Relativistic Electron Research Articles

Probing Magnetic Fields in Young Supernova Remnants with IXPE

Synchrotron emission from the shocked regions in supernova remnants provides, through its polarization, crucial details about the magnetic field strength and orientation in these regions. This, in turn, provides information on particle acceleration in these shocks. Due to the rapid losses of the highest-energy relativistic electrons, X-ray polarization measurements allow for investigations of the magnetic field to be carried outvery close to the sites of particle acceleration. Measurements of both the geometry of the field and the levels of turbulence implied by the observed polarization degree thus provide unique insights into the conditions leading to efficient particle acceleration in fast shocks. The Imaging X-ray Polarimetry Explorer (IXPE) has carried out observations of multiple young SNRs, including Cas A, Tycho, SN 1006, and RX J1713.7−3946. In each, significant X-ray polarization detections provide measurements of magnetic field properties that show some common behavior but also considerable differences between these SNRs. Here, we provide a summary of results from IXPE studies of young SNRs, providing comparisons between the observed polarization and the physical properties of the remnants and their environments.

Full inverse Compton Scattering: Total transfer of energy and momentum from electrons to photons

In this article we discuss a peculiar regime of Compton Scattering that assures the maximum transfer of energy and momentum from free electrons propagating in vacuum to the scattered photons. We name this regime Full Inverse Compton Scattering (FICS) because it is characterized by the maximum and full energy loss of the electrons in collision with photons: up to 100% of the electron kinetic energy is indeed transferred to the photon. In the case of relativistic electrons, characterized by a large Lorentz factor (γ≫1), FICS regime corresponds to an incident photon energy equal to mec22, i.e. approximately 255.5 keV. We interpret such an astonishing result as FICS being the time reversal of direct Compton Scattering of very energetic photons (of energy much greater than mec2) onto atomic electrons. Although the cross section of Compton scattering is decreasing with the energy of the incident photon, making the process less probable with respect to other reactions (pair production, nuclear reactions, etc.) when high energetic photons are bombarding a target, the kinematics straightforwardly implies that the back-scattered photons would have an energy reaching asymptotically mec22. FICS is instead the unique suitable working point in Compton scattering for achieving the total transfer of (kinetic) energy exactly from the electron to the photon. Experiencing transitions from the initial momentum to zero in the laboratory system, in FICS the electron is also subject to very large negative acceleration; this fact can lead to possible experiments of sensing the Unruh temperature and related photon bath. On the other side of the energy dynamic range, low relativistic electrons can be completely stopped by moderate energy photons (tens of keV), leading to full exchange of temperature between electron clouds and photon baths. Cosmic gamma ray sources can be affected in their evolution by this peculiar FICS regime of Compton scattering.

A VLA Study of Newly Discovered Southern Latitude Nonthermal Filaments in the Galactic Center: Polarimetric and Magnetic Field Properties

A population of structures unique to the Galactic Center (GC), known as the nonthermal filaments (NTFs), has been studied for over 40 yr, but much remains unknown about them. In particular, there is no widely accepted and unified understanding for how the relativistic electrons illuminating these structures are generated. One possibility is that there are compact and extended sources of cosmic rays, which then diffuse along magnetic flux tubes leading to the illumination of the NTFs through synchrotron emission. In this work, we present and discuss the polarimetric distributions associated with a set of faint NTFs in the GC that have only been studied in total intensity previously. We compare the derived polarized intensity, rotation measure, and intrinsic magnetic field distributions for these structures with the results obtained for previously observed GC NTFs. The results are then used to enhance our understanding of the large-scale polarimetric properties of the GC. We then use the derived polarimetric distributions to constrain models for the mechanisms generating the relativistic electrons that illuminate these structures.

