A waveguide Josephson array oscillator operating at a frequency of 100 GHz was designed and fabricated to evaluate its pumping capability as a local oscillator (LO) for our proposed SIS mixer-based amplifier, which incorporates SIS up- and down-converters. The LO power requirement was estimated by simulation to be several nanowatts when the SIS up-converter is optimized for noise temperature and conversion gain under the condition that the input port at the microwave frequencies is connected to a practical 50-W system. The fabricated waveguide Josephson array oscillator module has 31 Nb Josephson junctions placed at the center of a half-wavelength resonator of an Nb microstripline. The output power from the waveguide oscillator module was evaluated by connecting it to the waveguide SIS detector module in a 4-K cryostat. Photon-assisted tunneling steps were clearly observed, from which the detected power was estimated to be approximately 17 nW at 106 GHz. These results suggest that the Josephson array oscillator is capable of being used in the SIS mixer-based amplifier as the LO.

Fast photon detectors will be one of the key technologies to introduce new observing methods. Utilizing our recent development of low leakage SIS junctions with Nb/Al/AlOx/Al/Nb, we designed and fabricated an antenna-coupled SIS detector. The detector exhibited a leakage current of 2 pA at $$ T \le 0.8\,{\text{K}} $$ . Fourier transform spectrometer measurements show a clear resonance peak, although the peak frequency and bandwidth were shifted from the designed values. Even though the detector efficiency was low, the noise measurement suggests that the detector would allow us to read out single terahertz photon by introducing the photon-counting scheme.

We plan to realize a fast terahertz photon counting detector. The detector may resolve each photon signal from the source and enable us to introduce “photon statistics,” which will be a new measure for terahertz astronomy and astrophysics. An SIS junction with Nb/Al/AlOx/Al/Nb is considered for this detector, which should incorporate low leakage current of 1 pA. We have successfully developed an SIS junction to satisfy this requirement. The bandwidth is limited, while it is sufficient for laboratory experiments. We have designed an antenna-coupled SIS detector incorporating the low-leakage SIS junction for photon counting experiments. Additionally, the photo-response of the SIS junction is measured to discuss the photon counting capability in combination with the fast readout system.

Properties of the continua of eight PG QSOs from ASCA data

The Josephson Flux-Flow Oscillator (FFO) has been used as an on chip local oscillator at frequencies up to 650 GHz. The FFO linewidth of about 1 MHz was measured in the resonant regime V<915 /spl mu/V for niobium-aluminum oxide-niobium tunnel junctions, while considerably larger values were reported at higher voltages. To overcome this fundamental linewidth broadening we propose a novel on chip Cherenkov radiation flux-flow oscillator (CRFFO). It consists of a long Josephson junction and a superconducting slow wave transmission line that modifies essentially the junction dispersion relation. Two SIS detectors are connected both to the long Josephson junction and the transmission line to evaluate available microwave power. The output power coming both from the long junction and the transmission line is estimated at different bias conditions.

We present the results of X-ray observations of γ Cassiopeiae made with the ASCA satellite. Since the discovery of X-ray emission from this system, several possible models for the origin of X-rays have been proposed, including the corona of γ Cas, itself, a binary neutron-star companion, and a binary white-dwarf companion. However, there has been no clear way to distinguish which, if any, of these models is correct. In this paper we exploit the spectral resolution of the ASCA SIS detectors which, for the first time, enables the iron-line emission from this system to be studied in detail. The line and continuous spectrum of γ Cas is compared with the X-ray spectra of hot coronae of early type stars, low-mass X-ray binaries, high-mass X-ray binaries, and white-dwarf binaries. The observed properties of rapid fluctuations on timescales down to 10 s and a spectrum with a temperature of 10.6 keV, together with iron line emission, are very similar to those of binary white-dwarf systems. Assuming a white-dwarf binary origin for the X-rays, we estimated the luminosity of this system as a function of the binary separation using recent measurements of the circumstellar environment of γ Cas. Despite previous arguments that the X-ray luminosity is too large to be accounted for by accretion onto a white dwarf, we find that such a model is, in fact, consistent with the observed properties of the system.

