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Register a new userIonization Break-Out from Millisecond Pulsar Wind Nebulae: an X-ray Probe of the Origin of Superluminous Supernovae
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Magnetic spin-down of a millisecond neutron star has been proposed as the power source of hydrogen-poor superluminous supernovae (SLSNe-I). However, producing an unambiguous test that can distinguish this model from alternatives, such as circumstellar interaction, has proven challenging. After the supernova explosion, the pulsar wind inflates a hot cavity behind the expanding stellar ejecta: the nascent millisecond pulsar wind nebula. Electron/positron pairs injected by the wind cool through inverse Compton scattering and synchrotron emission, producing a pair cascade and hard X-ray spectrum inside the nebula. These X-rays ionize the inner exposed side of the ejecta, driving an ionization front that propagates outwards with time. Under some conditions this front can breach the ejecta surface within months after the optical supernova peak, allowing ~0.1-1 keV photons to escape the nebula unattenuated with a characteristic luminosity L_X ~ 1e43-1e45 erg/s. This ionization break-out may explain the luminous X-ray emission observed from the transient SCP 06F, providing direct evidence that this SLSN was indeed engine-powered. Luminous break-out requires a low ejecta mass and that the spin-down time of the pulsar be comparable to the photon diffusion timescale at optical maximum, the latter condition similar to that required for a supernova with a high optical fluence. These relatively special requirements may explain why most SLSNe-I are not accompanied by detectable X-ray emission. Global asymmetry of the supernova ejecta increases the likelihood of an early break-out along the direction of lowest density. Atomic states with lower threshold energies are more readily ionized at earlier times near optical maximum, allowing UV break-out across a wider range of pulsar and ejecta properties than X-ray break-out, possibly contributing to the blue/UV colors of SLSNe-I.
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During the search for counterparts of very-high-energy gamma-ray sources, we serendipitously discovered large, extended, low surface brightness emission from PWNe around pulsars with the ages up to ~100 kyrs, a discovery made possible by the low and stable background of the Suzaku X-ray satellite. A systematic study of a sample of 8 of these PWNe, together with Chandra datasets, has revealed us that the nebulae keep expanding up to for ~100 kyrs, although time scale of the synchrotron X-ray emission is only ~60 yr for typical magnetic fields of 100 microG. Our result suggests that the accelerated electrons up to ~80 TeV can escape from the PWNe without losing most energies. Moreover, in order to explain the observed correlation between the X-ray size and the pulsar spindwon age, the magnetic field strength in the PWNe must decrease with time.
Superluminous supernovae (SLSNe) are massive star explosions too luminous to be powered by traditional energy sources, such as radioactive 56Ni. These transients may instead be powered by a central engine, such as a millisecond pulsar or magnetar, whose relativistic wind inflates a nebula of high energy particles and radiation behind the expanding ejecta. We present 3D Monte Carlo radiative transfer calculations which follow the production and thermalization of high energy radiation from the nebula into optical radiation and, conversely, determine the gamma-ray emission that escapes the ejecta without thermalizing. We track the evolution of photons and matter in a coupled two-zone (wind/nebula and ejecta) model, accounting for the range of radiative processes. We identify a novel mechanism by which gamma-gamma pair creation in the upstream pulsar wind regulates the mean energy of particles entering the nebula over the first several years after the explosion, rendering our results on this timescale insensitive to the (uncertain) intrinsic wind pair multiplicity. To explain the observed late-time steepening of SLSNe optical light curves as being the result of gamma-ray leakage, the nebular magnetization must be very low, epsB <~ 1e-6-1e-4. For higher epsB, synchrotron emission quickly comes to dominate the thermalized nebula radiation, and being readily absorbed because of its lower photon energies, results in the SN optical light curve tracking the spin-down power even to late times >~ 1 yr, inconsistent with observations. For magnetars to remain viable contenders for powering SLSNe, we conclude that either magnetic dissipation in the wind/nebula is extremely efficient, or that the spin-down luminosity decays significantly faster than the canonical dipole rate ~1/t^2 in a way that coincidentally mimicks gamma-ray escape.
To look for possible phenomenological connections between pulsars timing properties and emissions from pulsar wind nebulae and their pulsars, we studied the power-law component of the X-ray emissions from 35 pulsar wind nebulae which have a detected pulsar in X-rays. Our major results are in the following: (1) The power-law component of the X-ray luminosities, in the energy range from 0.5 keV to 8 keV, of the nebulae and of the pulsar both show a strong correlation with the pulsar spin-down power ($dot{E}$), consistent with earlier studies. However, equally significant correlations with the magnetic field strength at the light cylinder ($B_{rm lc}$) are also found. The similar significance level of the correlations with $dot{E}$ and with $B_{rm lc}$ suggests that not only $dot{E}$ but also $B_{rm lc}$ plays an important role in understanding these power-law emissions. (2) Thermal X-ray emissions are detected in 12 pulsars among the 35 samples. With derived temperature as one additional variable, we found that the photon indices of pulsars non-thermal X-ray power-law spectra can be well described by a linear function of $log P$, $logdot{P}$ and temperature logarithm $log T$. It indicates that the surface temperature of neutron stars plays an important role in determining the energy distribution of the radiating pair plasma in pulsars magnetospheres.
Pulsar Wind Nebulae (PWNe) constitute an ideal astrophysical environment to test our current understanding of relativistic plasma processes. It is well known that magnetic fields play a crucial role in their dynamics and emission properties. At present, one of the main issues concerns the level of magnetic turbulence present in these systems, which in the absence of space resolved X-ray polarization measures cannot be directly constrained. In this work we investigate, for the first time using simulated synchrotron maps, the effect of a small scale fluctuating component of the magnetic field on the emission properties in X-ray. We illustrate how to include the effects of a turbulent component in standard emission models for PWNe, and which consequences are expected in terms of net emissivity and depolarization, showing that the X-ray surface brightness maps can provide already some rough constraints. We then apply our analysis to the Crab and Vela nebulae and, by comparing our model with Chandra and Vela data, we found that the typical energies in the turbulent component of the magnetic field are about 1.5 to 3 times the one in the ordered field.
Successful phenomenological models of pulsar wind nebulae assume efficient dissipation of the Poynting flux of the magnetized electron-positron wind as well as efficient acceleration of the pairs in the vicinity of the termination shock, but how this is realized is not yet well understood. The present paper suggests that the corrugation of the termination shock, at the onset of non-linearity, may lead towards the desired phenomenology. Non-linear corrugation of the termination shock would convert a fraction of order unity of the incoming ordered magnetic field into downstream turbulence, slowing down the flow to sub-relativistic velocities. The dissipation of turbulence would further preheat the pair population on short length scales, close to equipartition with the magnetic field, thereby reducing the initial high magnetization to values of order unity. Furthermore, it is speculated that the turbulence generated by the corrugation pattern may sustain a relativistic Fermi process, accelerating particles close to the radiation reaction limit, as observed in the Crab nebula. The required corrugation could be induced by the fast magnetosonic modes of downstream nebular turbulence; but it could also be produced by upstream turbulence, either carried by the wind or seeded in the precursor by the accelerated particles themselves.
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