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

Relativistic Astronomy. III. Test of Special Relativity via Doppler Effect

The Breakthrough Starshot program is planning to send transrelativistic probes to travel to nearby stellar systems within decades. Because the probe velocity is designed to be a good fraction of the light speed, Zhang & Li recently proposed that these transrelativistic probes can be used to study astronomical objects and to test special relativity. In this work, we propose some methods to test special relativity and constrain photon mass using the Doppler effect with the images and spectral features of astronomical objects as observed in the transrelativistic probes. We introduce more general theories to set up the framework of testing special relativity, including the parametric general Doppler effect and the Doppler effect with massive photons. We find that by comparing the spectra of a certain astronomical object, one can test Lorentz invariance and constrain photon mass. Additionally, using the imaging and spectrograph capabilities of transrelativistic probes, one can test time dilation and constrain photon mass. For a transrelativistic probe with velocity v ~ 0.2c, aperture D ~ 3.5 cm, and spectral resolution R ~ 100 (or 1000), we find that the probe velocity uncertainty can be constrained to σ v ~ 0.01c (or 0.001c), and the time dilation factor uncertainty can be constrained to (or 0.001), where is the time dilation factor and γ is the Lorentz factor. Meanwhile, the photon mass limit is set to m γ 10−33 g, which is slightly lower than the energy of the optical photon

Coherent Curvature Radiation Spectrum by Dynamically Fluctuating Bunches in Magnetospheres

Coherent curvature radiation by charged bunches has been discussed as the radiation mechanism for radio pulsars and fast radio bursts. Important issues for this radiation mechanism include how the bunches form and disperse in the magnetosphere of a pulsar or magnetar. More likely, bunches form and disperse continuously and it remains unclear what the spectral features are for these fluctuating bunches. In this work, we consider that the bunches in a magnetosphere have a formation rate of $\lambda_B$, a lifetime of $\tau_B$, and a typical Lorentz factor of $\gamma$, and analyze the spectral features of coherent curvature radiation by these fluctuating bunches. We find that the emission spectrum by a single fluctuating bunch is suppressed by a factor of $\sim(\lambda_B\tau_B)^2$ compared with that of a single persistent bunch, and there is a quasi-white noise in a wider band in the frequency domain. The high-frequency cutoff of the spectrum is at $\sim\max(\omega_{\rm peak},2\gamma^2/\tau_B)$, where $\omega_{\rm peak}$ is the peak frequency of curvature radiation. If the observed spectrum is not white-noise-like, the condition of $2\gamma^2\lambda_B\gtrsim \min(\omega_{\rm peak},2\gamma^2/\tau_B)$ would be required. Besides, the radiation by multiple fluctuating bunches along a field line is the incoherent summation of the radiation by single bunches if the bunch separation is longer than the wavelength. Conversely, a coherent summation should be involved. We also discuss the effects of bunch structures and the mechanism of bunch formation and dispersion.Comment: 12 pages, 6 figures. Accepted for publication in MNRA