relativity (GR) breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.

L. Sedov

R. Bruno, V. Carbone

In this review we will focus on a topic of fundamental importance for both astrophysics and plasma physics, namely the occurrence of large-amplitude low-frequency fluctuations of the fields that describe the plasma state. This subject will be treated within the context of the expanding solar wind and the most meaningful advances in this research field will be reported emphasizing the results obtained in the past decade or so. As a matter of fact, Helios inner heliosphere and Ulysses’ high latitude observations, recent multi-spacecrafts measurements in the solar wind (Cluster four satellites) and new numerical approaches to the problem, based on the dynamics of complex systems, brought new important insights which helped to better understand how turbulent fluctuations behave in the solar wind. In particular, numerical simulations within the realm of magnetohydrodynamic (MHD) turbulence theory unraveled what kind of physical mechanisms are at the basis of turbulence generation and energy transfer across the spectral domain of the fluctuations. In other words, the advances reached in these past years in the investigation of solar wind turbulence now offer a rather complete picture of the phenomenological aspect of the problem to be tentatively presented in a rather organic way.

N. Gehrels

M. Bachetti, F. Harrison, D. Walton et al.

The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their X-ray luminosities in the 0.5–10 kiloelectronvolt energy band range from 1039 to 1041 ergs per second. Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems. The most challenging sources to explain are those at the luminous end of the range (more than 1040 ergs per second), which require black hole masses of 50–100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3–30 kiloelectronvolt range of 4.9 × 1039 ergs per second. The pulsating source is spatially coincident with a variable source that can reach an X-ray luminosity in the 0.3–10 kiloelectronvolt range of 1.8 × 1040 ergs per second. This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects.

Charles A. Bowesman, Sergei N. Yurchenko, Ahmed F. Al-Refaie et al.

The TIRAMISU code, a new program for computing on-the-fly non-LTE molecular spectra and opacities for solving self-consistent radiative transfer problems in exoplanet atmospheres, is presented. The ultra-hot Jupiter KELT-20 b is used as a case study to identify the wavelength regions at which non-LTE effects may be detectable. It is shown that upper atmospheric OH in vibrational non-LTE should be observable primarily via hot bands in the mid-infrared and enhanced photodissociation in the visible. Varying the abundance of OH in non-LTE demonstrates a nonlinear relationship between the abundance and the strength of non-LTE effects. Using recent calculations of the photodissociation probabilities of OH, it is shown that non-LTE effects can increase the total photodissociation rate by 2 orders of magnitude in the upper atmosphere, which is likely to have a significant impact on atmospheric and chemical modelling. Increases and reductions in the molecular opacities under non-LTE conditions may lead to the mischaracterization of molecular abundances in retrievals that only consider opacities computed under LTE. Collisional data requirements to support future non-LTE modeling for a variety of exoplanet atmospheres and across a wide range of wavelengths are discussed.

Kurpas J., Pires A. M., Schwope A. D. et al.

We report on follow-up observations with XMM-Newton, the FORS2 instrument at the ESO-VLT, and FAST, aiming to characterise the nature of five thermally emitting isolated neutron star (INS) candidates recently discovered from searches in the footprint of the Spectrum Roentgen Gamma (SRG)/eROSITA All-sky Survey. We find that the X-ray spectra are predominantly thermal and can be described by low-absorbed blackbody models with effective temperatures ranging from 50 to 210 eV. In two sources, the spectra also show narrow absorption features at 300–400 eV. Additional non-thermal emission components are not detected in any of the five candidates. The soft X-ray emission, the absence of optical counterparts in four sources, and the consequent large X-ray-to-optical flux ratios > 3000 − 5400 confirm their INS nature. For the remaining source, eRASSU J144516.0–374428, the available data do not allow a confident exclusion of an active galactic nucleus nature. However, if the source is Galactic, the small inferred X-ray emitting region is reminiscent of a heated pulsar polar cap, possibly pointing to a binary pulsar nature. X-ray timing searches do not detect significant modulations in all candidates, implying pulsed fraction upper limits of 13–19% (0.001–13.5 Hz). The absence of pulsations in the FAST observations targeting eRASSU J081952.1–131930 and eRASSU J084046.2–115222 excludes periodic magnetospheric emission at 1–1.5 GHz with an 8σ significance down to 4.08 μJy and 2.72 μJy, respectively. The long-term X-ray emission of all sources does not imply significant variability. Additional observations are warranted to establish exact neutron star types. At the same time, the confirmation of the predominantly thermal neutron star nature in four additional sources highlights the power of SRG/eROSITA to complement the Galactic INS population.

