Evidence for Photoionization Driven Broad Absorption Line Variability

We present a qualitative analysis of the variability of quasar broad absorption lines using the large multi-epoch spectroscopic dataset of the Sloan Digital Sky Survey Data Release 10. We confirm that variations of absorption lines are highly coordinated among different components of the same ion or the same absorption component of different ions for C IV, Si IV and N V. Furthermore, we show that the equivalent widths of the lines decrease or increase statistically when the continuum brightens or dims. This is further supported by the synchronized variations of emission and absorption line equivalent width, when the well established intrinsic Baldwin effect for emission lines is taken into account. We find that the emergence of an absorption component is usually accompanying with dimming of the continuum while the disappearance of an absorption line component with brightening of the continuum. This suggests that the emergence or disappearance of a C IV absorption component is only the extreme case, when the ionic column density is very sensitive to continuum variations or the continuum variability amplitude is larger. These results support the idea that absorption line variability is driven mainly by changes in the gas ionization in response to continuum variations, that the line-absorbing gas is highly ionized, and in some extreme cases, too highly ionized to be detected in UV absorption lines. Due to uncertainties in the spectroscopic flux calibration, we cannot quantify the fraction of quasars with asynchronized continuum and absorption line variations.Comment: 41 pages, 15 figures, accepted to Ap

Multiple Unicast Capacity of 2-Source 2-Sink Networks

We study the sum capacity of multiple unicasts in wired and wireless multihop networks. With 2 source nodes and 2 sink nodes, there are a total of 4 independent unicast sessions (messages), one from each source to each sink node (this setting is also known as an X network). For wired networks with arbitrary connectivity, the sum capacity is achieved simply by routing. For wireless networks, we explore the degrees of freedom (DoF) of multihop X networks with a layered structure, allowing arbitrary number of hops, and arbitrary connectivity within each hop. For the case when there are no more than two relay nodes in each layer, the DoF can only take values 1, 4/3, 3/2 or 2, based on the connectivity of the network, for almost all values of channel coefficients. When there are arbitrary number of relays in each layer, the DoF can also take the value 5/3 . Achievability schemes incorporate linear forwarding, interference alignment and aligned interference neutralization principles. Information theoretic converse arguments specialized for the connectivity of the network are constructed based on the intuition from linear dimension counting arguments.Comment: 6 pages, 7 figures, submitted to IEEE Globecom 201