Breit interaction in dielectronic recombination of hydrogenlike xenon ions: storage-ring experiment and theory

Abstract Electron-ion collision spectroscopy of the KLL dielectronic recombination (DR) resonances of hydrogenlike xenon ions was performed at a heavy-ion storage ring with a resolving power that is competitive with x-ray spectroscopy of inner-shell transitions in highly charged ions. The $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 , $$KL_{1/2}L_{3/2}$$ K L 1 / 2 L 3 / 2 , and $$KL_{3/2}L_{3/2}$$ K L 3 / 2 L 3 / 2 resonance groups and even parts of their fine structure are individually resolved. The resonance strengths were measured on an absolute scale and compared with results from multi-configuration Dirac–Fock (MCDF) calculations. These are in excellent agreement with the experimental findings when QED effects on the resonance energies and the Breit interaction are considered. As already found for DR of hydrogenlike uranium (Bernhardt et al. in Phys Rev A 83:020701(R), 2011), this interaction is particularly strong for the $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 resonance group. For U$$^{91+}$$ 91 + , it increases the $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 DR resonance strength by 40%. For Xe$$^{53+}$$ 53 + , the increase is found to amount to 25%, confirming the prediction that the influence of the Breit interaction grows with increasing nuclear charge. A comprehensive appendix treats the derivation of experimental and theoretical merged-beams recombination rate coefficients for interacting beams of relativistic electrons and ions. Graphical abstract

Equation of state of the relativistic electron–positron–photon plasma at arbitrary temperature and degeneracy

We consider the plasma made of relativistic electrons and positrons in thermodynamic equilibrium with photons at arbitrary temperature and degeneracy. We establish various thermodynamic identities as a function of the electron and positron chemical potentials and temperature. The high-temperature regime in which the electrons and positrons are ultrarelativistic is recovered. The excess of electrons with respect to positrons or the excess of positrons with respect to electrons depends on the thermodynamic conditions. Doing so, we go beyond what is usually found in the literature where only the high temperature regime is usually tackled.

The Impact of the 8–10 March 2012 Geomagnetic Storm on Inner Zone Protons as Measured by Van Allen Probes

AbstractThe Relativistic Electron Proton Telescope (REPT) instrument on the Van Allen Probes observed a double‐peaked inner zone proton population throughout the 7 year lifetime of the mission. M. Hudson et al. (2023) showed that a strong SEP event accompanied by a CME‐shock in early March 2012 provided the Solar Energetic Proton (SEP) source for the higher L trapped proton population, which then diffused radially inward to be observed by REPT at . The study followed trajectories of SEP protons launched isotropically from a sphere at 7 Re for 2.5 hr in fields calculated by the LFM‐RCM global MHD model, which includes electric fields needed to model the transport and trapping of the protons by the shock, and then a radial diffusion simulation was run for 2 years using the result from the test‐particle simulation as the initial condition. The simulation result was compared with REPT measurement in November 2013 and showed reasonable agreement. However, the simulation overestimated the Phase Space Density by a factor of four due to lack of field line curvature scattering during the storm in the model. In this study, a test‐particle simulation is performed for 2 days following the injection and trapping of protons in March 2012 using TS05 fields to simulate the field line curvature scattering of the trapped SEP due to the buildup of the ring current during the geomagnetic storm. The resulting sample distribution was then weighted using the flux at the end of the two‐hour MHD‐test particle simulation. A radial diffusion simulation is then run using the initial profile that included the loss effect, with improved comparison with REPT measurements after 2 years.

Gain Switching of the Microbunching Instability to Produce Giant Bursts of Terahertz Coherent Synchrotron Radiation.

Relativistic electron bunches used to produce synchrotron radiation are systematically subjected to spontaneous appearance of microstructures, when a high number of electrons are used. In storage rings, this usually leads to an intense coherent emission in the terahetz range, with powers that are orders of magnitude higher than standard incoherent emission. However this emission generally displays an erratic behavior, which has strongly limited its domain of application so far. In this Letter-inspired by the process of gain switching in lasers-we present a new method for controlling the electron bunch dynamics during this instability. We show that it is possible to trigger the formation of the microstructures in the bunch, and also to considerably enhance the peak power of the coherent terahertz bursts. The experimental demonstration of this scheme-based on the modulation of a rf-cavity signal-is performed at the SOLEIL facility.