Tanagodchaporn Inyanya, Youn Kil Jung, Hongjing Yang et al.

Planet-formation theories suggest the presence of free-floating planets (FFPs) that are ejected from their formation sites. While these planets emit very little light, they can be identified through gravitational microlensing. Here, we report the discovery of a FFP candidate in the microlensing event KMT-2024-BLG-3237. The observed light curve exhibits strong finite-source effects characterized by a small amplitude (≲0.9 mag) and a short timescale (≲3 days). The analysis yields an Einstein timescale of t _E  = 0.54 ± 0.02 days and an angular Einstein radius of θ _E  = 6.30 ± 0.48 μ as. The measurements make it possible to estimate the lens mass as $M\,\simeq 102\,{M}_{\oplus }\,{({\pi }_{{\rm{rel}}}/16\,\mu {\rm{as}})}^{-1}$ , where π _rel is the relative lens-source parallax. Depending on the unknown π _rel , the lens could be a Neptune-mass planet ( π _rel  ≃ 0.1 mas) or a Saturn-mass planet ( π _rel  ≃ 16 μ as). A Bayesian analysis yields the lens mass $M={67.3}_{-42.5}^{+103.2}\,{M}_{\oplus }$ and the lens distance ${D}_{{\rm{L}}}={7.34}_{-2.11}^{+0.96}\,{\rm{kpc}}$ . This lens is thirteenth isolated microlens with a measurement of θ _E  < 10 μ as. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.

Aya Keller, Nicole Wolff, Karl van Bibber

Ultralight bosonic dark matter in its most general form can be detected through its decay or annihilation to a quasimonochromatic radio line. Assuming only that this line is consistent with the most general properties of the expected phase space of our Milky Way halo, we have developed and carried out a novel model-independent search for dark matter in the L and S bands. More specifically, the search selects for a line that exhibits a Doppler shift with position according to the solar motion through a static halo and similarly varies in intensity with position with respect to the Galactic center. Over the combined L - and S -band range 1020–2700 MHz, radiative annihilation of dark matter is excluded above 〈 σv 〉 ≈ 10 ^−30 cm ^3 s ^−1 , and for decay above λ  ≈ 10 ^−32 s ^−1 .

Doohyeok Lee, Jae Young Cho, Chang Hyon Ha et al.

Abstract The DAMA/LIBRA experiment has reported an annual modulation signal in NaI(Tl) detectors, which has been interpreted as a possible indication of dark matter interactions. However, this claim remains controversial, as several experiments have tested the modulation signal using NaI(Tl) detectors. Among them, the COSINE-100 experiment, specifically designed to test DAMA/LIBRA’s claim, observed no significant signal, revealing a more than 3σ discrepancy with DAMA/LIBRA’s results. Here we present COSINE-100U, an upgraded version of the experiment, which aims to expand the search for dark matter interactions by improving light collection efficiency and reducing background noise. The detector, consisting of eight NaI(Tl) crystals with a total mass of 99.1 kg, has been relocated to Yemilab, a new underground facility in Korea, and features direct PMT-coupling technology to enhance sensitivity. These upgrades significantly improve the experiment’s ability to probe low-mass dark matter candidates, contributing to the ongoing global effort to clarify the nature of dark matter.

