Unravelling the baffling mystery of the ultrahot wind phenomenon in white dwarfs

The presence of ultra-high excitation (UHE) absorption lines (e.g., O VIII) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. The occurrence of such features requires a dense environment with temperatures near $10^6$K, by far exceeding the stellar effective temperature. Here we report the discovery of a new hot wind white dwarf, GALEXJ014636.8+323615. Astonishingly, we found for the first time rapid changes of the equivalent widths of the UHE features, which are correlated to the rotational period of the star ($P=0.242035$d). We explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the UHE lines. The photometric and spectroscopic variability of GALEXJ014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. This is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultra hot wind phenomenon.Comment: Published in MNRAS Letter

Solar radiation forecasting using ad-hoc time series preprocessing and neural networks

In this paper, we present an application of neural networks in the renewable energy domain. We have developed a methodology for the daily prediction of global solar radiation on a horizontal surface. We use an ad-hoc time series preprocessing and a Multi-Layer Perceptron (MLP) in order to predict solar radiation at daily horizon. First results are promising with nRMSE < 21% and RMSE < 998 Wh/m2. Our optimized MLP presents prediction similar to or even better than conventional methods such as ARIMA techniques, Bayesian inference, Markov chains and k-Nearest-Neighbors approximators. Moreover we found that our data preprocessing approach can reduce significantly forecasting errors.Comment: 14 pages, 8 figures, 2009 International Conference on Intelligent Computin

The rapid evolution of the exciting star of the Stingray Nebula

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&

Highly indistinguishable and strongly entangled photons from symmetric GaAs quantum dots

The development of scalable sources of non-classical light is fundamental to unlocking thetechnological potential of quantum photonics. Semiconductor quantum dots are emerging asnear-optimal sources of indistinguishable single photons. However, their performance assources of entangled-photon pairs are still modest compared to parametric down converters.Photons emitted from conventional Stranski–Krastanov InGaAs quantum dots have shownnon-optimal levels of entanglement and indistinguishability. For quantum networks, bothcriteria must be met simultaneously. Here, we show that this is possible with a system thathas received limited attention so far: GaAs quantum dots. They can emit triggered polar-ization-entangled photons with high purity (g(2)(0) = 0.002±0.002), high indistinguish-ability (0.93±0.07 for 2 ns pulse separation) and high entanglement fidelity(0.94±0.01). Our results show that GaAs might be the material of choice for quantum-dotentanglement sources in future quantum technologie

Reddening, Absorption, and Decline Rate Corrections for a Complete Sample of Type Ia Supernovae leading to a Fully Corrected Hubble Diagram to v<30,000kms-1

Photometric BVI and redshift data corrected for streaming motions are compiled for 111 "Branch normal", 4 1991T-like, 7 1991bg-like, and 2 unusual SNe Ia. Color excesses E(B-V)host of normal SNe Ia, due to the absorption of the host galaxy, are derived by three independent methods leading to the intrinsic colors at maximum of (B-V)00=-0.024, and (V-I)00=-0.265 if normalized to a common decline rate of Dm_15=1.1. The strong correlation between redshift absolute magnitudes (based on Ho=60), corrected only for the extrinsic Galactic absorption, and the derived E(B-V)host leads to well determined, yet abnormal absorption-to-reddening ratios of R_BVI=3.65, 2.65, and 1.35. Comparison with the canonical Galactic values of 4.1, 3.1, 1.8 forces the conclusion that the law of interstellar absorption in the path length to the SN in the host galaxy is different from the local Galactic law. Improved correlations of the fully corrected absolute magnitudes with host galaxy type, decline rate, and intrinsic color are derived. The four peculiar 1991T-type SNe are significantly overluminous as compared to Branch-normal SNe Ia. The overluminosity of the seven 1999aa-like SNe is less pronounced. The seven 1991bg-types in the sample constitute a separate class of SNeIa, averaging in B two magnitudes fainter than the normal Ia. New Hubble diagrams in BVI are derived out to ~30,000kms-1 using the fully corrected magnitudes and velocities, corrected for streaming motions. Nine solutions for the intercept magnitudes in these diagrams show extreme stability at the 0.04 level using various subsamples of the data. The same precepts for fully correcting SN magnitudes we shall use for the luminosity recalibration of SNe Ia in the forthcoming final review of our HST Cepheid-SN experiment for the Hubble constant.Comment: 49 pages, 15 figures, 8 tables, accepted for publication in the Astrophysical Journa

Wireless wake-up sensor network for structural health monitoring of large-scale highway bridges

To realize in-situ structural health monitoring of critical infrastructure such as bridges, we present a powerful, but also low power and flexible, wireless sensor node utilizing a wake-up transceiver. The sensor node is equipped with several kinds of sensors, such as temperature, pressure and acceleration for in-situ monitoring of critical infrastructure. In addition to these commonly used sensors in wireless sensor networks, some nodes are equipped with global navigation satellite system receivers (GNSS) and others with tilt and acceleration sensors of very high accuracy that were developed by Nothrop Grumman LITEF GmbH. We present a low power wakeup multi-hop routing protocol that is able to transmit data with little overhead by supporting the use of wake-up receivers in combination with long-range communication radios. The wireless sensor nodes and the routing protocol are tested at a large-scale highway bridge in south-west Germany, where a prototype network was installed in June 2015 following a first test installation earlier in June at the same bridge. A gateway node equipped with a Global System for Mobile Communications (GSM) modem transfers the network data to a remote server located at the University of Freiburg