Brightening and Fading in the Youngest Galactic Supernova Remnant G1.9+0.3: 13 Years of Monitoring with the Chandra X-Ray Observatory

We report 13 years of Chandra monitoring of the youngest Galactic supernova remnant G1.9+0.3, the only remnant known to be increasing in brightness. We confirm the spatially integrated rate of flux increase of (1.2 ± 0.2)% yr−1 (1–7 keV), but find large spatial variations, from −3% yr−1 to +7% yr−1, over length scales as small as 10″ or smaller. We observe relatively little change in spectral slope, though one region shows significant hardening as it brightens by 1% yr−1. Such rates of change can be accommodated by any of several explanations, including steady blast-wave evolution, expansion or compression of discrete plasma blobs, magnetic turbulence, or variations in magnetic-field aspect angle. Our results do not constrain the mean magnetic-field strength, but a self-consistent picture can be produced in which the maximum particle energies are limited by the remnant age (applying both to electrons and to ions) to about 20 TeV, and the remnant-averaged magnetic-field strength is about 30 μG. The deceleration parameter m (average shock radius varying as t m ) is about 0.7, consistent with estimates from overall expansion dynamics and confirming an explosion date of about 1900 CE. Shock-efficiency factors ϵ e and ϵ B (fractions of shock energy in relativistic electrons and magnetic field) are 0.003 and 0.0002 in this picture. However, the large range of rates of brightness change indicates that such a global model is oversimplified. Temporal variations of photon index, expected to be small but measurable with longer time baselines, can discriminate among possible models.

Impact of EMIC Waves on Electron Flux Dropouts Measured by GPS Spacecraft: Insights From ELFIN

AbstractAlthough the effects of electromagnetic ion cyclotron (EMIC) waves on the dynamics of the Earth's outer radiation belt have been a topic of intense research for more than 20 years, their influence on rapid dropouts of electron flux has not yet been fully assessed. Here, we make use of contemporaneous measurements on the same ‐shell of trapped electron fluxes at 20,000 km altitude by Global Positioning System (GPS) spacecraft and of trapped and precipitating electron fluxes at 450 km altitude by Electron Losses and Fields Investigation (ELFIN) CubeSats in 2020–2022, to investigate the impact of EMIC wave‐driven electron precipitation on the dynamics of the outer radiation belt below the last closed drift shell of trapped electrons. During six of the seven selected events, the strong 1–2 MeV electron precipitation measured at ELFIN, likely driven by EMIC waves, occurs within 1–2 hr from a dropout of relativistic electron flux at GPS spacecraft. Using quasi‐linear diffusion theory, EMIC wave‐driven pitch angle diffusion rates are inferred from ELFIN measurements, allowing us to quantitatively estimate the corresponding flux drop based on typical spatial and temporal extents of EMIC waves. We find that EMIC wave‐driven electron precipitation alone can account for the observed dropout magnitude at 1.5–3 MeV during all events and that, when dropouts extend down to 0.5 MeV, a fraction of electron loss may sometimes be due to EMIC waves. This suggests that EMIC wave‐driven electron precipitation could modulate dropout magnitude above 1 MeV in the heart of the outer radiation belt.

The April 2023 SYM‐H = −233 nT Geomagnetic Storm: A Classical Event

AbstractThe 23–24 April 2023 double‐peak (SYM‐H intensities of −179 and −233 nT) intense geomagnetic storm was caused by interplanetary magnetic field southward component Bs associated with an interplanetary fast‐forward shock‐preceded sheath (Bs of 25 nT), followed by a magnetic cloud (MC) (Bs of 33 nT), respectively. These interplanetary structures were led by a coronal mass ejection erupted from the Sun in association with an M1.7 X‐ray flare. At the center of the MC, the plasma density exhibited an order of magnitude decrease, leading to a sub‐Alfvénic solar wind interval for ∼2.1 hr. Ionospheric Joule heating accounted for a significant part (∼81%) of the magnetospheric energy dissipation during the storm main phase. Equal amount of Joule heating in the dayside and nightside ionosphere is consistent with the observed intense and global‐scale DP2 (disturbance polar) currents during the storm main phase. The sub‐Alfvénic solar wind is associated with disappearance of substorms, a sharp decrease in Joule heating dissipation, and reduction in electromagnetic ion cyclotron wave amplitude. The shock/sheath compression of the magnetosphere led to relativistic electron flux losses in the outer radiation belt between L* = 3.5 and 5.5. Relativistic electron flux enhancements were detected in the lower L* ≤ 3.5 region during the storm main and recovery phases. Equatorial ionospheric plasma anomaly structures are found to be modulated by the prompt penetration electric fields. Around the anomaly crests, plasma density at ∼470 km altitude and altitude‐integrated ionospheric total electron content are found to increase by ∼60% and ∼80%, with ∼33% and ∼67% increases in their latitudinal extents compared to their quiet‐time values, respectively.

