C. E. Honingh

Atomic Carbon in M82: Physical Conditions Derived from Simultaneous Observations of the [C I] Fine-Structure Submillimeter-Wave Transitions

We report the first extragalactic detection of the neutral carbon [C I]3P2 →3P1 fine-structure line at 809 GHz. The line was observed toward M82 simultaneously with the 3P1 →3P0 line at 492 GHz, providing a precise measurement of the J = 2 → 1/J = 1 → 0 integrated line ratio of 0.96 (on a [K km s-1] scale). This ratio constrains the [C I] emitting gas to have a temperature of at least 50 K and a density of at least 104 cm-3. Already at this minimum temperature and density, the beam averaged C I column density is large, 2.1 × 1018 cm-2, confirming the high C I/CO abundance ratio of ≈ 0.5 estimated earlier from the 492 GHz line alone. We argue that the [C I] emission from M82 most likely arises in clouds of linear size around a few pc with a density of about 104 cm-3 or slightly higher and temperatures of from 50 K up to about 100 K.

First observations with CONDOR, a 1.5 THz heterodyne receiver

Context. The THz atmospheric “windows”, centered at roughly 1.3 and 1.5 THz, contain numerous spectral lines of astronomical importance, including three high-J CO lines, the [N II] line at 205 µm, and the ground transition of para-H2D + . The CO lines are tracers of hot (several 100 K), dense gas; [N II] is a cooling line of diffuse, ionized gas; the H2D + line is a non-depleting tracer of cold (∼20 K), dense gas. Aims. As the THz lines benefit the study of diverse phenomena (from high-mass star-forming regions to the WIM to cold prestellar cores), we have built the CO N + Deuterium Observations Receiver (CONDOR) to further explore the THz windows by ground-based observations. Methods. CONDOR was designed to be used at the Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared Astronomy (SOFIA). CONDOR was installed at the APEX telescope, and test observations were made to characterize the instrument. Results. The combination of CONDOR on APEX successfully detected THz radiation from astronomical sources. CONDOR operated with typical Trec = 1600 K and spectral Allan variance times of ∼30 s. CONDOR’s “first light” observations of CO 13−12 emission from the hot core Orion FIR4 revealed a narrow line with TMB ≈ 210 K and ∆V ≈ 5. 4k m s −1 . A search for [N II] emission from the ionization front of the Orion Bar resulted in a non-detection. Conclusions. The successful deployment of CONDOR at APEX demonstrates the potential for making observations at THz frequencies from ground-based facilities.

Terahertz hot electron bolometer waveguide mixers for GREAT

Supplementing the publications based on the first-light observations with the German Receiver for Astronomy at Terahertz frequencies (GREAT) on SOFIA, we present background information on the underlying heterodyne detector technology. We describe the superconducting hot electron bolometer (HEB) detectors that are used as frequency mixers in the L1 (1400 GHz), L2 (1900 GHz), and M (2500 GHz) channels of GREAT. Measured performance of the detectors is presented and background information on their operation in GREAT is given. Our mixer units are waveguide-based and couple to free-space radiation via a feedhorn antenna. The HEB mixers are designed, fabricated, characterized, and flight-qualified in-house. We are able to use the full intermediate frequency bandwidth of the mixers using silicon-germanium multi-octave cryogenic low-noise amplifiers with very low input return loss. Superconducting HEB mixers have proven to be practical and sensitive detectors for high-resolution THz frequency spectroscopy on SOFIA. We show that our niobium-titanium-nitride (NbTiN) material HEBs on silicon nitride (SiN) membrane substrates have an intermediate frequency (IF) noise roll-off frequency above 2.8 GHz, which does not limit the current receiver IF bandwidth. Our mixer technology development efforts culminate in the first successful operation of a waveguide-based HEB mixer at 2.5 THz and deployment for radioastronomy. A significant contribution to the success of GREAT is made by technological development, thorough characterization and performance optimization of the mixer and its IF interface for receiver operation on SOFIA. In particular, the development of an optimized mixer IF interface contributes to the low passband ripple and excellent stability, which GREAT demonstrated during its initial successful astronomical observation runs.

