Pulsar Wind Research Articles

The contribution of Galactic TeV pulsar wind nebulae to Fermi large area telescope diffuse emission

The large-scale diffuse γ − ray flux observed by Fermi Large Area Telescope (Fermi-LAT) in the 1–100 GeV energy range, parameterized as ∝ E−Γ, has a spectral index Γ that depends on the distance from the Galactic center. This feature, if attributed to the diffuse emission produced by cosmic rays interactions with the interstellar gas, can be interpreted as the evidence of a progressive cosmic ray spectral hardening towards the Galactic center. This interpretation challenges the paradigm of uniform cosmic rays diffusion throughout the Galaxy. We report on the implications of TeV Pulsar Wind Nebulae observed by the High Energy Stereoscopic System (H.E.S.S.) Galactic Plane Survey in the 1–100 TeV energy range for the interpretation of Fermi-LAT data. We argue that a relevant fraction of this population cannot be resolved by Fermi-LAT in the GeV domain providing a relevant contribution to the large-scale diffuse emission, ranging within ~4%–40% of the total diffuse γ-ray emission in the inner Galaxy. This additional component may account for a large part of the spectral index variation observed by Fermi-LAT, weakening the evidence of cosmic ray spectral hardening in the inner Galaxy.

On the nature of fast blue optical transients

ABSTRACT Short rise times of fast blue optical transients (FBOTs) require very light ejected envelopes, $M_{\rm ej} \le 10^{-1} \, \mathrm{M}_\odot$, much smaller than of a typical supernova. The detection by Chandra of X-ray emission in AT2020mrf of LX ∼ 1042 erg s−1 after 328 d implies total, overall dominant, X-ray energetics at the gamma-ray burst level of ∼6 × 1049 erg. We further develop a model of Lyutikov and Toonen, whereby FBOTs are the results of a late accretion-induced collapse of the product of double white dwarf (WD) merger between ONeMg WD and another WD. Small ejecta mass, and the rarity of FBOTs, results from the competition between mass-loss from the merger product to the wind, and ashes added to the core, on a time-scale of ∼103–104 yr. FBOTs proper come from central engine-powered radiation-dominated forward shock as it propagates through ejecta. All the photons produced by the central source deep inside the ejecta escape almost simultaneously, producing a short bright event. The high-energy emission is generated at the highly relativistic and highly magnetized termination shock, qualitatively similar to pulsar wind nebulae. The X-ray bump observed in AT2020mrf by SRG/eROSITA, predicted by Lyutikov and Toonen, is coming from the breakout of the engine-powered shock from the ejecta into the preceding wind. The model requires total energetics of just few × 1050 erg, slightly above the observed X-rays. We predict that the system is hydrogen poor.

Formation of periodic FRB in binary systems with eccentricity

ABSTRACT Long-term periodicity in the rate of flares is observed for two repeating sources of fast radio bursts (FRBs). In this paper, we present a hydrodynamical modelling of a massive binary consisting of a magnetar and an early-type star. We model the interaction of the pulsar wind from the magnetar with an intense stellar wind. It is shown that only during a fraction of the orbital period, radio emission can escape the system. This explains the duty cycle of the two repeating FRB sources with periodic activity. The width of the transparency window depends on the eccentricity, stellar wind properties, and the viewing angle. To describe the properties of the known sources, it is necessary to assume large eccentricities ≳0.5. We apply the maser cyclotron mechanism of the radio emission generation to model spectral properties of the sources. The produced spectrum is not wide: Δν/ν ∼ 0.2 and the typical frequency depends on the radius of the shock where the emission is generated. The shock radius changes along the orbit. This, together with changing parameters of the medium, allows us to explain the frequency drift during the phase of visibility. Frequency dependence of the degree of polarization at few GHz can be a consequence of a small-scale turbulence in the shocked stellar wind. It is much more difficult to explain huge (∼105 rad/m2) and variable value of the rotation measure observed for FRB 121102. We suggest that this can be explained if the supernova explosion, which produced the magnetar happened near a dense interstellar cloud with n ∼ 100 cm−3.

