Imaging and Spectral Observations of Quasi-Periodic Pulsations in a Solar Flare

We explore the Quasi-Periodic Pulsations (QPPs) in a solar flare observed by Fermi Gamma-ray Burst Monitor (GBM), Solar Dynamics Observatory (SDO), Solar Terrestrial Relations Observatory (STEREO), and Interface Region Imaging Spectrograph (IRIS) on 2014 September 10. QPPs are identified as the regular and periodic peaks on the rapidly-varying components, which are the light curves after removing the slowly-varying components. The QPPs display only three peaks at the beginning on the hard X-ray (HXR) emissions, but ten peaks on the chromospheric and coronal line emissions, and more than seven peaks (each peak is corresponding to a type III burst on the dynamic spectra) at the radio emissions. An uniform quasi-period about 4 minutes are detected among them. AIA imaging observations exhibit that the 4-min QPPs originate from the flare ribbon, and tend to appear on the ribbon front. IRIS spectral observations show that each peak of the QPPs tends to a broad line width and a red Doppler velocity at C I, O IV, Si IV, and Fe XXI lines. Our findings indicate that the QPPs are produced by the non-thermal electrons which are accelerated by the induced quasi-periodic magnetic reconnections in this flare.

On the Spin Period Distribution in Be/X-ray Binaries

There is a remarkable correlation between the spin periods of the accreting neutron stars in Be/X-ray binaries (BeXBs) and their orbital periods . Recently Knigge et al. (2011) showed that the distribution of the spin periods contains two distinct subpopulations peaked at $sim 10$ s and $sim 200$ s respectively, and suggested that they may be related to two types of supernovae for the formation of the neutron stars, i.e., core-collapse and electron-capture supernovae. Here we propose that the bimodal spin period distribution is likely to be ascribed to different accretion modes of the neutron stars in BeXBs. When the neutron star tends to capture material from the warped, outer part of the Be star disk and experiences giant outbursts, a radiatively-cooling dominated disk is formed around the neutron star, which spins up the neutron star, and is responsible for the short period subpopulation. In BeXBs that are dominated by normal outbursts or persistent, the accretion flow is advection-dominated or quasi-spherical. The spin-up process is accordingly inefficient, leading to longer periods of the neuron stars. The potential relation between the subpopulations and the supernova mechanisms is also discussed.