Quantum electrodynamics study of polarization effects on the properties of high harmonics of an intense ultrashort laser pulse interacting with relativistic electrons

Quantum electrodynamics study of polarization effects on the properties of high harmonics of an intense ultrashort laser pulse interacting with relativistic electrons

Theory of particle beams transport over curved plasma-discharge capillaries

We present a new approach that demonstrates the deflection and guiding of relativistic electron beams over curved paths by means of the magnetic field generated in a plasma-discharge capillary. The active bending plasma (ABP) represents a promising solution that has been recently demonstrated with a proof of principle experiment. An ABP device consists of a curved capillary where large discharges (of the order of kA) are propagated in a plasma channel. Unlike conventional bending magnets, in which the field is constant over the bending plane, in the ABP, the azimuthal magnetic field generated by the discharge grows with the distance from the capillary axis. This features makes the device less affected by the beam chromatic dispersion so that it can be used to efficiently guide particle beams with non-negligible energy spreads. The study we present in the following aims to provide a theoretical basis of the main ABP features by presenting an analytical description of a single-particle motion and rms beam dynamics. The retrieved relationships are verified by means of numerical simulations and provide the theoretical matrix formalism needed to completely characterize such a new transport device. Published by the American Physical Society 2024

The Principal Role of Chorus Ducting for Night‐Side Relativistic Electron Precipitation

AbstractNight‐side chorus waves are often observed during plasma sheet injections, typically confined around the equator and thus potentially responsible for precipitation of ≲100 keV electrons. However, recent low‐altitude observations have revealed the critical role of chorus waves in scattering relativistic electrons on the night‐side. This study presents a night‐side relativistic electron precipitation event induced by chorus waves at the strong diffusion regime, as observed by the ELFIN CubeSats. Through event‐based modeling of wave propagation under ducted or unducted regimes, we show that a density duct is essential for guiding chorus waves to high latitudes with minimal damping, thus enabling the strong night‐side relativistic electron precipitation. These findings underline both the existence and the important role of density ducts in facilitating night‐side relativistic electron precipitation.

The symbiotic recurrent nova V745 Sco at radio wavelengths

Abstract V745 Sco is a Galactic symbiotic recurrent nova with nova eruptions in 1937, 1989 and 2014. We study the behaviour of V745 Sco at radio wavelengths (0.6–37 GHz), covering both its 1989 and 2014 eruptions and informed by optical, X-ray, and γ-ray data. The radio light curves are synchrotron-dominated. Surprisingly, compared to expectations for synchrotron emission from explosive transients such as radio supernovae, the light curves spanning 0.6–37 GHz all peak around the same time (∼18–26 days after eruption) and with similar flux densities (5–9 mJy). We model the synchrotron light curves as interaction of the nova ejecta with the red giant wind, but find that simple spherically symmetric models with wind-like circumstellar material (CSM) cannot explain the radio light curve. Instead, we conclude that the shock suddenly breaks out of a dense CSM absorbing screen around 20 days after eruption, and then expands into a relatively low density wind ($\dot{M}_{out} \approx 10^{-9}-10^{-8}$ M⊙ yr−1 for vw = 10 km s−1) out to ∼1 year post-eruption. The dense, close-in CSM may be an equatorial density enhancement or a more spherical red giant wind with $\dot{M}_{in} \approx [5-10] \times 10^{-7}$ M⊙ yr−1, truncated beyond several × 1014 cm. The outer lower-density CSM would not be visible in typical radio observations of Type Ia supernovae: V745 Sco cannot be ruled out as a Type Ia progenitor based on CSM constraints alone. Complementary constraints from the free–free radio optical depth and the synchrotron luminosity imply the shock is efficient at accelerating relativistic electrons and amplifying magnetic fields, with εe and εB ≈ 0.01 − 0.1.