“The Goose” Pulsar Wind Nebula of PSR J1016–5857: The Birth of a Plerion

We report the results of X-ray (Chandra X-ray Observatory (CXO)) and radio (ATCA) observations of the pulsar wind nebula (PWN) powered by the young pulsar PSR J1016–5857, which we dub “the Goose” PWN. In both bands, the images reveal a tail-like PWN morphology that can be attributed to the pulsar’s motion. By comparing archival and new CXO observations, we measure the pulsar’s proper motion μ = 28.8 ± 7.3 mas yr−1, yielding a projected pulsar velocity v ≈ 440 ± 110 km s−1 (at d = 3.2 kpc); its direction is consistent with the PWN shape. Radio emission from the PWN is polarized, with the magnetic field oriented along the pulsar tail. The radio tail connects to a larger radio structure (not seen in X-rays), which we interpret as a relic PWN (also known as a plerion). The spectral analysis of the CXO data shows that the PWN spectrum softens from Γ = 1.7 to Γ ≈ 2.3–2.5 with increasing distance from the pulsar. The softening can be attributed to the rapid synchrotron burn-off, which would explain the lack of X-ray emission from the older relic PWN. In addition to nonthermal PWN emission, we detected thermal emission from a hot plasma, which we attribute to the host supernova remnant. The radio PWN morphology and the proper motion of the pulsar suggest that the reverse shock passed through the pulsar’s vicinity and pushed the PWN to one side.

SNR G292.0+1.8: A Remnant of a Low-mass-progenitor Stripped-envelope Supernova

We present a study of the Galactic supernova remnant (SNR) G292.0+1.8, a classic example of a core-collapse SNR that contains oxygen-rich ejecta, circumstellar material, a rapidly moving pulsar, and a pulsar wind nebula (PWN). We use hydrodynamic simulations of the remnant’s evolution to show that the SNR reverse shock is interacting with the PWN and has most likely shocked the majority of the supernova ejecta. In our models, such a scenario requires a total ejecta mass of ≲3 M ⊙ and implies that there is no significant quantity of cold ejecta in the interior of the reverse shock. In light of these results, we compare the estimated elemental masses and abundance ratios in the reverse-shocked ejecta to nucleosynthesis models, and further conclude that they are consistent with a progenitor star with an initial mass of 12–16 M ⊙. We conclude that the progenitor of G292.0+1.8 was likely a relatively low-mass star that experienced significant mass loss through a binary interaction and would have produced a stripped-envelope supernova explosion. We also argue that the region known as the “spur” in G292.0+1.8 arises as a result of the pulsar’s motion through the supernova ejecta, and that its dynamical properties may suggest a line-of-sight component to the pulsar’s velocity, leading to a total space velocity of ∼600 km s−1 and implying a significant natal kick. Finally, we discuss binary mass-loss scenarios relevant to G292.0+1.8 and their implications for the binary companion properties and future searches.

Additional Evidence for a Pulsar Wind Nebula in the Heart of SN 1987A from Multiepoch X-Ray Data and MHD Modeling

Since the day of its explosion, supernova (SN) 1987A has been closely monitored to study its evolution and to detect its central compact relic. In fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the SN. However, besides the detection in the Atacama Large Millimeter/submillimeter Array (ALMA) data of a feature that is compatible with the emission arising from a protopulsar wind nebula (PWN), the only hint of the existence of such an elusive compact object is provided by the detection of hard emission in NuSTAR data up to ∼20 keV. We report on the simultaneous analysis of multiepoch observations of SN 1987A performed with Chandra, XMM-Newton, and NuSTAR. We also compare the observations with a state-of-the-art three-dimensional magnetohydrodynamic simulation of SN 1987A. A heavily absorbed power law, consistent with the emission from a PWN embedded in the heart of SN 1987A, is needed to properly describe the high-energy part of the observed spectra. The spectral parameters of the best-fit power law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed nonthermal emission. The information extracted from our analysis is used to infer the physical characteristics of the pulsar and the broadband emission from its nebula, in agreement with the ALMA data. Analysis of the synthetic spectra also shows that, in the near future, the main contribution to the Fe K emission line will originate in the outermost shocked ejecta of SN 1987A.