Supersoft X-ray sources are a new class of luminous X-ray binaries discovered with the X-ray telescopes of the Einstein and ROSAT satellites and extensively studied in the optical with ground based telescopes, in the UV with IUE and HST and in X-rays with ROSAT, Beppo-SAX and ASCA (cf. Kahabka &amp; van den Heuvel 1997, van Teeseling 1997). The luminosities derived for a first sample of supersoft sources studied with moderate resolution X-ray spectroscopy (using Beppo-SAX LECS and ASCA SIS detectors, Parmar et al. 1997, Ebisawa et al. 1997) have been predicted to follow Iben’s stability line (Iben 1982), i.e. the location in the Hertzsprung-Russell diagram which separates the plateau phase from the cooling phase. This is not unreasonable as any system experiencing steady-state accretion, i.e. accretion at a rate equalling about the nuclear burning rate will be found close to the stability line. If the accretion rate exceeds this limit then the white dwarf gets bloated and disappears in X-rays. If the accretion rate falls below this limit the white dwarf envelope cools, the luminosity as well as the temperature ceases and the source enters unstable recurrent nuclear burning. From the population synthesis calculations of Yungelson (1996) follows that there exit for the Milky Way a few sources at any epoch which are more massive than 1.2 MQ. They are expected to be extremely X-ray bright and may be standard candles (cf. Table 1 and Figure 1 for the brightest known supersoft sources per galaxy Milky Way to NGC 55). Their spectral distribution is expected to be similar to that of the extremely hot galactic source RXJ0925.7-4758 (it peaks at 1 keV and the flux is distributed from 0.5 to 2 keV, see Figure 2 for the ASCA spectrum of RX J0925.7-4758 (and CAL 87) as derived by Ebisawa et al. 1997).

Interaction between two Josephson junctions and a microstrip resonator is investigated using a resonator shunted junction (RLCSJ) model. A sharp current step resulting from coupling with the resonator is observed in simulated I-V characteristics. There is strong phase-locking and decreasing of dynamic resistance at the current step. We design and fabricate two types of Josephson array oscillators using these effects to obtain stable phase locking and narrow linewidth for the oscillation. The fabricated oscillators are formed with nine shunted Nb/AlOx/Nb tunnel junctions and Nb microstrip resonators. The power of the oscillator delivered to the load resistor was estimated to be about 3.6 /spl mu/W at 330 GHz. A composite linewidth of the oscillators is also measured at about 19 MHz at 4.2 K using two oscillators with a log-periodic antenna and a SIS detector.

Radiation measurements with a double-junction stacked Josephson flux-flow oscillator on-chip coupled to an SIS detector are reported. Impedance matching between the oscillator and the detector has been achieved using a broad band coupling circuit. Radiation power in the detector up to 10 nW has been measured in the frequency range between 170 and 410 GHz. Coherent radiation from two stacked junctions has been observed both at the fundamental Josephson frequency and at its second harmonic. The distribution of the radiation power between first two harmonics allows us to distinguish between mutually coupled in-phase and out-of-phase flux-flow modes in the junctions. Coherent operation of stacked Josephson junction oscillators in the millimeter and sub-millimeter wave band is demonstrated.

We report on ASCA energy spectra of Cir X-1 taken near its zero phase on 1994\nAugust 4-5. The ASCA SIS detectors allow a more detailed study of the iron K\ncomplex than has been possible before. We find that prior to a sudden upward\nflux transition the dominant iron K feature appears to consist of a large edge\nfrom neutral or nearly-neutral iron. The depth of the edge corresponds to an\nabsorption column of about 1.5E24 cm^{-2}, while little absorption over that\nexpected from the Galaxy is seen at lower X-ray energies. The differential\nabsorption at high and low X-ray energies combined with the iron edge energy\nare strong evidence that partial covering is a crucial determinant of the\nbehaviour observed from Cir X-1. The continuum spectral variability observed by\nASCA can also be understood naturally in terms of partial covering column\nchanges. There is evidence for a relatively weak emission line from neutral\niron with an equivalent width of only about 65 eV. After the flux transition,\nthe strength of the edge feature is greatly reduced, suggesting a large\nreduction in the amount of partial covering. For a large region of\nstatistically acceptable chi-squared parameter space, the luminosity of Cir\nX-1, after correction for partial covering, need not change during the\ntransition. We discuss models for the partial covering and suggest that X-ray\nscattering by electrons may be important. Aspects of the Cir X-1 spectrum are\nvery similar to those of Seyfert 2 galaxies with Compton-thin tori.\n