Yisheng Tu, Zhi-Yun Li, Zhaohuan Zhu et al.

Jets and outflows are commonly observed in young stellar objects, yet their origins remain debated. Using 3D nonideal magnetohydrodynamic (MHD) simulations of a circumstellar disk threaded by a large-scale open poloidal magnetic field, we identify three components in the disk-driven outflow: (1) a fast, collimated jet, (2) a less collimated, slower laminar disk wind, and (3) a magneto-rotational instability (MRI)-active turbulent disk wind that separates the former two. At high altitudes, the MRI-active wind merges with the laminar disk wind, leaving only the jet and disk wind as distinct components. The jet is powered by a novel mechanism in the star formation context: a lightly mass-loaded outflow driven by toroidal magnetic pressure in the low-density polar funnel near the system’s rotation axis. A geometric analysis of the magnetic field structure confirms that magnetic tension does not contribute to the outflow acceleration, with magnetic pressure acting as the dominant driver. While the outflow in our model shares similarities with the magneto-centrifugal model—such as angular momentum extraction from the accreting disk—centrifugal forces play a negligible role in jet acceleration. In particular, the flow near the jet base does not satisfy the conditions for magneto-centrifugal wind launching. Additionally, the jet in our simulation exhibits strong spatial and temporal variability. These differences challenge the applicability of rotation–outflow velocity relations derived from steady-state, axisymmetric magneto-centrifugal jet models for estimating the jet’s launching radius. For the slower disk wind, vertical motion is driven by toroidal magnetic pressure, while centrifugal forces widen the wind’s opening angle.

V. Ginzburg

Chaitanya Chawak, Francisco Villaescusa-Navarro, Nicolás Echeverri-Rojas et al.

Recent works have discovered a relatively tight correlation between Ω _m and the properties of individual simulated galaxies. Because of this, it has been shown that constraints on Ω _m can be placed using the properties of individual galaxies while accounting for uncertainties in astrophysical processes such as feedback from supernovae and active galactic nuclei. In this work, we quantify whether using the properties of multiple galaxies simultaneously can tighten those constraints. For this, we train neural networks to perform likelihood-free inference on the value of two cosmological parameters (Ω _m and σ _8 ) and four astrophysical parameters using the properties of several galaxies from thousands of hydrodynamic simulations of the CAMELS project. We find that using properties of more than one galaxy increases the precision of the Ω _m inference. Furthermore, using multiple galaxies enables the inference of other parameters that were poorly constrained with one single galaxy. We show that the same subset of galaxy properties are responsible for the constraints on Ω _m from one and multiple galaxies. Finally, we quantify the robustness of the model and find that without identifying the model range of validity, the model does not perform well when tested on galaxies from other galaxy formation models.

Giulia Migliori, R. Margutti, B. D. Metzger et al.

We present the first deep X-ray observations of luminous fast blue optical transient (LFBOT) AT 2018cow at ∼3.7 yr since discovery, together with the reanalysis of the observation at δ t ∼ 220 days. X-ray emission is significantly detected at a location consistent with AT 2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first ∼100 days and would possibly signal the emergence of a new emission component, although a robust association with AT 2018cow can only be claimed at δ t ∼ 220 days, while at δ t ∼ 1350 days contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT 2018cow, which includes the detection of persistent, slowly fading UV emission with ν L _ν ≈ 10 ^39 erg s ^−1 . Similar to previous works (and in analogy with arguments for ultraluminous X-ray sources), these late-time observations are consistent with thin disks around intermediate-mass black holes (with M _• ≈ 10 ^3 –10 ^4 M _☉ ) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass black holes with M _• ≈ 10–100 M _☉ accreting at ≳the Eddington rate cannot be ruled out and provide a natural explanation for the inferred compact size ( R _out ≈ 40 R _☉ ) of the accretion disk years after the optical flare. Most importantly, irrespective of the accretor mass, our study lends support to the hypothesis that LFBOTs are accretion-powered phenomena and that, specifically, LFBOTs constitute electromagnetic manifestations of super-Eddington accreting systems that evolve to ≲Eddington over a ≈100-day timescale.