Optical Light Curve of 4FGL J0935.3+0901: A Flaring Black Widow Candidate

I obtained time-series photometry of the compact binary candidate for the Fermi source 4FGL J0935.3+0901. Superposed on the 2.44 hr orbital modulation are day-to-day variations and frequent flaring as seen in several redback and black widow millisecond pulsars (MSPs). The short orbital period favors a black widow. While the modulation of ≤1 mag is smaller than that of most black widows, it could indicate a low orbital inclination. Although a published optical spectrum shows strong emission lines, the light curve evinces pulsar heating of the companion star rather than accretion-disk emission of a transitional MSP. Emission lines and flaring occur in the same objects, probably powered by shocks between the relativistic pulsar wind and a wind driven off the companion star. I also recovered the period in photometry from the Zwicky Transient Facility (ZTF). A phase-connected ephemeris derived from MDM Observatory and ZTF data spanning 4 yr yields a period of 0.10153276(36) days and an epoch for the ascending node of the putative pulsar.

Unveiling the nature of the unidentified gamma-ray sources 4FGL J1908.6+0915e, HESS J1907+089/HOTS J1907+091, and 3HWC J1907+085 in the sky region of the magnetar SGR 1900+14

ABSTRACT Supernova remnants (SNRs), star formation regions (SFRs), and pulsar wind nebulae (PWNe) are prime candidates for Galactic PeVatrons. The non-thermal high-energy (HE, ε > 100 MeV) and very high-energy (VHE, ε > 100 GeV) γ-ray emission from these sources should be a promising manifestation of acceleration processes. We investigate the possibility to explain the HE and VHE γ-ray emission from the sky region of the magnetar SGR 1900+14 as a signature of cosmic rays accelerated in above-mentioned sources. To this end, we simulate the γ-ray emission from the extended Fermi-LAT HE source 4FGL J1908.6+0915e, the extended VHE H.E.S.S. source candidate HOTS J1907+091, and the point-like HAWC TeV source 3HWC J1907+085, which are spatially coincident with the SNR G42.8+0.6, the magnetar SGR 1900+14, and the star-forming region W49A. The simulations are performed within the hadronic and leptonic models. We show that the observed γ-ray emission from the region of the magnetar SGR 1900+14 can, in principle, include contributions of different intensities from all three types of (potentially confused) sources. The considered in detail cases of a magnetar-connected but still undetected SNR and a PWN are the most promising ones, but with a serious requirement on the energy reserve of radiated CR particles – of the order of $10^{51}d_{\textrm {10kpc}}^{2}$ erg for sources at a distance of d ∼ 10 kpc. Such energy reserve can be provided by the magnetar-related hypernova and/or magnetar wind nebula remnant created by the newborn millisecond magnetar with the large supply of rotational energy Erot ∼ 1052 erg.

Interpretation of the puzzling gamma-ray spectrum of the Geminga halo

The gamma-ray halo around Geminga is formed owing to the slow diffusion of the electrons released by the Geminga pulsar wind nebula (PWN). The latest observations from the HAWC and H.E.S.S. Collaborations exhibit complex features in the TeV gamma-ray spectrum of the Geminga halo. We first show that the new results cannot be interpreted by the commonly used simple model, where a single power-law injection spectrum and an energy index of $\ensuremath{\delta}=1/3$ for the diffusion coefficient are assumed. We then propose a two-population electron injection model based on the x-ray observations of the Geminga PWN, which consists of a population of freshly accelerated electrons escaping from the PWN through rapid outflows and a population trapped longer inside the PWN before escaping. The two-population model interprets the HAWC and H.E.S.S. data well, and the goodness of fit improves significantly compared with the single power-law injection model. It also predicts a different energy dependency of the gamma-ray profile from the single power-law injection model, which could be tested by the Large High Altitude Air Shower Observatory in the coming future. We note that a $\ensuremath{\delta}$ slightly larger than 1 is needed to fit the HAWC and H.E.S.S. data consistently. We also discuss the possible improvements by adopting the two-zone diffusion model.

The Eel Pulsar Wind Nebula: A PeVatron-candidate Origin for HAWC J1826−128 and HESS J1826−130

