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Synchrotron Radiation from Electrons with a Pitch-angle Distribution
In most astrophysical processes involving synchrotron radiation, the pitch-angle distribution of the electrons is assumed to be isotropic. However, if electrons are accelerated anisotropically, e.g., in a relativistic shock wave with an ordered magnetic field or in magnetic reconnection regions, the electron pitch angles might be anisotropic. In this Letter, we study synchrotron radiation from electrons with a pitch-angle distribution with respect to a large-scale uniform magnetic field. Assuming that the pitch-angle distribution is normal with a scatter of σ p and that the viewing direction is where the pitch-angle direction peaks, we find that for electrons with a Lorentz factor γ, the observed flux satisfies F ν ∝ ν 2/3 for ν ν cr (ν cr is the critical frequency of synchrotron), if σ p 1/γ is satisfied. On the other hand, if σ p 1/γ, the spectrum below ν cr is a broken power law with a break frequency , e.g., for ν ν br and for . Thus, the ultimate synchrotron line of death is F ν ∝ ν 2/3. We discuss the application of this theory to blazars and gamma-ray bursts
Dispersion Measure Variation of Repeating Fast Radio Burst Sources
The repeating fast radio burst (FRB) 121102 was recently localized in a dwarf
galaxy at a cosmological distance. The dispersion measure (DM) derived for each
burst from FRB 121102 so far has not shown significant evolution, even though
an apparent increase was recently seen with newly detected VLA bursts. It is
expected that more repeating FRB sources may be detected in the future. In this
work, we investigate a list of possible astrophysical processes that might
cause DM variation of a particular FRB source. The processes include (1) the
cosmological scale effects such as Hubble expansion and large-scale structure
fluctuations; (2) the FRB local effects such as gas density fluctuation,
expansion of a supernova remnant, a pulsar wind nebula, and an HII region; and
(3) the propagation effect due to plasma lensing. We find that the DM
variations contributed by the large-scale structure are extremely small, and
any observable DM variation is likely caused by the plasma local to the FRB
source. Besides mechanisms that produce decreasing DM with time, we suggest
that an FRB source in an expanding supernova remnant around a nearly neutral
ambient medium during the deceleration (Sedov-Taylor and snowplow) phases or in
a growing HII region can introduce DM increasing. Some effects (e.g. an FRB
source moving in an HII region or plasma lensing) can give either positive or
negative DM variations. Future observations of DM variations of FRB 121102 and
other repeating FRB sources can bring important clues for the physical origin
of these sources.Comment: 12 pages. Accepted for publication in Ap
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