Session FF03: V: High Energy and Particle Astrophysics

Latest Results from the XMM-SERVS X-ray Survey of the LSST Deep-Drilling Fields

Cosmic X-ray surveys over the past two decades have played a critical role in transforming our understanding of growing supermassive black holes (SMBHs) in the distant universe. I will describe new results from one key survey advancing this effort: the 13.1 deg² XMM-SERVS survey. XMM-SERVS has successfully mapped three LSST Deep-Drilling Fields (DDFs) at 50 ks XMM-Newton depth, focusing on the SERVS areas of W-CDF-S, XMM-LSS, and ELAIS-S1. These fields have first-rate multiwavelength coverage already and are LSST/DES DDFs, MOONS/PFS/4MOST massive spectroscopy fields, prime TolTEC/ALMA fields, and SDSS-V/4MOST multi-object reverberation-mapping fields. About 12,000 X-ray sources have been detected and characterized, the majority of which (86%) are active galactic nuclei (AGNs), and these are presently being studied. I will summarize science investigations including multiwavelength fitting of infrared-to-X-ray spectral energy distributions, identification of the most highly X-ray obscured AGNs, dwarf AGN studies, combined radio/X-ray AGN investigations, and mapping of cosmic SMBH growth as a function of galaxy stellar mass. I will also briefly highlight how the proposed STAR-X MIDEX mission could obtain much more sensitive X-ray coverage of the LSST DDFs as it conducts its 12 deg² Deep and 300 deg² Medium-Deep time-domain surveys.   

Hunting for Galactic PeVatrons in the X-ray Regime

Cosmic rays with energies up to a few PeV (10¹⁵ PeV) are believed to originate from our own galaxy. However, the origin of Galactic cosmic-rays has remained a mystery for over a century since their discovery. The H.E.S.S. observatory discovered a PeVatron within 10 parsecs of the center of our galaxy, which suggests that the supermassive balck hole Sgr A* may be responsible. Despite being one of the least active supermassive black holes, Sgr A* may have acted as a potent particle accelerator during its active stage in the past. In this presentation, I will describe our attempts to find multi-wavelength observational evidence that Sgr A* used to be a PeVatrons, and how we can reconstruct Sgr A* activity history in the past few million years. Finally, I will introduce our ongoing efforts to uncover and identify Galactic PeVatrons beyond the Galactic center and to test whether they bear a nature of supernova remnants, pulsar wind nebula or else.   

Unstable quasi-normal modes in anisotropic neutron stars

Stellar pulsation modes carry information about the interior structure of a star and serve as a way to probe the properties of matter under extreme conditions, like in the neutron star core. In general relativity, the non-radial modes lose energy over time due to the emission of gravitational waves and are therefore described as quasi-normal modes, i.e., confined waves in an open system. These modes are characterized by complex frequencies, where the sign of the imaginary part determines the stability of the mode: either an exponential decay (stable) or an exponential growth (unstable). It has been shown that in an isotropic star, all unstable modes have no oscillations and cannot cause outgoing gravitational radiation. However, this is not proven for the anisotropic case, in which the fluid pressure has a directional dependence. In this talk, I will present our work on a numerical computation of the quasi-normal mode frequencies of an anisotropic neutron star. We derive the pulsation formalism from perturbed Einstein field equations, which we verify with existing results in various known limits. In particular, the real part of the f-mode frequencies deviates from that of the relativistic Cowling approximation by 10%, while the p-mode deviations decrease below 1% as we go higher in the mode order. The trend of this deviation agrees with the known isotropic cases. We then apply the formalism to several anisotropic neutron star models. Our results show that some p-modes become unstable as the anisotropy increases, while they still have non-zero oscillation frequencies. This suggests we have oscillatory unstable modes. I will further discuss the possible implications of such instabilities.   

On "zebra" radio emission of the Crab pulsar. Solving the 15 year old puzzle

The Crab pulsar produces peculiar radio emission during the high-frequency interpulse (HFIP). Its spectrum resembles a "zebra" pattern and consists of emission bands in the 5-30 GHz range. The bands are proportionally-spaced with "delta omega ~ 0.06 omega". Despite extensive theoretical efforts, no satisfactory mechanism explaining the HFIP puzzle has been proposed for over 15 years. Here, we propose a model that resolves this long-standing puzzle.   

