GRB beaming and gravitational-wave observations

Using the observed rate of short-duration gamma-ray bursts (GRBs) it is possible to make predictions for the detectable rate of compact binary coalescences in gravitational-wave detectors. These estimates rely crucially on the growing consensus that short gamma-ray bursts are associated with the merger of two neutron stars or a neutron star and a black hole, but otherwise make no assumptions beyond the observed rate of short GRBs. In particular, our results do not assume coincident gravitational wave and electromagnetic observations. We show that the non-detection of mergers in the existing LIGO/Virgo data constrains the progenitor masses and beaming angles of gamma-ray bursts. For future detectors, we find that the first detection of a NS-NS binary coalescence associated with the progenitors of short GRBs is likely to happen within the first 16 months of observation, even in the case of a modest network of observatories (e.g., only LIGO-Hanford and LIGO-Livingston) operating at modest sensitivities (e.g., advanced LIGO design sensitivity, but without signal recycling mirrors), and assuming a conservative distribution of beaming angles (e.g. all GRBs beamed at \theta=30 deg). Less conservative assumptions reduce the waiting time until first detection to weeks to months. Alternatively, the compact binary coalescence model of short GRBs can be ruled out if a binary is not seen within the first two years of operation of a LIGO-Hanford, LIGO-Livingston, and Virgo network at advanced design sensitivity. We also demonstrate that the rate of GRB triggered sources is less than the rate of untriggered events if \theta<30 deg, independent of the noise curve, network configuration, and observed GRB rate. Thus the first detection in GWs of a binary GRB progenitor is unlikely to be associated with a GRB

Viewing angle of binary neutron star mergers

The joint detection of the gravitational wave (GW) GW170817 and its electromagnetic (EM) counterparts GRB170817A and kilonova AT 2017gfo has triggered extensive study of the EM emission of binary neutron star mergers. A parameter which is common to and plays a key role in both the GW and the EM analyses is the viewing angle of the binary's orbit. If a binary is viewed from different angles, the amount of GW energy changes (implying that orientation and distance are correlated) and the EM signatures can vary, depending on the structure of the emission. Information about the viewing angle of the binary orbital plane is therefore crucial to the interpretation of both the GW and the EM data, and can potentially be extracted from either side. In the first part of this study, we present a systematic analysis of how well the viewing angle of binary neutron stars can be measured from the GW data. We show that if the sky position and the redshift of the binary can be identified via the EM counterpart and an associated host galaxy, then for 50$\%$ of the systems the viewing angle can be constrained to $\leq 7^{\circ}$ uncertainty from the GW data, independent of electromagnetic emission models. On the other hand, if no redshift measurement is available, the measurement of the viewing angle with GW alone is not informative, unless the true viewing angle is close to $90^{\circ}$. This holds true even if the sky position is measured independently. Then, we consider the case where some constraints on the viewing angle can be placed from the EM data itself. We show that the EM measurements can then be used in the analysis of GW data to improve the precision of the luminosity distance, and hence of the Hubble constant, by a factor of 2 to 3.Comment: Accepted by Physical Review

The ultrafast nonlinear response of air molecules and its effect on femtosecond laser plasma filaments in atmosphere

The nonlinear propagation of an intense ultrafast laser pulse in atmosphere or other gas media leads to filamentation, a phenomenon useful for applications such as remote sensing, spectral broadening and shaping of ultrashort laser pulses, terahertz generation, and guiding of electrical discharges. Axially extended optical filaments result from the dynamic balance between nonlinear self-focusing in the gas and refraction from the free electron distribution generated by laser ionization. In the air, self-focusing is caused by two nonlinear optical processes: (1) the nearly-instantaneous, electronic response owing to the distortion of electron orbitals, and (2) the delayed, orientational effect due to the torque applied by the laser field on the molecules with anisotropic polarizability. To study their roles in filamentary propagation as well as influences on plasma generation in atmosphere, these effects were experimentally examined by a sensitive, space- and time-resolved technique based on single-shot supercontinuum spectral interferometry (SSSI), which is capable of measuring ultrafast refractive index shift in the optical medium. A proof-of-principle experiment was first performed in optical glass and argon gas, showing good agreement between the laser pulse shape and the refractive index temporal evolution owing to pure instantaneous n2 effect. Then the delayed occurrence of the molecular alignment in the temporal vicinity of the femtosecond laser pulse, as well as the subsequent periodic &ldquo;alignment revivals&rdquo; due to the coherently excited rotational wavepacket were measured in various linear gas molecules, and the results agreed well with quantum perturbation theory. It was found that the magnitude of orientational response is much higher than the electronic response in N2 and O2, which implies that the molecular alignment is the dominant nonlinear effect in atmospheric propagation when the pulse duration is longer than &sim;40 fs, the rotational response timescale of air molecules. Realizing the possibility of manipulating plasma generation by aligning air molecules, the molecular orientational effect was further investigated by a technique developed to directly measure, for the first time, the radial and axial plasma density in a meter-long filament. The experiment was performed using both &sim;40 fs and &sim;120 fs laser pulse durations while keeping the peak power fixed under various focusing conditions, and the alignment-assisted filamenation with &sim;2&ndash;3 times plasma density and much longer axial length was consistently observed with the longer pulse, which experienced larger refractive index shift and thus stronger self-focusing. Simulations reproduced the axial electron density measurements well for both long and short pulse durations, when using a peak magnitude of instantaneous response as <15% of the rotational response

Eyeriss v2: A Flexible Accelerator for Emerging Deep Neural Networks on Mobile Devices

A recent trend in DNN development is to extend the reach of deep learning applications to platforms that are more resource and energy constrained, e.g., mobile devices. These endeavors aim to reduce the DNN model size and improve the hardware processing efficiency, and have resulted in DNNs that are much more compact in their structures and/or have high data sparsity. These compact or sparse models are different from the traditional large ones in that there is much more variation in their layer shapes and sizes, and often require specialized hardware to exploit sparsity for performance improvement. Thus, many DNN accelerators designed for large DNNs do not perform well on these models. In this work, we present Eyeriss v2, a DNN accelerator architecture designed for running compact and sparse DNNs. To deal with the widely varying layer shapes and sizes, it introduces a highly flexible on-chip network, called hierarchical mesh, that can adapt to the different amounts of data reuse and bandwidth requirements of different data types, which improves the utilization of the computation resources. Furthermore, Eyeriss v2 can process sparse data directly in the compressed domain for both weights and activations, and therefore is able to improve both processing speed and energy efficiency with sparse models. Overall, with sparse MobileNet, Eyeriss v2 in a 65nm CMOS process achieves a throughput of 1470.6 inferences/sec and 2560.3 inferences/J at a batch size of 1, which is 12.6x faster and 2.5x more energy efficient than the original Eyeriss running MobileNet. We also present an analysis methodology called Eyexam that provides a systematic way of understanding the performance limits for DNN processors as a function of specific characteristics of the DNN model and accelerator design; it applies these characteristics as sequential steps to increasingly tighten the bound on the performance limits.Comment: accepted for publication in IEEE Journal on Emerging and Selected Topics in Circuits and Systems. This extended version on arXiv also includes Eyexam in the appendi