Development of a detailed system model of the Eleven feed receiver using the CAESAR software

A mathematically general method is presented for exporting the far-field patterns, radiation efficiency and impedance data, for multi-port antennas, from EM solvers to the microwave circuit simulator of the CAESAR software in order to perform the analysis of a multi-port antenna-receiver system. This hybrid modelling approach offers a few important advantages over the early developed modelling methods and standard simulation tools; including the capability to predict the correlated noise contributions due to both internal and external noise sources, while exploiting the translation/rotation symmetry of the antenna structure to reduce computation time. This approach is validated using folded dipole antennas and applied to model a more complex antenna-receiver system the Eleven antenna feed operating from 2 to 12 GHz which consists of four log-periodic dipole arrays co-integrated with LNAs, a balun and hybrid combining network. The Eleven feed receiver model is verified by comparing the simulation results with the measurements performed with a practical system

Hard X-ray Flares Preceding Soft X-ray Outbursts in Aquila X-1: A Link between Neutron Star and Black Hole State Transitions

We have analyzed {\it Rossi X-ray Timing Explorer} (RXTE) data of the neutron star transient Aquila X-1 obtained during its outbursts in May/June 1999 and September/October 2000. We find that in the early rise of these outbursts, a hard flare in the energy range above 15 keV preceded the soft X-ray peak. The hard X-ray flux of the hard flares at maximum was more than a factor of three stronger than at any other point in the outbursts. The rise of the hard X-ray flare to this maximum, was consistent with a monotonically brightening low/hard state spectrum. After the peak of the hard flare, a sharp spectral transition occurred with spectral pivoting in the range 8--12 keV. Our timing analysis shows that during the hard flare the power spectra were mainly composed of band-limited noise and a $\sim$ 1--20 Hz QPO, which correlate in frequency. Immediately after the hard flare, the power spectra turned into power law noise. The spectral and timing properties during and after the hard flares are very similar to those in black hole transients during the early rise of an outburst. We suggest that these hard flares and spectral transitions in Aql X-1 are of the same origin as those observed in black hole transients. This leads to the association of the 1--20 Hz QPOs and band-limited noise in Aql X-1 with those in black hole transients. We discuss the impact of this discovery on our understanding of soft X-ray transient outbursts, state transitions and variability in X-ray binaries.Comment: 13 pages including 4 figures, accepted for publication in ApJL. For more details, see http://zon.wins.uva.nl/~yuwf/xraytransients.htm

Detecting Network Intrusion beyond 1999:Applying Machine Learning Techniques to a Partially Labeled Cybersecurity Dataset

This paper demonstrates how different machine learning techniques performed on a recent, partially labeled dataset (based on the Locked Shields 2017 exercise) and which features were deemed important. Moreover, a cybersecurity expert analyzed the results and validated that the models were able to classify the known intrusions as malicious and that they discovered new attacks. In a set of 500 detected anomalies, 50 previously unknown intrusions were found. Given that such observations are uncommon, this indicates how well an unlabeled dataset can be used to construct and to evaluate a network intrusion detection system

Understanding the Slow Depletion of Memory CD4+ T Cells in HIV Infection

Using a simple mathematical model, Andrew Yates and colleagues show that a runaway cycle of T cell activation and infection cannot explain the slow rate of CD4 decline during chronic HIV infection

Disc-jet coupling in an atoll-type neutron star X-ray binary: 4U 1728-34 (GX 354-0)

We have analysed 12 simultaneous radio (VLA) and X-ray (RXTE) observations of the atoll-type X-ray binary 4U 1728-34, performed in two blocks in 2000 and 2001. We have found that the strongest and most variable emission seems to be associated with repeated transitions between hard (island) and softer (lower banana) X-ray states, while weaker, persistent radio emission is observed when the source is steadily in the hard X-ray state. There is a significant positive ranking correlation between the radio flux density at 8.46 GHz and the 2-10 keV X-ray flux. Moreover, significant positive ranking correlations between radio flux density and X-ray timing features (i.e. break and low-frequency Lorentzian frequencies) have been found. These correlations represent the first evidence for a coupling between disc and jet in an atoll-type X-ray binary. Furthermore, drawing an analogy between the hard (island) state and the low/hard state of black hole binaries, we confirm previous findings that accreting neutron stars are a factor of ~30 less `radio loud' than black holes.Comment: accepted for publication in MNRAS Letter

Climate change and hunter-gatherers in montane eastern DR Congo

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