4.7-THz Superconducting Hot Electron Bolometer Waveguide Mixer

We present the first superconducting hot electron bolometer (HEB) waveguide mixer operating at 4.7 THz. The 5.5-nm-thick, 300-nm-long, and 3600-nm-wide NbN HEB microbridge is integrated into a normal metal (Au) planar circuit on a 2 μm thick silicon substrate. This circuit is integrated in a 24 μm × 48 μm × 21 μm waveguide cavity and a 14 μm × 7 μm × 200 μm substrate channel, which is directly machined into a CuTe alloy block. The power spectrum of the HEB mixer, measured with a Fourier transform spectrometer, is in good agreement with the results of 3-D EM circuit simulation. Measured mixer performance shows a state-of-the-art double sideband noise temperature of 1100 K, averaged over the IF bandwidth of 0.2-3.5 GHz. The 3-dB noise roll-off is 3.5 GHz. This mixer is used in the German REceiver for Astronomy at Terahertz frequencies (GREAT) at the airborne Stratospheric Observatory for Far Infrared Astronomy (SOFIA).

SMART: The KOSMA Sub-Millimeter Array Receiver for Two frequencies

We present the first results obtained with our new dual frequency SIS array receiver SMART The instrument is operational since September 2001 at the KOSMA 3m telescope on Gornergrat near Zermatt/Switzerland. The receiver consists of two 2×4 pixel subarrays. One subarray operates at a frequency of 490 GHz, the other one at 810 GHz. Both subarrays are pointed at the same positions on the sky. We can thus observe eight spatial positions in two frequencies simultaneously. For the first year of operation we installed only one half of each subarray, i.e. one row of 4 mixers at each frequency. The receiver follows a very compact design to fit our small observatory. To achieve this, we placed most of the optics at ambient temperature, accepting the very small sensitivity loss caused by thermal emission from the optical surfaces. The optics setup contains a K-mirror type image rotator, two Martin-Puplett diplexers and two solid state local oscillators, which are multiplexed using collimating Fourier gratings. To reduce the need for optical alignment, we machined large optical subassemblies monolithically, using CNC milling techniques. We use the standard KOSMA fixed tuned waveguide SIS mixers with Nb junctions at 490 GHz, and similar Nb mixers with Al tuning circuits at 810 GHz. We give a short description of the front end design and present focal plane beam maps, receiver sensitivity measurements, and the first astronomical data obtained with the new instrument.

[12Cii] and [13C ii] 158 μ m emission from NGC 2024: Large column densities of ionized carbon

Context. We analyse the NGC 2024 Hii region and molecular cloud interface using [ 12 Cii] and [ 13 Cii] observations. Aims. We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the Hii region. Methods. Observations of [ 12 Cii] and [ 13 Cii] emission at 158 µm with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the PDR. Results. The [ 12 Cii] emission closely follows the distribution of the 8 µm continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [ 13 Cii] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [ 12 Cii] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [ 13 Cii] cannot yet be explained. Conclusions. Our analysis of the two isotopes results in a total column density of N(H)≈ 1.6×10 23 cm −2 in the gas emitting the [Cii] line. A large fraction of this gas has to be at a temperature of several hundred K. The self-absorption is caused by a cooler (T≤100 K) foreground component containing a column density of N(H)≈ 10 22 cm −2 .

