Wideband Ku Receiver
Overview
The current GBT Ku-band receiver is a two-beam system with dual polarization in each beam, operating from 12-15.4 GHz. The Ku band receiver is requested for 10-15% of all present science proposals. This frequency band has great potential to be a “work-horse” band for the GBT. It straddles the break point between high and low frequency observing in GB, and the dual beam makes it possible to observe over a wide range of weather conditions.
As part of the EVLA upgrade, a new Ku receiver has been developed which represents both a significant increase in the bandwidth (12-18 GHz) and a decrease in the receiver temperature (~1.5x) over the 1990’s design of the GBT Ku-band receiver. As a result, NRAO is replacing the existing GBT Ku receiver with the higher performance system of the EVLA.
Key Science
The Ku receiver will have a large scientific impact, allowing for improved scientific performance across many scientific areas. Nonetheless the instrument has four key science projects themes toward which its development will be focussed.
I. Fundamental Physics – Probing Gravity in the Strong Field Regime: The main science driver for this project is the detection of pulsars in orbit around the massive black hole (Sgr A*) at the center of our Galaxy. The ultimate goal of this effort would be to accurately time such pulsars in order to test Einstein’s general theory of relativity in the strong field regime, where deviations from the predictions of general relativity are theoretically most likely. Testing strong gravity and the direct detection of gravitational waves were among the major science frontiers identified in the Astro2010 Decadal Survey process. The frequency range of 10-20 GHz has been demonstrated by several authors (e.g. Macquart, Kanekar, Frail & Ransom, 2010, ApJ, 715, 939) to be ideal to overcome the strong interstellar scattering in the Galactic Center (GC) direction and to maximize the likelihood of detecting normal pulsars in close orbit around Sgr A*. The receiver will be our "last best hope" of finding those pulsars before something like SKA-High is built.
II. Fundamental Physics – Changes in the Fundamental Constants: The new Ku-band receiver will be excellent at probing changes in fundamental constants such as the proton-electron mass ratio μ and the fine structure constant α. As an example, a comparison between the redshifts of the ammonia NH3 23.7 GHz inversion line and the CS (1-0) rotational line from a single galaxy is extremely sensitive to changes in the proton-electron mass ratio (Flambaum & Kozlov 2007, Phys. Rev. Lett, 98, 240801). The 10-18 GHz frequency range is the most important for this purpose, as it corresponds to the redshift range 0.3 < z < 1.37, i.e. a combination of a large lookback time (3.3 – 9 Gyrs) and sufficiently low redshifts that galaxies contain significant amounts of molecular gas, making it possible to detect the redshifted ammonia line.
III. Chemical Origin of Life: In addition to a search for new molecular species (e.g. carbodiimide, with transitions at 15.9 and 16.4 GHz), in the range of 15-18 GHz there are also several transitions of known interstellar molecular species that will help us constrain the physical and chemical environment of astronomical regions. In this region, there are over 1600 potential interstellar transition that may be detected. For instance there are low energy transitions of well known molecules like NH3, transitions of deuterated species like NH2D, D2CO and DNCO, transitions of abundant isotopic species such as H2C(34)S, (33)SO2 and (34)SO2, etc.
IV. Galaxies Across Cosmic Time: The wide-band 12-18 GHz receiver will be extremely interesting for spectroscopy in redshifted molecular lines. It will allow "blind'' searches for redshifted CO (1-0) emission from galaxies at 5.4 < z < 8.6, well into the epoch of reionization. Dusty galaxies at these redshifts would not detectable by the standard Lyman-break technique due to dust-obscuration of their UV continuum emission, but could be detected in redshifted CO emission. the receiver will also allow targeted searches for redshifted CO (1-0) emission from individual Lyman-α emitters and quasars at z >~ 6, allowing one to relate the molecular gas mass to the star formation rate in normal galaxies at these redshifts.