HAWC J1826−128 is one of the brightest Galactic TeV γ-ray sources detected by the High Altitude Water Cherenkov (HAWC) observatory, with photon energies extending up to nearly ∼100 TeV. This HAWC source spatially coincides with the H.E.S.S. TeV source HESS J1826−130 and the “Eel” pulsar wind nebula (PWN), which is associated with the GeV pulsar PSR J1826−1256. In the X-ray band, Chandra and XMM-Newton revealed that the Eel PWN is composed of both a compact nebula (∼15″) and diffuse X-ray emission (∼6′ × 2′) extending away from the pulsar. Our NuSTAR observation detected hard X-ray emission from the compact PWN up to ∼20 keV and evidence of the synchrotron burn-off effect. In addition to the spatial coincidence between HESS J1826−130 and the diffuse X-ray PWN, our multiwavelength spectral energy distribution (SED) analysis using X-ray and γ-ray data establishes a leptonic origin of the TeV emission associated with the Eel PWN. Furthermore, our evolutionary PWN SED model suggests (1) a low PWN B-field of ∼1 μG, (2) a significantly younger pulsar age (t ∼ 5.7 kyr) than the characteristic age (τ = 14.4 kyr), and (3) a maximum electron energy of PeV. The low B-field, as well as the putative supersonic motion of the pulsar, may account for the asymmetric morphology of the diffuse X-ray emission. Our results suggest that the Eel PWN may be a leptonic PeVatron particle accelerator powered by the ∼6 kyr old pulsar PSR J1826−1256 with a spin-down power of 3.6 × 1036 erg s−1.

Spatially resolved X-ray spectra of the galactic SNR G18.95-1.1: SRG/eROSITA view

Aims. We study the X-ray emission of the galactic supernova remnant (SNR) G18.95-1.1 with the eROSITA telescope on board the Spectrum Röntgen Gamma (SRG) orbital observatory. In addition to the pulsar wind nebula that was previously identified and examined by ASCA and Chandra, we study the X-ray spectra of the bright SNR ridge, which is resolved into a few bright clumps. Methods. The wide field of view and the large collecting area in the 0.2-2.3 keV energy range of SRG/eROSITA allowed us to perform spatially resolved spectroscopy of G18.95-1.1. Results. The X-ray ridge of G18.95-1.1 is asymmetric, indicating either supernova ejecta asymmetry or their interaction with a cloud. The X-ray dim northern regions outside the pulsar wind nebula can be described by a thin thermal plasma emission with a temperature ~0.3keV and a solar composition. The X-ray spectra of a few bright clumps located along the southern ridge may be satisfactorily approximated by a single thermal component of the Si-rich ejecta at the collisional ionization equilibrium with a temperature of about 0.3 keV. The bright ridge can be alternatively fit with a single component that is not dominated by equilibrium ejecta with T ~ 0.6 keV. The high ratio of the derived Si/O abundances indicates that the ejecta originated in deep layers of the progenitor star. The plasma composition of a southern Si-rich clump and the bright ridge are similar to what was earlier found in the Vela shrapnel A and G.

Ion Acoustic Shock Wave Formation and Ion Acceleration in the Interactions of Pair Jets with Electron–ion Plasmas

Astrophysical jets are ubiquitous in the universe and often associated with compact objects, and their interactions with the ambient medium not only dissipate their own energy but also provide ideal circumstances for particle acceleration. By means of theoretical analysis and particle-in-cell simulations, here we study the ion acoustic shock wave (IASW) formation and consequent ion acceleration when electron–positron (e − e +) jets are injected into ambient electron–ion plasmas. It is found that the Buneman instability can be excited first, which induces the formation of an ion acoustic wave (IAW). As the amplitude of the IAW increases, its waveform is steepened and subsequently an IASW is formed. Some ions in the ambient plasmas will be reflected when they encounter the IASW, and thus can be accelerated to form an energetic ion beam. For an initial e − e + jet with the Lorentz factor γ 0 = 100 and the ion–electron mass ratio m i /m e = 1836, the ions can be accelerated up to 580 MeV. This study deepens our understanding of the fireball model of gamma-ray bursts, the shock model of pulsar wind nebulae, the origin of cosmic rays, and other related astrophysical processes.