NbTiN Hot Electron Bolometer Waveguide Mixers on

We report on NbTiN hot electron bolometer (HEB) mixer design and fabrication for the 1.4, 1.9 and 2.5 THz frequency bands. The mixers under discussion are our contribution to the multi-band single-pixel receivers of the German Receiver for Astronomy at Terahertz Frequencies (GREAT), which is a first light instrument for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), and the focal plane array receiver on the balloon-borne Stratospheric Terahertz Observatory (STO). We measure device noise vs. intermediate frequency (IF) and analyse the receiver system output power stability and IF band ripple with newly developed SiGe low-noise amplifiers from the S. Weinreb group (Caltech). The mixers use waveguide technology with the device coupled to the fundamental waveguide mode via an integrated probe antenna. The device is electrically connected through beam leads, which reliably suspend the 2 μm thin Si3N4 membrane with micrometer mounting precision. Electron beam lithography defines the 400 nm long and 4 nm thick NbTiN microbridges and a novel deep reactive-ion etch is used for shaping of the substrates.

{\rm Si}_{3}{\rm N}_{4}

We present measurements and simulations of mixer performance around 660 GHz and around 800 GHz. We use Nb-Al/sub 2/O/sub 3/-Nb tunnel junctions with areas of 0.9 /spl mu/m/sup 2/ and 0.7 /spl mu/m/sup 2/, and RA-products of 14.5 /spl Omega/(/spl mu/m)/sup 2/ and 13 /spl Omega/(/spl mu/m)/sup 2/ for 660 GHz and 800 GHz. Both junctions have an integrated tuning structure made of niobium that consists of a series resonant stub and a quarter lambda transformer. The waveguide mixerblock has no additional adjustable tuning elements. It contains just a waveguide cavity and a substrate channel across it. A horn is carefully adjusted to the cavity and flanged to the block. The measured receiver noise temperatures from 630-690 GHz are below 190 K with a best value of 120 K at 655 GHz. From 780-820 GHz they are below 950 K with a best value of 780 K at 792 GHz. When the operating temperature is reduced from 4.2 K to 2.5 K, a reduction in noise temperature from 830 K to 660 K is observed at 810 GHz. The mixer performance is simulated using the quantum theory of mixing. The simulated performance shows a fairly good agreement with the measured one.

Membranes at THz Frequencies

We present a new multi-pixel high resolution ( R ≳ 10 7 ) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receiver uses 2 × 7-pixel subarrays in orthogonal polarization, each in an hexagonal array around a central pixel. We present the first results for this new instrument after commissioning campaigns in May and December 2015 and after science observations performed in May 2016. The receiver is designed to ultimately cover the full 1.8−2.5 THz frequency range but in its first implementation, the observing range was limited to observations of the [CII] line at 1.9 THz in 2015 and extended to 1.83−2.07 THz in 2016. The instrument sensitivities are state-of-the-art and the first scientific observations performed shortly after the commissioning confirm that the time efficiency for large scale imaging is improved by more than an order of magnitude as compared to single pixel receivers. An example of large scale mapping around the Horsehead Nebula is presented here illustrating this improvement. The array has been added to SOFIA’s instrument suite already for ongoing observing cycle 4.

Low noise broadband fixed tuned SIS waveguide mixers at 660 and 800 GHz

We present the upGREAT THz heterodyne arrays for far-infrared astronomy. The low-frequency array (LFA) is designed to cover the 1.9-2.5 THz range using 2 × 7-pixel waveguide-based HEB mixer arrays in a dual polarization configuration. The high-frequency array (HFA) will perform observations of the [OI] line at ~ 4.745 THz using a 7-pixel waveguide- based HEB mixer array. This paper describes the common design for both arrays, cooled to 4.5 K using closed-cycle pulse tube technology. We then show the laboratory and telescope characterization of the first array with its 14 pixels (LFA), which culminated in the successful commissioning in May 2015 aboard the SOFIA airborne observatory observing the [CII] fine structure transition at 1.9005 THz. This is the first successful demonstration of astronomical observations with a heterodyne focal plane array above 1 THz and is also the first time high-power closed-cycle coolers for temperatures below 4.5 K are operated on an airborne platform.