| Carlos A. Durán||SCHEDULED|
After the Herschel Space Telescope ceased operations in 2013, the
astronomical community has been lacking access to those parts of the
terahertz spectrum that are not visible from ground-based observatories.
The atmosphere blocks most of the radiation at those frequency bands,
even at high geographical altitude facilities like the Atacama desert (>
5000 m altitude), where APEX and ALMA operate. 4GREAT, an extension of
the German Receiver at Terahertz frequencies (GREAT) operated aboard the
Stratospheric Observatory for Infrared Astronomy (SOFIA), has been
developed in response to those needs. This works describes its design,
test, commissioning and scientific capabilities.
4GREAT is a heterodyne receiver that comprises four different detector
bands and their associated subsystems, which can be simultaneously
operated fully independently in one system. All four detector beams are
co-aligned on the sky.
The four frequency bands of 4GREAT cover 492-627, 893-1073, 1239-1515
and 2495-2690 GHz respectively. Various astrophysically important
spectral lines are observable in each band, and in some cases different
transitions of the same species, for example CO, lie in two or more
bands of 4GREAT. The very important ground state transitions of various
molecules can be observed, including NH3 , H218O, CH, OH, OH+ , NH, NH2,
and the deuterated isotopologues HDO, and OD, as well as fine structure
lines from neutral atomic carbon, [CI], and ionized nitrogen, [NII].
The expanded capabilities of GREAT with 4GREAT are now being used for a
variety of spectroscopic studies. Its potential has been demonstrated by
an absorption study of two ground state transitions or the methylidyne
radical (CH) in diffuse molecular gas. As CH traces the unobservable
molecular hydrogen, such observations are of fundamental importance for
diffuse cloud astrochemistry.
[Referees: Prof. Dr. Karl M. Menten, Prof. Dr. Frank Bertoldi, Prof. Dr.
Ian Brock, Prof. Dr. Hubert Schorle]