Da-Cheng Yan, Yan Yan, Zhou Rui

Abstract In this work, we investigate six helicity amplitudes of the four-body $$B_{(s)} \rightarrow (\pi \pi )(K\bar{K})$$ B ( s ) → ( π π ) ( K K ¯ ) decays via an angular analysis in the perturbative QCD (PQCD) approach. The $$\pi \pi $$ π π invariant mass spectrum is dominated by the vector resonance $$\rho (770)$$ ρ ( 770 ) together with scalar resonance $$f_0(980)$$ f 0 ( 980 ) , while the vector resonance $$\phi (1020)$$ ϕ ( 1020 ) and scalar resonance $$f_0(980)$$ f 0 ( 980 ) are expected to contribute in the $$K\bar{K}$$ K K ¯ invariant mass range. We extract the two-body branching ratios $$\mathcal{B}(B_{(s)}\rightarrow \rho \phi )$$ B ( B ( s ) → ρ ϕ ) from the corresponding four-body decays $$B_{(s)}\rightarrow \rho \phi \rightarrow (\pi \pi )(K \bar{K})$$ B ( s ) → ρ ϕ → ( π π ) ( K K ¯ ) based on the narrow width approximation. The predicted $$\mathcal{B}(B^0_{s}\rightarrow \rho \phi )$$ B ( B s 0 → ρ ϕ ) agrees well with the current experimental data within errors. The longitudinal polarization fractions of the $$B_{(s)}\rightarrow \rho \phi $$ B ( s ) → ρ ϕ decays are found to be as large as $$90\%$$ 90 % , basically consistent with the previous two-body predictions within uncertainties. In addition to the direct CP asymmetries, the triple-product asymmetries (TPAs) originating from the interference among various helicity amplitudes are also presented for the first time. Since the $$B_s^0\rightarrow \rho ^0\phi \rightarrow (\pi ^+\pi ^-)(K^+K^-)$$ B s 0 → ρ 0 ϕ → ( π + π - ) ( K + K - ) decay is induced by both tree and penguin operators, the values of the $$\mathcal{A}^\textrm{CP}_\textrm{dir}$$ A dir CP and $$\mathcal{A}^{1}_{\text {T-true}}$$ A T-true 1 are calculated to be $$(21.8^{+2.7}_{-3.3})\%$$ ( 21 . 8 - 3.3 + 2.7 ) % and $$(-10.23^{+1.73}_{-1.56})\%$$ ( - 10 . 23 - 1.56 + 1.73 ) % respectively. While for pure penguin decays $$B^0\rightarrow \rho ^0\phi \rightarrow (\pi ^+\pi ^-)(K^+K^-)$$ B 0 → ρ 0 ϕ → ( π + π - ) ( K + K - ) and $$B^+\rightarrow \rho ^+\phi \rightarrow (\pi ^+\pi ^0)(K^+K^-)$$ B + → ρ + ϕ → ( π + π 0 ) ( K + K - ) , both the direct CP asymmetries and “true” TPAs are naturally expected to be zero in the standard model (SM) due to the absence of the weak phase difference. The “fake” TPAs requiring no weak phase difference are usually none zero for all considered decay channels. The sizable “fake” $$\mathcal{A}^{1}_{\text {T-fake}}=(-20.92^{+6.26}_{-2.80})\%$$ A T-fake 1 = ( - 20 . 92 - 2.80 + 6.26 ) % of the $$B^0\rightarrow \rho ^0\phi \rightarrow (\pi ^+\pi ^-)(K^+K^-)$$ B 0 → ρ 0 ϕ → ( π + π - ) ( K + K - ) decay is predicted in the PQCD approach, which provides valuable information on the final-state interactions. The above predictions can be tested by the future LHCb and Belle-II experiments.

