Abstract: We present spectrally resolved observations of triatomic carbon (C3) in several ro-vibrational transitions between the vibrational ground state and the low-energy ν2 bending mode at frequencies between 1654–1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N, using Herschel's HIFI instrument. We detect C3 in absorption arising from the warm envelope surrounding the hot core, as indicated by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C3 absorption due to diffuse foreground clouds. From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (Trot) of ~50-70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on the derived Trot, we get column densities N(C3) ~ 7–9 × 1014 cm-2 and abundance x(C3) ~ 10-8 with respect to H2. For W31C, using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x(C3) = 10-8, Tkin = 30–50 K, N(C3) = 1.5 × 1015 cm-2 fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis.

Abstract: In this paper the absorbed LO power requirements and the noise performance of NbN based phonon-cooled hot electron bolometric (HEB) quasioptical mixers are investigated for RF frequencies in the 0.55-1.1 range The minimal measured DSB noise temperatures are about 500 K at 640 GHz, 600 K at 750 GHz, 850 K at 910 GHz and 1250 K at 1.1 THz. The increase in noise temperature at 1.1THz is attributed to water absorption. The absorbed LO power is measured using a calorimetric approach. The results are subsequently corrected for lattice heating. These values are compared to results of a novel one dimensional hot spot mixer models and to a more traditional isotherm method which tends to underestimate the absorbed LO power for small bias powers. Typically a LO power between 50nW and 100nW is needed to pump the device to the optimal operating point.

Abstract: In this paper, we report about the development, fabrication, and characterization of a balanced waveguide hot electron bolometer (HEB) receiver for the Atacama Pathfinder EXperiment telescope covering the frequency band of 1.25–1.39 THz. The receiver uses a quadrature balanced scheme and two HEB mixers, fabricated from 4- to 5-nm-thick NbN film deposited on crystalline quartz substrate with an MgO buffer layer in between. We employed a novel micromachining method to produce all-metal waveguide parts at submicrometer accuracy (the main-mode waveguide dimensions are 90×180 μm). We present details on the mixer design and measurement results, including receiver noise performance, stability and “first-light” at the telescope site. The receiver yields a double-sideband noise temperature averaged over the RF band below 1200 K, and outstanding stability with a spectroscopic Allan time more than 200 s.