An official website of the United States government
Abstract Binary neutron star mergers can form short-lived magnetar-like remnants whose magnetically dominated polar towers reachB ∼ 1015–1016G, but the microphysical composition of these outflows remains poorly understood. Combining tower geometries from general-relativistic magnetohydrodynamics simulations with an analytic treatment of QED and hadronic processes, we argue that magnetic reconnection is the most viable particle acceleration channel in this strongly radiative regime, where the current sheets thin to collisionless scales. Purely leptonic pair loading—including the resonant inverse-Compton scattering of soft photons—is bottlenecked by rapid pitch-angle damping and the tendency of one-photon magnetic conversion to populate low Landau levels. Once protons reach mildly relativistic energies (γp ≳ 1.3), however, inelastic proton–proton (pp) collisions inject large-pitch-angle pions that driveπ0 → 2γ → e±cascades with multiplicity atB = 1015G, supplying the perpendicular momentum the leptonic channel cannot maintain. This hadronic route dominates pair loading and channels most of the dissipated magnetic energy into thee±population that could power the nonthermal emission emerging at larger radii. Charged-pion decay, modulated byπ±synchrotron cooling, further seeds a nonthermal neutrino tail up to ∼300 (σp/5) MeV, spectrally distinct from the thermal cooling burst and detectable from sources within ∼100 kpc.
more »
« less
This content will become publicly available on April 23, 2027
Synchrotron-cooled Plasma Distribution in the Outer Magnetosphere of a Neutron Star
Abstract The guiding center formalism is employed to analyze the motion of a charged relativistic particle in an inhomogeneous magnetic field subject to magnetic mirroring and energy loss due to cooling. The governing equation for the evolution of the magnetic moment is derived. An example representing a neutron star's (pulsar or magnetar) magnetosphere is presented to illustrate typical particle orbits. Notably, radiative losses are most pronounced near a trapped particle’s turning point. Depending on the initial particle’s pitch angle, energy loss can become catastrophic, resulting in the rapid migration of the particle into the loss cone and subsequent precipitation onto a neutron star. Conversely, particles with larger pitch angles remain temporarily trapped and form a gradually decaying “cooled-loss-cone” or “funnel” distribution, characterized by the maximum momentum space particle density being located at the edge of the loss cone. The size of the loss cone is energy dependent and scales asαc ∝ γ3/10. Synchrotron losses are strongest in a well-localized region of the magnetosphere, a few hundred to a thousand stellar radii under typical pulsar and magnetar conditions. This region is a plausible site for synchrotron radiation originating in the outer magnetosphere, and could also be responsible for nonpolar coherent pulsar emission, as well as weak fast radio bursts.
more »
« less
- Award ID(s):
- 2409249
- PAR ID:
- 10685103
- Publisher / Repository:
- AAS
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 1002
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 30
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract We investigate a 15‐day period in October 2011. Auroral observations by the Special Sensor Ultraviolet Spectrographic Imager instrument onboard the Defense Meteorological Satellite Program F16, F17, and F18 spacecraft indicate that the polar regions were covered by weak cusp‐aligned arc (CAA) emissions whenever the interplanetary magnetic field (IMF) clock angle was small, |θ| < 45°, which amounted to 30% of the time. Simultaneous observations of ions and electrons in the tail by the Cluster C4 and Geotail spacecraft showed that during these intervals dense (≈1 cm−3) plasma was observed, even as far from the equatorial plane of the tail as |ZGSE| ≈ 13RE. The ions had a pitch angle distribution peaking parallel and antiparallel to the magnetic field and the electrons had pitch angles that peaked perpendicular to the field. We interpret the counter‐streaming ions and double loss‐cone electrons as evidence that the plasma was trapped on closed field lines, and acted as a source for the CAA emission across the polar regions. This suggests that the magnetosphere was almost entirely closed during these periods. We further argue that the closure occurred as a consequence of dual‐lobe reconnection. Our finding forces a significant re‐evaluation of the magnetic topology of the magnetosphere during periods of northwards IMF.more » « less
-
