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

Characterization of interplay among DNA repair, histone H3 lysine 9 dimethylation and small interfering RNA pathway in Caenorhabditis elegans

Heterochromatin assembly on repetitive sequences has been proposed to maintain genome integrity in different species. Histone H3 lysine 9 dimethylation (H3K9me2) has been shown to play an important role in mediating the formation of heterochromatin. In our lab, we use Caenorhabditis elegans as a model system to study the biological significance of the enrichment of H3K9me2 on repetitive elements and unpaired chromatin. Research of several other labs implicated that distribution of H3K9me2 on repetitive sequences limits transposon transcription and mutation occurrence near repetitive sequences. Additionally, our lab has reported that CSR-1 small RNA pathway (CSR-1, EKL-1, EGO-1 and DRH-3) regulates the deposition of H3K9me2 on unpaired chromatin during meiotic prophase. In this study, I identified a putative DNA annealing helicase, SMRC-1, that associates with the H3K9me2 methyltransferase MET-2 and a modified form of EKL-1. The majority of this thesis focuses on understanding the interaction between MET-2 and SMRC-1. Characterization of smrc-1 mutants revealed that SMRC-1 is important for resolving replication stress. Thus the association between SMRC-1 and MET-2 might suggest H3K9me2 functions as an epigenetic regulator of double strand break formations genome wide. Additionally, I found that a modified form of EKL-1 might interact with SMRC-1. Based on these findings, I propose a model whereby SMRC-1 bridges the CSR-1 small RNA pathway with H3K9me2

Thermography Detection on the Fatigue Damage

It has always been a great temptation in finding new methods to in-situ “watch” the material fatigue-damage processes so that in-time reparations will be possible, and failures or losses can be minimized to the maximum extent. Realizing that temperature patterns may serve as fingerprints for stress-strain behaviors of materials, a state-of-art infrared (IR) thermography camera has been used to “watch” the temperature evolutions of both crystalline and amorphous materials “cycle by cycle” during fatigue experiments in the current research. The two-dimensional (2D) thermography technique records the surface-temperature evolutions of materials. Since all plastic deformations are related to heat dissipations, thermography provides an innovative method to in-situ monitor the heat-evolution processes, including plastic-deformation, mechanical-damage, and phase-transformation characteristics. With the understanding of the temperature evolutions during fatigue, thermography could provide the direct information and evidence of the stress-strain distribution, crack initiation and propagation, shear-band growth, and plastic-zone evolution, which will open up wide applications in studying the structural integrity of engineering components in service. In the current research, theoretical models combining thermodynamics and heat-conduction theory have been developed. Key issues in fatigue, such as in-situ stress-strain states, cyclic softening and hardening observations, and fatigue-life predictions, have been resolved by simply monitoring the specimen-temperature variation during fatigue. Furthermore, in-situ visulizations as well as qualitative and quantitative analyses of fatigue-damage processes, such as Lüders-band evolutions, crack propagation, plastic zones, and final fracture, have been performed using thermography results. As a method requiring no special sample preparation or surface contact by sensors, thermography provides an innovative and convenient method to in-situ monitor and analyze the mechanical-damage processes of materials and components