| 1. |
- Brandenburg, Axel, et al.
(author)
-
Hemispheric Handedness in the Galactic Synchrotron Polarization Foreground
- 2020
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 896:1
-
Journal article (peer-reviewed)abstract
- The large-scale magnetic field of the Milky Way is thought to be created by an alpha omega dynamo, which implies that it should have opposite handedness north and south of the Galactic midplane. Here we attempt to detect a variation in handedness using polarization data from the Wilkinson Microwave Anisotropy Probe. Previous analyzes of the parity-even and parity-odd parts of linear polarization of the global dust and synchrotron emission have focused on quadratic correlations in spectral space of, and between, these two components. Here, by contrast, we analyze the parity-odd polarization itself and show that it has, on average, opposite signs in northern and southern Galactic hemispheres. Comparison with a Galactic mean-field dynamo model shows broad qualitative agreement and reveals that the sign of the observed hemispheric dependence of the azimuthally averaged parity-odd polarization is not determined by the sign of alpha, but by the sense of differential rotation.
|
|
| 2. |
- Wang, Z., et al.
(author)
-
Monitoring the Spatio-temporal Evolution of a Reconnection X-line in Space
- 2020
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 899:2
-
Journal article (peer-reviewed)abstract
- Inherently, magnetic reconnection-the process responsible for stellar flares and magnetospheric substorms-is very dynamic in space, owing to magnetic fluctuations and unsteady inflows. However, this process was always explained as a static picture in spacecraft measurements, neglecting the temporal evolution. This picture is not correct. Here we provide the first dynamic picture of magnetic reconnection in space, by monitoring the spatio-temporal evolution of a reconnection X-line at the magnetopause. Surprisingly, we find that the angle of a reconnection X-line can change from 448 to 249 during tens of milliseconds, which is significantly smaller than the characteristic timescale of the reconnection process (t=d(i)/V-A similar to 410 ms). Meanwhile, the spacecraft moves from the inflow region to the outflow region (spatial evolution). This result demonstrates that the magnetic reconnection in space can develop rapidly during tens of milliseconds, and thus that the concept of dynamic reconnection should be invoked instead of a static diagram.
|
|
| 3. |
- Xu, J., et al.
(author)
-
Observation of Energy Conversion Near the X-line in Asymmetric Guide-field Reconnection
- 2020
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 895:1
-
Journal article (peer-reviewed)abstract
- With high-resolution data of the recently launched Magnetospheric Multiscale mission, the present study reports an asymmetric guide-field reconnection at the dayside magnetopause, which has a density asymmetry N-MSP/N-MSH similar to 15 on the two sides of the current sheet and a guide field B-g similar to 1.4B(L, MSH) in the out-of-plane direction, exhibiting all the two-fluid features including Alfvenic plasma jets and Hall field systems. Using the First Order Taylor Expansion method, we identify the X-line of this reconnection. Different from previous observations, the energy conversion dominantly occurs sunward of the X-line, while the local waves arise simultaneously. The qualitative analysis about the electron energization in this reconnection is proposed, suggesting that the guide field may play an important role in modifying the location where the energy conversion occurs together with the electric field near the X-line.
|
|
| 4. |
- Bruno, A., et al.
(author)
-
Solar-cycle Variations of South Atlantic Anomaly Proton Intensities Measured with the PAMELA Mission
- 2021
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 917:2
-
Journal article (peer-reviewed)abstract
- We present a study of the solar-cycle variations of >80 MeV proton flux intensities in the lower edge of the inner radiation belt, based on the measurements of the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) mission. The analyzed data sample covers an similar to 8 yr interval from 2006 July to 2014 September, thus spanning from the decaying phase of the 23rd solar cycle to the maximum of the 24th cycle. We explored the intensity temporal variations as a function of drift shell and proton energy, also providing an explicit investigation of the solar modulation effects at different equatorial pitch angles. PAMELA observations offer new important constraints for the modeling of low-altitude particle radiation environment at the highest trapping energies.
|
|
| 5. |
- Divin, Andrey, et al.
(author)
-
A Fully Kinetic Perspective of Electron Acceleration around a Weakly Outgassing Comet
- 2020
-
In: Astrophysical Journal Letters. - : IOP PUBLISHING LTD. - 2041-8205 .- 2041-8213. ; 889:2
-
Journal article (peer-reviewed)abstract
- The cometary mission Rosetta has shown the presence of higher-than-expected suprathermal electron fluxes. In this study, using 3D fully kinetic electromagnetic simulations of the interaction of the solar wind with a comet, we constrain the kinetic mechanism that is responsible for the bulk electron energization that creates the suprathermal distribution from the warm background of solar wind electrons. We identify and characterize the magnetic field-aligned ambipolar electric field that ensures quasi-neutrality and traps warm electrons. Solar wind electrons are accelerated to energies as high as 50-70 eV close to the comet nucleus without the need for wave-particle or turbulent heating mechanisms. We find that the accelerating potential controls the parallel electron temperature, total density, and (to a lesser degree) the perpendicular electron temperature and the magnetic field magnitude. Our self-consistent approach enables us to better understand the underlying plasma processes that govern the near-comet plasma environment.
|
|
| 6. |
- Li, K. -A, et al.