Abstract Relativistic magnetic reconnection is a key mechanism for dissipating magnetic energy and accelerating particles in astrophysics. In the absence of radiative cooling, recent particle-in-cell (PIC) simulations have shown that high-energy particles gain most of their energy in the upstream region, during a short-lived “free phase” where they meander between the two sides of the layer; when they get captured/trapped by the downstream flux ropes, they undergo a “trapped phase,” where no significant energization occurs. Here, we perform a suite of 3D PIC simulations of relativistic reconnection, including inverse Compton (IC) losses in the weakly cooled regime in which the radiation-reaction-limited Lorentz factorγradexceeds the magnetizationσ. We show that electron cooling losses do not appreciably alter the reconnection rate, the structure of the layer, and the physics of particle acceleration in the free phase, so the spectrum of free electrons isdNfree/dγ ∝ γ−1, as in the uncooled case. The spectrum of trapped electrons above the cooling breakγcool(in the rangeγcool < γ < γrad) isdN/dγ ∝ γ−3, steeper than the scalingdN/dγ ∝ γ−2of uncooled simulations. This confirms that no significant particle energization occurs during the trapped phase. Our results validate the model by Zhang et al. for particle acceleration in 3D relativistic reconnection, and imply that radiative emission models of reconnection-powered astrophysical sources should employ a two-zone structure that differentiates between free, rapidly accelerating particles and trapped, passively cooling particles.more » « less
-
Abstract Electron cyclotron harmonic waves (ECH) play a key role in scattering and precipitation of plasma sheet electrons. Previous analysis on the resonant interaction between ECH waves and electrons assumed that these waves are generated by a loss cone distribution and propagate nearly perpendicular to the background magnetic field. Recent spacecraft observations, however, have demonstrated that such waves can also be generated by low energy electron beams and propagate at moderately oblique angles . To quantify the effects of this newly observed ECH wave mode on electron dynamics in Earth's magnetosphere, we use quasi‐linear theory to calculate the associated electron pitch angle diffusion coefficient. Utilizing THEMIS spacecraft measurements, we analyze in detail a few representative events of beam‐driven ECH waves in the plasma sheet and the outer radiation belt. Based on the observed wave properties and the hot plasma dispersion relation of these waves, we calculate their bounce‐averaged pitch angle, momentum and mixed diffusion coefficients. We find that these waves most efficiently scatter low‐energy electrons (10–500 eV) toward larger pitch angles, on time scales of to seconds. In contrast, loss‐cone‐driven ECH waves most efficiently scatter higher‐energy electrons (500 eV–5 keV) toward lower pitch‐angles. Importantly, beam‐driven ECH waves can effectively scatter ionospheric electron outflows out of the loss cone near the magnetic equator. As a result, these outflows become trapped in the magnetosphere, forming a near‐field‐aligned anisotropic electron population. Our work highlights the importance of ECH waves, particularly beam‐driven modes, in regulating magnetosphere‐ionosphere particle and energy coupling.more » « less
-
Abstract Recent advances in numerical simulations of magnetically arrested accretion onto supermassive black holes have shed light on the formation and dynamics of magnetospheric current sheets near the black hole horizon. By considering the pair magnetizationσein the upstream region and the mass accretion rateṁ(in units of the Eddington mass accretion rate) as free parameters we estimate the strength of the magnetic field and develop analytical models, motivated by recent three-dimensional particle-in-cell simulations, to describe the populations of relativistic electrons and positrons (pairs) in the reconnection region.Applying our model to M87*, we numerically compute the non-thermal photon spectra for various values ofσe. We show that pairs that are accelerated up to the synchrotron radiation-limited energy while meandering across both sides of the current sheet, can produce MeV flares with luminosity of ∼ 1041 erg s-1— independent ofσe— for a black hole accreting atṁ=10-5. Pairs that are trapped in the transient current sheet can produce X-ray counterparts to the MeV flares, lasting about a day for current sheets with length of a few gravitational radii. We also show that the upstream plasma can be enriched due to photon-photon pair creation, and derive a new equilibrium magnetization ofσe∼ 103-104forṁ= 10-6- 10-5. Additionally, we explore the potential of magnetospheric current sheets to accelerate protons to ultra-high energies, finding that while acceleration to such energies is limited by various loss mechanisms, such as synchrotron and photopion losses from the non-thermal emission from pairs, maximal proton energies in the range of a few EeV are attainable in magnetospheric sheets forming around supermassive sub-Eddington accreting black holes.more » « less
