November 25, 2002
Observations made at XBAND October 30,2002
The spectrum of a continuum source shows an apparent decrease in system sensitivity between 9700 and 9900 MHz, and a strong loss of signal at 9760 MHz. There is a faint feature at 9880 MHz which, if from the instrument, could indicate problems in making reliable line observations. The feature should be examined with additional observations. Also, the spurious line seen in one of the OFF/OFF scans should be examined, to check whether the feature was caused by an interference spike.
The inferred sensitivity of the GBT at 9.8 GHz is 1.81 K/Jy and 1.69 K/Jy for receiver channels one and two, respectively.
The velocity observed for the H97 hydrogen recombination line, 23.8 km/s, is consistent with the radial velocity (+25 km/s) found for a nearby transition in this source. The line width is in good agreement with other observations.
Channel Two shows a large anomalous narrow feature in the central channels.
This configuration of the Spectrometer should be examined using additional observations before it is released for general use at XBAND.
Figure 1 (tday12#21ava.ps) shows the average of the scans #21 - #31.
The spectrum is dominated by a drift from higher to lower intensity,
of about 10% from 9700 MHz to 9900 MHz, and by a large decrease in
apparent sensitivity (a "suck-out") at 9760 MHz. The emission line
feature at 9830 MHz is the expected hydrogen recombination line
H-87/alpha. The faint emission feature at 9880 MHz is unidentified
(Barry Turner) and it is not clear if it is an artifact of the
instrument.
If we focus on the region between 9720 and 9744 MHz, where the antenna temperature is relatively constant, we find that the mean value for CH:1(LL) is 11.98 +/- 0.42, compared with 11.16 +/- 0.42 for CH:2(RR), a difference of 7%. The rms values are comparable, although CH:1(RR) is somewhat higher. Choosing a narrow range in frequency where the change in Ta* is small, we get for the rms in an individual scan the values 37.5 and 31.4 mK for CH:1 and CH:2, respectively. The expected value of the rms is 17.0 mK, indicating that the baseline structure is limiting the sensitivity. Averaging the six scans together yields rms values over the same range of 32.0 and 24.8 mK. These exceed the improvement expected (to 15.3 and 12.8) by a factor of two, additional evidence that the systematics of the baseline seen toward this strong source dominate the receiver noise.
Churchwell et al (1976, A&A, 48,331) give for the flux density of NGC 7027 at these frequencies the value 6.6 +/- 0.1 Jy. The implied system sensitivity is then 1.81 and 1.69 K/Jy for RX:1 and RX:2 respectively. The observations were made at an elevation of 56 degrees, without the benefit of the active surface.
During the setup two scans (#17 and #19) were made using OFF/ON pairs
with the OFF and the ON taking 30 seconds each. Figure 2 (tday12#17pla.ps)
compares the average of the two short scans with the first of the 5 minute
scans. The baseline shapes are identical in detail, showing that no
improvement was obtained by position-switching at a rate that was
ten times faster.
Figure 3 (tday12#21avf.ps) shows the profile of the hydrogen recombination
line obtained by averaging all scans and the two polarizations. Table 1
compares the properties of the line with those found in an observation
made using the MPIfR 100-meter telescope (Churchwell et al. 1976,
A&A,48,331). Note that the Churchwell data are for the nearby H90/alpha
transition at 8873 MHz. The results are in satisfactory agreement except
for the antenna temperature, where the GBT signal is greater by a factor
of 1.7
Table 1
Quantity GBT Bonn
Velocity km/s 23.8 25 +/- 2
FWHM km/s 50.4 49 +/- 5
Peak Temp K 0.25 0.150 +/- 0.006
Figure 4:
Figure 5:
Figure 5 (tday12#33ava.ps) shows the data for five scans averaged, for
each polarization. A linear baseline has been removed, and the residuals
show an rms of 11.6 for CH1 and 11.2 for CH2. The spectra are flat,
with structure of amplitude approximately 20 mK. The following
table shows that the rms does integrate down with time in the expected
manner. The data are also shown in Figure 6 (tday12rmspl.ps).
Table 2.
Integration RMS, in mK
Time (secs) Expected RX1 RX2 Both
264 17.0 16.5 16.5
528 12.0 11.3 11.5 11.8
1056 8.51 8.02
1320 7.61 7.39 7.56
2640 5.38 5.20
The observed rms values are smaller than expected by about 3%.
This agreement is satisfactory, given that the blanking time
was greater than it should have been (50 msec instead of 2 msec)
and we may not have estimated the effective integration time
accurately.