Intra-pulse variability induced by plasmoid formation in pulsar magnetospheres

Context. Pulsars show irregularities in their pulsed radio emission that originate from propagation effects and the intrinsic activity of the source. Aims. In this work, we investigate the role played by magnetic reconnection and the formation of plasmoids in the pulsar wind current sheet as a possible source of intrinsic pulse-to-pulse variability in the incoherent, high-energy emission pattern. Methods. We used a two-dimensional particle-in-cell simulation of an orthogonal pulsar magnetosphere restricted to the plane perpendicular to the star spin axis. We evolved the solution for several tens of pulsar periods to gather a statistically significant sample of synthetic pulse profiles. Results. The formation of plasmoids leads to strong pulse-to-pulse variability in the form of multiple short, bright subpulses, which appear only on the leading edge of each main pulse. These secondary peaks of emission are dominated by the dozen plasmoids that can grow up to macroscopic scales. They emerge from the high end of the hierarchical merging process occurring along the wind current layer. The flux of the subpulses is correlated with their width in phase. Although the full-scale separation is not realistic, we argue that the simulation correctly captures the demographics and the properties of the largest plasmoids, and therefore of the brightest subpulses. Conclusions. The prediction of subpulses at specific pulse phases provides a new observational test of the magnetic reconnection scenario as the origin of the pulsed incoherent emission. High-time-resolution observations of the Crab pulsar in the optical range may be the most promising source to target for this purpose.

The Bow Shock and Kinematics of PSR J1959+2048

Pulsar Hα bow shocks provide rare opportunities to constrain the energetics and kinematics of the relativistic pulsar wind. We have acquired optical imaging and integral field unit spectroscopy of the bow shock of the millisecond pulsar PSR J1959+2048, measuring the shock symmetry axis at a position angle = 213.2 ± 0.°2 and showing that this slow nonradiative shock has a broad-to-narrow line component ratio I b /I n = 4. The data show that the pulsar’s velocity lies 2.°2 out of the plane of the sky. Coupled with a new fit for its timing proper motion, giving μ tot = 30.05 mas yr−1 and a Very Long Baseline Array (VLBA) interferometric parallax measurement giving kpc (90% range), we have unusually complete information on the pulsar kinematics. The bow shock constraints on the wind momentum flux imply that, at the best-fit parallax distance, the pulsar moment of inertia must be very large and/or the Hα efficiency at its modest shock velocity must be very high.

Magnetic Field Structure and Faraday Rotation of the Plerionic Supernova Remnant G21.5–0.9

We present a polarimetric study of the pulsar wind nebula (PWN) in supernova remnant G21.5−0.9, using archival Very Large Array data taken at 5 and 7.4 GHz. The rotation measure (RM) map of the PWN shows a symmetric pattern that aligns with the presumed pulsar spin axis direction, implying a significant contribution to the RM from the nebula. We suggest that the spatial variation of the internal RM is mostly caused by the nonuniform distribution of electrons originating from the supernova ejecta. Our high-resolution radio polarization map reveals a global radial B-field. We show that a simple model with overall radial field and turbulence on a small scale can reproduce many observed features of the PWN, including the polarization pattern and polarized fraction. The modeling results also reject a strong large-scale toroidal B-field, suggesting that the toroidal field observed in the inner PWN cannot propagate to the entire nebula. Lastly, our model predicts that the internal Faraday rotation would break the linear relation between the polarization angle and the square of the wavelength, and cause severe depolarization at low frequencies.

On the Potential of Bright, Young Pulsars to Power Ultrahigh Gamma-Ray Sources

The recent discovery of a new population of ultrahigh-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons—cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1° or more, most of them contain several potential counterparts, including supernova remnants, young stellar clusters, and pulsar wind nebulae (PWNe), which can perform as PeVatrons and thus power the surrounding diffuse ultrahigh-energy gamma-ray structures. In the case of PWNe, gamma-rays are produced by electrons, accelerated at the pulsar wind termination shock, through the inverse Compton scattering of 2.7 K cosmic microwave background (CMB)radiation. The high conversion efficiency of pulsar rotational power to relativistic electrons, combined with the short cooling timescales, allow gamma-ray luminosities up to the level of . The pulsar spin-down luminosity, , also determines the absolute maximum energy of individual photons: . This fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young PWNe with typical magnetic field of ≈100 μG with . We discuss the implications of Eγ ,max by comparing it with the highest-energy photons reported by LHAASO from a dozen of ultrahigh-energy sources. Whenever a PWN origin of the emission is possible, we use the LHAASO measurements to set upper limits on the nebular magnetic field.