Unforeseen advantage of looser focusing in vacuum laser acceleration

Acceleration of electrons in vacuum directly by intense laser fields holds great promise for the generation of high-charge, ultrashort, relativistic electron bunches. While the energy gain is expected to be higher with tighter focusing, this does not account for the reduced acceleration range, which is limited by diffraction. Here, we present the results of an experimental investigation that exposed nanotips to relativistic few-cycle laser pulses. We demonstrate the vacuum laser acceleration of electron beams with 100s pC charge and 15 MeV energy. Two different focusing geometries, with normalized vector potential a0 of 9.8 and 3.8, produced comparable overall charge and electron spectra, despite a factor of almost ten difference in peak intensity. Our results are in good agreement with 3D particle-in-cell simulations, which indicate the importance of dephasing.

Evidence for hybrid gamma-ray emission from the supernova remnant G150.3+4.5

The supernova remnant (SNR) G150.3+4.5 was first identified in radio, exhibiting a hard GeV spectrum and a ~1.5º radius. Radio observations revealed a bright arc with an index of ~−0.40, which stands in contrast to the index of ~−0.69 for the rest. This arc is coincident with the point-like Fermi source 4FGL J0426.5+5434 and KM2A source 1LHAASO J0428+5531. The rest of the SNR has a hard GeV spectrum and a soft TeV spectrum, implying a spectral cut-off or break near 1 TeV. Since there is no X-ray counterpart and no pulse signal detected, the gamma-ray (γ-ray) emission mechanism from the SNR and the point-like source appear puzzling. In this work, we reanalyse the γ-ray emission using 14 yr data recorded by Fermi Large Area Telescope and find that the spectrum of the northern half-sphere is compatible with a broken power law with a break at 146 ± 11 eV and photon indices of ΓNorthlobe = 1.54 ± 0.04stat ± 0.07syst (2.28 ± 0.08stat ± 0.12syst) below (above) the break. In addition, the southern half-sphere can be described well with a single power law with ΓSouthlobe =1.95 ± 0.07stat ± 0.09syst. Since the southern half-sphere is well correlated with CO emission, we propose that the γ-ray emission of the northern half-sphere could be dominated by relativistic electrons via inverse-Compton processes, while the southern half-sphere is dominated by cosmic rays via hadronic processes. 4FGL J0426.5+5434 may result from the illumination of a cloud by escaping cosmic rays or recent shock-cloud interaction. Observations from LHAASO-KM2A thus favour the possibility of a cosmic-ray PeVatron candidate, however, leptonic scenarios cannot be ruled out. Further multi-wavelength observations are warranted to confirm the hadronic nature of 1LHAASO J4028+5531.

Initial Results of Low Earth Orbit Space Radiation Dosimeter on Board the Next Generation Small Satellite-2

As human exploration goals shift from missions in low Earth orbit (LEO) to long-duration interplanetary missions, radiation protection remains one of the key technological issues that must be resolved. The low Earth orbit space radiation dosimeter (LEO-DOS) instrument to measure radiation levels and create a global dose map in the LEO on board the the next generation small satellite-2 (NEXTSat-2) was launched successfully on May 25, 2023 using the Nuri KSLV-III in Korea. The NEXTSat-2 orbits the Earth every 100 minutes, in an orbit with an inclination of 97.8° and an altitude of about 550 km above sea level. The LEO-DOS is equipped with a particle dosimeter (PD) and a neutron spectrometer (NS), which enable the measurement of dosimetric quantities such as absorbed dose (D), dose equivalent (H) for charged particles and neutrons. To verify the observations of LEO-DOS, we conducted a radiation dose estimation study based on the initial results of LEO-DOS, measured from June 2023 to September 2023. The study considered four source categories: (i) galactic cosmic ray particles; (ii) the South Atlantic Anomaly region of the inner radiation belt (IRB); (iii) relativistic electrons and/or bremsstrahlung in the outer radiation belt (ORB); and (iv) solar energetic particle (SEP) events.

