arXiv subjects

W. Peters

Publications and source records attributed to W. Peters.

9 recordsLinked to original sources

arXiv2025

Exploring Compact Symmetric Objects with Complex Morphologies

Compact symmetric objects (CSOs) are a unique class of jetted active galactic nuclei (AGN) defined by sub-kpc radio emission, showing radio structure on both sides of the central engine. CSOs tend to exhibit little to no relativistic beaming, thereby allowing us to determine their physical characteristics, such as the magnetic field strength and particle energy density. Selected with a literature search, we describe VLBI observations, imaging, and analyses of 167 CSO candidates. We identified 65 new bona fide CSOs, thus almost doubling the number of known bona fide CSOs to 144. With our greater breadth of sources, we confirm that edge-dimmed CSOs (CSO-1s) may represent a more diverse population than originally expected. We highlight a number of CSOs with complex morphologies, including candidates for supermassive binary black holes (SBBHs) and CSOs that appear to have morphologies akin to wide-angle tail (WAT) galaxies, which could perhaps indicate that some CSOs are experiencing a galactic merger.

astro-ph.GA↗

arXiv2024

Sporadic radio pulses from a white dwarf binary at the orbital period

Recent observations have revealed rare, previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes, and with periodicity of minutes to an hour. These transient radio signals must originate from sources in the Milky Way, and from coherent emission processes in astrophysical plasma. They are theorized to be produced in the extreme and highly magnetised environments around white dwarfs or neutron stars. However, the astrophysical origin of these signals remains contested, and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source, ILT J1101+5521, whose roughly minute-long pulses arrive with a periodicity of 125.5 minutes. We find that ILT J1101+5521 is an M dwarf -- white dwarf binary system with an orbital period that matches the period of the radio pulses, which are observed when the two stars are in conjunction. The binary nature of ILT J1101+5521 establishes that some long-period radio transients originate from orbital motion modulating the observed emission, as opposed to an isolated rotating star. We conclude that ILT J1101+5521 is likely a polar system where magnetic interaction has synchronised the rotational and orbital periods of the white dwarf. Magnetic interaction and plasma exchange between two stars has been theorized to generate sporadic radio emission, making ILT J1101+5521 a potential low-mass analogue to such mechanisms.

astro-ph.HE↗

arXiv2023

Unstable phenomena in stable magnetospheres: searching for radio flares from magnetic OBA stars using VCSS

Although the majority of hot magnetic stars have extremely stable, $\sim$kG strength surface magnetic fields with simple topologies, a subset undergo small-scale explosions due to centrifugal breakout (CBO). The resulting small-scale flares are typically below the sensitivity of current magentospheric diagnostics and do not generate detectable transient signatures. However, a recently reported radio flare from the hot magnetic star CU Vir suggests that some of the most energetic events do reach detectable levels. Motivated by this, we searched for transient radio sources in the first two epochs of the VLITE Commensal Sky Survey (VCSS) at the position of 761 hot magnetic stars. We report three detections. A false association analysis shows a less than 1% probability that the sources are imaging artifacts. We then examine the stellar parameters of the three stars to understand if they are likely to produce flares. We conclude that while at this stage we cannot make a definitive association of the detections with the stars, the current data are consistent with the hypothesis that the flares originate in the stellar magnetospheres.

astro-ph.SR↗

arXiv2005

Search for particle-bound 26-O and 28-F in p-stripping reactions

We have searched for particle-bound 26-O and 28-F isotopes in the reaction products of secondary 27-F and 29-Ne beams, respectively. No events have been observed. Upper limits for the respective production cross sections by one-p-stripping reactions are established under the assumption that 26-O and 28-F are particle bound. Since the experimental upper limits are much lower than common estimates we conclude that neither 26-O nor 28-F are likely particle bound.

nucl-ex↗

arXiv1999

Coherent Photoproduction of Dileptons on Light Nuclei - a New Means to Learn about Vector Mesons

In the last years much work has been done to learn about the properties of the $ρ$-meson in nuclear matter. In a calculation of the in-medium spectralfunction of the $ρ$ we find that it is strongly modified by baryonic resonances, especially the $D_{13}(1520)$, through its coupling to resonance-hole states. In order to test the predictions of a model for the in-medium properties of the $ρ$-meson so far mostly heavy-ion collision have been used. We argue that the coherent photoproduction of $ρ$-mesons yields additional information about the nature of the medium modification. It can be shown that the production amplitude is sensitive on the momentum dependence of the $ρ$-selfenergy. This can be used to distinguish experimentally between various models for the $ρ$ in matter.

nucl-th↗

arXiv1998

Coherent Photoproduction of Eta-Mesons on Spin-Zero Nuclei in a Relativistic, Non-local Model

The coherent photoproduction of $η$-mesons on spin-zero nuclei is studied in a relativistic, non-local model, which we have previously applied to the coherent photoproduction of pions. We find that different off-shell extrapolations of the elementary production operator lead to large effects in the cross section. We also show that the almost complete suppression of the N(1535) seen in earlier studies on this reaction is a result of the local or factorization approximation used in these works. Non-local effects can lead to a considerable contribution from this resonance. The relative size of the N(1535) contribution depends on the structure of the nucleus under consideration. We give an estimate for the contribution of the N(1520) resonance and discuss the effect of an $η$-nucleus optical potential

nucl-th↗

arXiv1998

Coherent Photoproduction of Pions on Spin-Zero Nuclei in a relativistic, non-local Model

A relativistic and non-local model for the coherent photoproduction of pions on spin-zero nuclei is presented. The production operator is derived from an effective Lagrangian model that contains all the degrees of freedom known to contribute to the elementary process. Using the framework of DWIA, the matrix elements of this operator are evaluated using relativistic bound state wave functions. Final state interactions are accounted for via a pion-nucleus optical potential. The effects of in-medium modifications of the production operator are discussed. We give results for $^{12}C$ and $^{40}Ca$, and compare our calculations to the data of the A2 collaboration at MAMI.

nucl-th↗

arXiv1997

The Spectral Function of the Rho Meson in Nuclear Matter

We calculate the modification of a rho meson in nuclear matter through its coupling to resonance-hole states. Starting from a recently proposed model, we include all four star resonances up to 1.9 GeV. In contrast to previous works, we include not only resonances that couple to the rho in a relative p-wave, but also those that couple to an s-wave state. In addition, we solve the equation for the rho spectral function self-consistently. We find that s-wave resonances affect the in medium spectral function of the rho strongly. In the transverse channel the rho meson is, especially at non zero momentum, completely washed out and can in the presence of nuclear matter no longer be viewed as a resonant excitation of the vacuum. Instead, our model shows a continuum of possible excitations with the quantum numbers of a transversely polarized rho. In the longitudinal channel, however, the rho retains its resonant character in our calculation. As a consequence of the self-consistent treatment we also find a strong enhancement of the widths of the included nucleon resonances in medium.

nucl-th↗

arXiv1995

Production of Eta-Mesons in Collisions of Nucleons and Delta-Resonances

We calculate the cross section for the production of $η$-mesons via \mbox{$ΔN \to N N η$} in a relativistic One-Boson-Exchange-Model. Using this cross section we then determine the probability for the production of an $η$-meson by a $Δ$-resonance moving in nuclear matter. The result is compared to prescriptions in BUU-calculations in which \et-production proceeds both through a direct channel and through the sequential process $Δ\to N π; πN \to N η$.

nucl-th↗