P. Verhoeve

Simultaneous absolute timing of the Crab pulsar at radio and optical wavelengths

Context. The Crab pulsar emits across a large part of the electromagnetic spectrum. Determining the time delay between the emission at different wavelengths will allow to better constrain the site and mechanism of the emission. We have simultaneously observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with μs time resolution and at 2 GHz using the Nancay radio telescope with an instrument doing coherent dedispersion and able to record giant pulses data. Aims. We have studied the delay between the radio and optical pulse using simultaneously obtained data therefore reducing possible uncertainties present in previous observations. Methods. We determined the arrival times of the (mean) optical and radio pulse and compared them using the tempo2 software package. Results. We present the most accurate value for the optical-radio lag of 255±21 μs and suggest the likelihood of a spectral dependence to the excess optical emission asociated with giant radio pulses.

High-resolution x-ray spectra measured using tantalum superconducting tunnel junctions

The spectral response of a 100×100 μm2 tantalum based superconducting tunnel junction to 5.9 keV x-ray photons from a 55Fe source has been studied. In full illumination the energy resolution for the Mn Kα line complex is 56 eV, dominated by spatial nonuniformity in the response of the detector. When illuminating selectively a 5–10 μm diam spot in the center of the detector, the energy resolution improves to 22 eV, corresponding to 15.7 eV for the individual Mn Kα1 and Mn Kα2 lines. This exceeds the predicted theoretical energy resolution of 7.3 eV for this type of device by only a factor of ∼2.

Superconducting tunnel junctions as photon counting detectors in the infrared to the ultraviolet

Photon counting experiments with Ta/Al superconducting tunnel junctions are presented. Single photon detection is demonstrated in the wavelength range /spl lambda/=200-2000 nn with a resolving power /spl lambda///spl Delta//spl lambda/=22-4. The response of the detector shows good linearity with photon energy.

Generalized proximity effect model in superconducting bi- and trilayer films

This article presents a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi- and trilayer films. It is valid for any kind of bilayer S1-S2, whatever the quality of the materials S1 and S2, the quality of the S1-S2 interface, and the layer thicknesses. The trilayer model is valid for a thin S3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, which apply to very thin or very thick layers or to perfectly transparent S1-S2 interfaces, are too restrictive to apply to an arbitrary bilayer. The new model is applied to practical junctions, with layer thicknesses intermediate between the “thick” and the “thin” approximation.

Direct determination of quasar redshifts

We present observations of 11 quasars, selected in the range z 2.2-4.1, obtained with ESAs Superconducting Tunnel Junction (STJ) camera on the WHT. Using a single template QSO spectrum, we show that we can determine theredshifts of these objects to about 1%. A follow-up spectroscopic observation of one QSO for which our best-fit redshift (z = 2.976) differs significantly from the tentative literature value (z 2.30) confirms that the latter was incorrect.

Resolution limitation due to phonon losses in superconducting tunnel junctions

We report greatly enhanced energy resolving power of Ta∕Al superconducting tunnel junction (STJ) photon detectors. The improvement in resolution has led to the observation of features in the energy dependence of the resolving power, which are not predicted by the currently prevailing formulas. We show that these effects are related to phonon noise generated during energy down-conversion in the superconducting electrodes, and the dependence of phonon energy loss on the distance of a photon absorption site from the escape interface. The predictions of the model agree well with experimental data in the energy range 0.6–6eV.

Superconducting tunnel junctions as photon-counting imaging spectrometers from the optical to the x-ray band

Superconducting tunnel junctions (STJs) have been exten- sively investigated as photon detectors. They combine intrinsic energy resolution with good detection efficiency and high responsivity, thus pro- viding energy-resolved photon-counting performance in a wide energy range, from the near IR (NIR) to the x rays. On this basis, STJs offer advantages with respect to alternative photon detection systems, such as an intrinsic energy resolution, higher speed, and when compared to wavelength dispersive systems, increased detection efficiency. We over- view the STJ development in the Astrophysics Division of the European Space Agency (ESA), including STJ fabrication and operation, spectro- scopic performance of single STJs, small arrays, and STJ-absorber structures in the UV-NIR and x-ray bands. As a first application we de- scribe S-Cam, a cryogenic camera for ground-based optical astronomy exploiting a 636 array of Ta STJs. This camera has undergone four campaigns at the 4.2-m William Herschel Telescope at La Palma (Spain), and future generations of the camera are under active develop- ment. For the soft x-ray band (50 to 3000 eV) a STJ-based instrument with an active area of 73 7m m 2 is proposed as part of the payload of the X-ray Evolving Universe Spectroscopy (XEUS) mission, which is cur- rently under study at ESA. Future developments include devices based on lower Tc superconductors for improved energy resolution as well as larger format detector arrays combined with alternative readout schemes.

Critical temperature of superconducting bilayers: Theory and experiment

A generalized model for the critical temperature TC of superconducting bilayers is presented, which is valid with no restrictions to film thicknesses, TC of the layers, and interface resistivity. The model is verified experimentally on a series of Nb–Al and Ta–Al bilayers with Nb, Ta layer thicknesses of 100 nm and Al layer thicknesses ranging from 5 to 200 nm. Excellent agreement between theory and experiment was found for the energy gap and the TC of bilayers. The results are important for designing practical superconducting devices.

SINGLE PHOTON DETECTION AT VISIBLE AND X-RAY WAVELENGTHS WITH NB-AL SUPERCONDUCTING TUNNEL JUNCTIONS

Photon counting experiments at wavelengths ranging from near infrared to x-ray with niobium based superconducting tunnel junctions with aluminum trapping layers are presented. Single photons can be detected up to a wavelength of 1 μm. The response in the ultraviolet to near-infrared region is characterized by a good energy linearity (<2.5%), a capability to handle event rates up to ∼3 kHz, and moderate energy resolving power (E/ΔE≈7 for E=4 eV). The x-ray response at 6 keV is characterized by anomalously high signals compared to the low energy response, a severe energy nonlinearity and a relatively poor energy resolution of ∼140 eV, full width at half maximum.

S-Cam: A spectrophotometer for optical astronomy: Performance and latest results

S-Cam is a cryogenic optical camera for ground-based astronomy designed around a 6×6 array of superconducting tunnel junctions. It has been conceived as a technology demonstrator, aimed at proving the potential of a new generation of single photon counting detectors for ground-based telescopes and as a possible precursor to a future space-based instrumentation. The camera is based on a 6×6 array of Ta–Al Josephson junctions, operating at about 350 mK and individually read out. For each detected photon, the absorption position, the arrival time, and the corresponding energy are measured. This allows for recording an image and simultaneously obtaining spectrophotometric information from the observed objects. In this article we provide an overview of the latest cryogenic detector performance, an up-to-date description of the S-Cam system, and a summary of the results obtained both during testing at ESTEC and during actual observations at the William Herschel Telescope in La Palma (Canary Islands, Spain). An ...