Modeling the Solid Neutron-Star Crust with SPH

The crust of a neutron star is a Coulomb crystal that is composed of a lattice of iron-type nuclei. As microphysical calculations show, it is the strongest material known in nature. This leads to potentially observable phenomena such as toroidal oscillations of the crust after giant X-ray flares, continuous gravitational waves from neutron-star mountains, and resonant shattering flares due to crustal failure in a neutron star merger event. Observations of these phenomena can in turn give insight into the properties of dense nuclear matter such as the nuclear equation of state.

However, the dynamical evolution of neutron stars, when modeled numerically, is usually described as a purely fluid dynamics problem. Here, we present the first 3D simulations of neutron-star crustal toroidal oscillations including material strength with the Los Alamos National Laboratory SPH code FleCSPH. The code has been extended by constitutive models for terrestrial and astrophysical solids as well as a fixed general relativistic background metric for compact stars.

In the first half of the talk, we present the numerical implementation of solid material modeling together with standard tests. The second half is on the simulation of crustal oscillations in the fundamental toroidal mode. Here, the crust is initially modeled as a thin layer on top of the neutron star. As it is very sensitive to density fluctuations and numerical interactions with the

core, we discuss different strategies to decouple the crust and core dynamics from each other and suppress numerical variations in crustal density.   

Overview of Astrophysical MeV-Gap Gamma Ray and Dark Matter Detection with GRAMS

Gamma-Ray and AntiMatter Survey (GRAMS), one of the NASA Physics of the Cosmos suborbital missions, is an experiment designed to detect astrophysical gamma rays falling within the MeV range of 0.1 to 100, called MeV-Gap, and also to serve as an indirect method for detecting dark matter. These MeV-gap gamma rays haven't been extensively studied due to limitations in existing instruments to detect them. GRAMS also has the potential to be the first to detect MeV gamma rays from evaporating Primordial Black Holes (PBHs) and neutron star mergers associated with gravitational waves, as well as from galactic neutron star merger remnants. Our focus is on the GRAMS detector design, which has a significant sensitivity improvement, more than an order of magnitude, in detecting MeV-gap gamma rays and is highly sensitive to antideuterons and antiheliums, aiding in the detection of dark matter annihilation or decay. Currently, we are building a small-scale liquid argon time projection chamber detector. I will be presenting the research and development status, including the preparation for the first prototype balloon flight in 2025/2026.   

Search for fast magnetic monopoles with NOvA Far Detector.

The NOvA experiment at Fermilab consists of two functionally identical liquid scintillator detectors, the Near Detector and the Far Detector, to study neutrino oscillations using GeV-scale neutrinos from the Fermilab NuMI beam. Due to its location close to the earth's surface, surface area of 4000 m², and little overburden, the NOvA Far Detector is sensitive to an extensive range of magnetic monopole masses and velocities. With the help of the Far Detector, we are looking for signals of relic monopoles in the cosmic ray flux that might have been produced in the early universe. We have developed a software trigger that continously searches the data stream composed of mostly a kHz of cosmic rays for signals that resemble a magnetic monopole, to ensure a monopole crossing the detector will be recorded for later analysis. The magnetic monopoles are expected to deposit enormous energy through ionization. We have set up a test stand to study the detector electronics response to such a huge signal. In this talk, I will present the status of the search for fast-moving magnetic monopoles with NOvA Far Detector.   

The Unusual Energy-dependence of Be-10/Be-9 Ratio in Cosmic Rays

Recently, Lipari [1] has analyzed the dependence of the ratio Be – 10 / Be – 9 in cosmic rays over the energy interval 0.5 GeV to about 12 GeV per nucleon and concludes that the ratio increases significantly more slowly than expected due to the relativistic time-dilatation of the lifetime of Be – 10. This finding was based on the model in which the spallation of C, O and other heavier nuclei in the collisions with gas in the interstellar medium is exclusively responsible for the production the Be nuclei and the residence-time of cosmic rays in the ISM decreases with energy. We have analyzed the same data assuming that the sources of cosmic rays are spatially and temporally discrete, and a substantial part of the spallation of cosmic rays occurs in the sources before they leak into the ISM. We assume that the residence time of cosmic rays in the sources in energy-dependent, but their subsequent leakage from the Galaxy is independent of energy up to considerably higher energies ~ PeV. In the latter model the isotopic ratio of Be is nearly consistent with the expectations of relativity.