(author)
-
The Stellar β-decay Rate of 134Cs and Its Impact on the Barium Nucleosynthesis in the s-process
- 2021
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 919:2, s. L19-
-
Journal article (peer-reviewed)abstract
- We have calculated the stellar β-decay rate of the important s-process branching point 134Cs based on the state-of-the-art shell model calculations. At typical s-process temperatures (T ∼ 0.2-0.3 GK), our new rate is one order of magnitude lower than the widely used rate from Takahashi and Yokoi (hereafter TY87). The impact on the nucleosynthesis in AGB stars is investigated with various masses and metallicities. Our new decay rate leads to an overall decrease in the 134Ba/136Ba ratio, and well explains the measured ratio in meteorites without introducing the i-process. We also derive the elapsed time from the last AGB nucleosynthetic event that polluted the early solar system to be >28 Myr based on the 135Cs/133Cs ratio, which is consistent with the elapsed times derived from 107Pd and 182Hf. The s-process abundance sum of 135Ba and 135Cs is found to increase, resulting in a smaller r-process contribution of 135Ba in the solar system.
|
|
| 7. |
- Tews, Ingo, et al.
(author)
-
On the Nature of GW190814 and Its Impact on the Understanding of Supranuclear Matter
- 2021
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 908:1
-
Journal article (peer-reviewed)abstract
- The observation of a compact object with a mass of 2.50-2.67M99.9%. Even if we weaken previously employed constraints on the maximum mass of neutron stars, the probability of a binary black hole origin is still similar to 81%. Furthermore, we study the impact that this observation has on our understanding of the nuclear equation of state by analyzing the allowed region in the mass-radius diagram of neutron stars for both a binary black hole or neutron star-black hole scenario. We find that the unlikely scenario in which the secondary object was a neutron star requires rather stiff equations of state with a maximum speed of sound c(x) >= root 0.6 times the speed of light, while the binary black hole scenario does not offer any new insight.
|
|
| 8. |
- Wang, R., et al.
(author)
-
Electrostatic Turbulence and Debye-scale Structures in Collisionless Shocks
- 2020
-
In: Astrophysical Journal Letters. - : IOP PUBLISHING LTD. - 2041-8205 .- 2041-8213. ; 889:1
-
Journal article (peer-reviewed)abstract
- We present analysis of more than 100 large-amplitude bipolar electrostatic structures in a quasi-perpendicular supercritical Earth's bow shock crossing, measured by the Magnetospheric Multiscale spacecraft. The occurrence of the bipolar structures is shown to be tightly correlated with magnetic field gradients in the shock transition region. The bipolar structures have negative electrostatic potentials and spatial scales of a few Debye lengths. The bipolar structures propagate highly oblique to the shock normal with velocities (in the plasma rest frame) of the order of the ion-acoustic velocity. We argue that the bipolar structures are ion phase space holes produced by the two-stream instability between incoming and reflected ions. This is the first identification of the ion two-stream instability in collisionless shocks.
|
|
| 9. |
- Warnecke, J., et al.
(author)
-
Investigating Global Convective Dynamos with Mean-field Models : Full Spectrum of Turbulent Effects Required
- 2021
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 919:2
-
Journal article (peer-reviewed)abstract
- The role of turbulent effects for dynamos in the Sun and stars continues to be debated. Mean-field (MF) theory provides a broadly used framework to connect these effects to fundamental magnetohydrodynamics. While inaccessible observationally, turbulent effects can be directly studied using global convective dynamo (GCD) simulations. We measure the turbulent effects in terms of turbulent transport coefficients, based on the MF framework, from an exemplary GCD simulation using the test-field method. These coefficients are then used as an input into an MF model. We find a good agreement between the MF and GCD solutions, which validates our theoretical approach. This agreement requires all turbulent effects to be included, even those which have been regarded as unimportant so far. Our results suggest that simple dynamo models, as are commonly used in the solar and stellar community, relying on very few, precisely fine-tuned turbulent effects, may not be representative of the full dynamics of dynamos in global convective simulations and astronomical objects.
|
|
| 10. |
- Zdziarski, Andrzej A., et al.
(author)
-
Hybrid Comptonization and Electron-Positron Pair Production in the Black-hole X-Ray Binary MAXI J1820+070
- 2021
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 914:1
-
Journal article (peer-reviewed)abstract
- We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform an analysis of two joint spectra from NuSTAR and INTEGRAL, covering the range of 3-650 keV, and of an average joint spectrum over the rise of the hard state, covering the 3-2200 keV range. The spectra are well modeled by Comptonization of soft seed photons. However, the distributions of the scattering electrons are not purely thermal; we find they have substantial high-energy tails, well modeled as power laws. The photon tail in the average spectrum is detected well beyond the threshold for electron-positron pair production, 511 keV. This allows us to calculate the rate of the electron-positron pair production and put a lower limit on the size of the source from pair equilibrium. At the fitted Thomson optical depth of the Comptonizing plasma, the limit is about 4 gravitational radii. If we adopt the sizes estimated by us from the reflection spectroscopy of >20 gravitational radii, the fractional pair abundance becomes much less than unity. The low pair abundance is confirmed by the lack of both an annihilation feature and of a pair absorption cutoff above 511 keV in the average spectrum.
|
|
