Open Access

High-latitude intermediate-velocity clouds (IVCs) are part of the Milky Way’s H I halo and originate from either a galactic fountain process or extragalactic gas infall. They are partly molecular and can most of the time be identified in CO. Some of these regions also exhibit high-velocity cloud gas, which is mostly atomic, and gas at local velocities (LVCs), which is partly atomic and partly molecular. We conducted a study on the IVCs Draco and Spider, both were exposed to a very weak UV field, using the spectroscopic receiver upGREAT on the Stratospheric Observatory for Infrared Astronomy (SOFIA). The 158 µm fine-structure line of ionized carbon ([C II]) was observed, and the results are as follows: In Draco, the [C II] line was detected at intermediate velocities (but not at local or high velocities) in four out of five positions. No [C II] emission was found at any velocity in the two observed positions in Spider. To understand the excitation conditions of the gas in Draco, we analyzed complementary CO and H I data as well as dust column density and temperature maps from Herschel. The observed [C II] intensities suggest the presence of shocks in Draco that heat the gas and subsequently emit in the [C II] cooling line. These shocks are likely caused by the fast cloud’s motion toward the Galactic plane that is accompanied by collisions between H I clouds. The nondetection of [C II] in the Spider IVC and LVC as well as in other low-density clouds at local velocities that we present in this paper (Polaris and Musca) supports the idea that highly dynamic processes are necessary for [C II] excitation in UV-faint low-density regions.

4GREAT is an extension of the German Receiver for Astronomy at Terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are co-aligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525 and 2490-2590 GHz, respectively. This paper presents the design and characterization of the instrument, and its in-flight performance. 4GREAT saw first light in June 2018, and has been offered to the interested SOFIA communities starting with observing cycle 6.

In recent years, a plethora of observations with high spectral resolution of sub-millimetre and far-infrared transitions of methylidene (CH), conducted with Herschel and SOFIA, have demonstrated this radical to be a valuable proxy for molecular hydrogen that can be used for characterising molecular gas within the interstellar medium on a Galactic scale, including the CO-dark component. We report the discovery of the 13CH isotopologue in the interstellar medium using the upGREAT receiver on board SOFIA. We have detected the three hyperfine structure components of the ≈2 THz frequency transition from its X2Π1∕2 ground-state towards the high-mass star-forming regions Sgr B2(M), G34.26+0.15, W49(N), and W51E and determined 13CH column densities. The ubiquity of molecules containing carbon in the interstellar medium has turned the determination of the ratio between the abundances of the two stable isotopes of carbon, 12C/13C, into a cornerstone for Galactic chemical evolution studies. Whilst displaying a rising gradient with galactocentric distance, this ratio, when measured using observations of different molecules (CO, H2CO, and others), shows systematic variations depending on the tracer used. These observed inconsistencies may arise from optical depth effects, chemical fractionation, or isotope-selective photo-dissociation. Formed from C+ either through UV-driven or turbulence-driven chemistry, CH reflects the fractionation of C+, and does not show any significant fractionation effects, unlike other molecules that were previously used to determine the 12C/13C isotopic ratio. This makes it an ideal tracer for the 12C/13C ratio throughout the Galaxy. By comparing the derived column densities of 13CH with previously obtained SOFIA data of the corresponding transitions of the main isotopologue 12CH, we therefore derive 12C/13C isotopic ratios toward Sgr B2(M), G34.26+0.15, W49(N) and W51E. Adding our values derived from 12∕13CH to previous calculations of the Galactic isotopic gradient, we derive a revised value of 12C/13C = 5.87(0.45)RGC + 13.25(2.94).

We present a new multi-pixel high resolution (R ≳ 107) 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.

Based on our reconfigurable FPGA spectrometer technology, we have developed a read-out system, operating in the frequency domain, for arrays of Microwave Kinetic Inductance Detectors (MKIDs). The readout consists of a combination of two digital boards: A programmable DAC-/FPGA-board (tone-generator) to stimulate the MKIDs detectors and an ADC-/FPGA-unit to analyze the detectors response. Laboratory measurement show no deterioration of the noise performance compared to low noise analog mixing. Thus, this technique allows capturing several hundreds of detector signals with just one pair of coaxial cables.