Results from the X-ray spectral analysis of the ASCA PV phase observation of the Seyfert 1 galaxy IC 4329A are presented. We find that the 0.4 - 10 keV spectrum of IC 4329A is best described by the sum of a steep ($\Gamma \sim 1.98$) power-law spectrum passing through a warm absorber plus a strong reflection component and associated Fe K line, confirming recent results (Madejski et al. 1995, Mushotsky et al. 1995). Further cold absorption in excess of the Galactic value and covering the entire source is also required by the data, consistent with the edge-on galactic disk and previous X-ray measurements. The effect of the warm absorber at soft X-ray energies is best parameterized by two absorption edges, one consistent with OVI, OVII or NVII, the other consistent with OVIII. A description of the soft excess in terms of blackbody emission, as observed in some other Seyfert 1 galaxies, is ruled out by the data. A large amount of reflection is detected in both the GIS and SIS detectors, at similar intensities. We find a strong correlation between the amount of reflection and the photon index, but argue that the best solution with the present data is that given by the best statistical fit. The model dependence of the Fe K line parameters is also discussed. Our best fit gives a slightly broad ($\sigma \simeq 0.11 \pm 0.08$ keV) and redshifted (E $\simeq 6.20 \pm 0.07$ keV) Fe K line, with equivalent width $\simeq$ 89 $\pm$ 33 eV. The presence of a weak Fe K line with a strong reflection can be reconciled if one assumes iron underabundances or ionized reflection. We also have modeled the line with a theoretical line profile produced by an accretion disk. This yields results in better agreement with the constraints obtained from the reflection component.

We present a simple method to determine the spectral response of an SIS detector in the millimeter and submillimeter wavelength range from its current response to a chopped cold-load. This direct detection response is also a good indicator of quantum efficiency and mixer noise temperature when using the SIS junction in heterodyne mode. A simple experimental setup without local oscillator, cryogenic IF-amplifier or any quasioptical interferometer allows a quick diagnosis of integrated planar impedance matching structures.

view Abstract Citations (92) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Corona(e) of AR Lacertae. I. The Temperature and Abundance Distribution Singh, K. P. ; White, N. E. ; Drake, S. A. Abstract X-ray spectra of the RS CVn binary AR Lac, obtained from simultaneous observations with the ROSAT PSPC and the ASCA SIS and GIS detectors, have been analyzed to study the coronal temperature and abundance distribution. The spectra were jointly fitted with plasma emission models to the following possible temperature distributions: (a) one with discrete multitemperature emission components, (b) a continuous emission measure with a power-law dependence on temperature, and (c) a continuous emission distribution parameterized by the sum of a sixth-order Chebyshev polynomial. We find that (i) solar abundance plasma models with either discrete or continuous emission measure (CEM) distributions are rejected, (ii) the best fit is obtained with a two-temperature (2T) plasma emission model with an underabundance of the elements 0, Mg, Si, S, Ar, Ca, and Fe by a factor of 3-4 relative to the solar photospheric values, and (iii) the best-fit CEM distribution also has similarly reduced abundances but fits the data less well than the 2T model. These results are confirmed even when the Fe-L region, which is subject to uncertainties in the atomic physics, is excluded from the fit. We consider optical depth effects as unlikely to be the explanation for the observed weakness of the line complexes relative to the continuum. Analysis of the spectral data during the primary and secondary eclipses shows that the emission measure of the high-temperature component in the 2T models appears to be more affected by the primary eclipse than the low-temperature component, suggesting that part of the former is concentrated in structures that are spatially more compact. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176695 Bibcode: 1996ApJ...456..766S Keywords: STARS: BINARIES: ECLIPSING; STARS: ABUNDANCES; STARS: CORONAE; STARS: INDIVIDUAL CONSTELLATION NAME: AR LACERTAE; X-RAYS: STARS full text sources ADS | data products SIMBAD (5) HEASARC (1) Related Materials (1) Part 2: 1996ApJ...473..470S

Photon-induced current correlations in SIS detectors can result in an output noise that is greater or less than shot noise. Evidence of these correlations had been observed for 100 GHz rf by accurate noise measurements as reported in our previous work. We now present a detailed analysis of these current correlations for frequencies between 100 and 500 GHz. We also report new measurements of photon-induced noise in a 490 GHz SIS mixer, and discuss the Gaussian beam techniques used to eliminate the thermal background radiation. For small 490 GHz rf power, the output noise is equal to shot noise. The results of the 100 and 490 GHz photon noise measurement are summarized in context to shot noise and the effect of the current correlations predicted by the theoretical model. >