Samidha Shetty, Gordon Brittan, Prasanta S. Bandyopadhyay

Empirical Bayes-based Methods (<i>EBM</i>) is an increasingly popular form of Objective Bayesianism (<i>OB</i>). It is identified in particular with the statistician Bradley Efron. The main aims of this paper are, first, to describe and illustrate its main features and, second, to locate its role by comparing it with two other statistical paradigms, Subjective Bayesianism (<i>SB</i>) and Evidentialism<i>. EBM</i>’s main formal features are illustrated in some detail by schematic examples. The comparison between what Efron calls their underlying “philosophies” is by way of a distinction made between confirmation and evidence. Although this distinction is sometimes made in the statistical literature, it is relatively rare and never to the same point as here. That is, the distinction is invariably spelled out intra- and not inter-paradigmatically solely in terms of one or the other accounts. The distinction made in this paper between confirmation and evidence is illustrated by two well-known statistical paradoxes: the base-rate fallacy and Popper’s paradox of ideal evidence. The general conclusion reached is that each of the paradigms has a basic role to play and all are required by an adequate account of statistical inference from a technically informed and fine-grained philosophical perspective.

Vassilis Amiridis, Stelios Kazadzis, Antonis Gkikas et al.

The Mediterranean, and particularly its Eastern basin, is a crossroad of air masses advected from Europe, Asia and Africa. Anthropogenic emissions from its megacities meet over the Eastern Mediterranean, with natural emissions from the Saharan and Middle East deserts, smoke from frequent forest fires, background marine and pollen particles emitted from ocean and vegetation, respectively. This mixture of natural aerosols and gaseous precursors (Short-Lived Climate Forcers—SLCFs in IPCC has short atmospheric residence times but strongly affects radiation and cloud formation, contributing the largest uncertainty to estimates and interpretations of the changing cloud and precipitation patterns across the basin. The SLCFs’ global forcing is comparable in magnitude to that of the long-lived greenhouse gases; however, the local forcing by SLCFs can far exceed those of the long-lived gases, according to the Intergovernmental Panel on Climate Change (IPCC). Monitoring the spatiotemporal distribution of SLCFs using remote sensing techniques is important for understanding their properties along with aging processes and impacts on radiation, clouds, weather and climate. This article reviews the current state of scientific know-how on the properties and trends of SLCFs in the Eastern Mediterranean along with their regional interactions and impacts, depicted by ground- and space-based remote sensing techniques.

J. Tacza, G. Li, J.‐P. Raulin

Abstract The suppression of high‐energy cosmic rays, known as Forbush decreases (FDs), represents a promising factor in influencing the global electric circuit (GEC) system. Researchers have delved into these effects by examining variations, often disruptive, of the potential gradient (PG) in ground‐based measurements taken in fair weather regions. In this paper, we aim to investigate deviations observed in the diurnal curve of the PG, as compared to the mean values derived from fair weather conditions, during both mild and strong Forbush decreases. Unlike the traditional classification of FDs, which are based on ground level neutron monitor data, we classify FDs using measurements of the Alpha Magnetic Spectrometer (AMS‐02) on the International Space Station. To conduct our analysis, we employ the superposed epoch method, focusing on PGs collected between January 2010 and December 2019 at a specific station situated at a low latitude and high altitude: the Complejo Astronómico El Leoncito (CASLEO) in Argentina (31.78°S, 2,550 m above sea level). Our findings reveal that for events associated with FDs having flux amplitude (A) decrease ≤10%, no significant change in the PG is observed. However, for FDs with A > 10%, a clear increase in the PG is seen. For these A > 10% events, we also find a good correlation between the variation of Dst and Kp indices and the variation of PG.