One‐Step Local Acceleration Process of Ultra‐Relativistic Electrons in the Center of the Outer Radiation Belt: Observations

AbstractUltra‐relativistic (>3 MeV) electrons are considered as a novel, separate population in the Earth's radiation belts since their loss and acceleration features are distinct from relativistic (∼MeV) electrons. The dominant acceleration mechanism of ultra‐relativistic electrons remains a subject of ongoing debate. Some studies suggest that the acceleration mechanism of ultra‐relativistic electrons is energy‐dependent: local acceleration dominants the enhancement of ∼3–5 MeV electrons while two‐step acceleration process leading by radial diffusion effects for ∼7 MeV electrons. However, a recent study (https://doi.org/10.1126/sciadv.abc0380) theoretically demonstrated that local acceleration could accelerate electrons up to >7 MeV directly under the extreme plasma depletion. In this study, we report four enhancement events of ultra‐relativistic electrons that occurred in September and October 2017. Analysis of phase space density (PSD) radial profiles and contours demonstrate that local acceleration plays the dominant role in the enhancements of ∼7 MeV electrons in mid‐September and mid‐October, supported by persistently growing peaks in electron PSD. While enhancements of ultra‐relativistic electrons in other two events still show energy‐dependent phenomenon, our results provide the observation evidence that one‐step local acceleration process can lead to the enhancement of ∼7 MeV electrons in some events. We suggest that the acceleration process of ultra‐relativistic electrons may be subject to the efficiency of local acceleration.

Experimental measurements of gamma-photon production and estimation of electron/positron production on the PETAL laser facility

This article reports the first measurements of high-energy photons produced with the high-intensity PETawatt Aquitaine Laser (PETAL) laser. The experiments were performed during the commissioning of the laser. The laser had an energy of about 400 J, an intensity of 8 × 1018 W·cm−2, and a pulse duration of 660 fs (FWHM). It was shot at a 2 mm-thick solid tungsten target. The high-energy photons were produced mainly from the bremsstrahlung process for relativistic electrons accelerated inside a plasma generated on the front side of the target. This paper reports measurements of electrons, protons and photons. Hot electrons up to ≈35 MeV with a few-MeV temperature were recorded by a spectrometer, called SESAME (Spectre ÉlectronS Angulaire Moyenne Énergie). K- and L-shells were clearly detected by a photon spectrometer called SPECTIX (Spectromètre Petal à Cristal en TransmIssion pour le rayonnnement X). High-energy photons were diagnosed by CRACC-X (Cassette de RAdiographie Centre Chambre-rayonnement X), a bremsstrahlung cannon. Bremsstrahlung cannon analysis is strongly dependent on the hypothesis adopted for the spectral shape. Different shapes can exhibit similar reproductions of the experimental data. To eliminate dependence on the shape hypothesis and to facilitate analysis of the data, simulations of the interaction were performed. To model the mechanisms involved, a simulation chain including hydrodynamic, particle-in-cell, and Monte Carlo simulations was used. The simulations model the preplasma generated at the front of the target by the PETAL laser prepulse, the acceleration of electrons inside the plasma, the generation of MeV-range photons from these electrons, and the response of the detector impacted by the energetic photon beam. All this work enabled reproduction of the experimental data. The high-energy photons produced have a large emission angle and an exponential distribution shape. In addition to the analysis of the photon spectra, positron production was also investigated. Indeed, if high-energy photons are generated inside the solid target, some positron/electron pairs may be produced by the Bethe–Heitler process. Therefore, the positron production achievable within the PETAL laser facility was quantified. To conclude the study, the possibility of creating electron/positron pairs through the linear Breit–Wheeler process with PETAL was investigated.

Galaxy populations and redshift dependence of the correlation between infrared and radio luminosity

We argue that the difference in infrared-to-radio luminosity ratio between local and high-redshift star-forming galaxies reflects the alternative physical conditions –including magnetic field configurations– of the dominant population of star-forming galaxies in different redshift ranges. We define three galactic types, based on our reference model, with reference to ages of stellar populations. “Normal” late-type galaxies dominate the star formation in the nearby Universe; “starburst” galaxies take over at higher redshifts, up to z ∼ 1.5; while “protospheroidal” galaxies dominate at high redshift. A reanalysis of data from the COSMOS field combined with literature results shows that, for each population, the data are consistent with an almost redshift-independent mean value of the parameter qIR, which quantifies the infrared–radio correlation. However, we find a hint of an upturn of the mean qIR at z ≳ 3.5 consistent with the predicted dimming of synchrotron emission due to cooling of relativistic electrons by inverse Compton scattering off the cosmic microwave background. The typical stellar masses increase from normal, to starburst, and to protospheroidal galaxies, accounting for the reported dependence of the mean qIR on stellar mass. Higher values of qIR found for high-z strongly lensed dusty galaxies selected at 500 μm might be explained by differential magnification.