Inhomogeneities in a synchrotron source can severely affect the conclusions drawn from observations regarding the source properties. However, their presence is not always easy to establish, since several other effects can give rise to similar observe
d characteristics. It is argued that the recently observed broadening of the radio spectra and/or light curves in some of the type Ib/c supernovae is a direct indication of inhomogeneities. As compared to a homogeneous source, this increases the deduced velocity of the forward shock and the observed correlation between total energy and shock velocity could in part be due to a varying covering factor. The X-ray emission from at least some type Ib/c supernovae is unlikely to be synchrotron radiation from an electron distribution accelerated in a non-linear shock. Instead it is shown that the observed correlation during the first few hundred days between the radio, X-ray and bolometric luminosities indicates that the X-ray emission is inverse Compton scattering of the photospheric photons. Inhomogeneities are consistent with equipartition between electrons and magnetic fields in the optically thin synchrotron emitting regions.
The high frequency component in blazars is thought to be due to inverse Compton scattered radiation. Recent observations by Fermi-LAT are used to evaluate the details of the scattering process. A comparison is made between the usually assumed single
scattering scenario and one in which multiple scatterings are energetically important. In the latter case, most of the radiation is emitted in the Klein-Nishina limit. It is argued that several of the observed correlations defining the blazar sequence are most easily understood in a multiple scattering scenario. Observations indicate also that, in such a scenario, the blazar sequence is primarily governed by the energy density of relativistic electrons rather than that of the seed photons. The pronounced X-ray minimum in the spectral energy distribution often observed in the most luminous blazars is discussed. It is shown how this feature can be accounted for in a multiple scattering scenario by an extension of standard one-zone models.
We present high spatial resolution optical imaging and polarization observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula (PWN) performed with HST, and compare them with X-ray data obtained with the Chandra X-ray Observatory.
We have studied the bright region southwest of the pulsar where a bright blob is seen in 1999. We show that it may be a result of local energy deposition around 1999, and that the emission from this then faded away. Polarization data from 2007 show that the polarization properties show dramatic spatial variations at the 1999 blob position arguing for a local process. Several other positions along the pulsar-blob orientation show similar changes in polarization, indicating previous recent local energy depositions. In X-rays, the spectrum steepens away from the blob position, faster orthogonal to the pulsar-blob direction than along this axis of orientation. This could indicate that the pulsar-blob orientation is an axis along where energy in the PWN is mainly injected, and that this is then mediated to the filaments in the PWN by shocks. We highlight this by constructing an [S II]-to-[O III]-ratio map. We argue, through modeling, that the high [S II]/[O III] ratio is not due to time-dependent photoionization caused by possible rapid Xray emission variations in the blob region. We have also created a multiwavelength energy spectrum for the blob position showing that one can, to within 2sigma, connect the optical and X-ray emission by a single power law. We obtain best power-law fits for the X-ray spectrum if we include extra oxygen, in addition to the oxygen column density in the interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen is most naturally explained by the oxygen-rich ejecta of the supernova remnant. The oxygen needed likely places the progenitor mass in the 20 - 25 Msun range.
Recent observations of the Crab pulsar show no evidence for a spectral break in the infrared regime. It is argued that the observations are consistent with a power-law spectrum in the whole observable infrared - optical range. This is taken as the st
arting point for an evaluation of how self-consistent incoherent synchrotron models fare in a comparison with observations. Inclusion of synchrotron self-absorption proves important as does the restriction on the observed size of the emission region imposed by the relativistic beaming thought to define the pulse profile. It is shown that the observations can be used to derive two independent constraints on the distance from the neutron star to the emission region; in addition to a direct lower limit, an indirect measure is obtained from an upper limit to the magnetic field strength. Both of these limits indicate that the emission region is located at a distance considerably larger than the light cylinder radius. The implications of this result are discussed and it is emphasized that, in order for standard incoherent synchrotron models to fit inside the light cylinder, rather special physical conditions need to be invoked.
The optical-UV component in GRB 060218 is assumed to be due to optically thick cyclotron emission. The key aspect of this model is the high temperature of the absorbing electrons. The heat input derives from nuclei accelerated in semi-relativistic in
ternal shocks, like in ordinary gamma-ray bursts. Coulomb collisions transfer part of that energy to electrons. Inverse Compton cooling on the X-ray photons leads to electron temperatures around 100 keV. Such a high brightness temperature for the optical-UV emission implies an emitting area roughly equal to that of the thermal X-ray component. This suggests a model in which the radio, optical-UV and thermal X-ray emission are closely related: Although the optical-UV and thermal X-ray emission are two separate spectral components, it is argued that they both come from the photosphere of a quasi-spherical, continuous outflow, whose interaction with the circumstellar medium gives rise to the radio emission. The properties of GRB 060218, as measured in the co-moving frame, are similar to those of ordinary gamma-ray burst; i.e., the main difference is the much lower value of the bulk Lorentz factor in GRB 060218. The cyclotron absorption implies a magnetic field in rough equipartition with the matter energy density in the outflow. Hence, the magnetic field could have a dynamically important role, possibly with a magnetar as the central engine.