Spitzer and Herschel studies of dust in supernova remnants in the Small Magellanic Cloud

ABSTRACT With the entire Small Magellanic Cloud (SMC) mapped by the Spitzer Space Telescope and Herschel Space Observatory, we were able to search 8–250 $\mu$m images in order to identify infrared (IR) emission associated with SMC supernova remnants (SNRs). A valid detection had to correspond with known X-ray, H α, and radio emission from the SNRs. From the 24 known SNRs, we made five positive detections with another five possible detections. Two detections are associated with pulsars or pulsar wind nebula, and another three detections are part of the extended nebulous emission from the SNRs. We modelled dust emission where fast moving electrons are predicted to collide and heat dust grains which then radiate in IR. With known distance (62.44 ± 0.47 kpc), measured SNR sizes, electron densities, temperatures from X-ray emission as well as hydrogen densities, the modelling of SMC SNRs is straightforward. If the higher range of hydrogen and electron densities were to be accepted, we would expect almost all SMC SNRs to be detected in the IR, at least at 24 $\mu$m, but the actual detection rate is only 25 per cent. One possible and common explanation for this discrepancy is that small grains have been destroyed by the SNRs shockwave. However, within the uncertainties of hydrogen and electron densities, we find that IR dust emission can be explained reasonably well, without invoking dust destruction. There is no conclusive evidence that SNRs destroy swept-up ISM dust.

Efficiency of nonthermal particle acceleration in magnetic reconnection

The nonthermal particle acceleration during magnetic reconnection remains a fundamental topic in several astrophysical phenomena, such as solar flares, pulsar wind, and magnetars, for more than half a century, and one of the unresolved questions is its efficiency. Recently, nonthermal particle acceleration mechanisms during reconnection have been extensively studied by particle-in-cell simulations, yet it is an intriguing enigma as to how the magnetic field energy is divided into thermally heated plasmas and nonthermal particles. Here, we study both non-relativistic and relativistic magnetic reconnections using large-scale particle-in-cell simulation for a pair plasma and indicate that the production of the nonthermal particle becomes efficient with increasing the plasma temperature. In the relativistic hot plasma case, we determine that the heated plasmas by reconnection can be approximated by a kappa distribution function with the kappa index of approximately 3 or less (equivalent to 2 or less for the power-law index), and the nonthermal energy density of reconnection is approximately over 95% of the total internal energy in the downstream exhaust.

Temporal Scattering, Depolarization, and Persistent Radio Emission from Magnetized Inhomogeneous Environments near Repeating Fast Radio Burst Sources

Abstract Some repeating fast radio burst (FRB) sources exhibit complex polarization behaviors, including frequency-dependent depolarization, variation of rotation measure (RM), and oscillating spectral structures of polarized components. Very recently, Feng et al. reported that active repeaters exhibit conspicuous frequency-dependent depolarization and a strong correlation between RM scatter (σ RM) and the temporal scattering time (τ s), σ RM ∝ τ s 1.0 ± 0.2 , both of which can be well described by multipath propagation through a magnetized inhomogeneous plasma screen. This observation strongly suggests that the temporal scattering and RM scatter originate from the same region. Besides, a particular finding of note in Feng et al. is that the FRBs with compact persistent radio sources (PRSs) tend to have extreme σ RM. In this work, we focus on some theoretical predictions of the relations among temporal scattering, depolarization by RM scatter, and PRSs contributed by the magnetized plasma environment close to a repeating FRB source. The behaviors of the RM scatter imply that the magnetized plasma environment is consistent with a supernova remnant or pulsar wind nebula, and the predicted σ RM–τ s relation is σ RM ∝ τ s ( 0.54 − 0.83 ) for different astrophysical scenarios. We further make a general discussion of PRSs that does not depend on specific astrophysical scenarios. We show that the specific luminosity of a PRS should have a positive correlation with the RM contributed by the plasma screen. This is consistent with the observations of FRB 121102 and FRB 190520B.

Pulsar wind nebula origin of the LHAASO-detected ultra-high energy γ-ray sources

The recent measurement by LHAASO of gamma-ray emission extending up to hundreds of TeV from multiple Galactic sources represents a major observational step forward in the search for the origin of the Galactic cosmic rays. The burning question is if this ultra-high-energy emission is associated with the acceleration of protons and/or nuclei to PeV energies, or if it can be associated with PeV-electron accelerators. A strong Klein-Nishina suppression of inverse Compton emission at these energies is unavoidable; nevertheless, we show here that inverse Compton emission can provide a natural explanation for the measured emission and that an association with the established PeV-electron accelerating source class of pulsar wind nebulae is also rather natural. However, a clear distinction between different models requires taking multi-wavelength data into account, having good knowledge of the local environmental conditions, and, in some cases, performing multi-source modelling.