Science Background and More

In the SAMBA Survey program, we seek to discover new maser sources within the nuclei of galaxies, where experience has shown H2O masers to be associated with supermassive black holes.  In some nuclei, these masers pepper the accretion disks around the black holes, lying as close as 0.1 pc to the center.  Once discovered, the angular distribution of maser features throughout individual disks may be mapped with Very Long Baseline Interferometry (VLBI).
Depending on the specifics of the galactic nucleus (e.g., orientation to the line of sight and black hole mass), from these maps, one can estimate important physical quantities with unusual precision including disk orientation, shape, and degree of warping.  No other astronomical technique can provide images of accretion disks so close to supermassive black holes.  
Once a disk has been mapped, over a period of years, one can track individual masers as they orbit the black holes and use this data to estimate the distances to the black holes.  This is particularly important because the technique is entirely geometric and independent of the luminosity calibrations common to nearly all other distance measurement techniques.  So far a "geometric distance" is available for the galaxy NGC4258 (see Nature magazine, August 5, 1999).  The fractional uncertainty in its distance is currently only seven percent, and we are working to reduce it to about three percent - what is now the most accurate distance to any galaxy will soon be even better!  With luck, geometric distances to NGC4258 and several more galaxies will ultimately provide anchors in the local universe against which calibration of the extragalactic distance scale can be checked.  Controversy still exists in that calibration at the 10 - 20 percent level (e.g., it depends on the rather uncertain distance to the Large Magellanic cloud, a satellite galaxy to our own).



We use two instruments: the facility spectrometer at the Green Bank Telescope of the National Radio Astronomy Observatory, and our own custom built SAO4K autocorrelator for operations elsewhere. The GBT spectrometer supports simultaneously two 200 MHz bands in two polarizations and in two beams on the sky, with 8192 spectral channels each. When working with the GBT spectrometer, we use a double beam switched observing mode to construct total power spectra. The SAO4K supports one baseband channel (one polarization and beam) with bandwidths up to 400 MHz and 4096 spectral channels.  This corresponds to about 1.3 km/s at the rest frequency of the H2O line (22.2 GHz). When we use the SAO4K spectrometer, we use a position switched observing mode.


The Match Between Equipment and Science

The GBT and SAO4K spectrometers are unusual in their wide bandwidth capability.  Known H2O masers in active galactic nuclei (AGN) display typically sub-Jy spectral lines distributed over hundreds of thousands of km/s, where the bandwidth is dictated by the rotation speed of the disk at radii where maser action is excited.  That speed is impossible to predict in advance of discovery, so the widest possible instantaneous bandwidth is critical for efficient surveys of large numbers of galaxies.  Except for studies conducted with the Nobeyama 45-m telescope since 1992 (see Nakai, Inoue, & Miyoshi 1993, January, Nature), most past studies of H2O masers have been burdened by the narrow bandwidth of the available autocorrelators.
We have deployed SAO4K at the 70-m diameter antennas of the NASA Deep Space Network (DSN), which are among the most sensitive in the world at 22 GHz.  Because the masers are so weak, the combination of sensitivity and bandwidth is critical.  Unfortunately, it is also hard to come by.  There are only three antennas in the northern hemisphere at which maser surveys are feasible.  We use the Green Bank Telescope and the 70-m NASA antenna near Robledo de Chavela in Spain. (The third northern telescope is at Effelsberg, Germany.)  In the southern hemisphere, we use NASA's sister antenna near Canberra, Australia - it is the only competitive facility.  (The aperture of the well-known 64-m Parkes antenna is at least four times less sensitive.)
Although the DSN telescopes have wide bandwidth receivers, they do not have wide bandwidth spectrometers with high frequency resolution (i.e., on the order of a maser line's width).  Through the marriage of the SAO4K and the 70-m antennas of the NASA DSN (or through use of facilities at the GBT) it is practical to search for new masers and to study in detail the supermassive black holes that power them.



Reduction of SAO4K and GBT Spectrometer data is accomplished via scripts written in IDL, which provides advantages over the NRAO AIPS++ package.
Operation of SAO4K depends on a general software package developed by Tom Kuiper of NASA's Jet Propulsion Laboratory (JPL) to run other Spaceborne correlators at DSN facilities.
The correlator is serviced by a laptop running a server.  The server communicates with a client running on local radio science workstations.  This client controls other servers at the DSN stations, such as those that manage the radio astronomy instruments and antenna motion.

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Last modified on 3/28/04
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