Science Background and More
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| 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). |
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| 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. |
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| 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). |
Equipment
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| 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
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| 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. |
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| 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.) |
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| 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. |
Software
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| 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. |
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| 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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