Spectral surveys provide the only way to determine the full molecular inventory of an object and hence build a comprehensive view of the state of the molecular gas and its role in star formation and the structure and evolution of the ISM. Of course spectral surveys also provide the most efficient method of identifying new and unexpected species that have to be include in the chemical networks. The most extensive and complete survey of an extragalactic system has been the continuous spectral survey from 129 GHz to 175 GHz carried out by Martín et al. (2006) toward NGC253. This first spectral line surveys at 2 mm towards the prototypical starbursts galaxies NGC253 have shown an unexpected chemical richness.

Radio pulsars in relativistic binary systems are unique tools to study the curved space-time around massive compact objects. The discovery of a pulsar closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr A⋆, would provide a superb test-bed for gravitational physics. To date, the absence of any radio pulsar discoveries within a few arc minutes of Sgr A⋆ has been explained by one principal factor: extreme scattering of radio waves caused by inhomogeneities in the ionized component of the interstellar medium in the central 100 pc around Sgr A⋆. Scattering, which causes temporal broadening of pulses, can only be mitigated by observing at higher frequencies. Here we describe recent searches of the Galactic centre region performed at a frequency of 18.95 GHz with the Effelsberg radio telescope.

The Anomalous X‐ray Pulsar 4U 0142+61 is the only neutron star where it is believed that one of the long searched‐for ‘fallback’ disks has been detected in the mid‐IR by Wang et al. [1] using Spitzer. Such a disk originates from material falling back to the NS after the supernova. We search for cold circumstellar material in the 90 GHz continuum using the Plateau de Bure Interferometer. No millimeter flux is detected at the position of 4U 0142+61, the upper flux limit is 150 μJy corresponding to the 3σ noise rms level. The re‐processed Spitzer MIPS 24μm data presented previously by Wang et al. [2] show some indication of flux enhancement at the position of the neutron star, albeit below the 3σ statistical significance limit. At far infrared wavelengths the source flux densities are probably below the Herschel confusion limits.

Microwave Kinetic Inductance Detectors have great potential for large very sensitive detector arrays for use in, for example, ground and spaced based sub?mm imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing 1000s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present the use of a readout based on a Fast Fourier transform Spectrometer, showing no deterioration of the noise performance compared to low noise analog mixing while allowing high multiplexing ratios (>100). We present use of this technique to multiplex 44 MKIDs, while this and similar setups are regularly now being used in our array development. This development will help the realization of large cameras, particularly in the short term for ground based astronomy.

We report on submillimetre bolometer observations of the isolated neutron star RX J1856.5−3754 using the Large Apex Bolometer Camera bolometer array on the Atacama Pathfinder Experiment telescope. No cold dust continuum emission peak at the position of RX J1856.5−3754 was detected. The 3σ flux density upper limit of 5 mJy translates into a cold dust mass limit of a few earth masses. We use the new submillimetre limit, together with a previously obtained H-band limit, to constrain the presence of a gaseous, circumpulsar disc. Adopting a simple irradiated disc model, we obtain a mass accretion limit of Graphic and a maximum outer disc radius of ∼1014 cm. By examining the projected proper motion of RX J1856.5−3754, we speculate about a possible encounter of the neutron star with a dense fragment of the CrA molecular cloud a few thousand years ago.

We review the development of our digital broadband Fast Fourier Transform Spectrometers (FFTS). In just a few years, FFTS back-ends - optimized for a wide range of radio astronomical applications - have become a new standard for heterodyne receivers, particularly in the mm and sub-mm wavelength range. They offer high instantaneous bandwidths with many thousands spectral channels on a small electronic board (100 x 160 mm). Our FFT spectrometer make use of the latest versions of GHz analog-to-digital converters (ADC) and the most complex field programmable gate array (FPGA) chips commercially available today. These state-of-the-art chips have made possible to build digital spectrometers with instantaneous bandwidths up to 1.8 GHz and 8192 spectral channels.

