Faster and parameter-free discord search in quasi-periodic time series

Time series discord has proven to be a useful concept for time-series anomaly identification. To search for discords, various algorithms have been developed. Most of these algorithms rely on pre-building an index (such as a trie) for subsequences. Users of these algorithms are typically required to choose optimal values for word-length and/or alphabet-size parameters of the index, which are not intuitive. In this paper, we propose an algorithm to directly search for the top-K discords, without the requirement of building an index or tuning external parameters. The algorithm exploits quasi-periodicity present in many time series. For quasi-periodic time series, the algorithm gains significant speedup by reducing the number of calls to the distance function.

High temporal resolution observations of the pulsar microstructure at 102 MHz

We report observations of the microstructure of 7 pulsars at 102 MHz with a time resolution of up to 1 μs. We have detected fine pulse structure with a μs time-scale for all seven pulsars. We observe that interstellar scattering does not broaden the μs time-scale pulsar microstructure, and find structure of a time-scale much shorter than the interstellar scatter broadening. In addition to the previously known structure with time-scale τ_μ of tens of μs, we have detected structures with characteristic time-scale of a few μs, nearly equal to the reciprocal of the analysis bandwidth. This suggests that even shorter micropulses may exist.
Micropulses usually cluster in groups, each of duration tens to hundreds of μs, which may be identified as microstructure observed previously with smaller time resolution.
In addition to the previously known quasi-periodicity of hundreds of milliseconds, we reveal a structures with characteristic period of tens of μs. We observe structures with short quasi-periodicity to have a limited bandwidth. The characteristic period P_μ of these structures in PSR B0950+08 does not have a common value for different frequency regions in the same single pulse.
Scattering was revealed as a train of quasi-periodic micropulses with an exponentially damped envelope. The scatter-broadening and decorrelation bandwidth were measured. We report two time-scales of decorrelation bandwidth of pulsar PSR B0950+08 and propose two branches of Δν_d(ν) dependence.
The dispersion measure of PSR B0950+08 as DM= 2.9686 ± 0.0001 pc cm⁻³ has been determined.