Three Nb/AlO/sub x//Nb SIS detectors, designed to operate in the 400-550, 550-700, and 600-750 GHz bands, have been studied in direct detection mode using a Fourier-transform spectrometer. All three detectors were of quasi-optical type and had on-chip-integrated-fixed tuned SIS junctions. The tuning ranges of the detectors were selected to cover the interesting region around the superconducting gap frequency of Nb (about 700 GHz). Measurements show detector responses at frequencies above the gap frequency, i.e., up to /spl ap/920 GHz, and that cooling the detectors to 3.1 K improved the direct detection responses about 15% below 700 GHz and about 50% for frequencies up to 800 GHz, compared to the responses at 4.2 K. The 500 GHz SIS detector was also studied in a 440-520 GHz heterodyne receiver set up. Good agreement between modeled tuning circuit characteristics, tuning range of the mixer and the direct detection response bandwidths were found. However, it is essential that the dispersion of the field penetration depth into the superconductor is included in the modeling of the tuning circuits when the detector is operated at frequencies above the superconducting gap. >

The dominant source of noise in an SIS mixer is the noise in the photon-induced current. We have made accurate measurements of noise induced in SIS junctions by 95 GHz photons. The noise is measured at 1.5 GHz using a low-noise cryogenic measurement system. The measured photon-induced noise is compared to the noise predicted by Tucker's theory augmented by a vacuum/thermal noise term. For small to moderate rf powers, at which SIS mixers are operated, the measured noise is nearly perfectly predicted by this theory for all the devices measured. Measurements of series arrays of SIS junctions also agree with this theory showing that the noise of each SIS junction in the array is independent. At large rf powers, the measured noise was higher than the predicted noise, in devices with smaller capacitance. We also measured the noise in single junctions and arrays with no rf radiation. These measurements agreed very well with the predicted shot noise for most bias conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A double-dipole antenna backed by a ground plane has been fabricated for submillimeter wavelengths. The double-dipole antenna is integrated on a thin dielectric membrane with a planar detector at its center. Measured feed patterns at 246 GHz agree well with theory and demonstrate a rotationally symmetric pattern with high coupling efficiency to Gaussian beams. The input impedance is around 50 Omega and will match well to a Schottky diode or SIS detector. The double-dipole antenna served as the feed for a small machined parabolic reflector. The integrated reflector had a measured gain of 37 dB at 119 mu m. This makes the double-dipole antenna ideally suited as a feed for high-resolution tracking or for long-focal-length Cassegrain antenna systems. >

An integrated corner-reflector antenna has been designed, fabricated and measured at millimeter-wave frequencies. The structure consists of a traveling-wave antenna integrated on a 1.2- mu m dielectric membrane, and suspended in a longitudinal cavity etched in a silicon wafer. A novel traveling wave antenna design, the modified-bend antenna, with an antenna length of 1.2 lambda and spacing 0.96 lambda from the apex, results in a wideband input impedance centered at 140 Omega and low cross-polarization levels. Measurements at 180-270 GHz show a well-defined pattern with low sidelobe levels, and a main-beam efficiency of 93% and 83% at 180 and 222 GHz, respectively. The monolithic approach allows the integration of a matching network and a Schottky-diode or SIS detector at the base of the antenna to yield a low-noise monolithic millimeter-wave receiver.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Superconductors exhibit two basic modes of radiation detection —a thermal or bolometer mode and a rectification mode which depends on a nonlinear voltage-current characteristic. The SIS detector has been shown to have a quantum limited current responsivity of e/hf. By analogy, weak link detectors may have a quantum limited voltage responsivity of about 2eR/hf. Granular films behave like arrays of weak links.

An experiment on the coupling of the flux-flow type Josephson oscillator to SIS detectors by inductive coupling scheme has been carried out. Radiated powers from the Josephson oscillator were detected by an array of Josephson tunnel junctions fabricated on top of the upper electrode of the oscillator. The thickness of the common electrode was made thin enough to allow inductive coupling through the electrode. The maximum value of the obtained voltage amplitude of the emitted wave from the oscillator was 1.7mV, which is in reasonable agreement with the theoretical one. The present method can be an efficient coupling scheme between the Josephson oscillator and external circuits.