Very large arrays of Microwave Kinetic Inductance Detectors (MKIDs) have the potential to revolutionize ground and space based astronomy. They can offer in excess of 10.000 pixels with large dynamic range and very high sensitivity in combination with very efficient frequency division multiplexing at GHz frequencies. In this paper we present the development of a 400 pixel MKID demonstration array, including optical coupling, sensitivity measurements, beam pattern measurements and readout. The design presented can be scaled to any frequency between 80 GHz and >5 THz because there is no need for superconducting structures that become lossy at frequencies above the gap frequency of the materials used. The latter would limit the frequency coverage to below 1 THz for relatively high gap materials such as NbTiN. An individual pixels of the array consist of a distributed Aluminium CPW MKID with an integrated twin slot antenna at its end. The antenna is placed in the in the second focus of an elliptical high purity Si lens. The lens-antenna coupling design allows room for the MKID resonator outside of the focal point of the lens. The best dark noise equivalent power of these devices is measured to be NEP = 7×10-19 W/[square root]Hz and the optical coupling efficiency is around 30%, in which no antireflection coating was used on the Si lens. For the readout we use a commercial arbitrary waveform generator and a 1.5 GHz FFTS. We show that using this concept it is possible to read out in excess of 400 pixels with 1 board and 1 pair of coaxial cables.

We present our second generation of broadband Fast Fourier Transform Spectrometer (FFTS), optimized for a wide range of radio astronomical applications. The new digitizer and analyzer boards make use of the latest versions of GHz analogto-digital converters and the most complex field programmable gate array chips commercially available today. These state-ofthe-art chips have made possible to build digital spectrometers with instantaneous bandwidths up to 1.8 GHz and 8192 spectral channels.

To make best use of the exceptional good weather conditions at Chajnantor we developed CHAMP+, a two time seven pixel dual-color heterodyne array for operation in the 350 and 450 µm atmospheric windows. CHAMP+ uses state-of-the-art SIS-mixers provided by our collaborators at SRON. To maximize its performance, optical single sideband filter are implemented for each of the two subarrays, and most of the optics is operated cold (20K) to minimize noise contributions. The instrument can be operated remotely, under full computer control of all components. The autocorrelator backend, currently in operation with 2 × 1GHz of bandwidth for each of the 14 heterodyne channels, will be upgraded by a new technologies FFT spectrometer array in mid 2008. CHAMP+ has been commissioned successfully in late 2007. We will review the performance of the instrument "in the field," and present its characteristics as measured on-sky.

Context.We present the technology and first scientific results of a new generation of very flexible and sensitive spectrometers, well-suited for the needs of spectral-line radio and (sub)millimeter astronomy: Fast Fourier Transform Spectrometers (FFTS), which are in operation at the Atacama Pathfinder EXperiment (APEX) telescope. Aims. The FFTS for APEX is a novel high-resolution 2 x 1 GHz bandwidth digital spectrometer backend. Due to its high frequency resolution, and the demonstrated capability of operating at high altitude, the FFTS became the facility spectrometer for spectral line observations at APEX. Methods. The FFTS is based on one of the currently most powerful digitizer/analyzer boards available from Acqiris, Switzerland. The board incorporates two 1 Gsamples/s analog-to-digital converters (ADCs) with 8-bit resolution which feed an on-board complex field programmable gate array (FPGA) chip. The enormous processing power by today's FPGAs allow for a complete real-time FFT signal processing pipeline to decompose a 1 GHz band into 16 384 spectral channels in just one chip. Results. Since May 2005 an MPIfR FFTS has been extensively used in all regular spectroscopic science observations. The performance at APEX was demonstrated to be very reliable and as good as measured in the first laboratory tests which finally led to the request to provide a second, facility type FFTS for APEX. The unit was delivered and commissioned in March this year. Conclusions. Using a commercially available digitizer board, it was possible to develop a complete FFTS in only a few months. Successful observations at APEX demonstrate that this new generation of FPGA-based spectrometers easily matching and superseding the performance of older technology spectrometers and can built up much more easily. Furthermore, the by now available class of new high-speed ADCs and the continuous increase of FPGA processing power makes it very likely that FFTS can be pushed to broader bandwidth and even more spectral channels in the near future.