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```Looking at What We Can’t See:
ST 562 Radio Astronomy For Teachers
By: Cecilia Huang and Joleen Welborn
The Tools
Telescope
One telescope detects and records radio waves at different
frequencies. We took observations at
1420.4 MHz, the emission frequency of neutral
hydrogen.
http://www.cassicorp.com
• N2I2: Interferometer
Two 3.05 m diameter radio telescopes situated
24 m apart will detect at different frequencies
as well, but can also be used to calculate RA
and Declination. Operates in 3 possible modes:
tracking, meridian and non-meridian (drift), and as a single
dish. Together, the telescopes act as if they were a single
telescope with higher resolution.
http://www.nrao.edu/epo/amateur/N2I2.pdf
N2I2 Drift Scan of Sun
• We took a reading using the N2I2 interferometer in drift
mode. We lined the telescope up and let the object drift
into the beam. The fringes of the sun are predictably
regular and calculations of the fringe period match the
theoretical. We did this to make sure the equipment is
functioning well.
Sun Scan
5
4.5
4
3.5
Power
3
2.5
2
1.5
1
0.5
0
1
51
101
151
201
Time (Sec)
251
301
351
Calculating Fringe Period
~3 fringes
over 100
seconds
Divide number of fringes over that period of time. This
gives the fringe period, or the number of fringes per
second. The number we get can be plugged into the
following equation determine what the RA and DEC is.
t = λ / by ωecos δ
Sun Scan
If this matches the actual position of the sun, we can
11:30 a.m. July 24, 2006
conclude the equipment is working properly.
5
4.5
4
3.5
Power
3
2.5
2
1.5
1
0.5
0
1
51
101
151
201
Tim e (Sec)
251
301
351
Choosing the Project
• Because of the fascinating nature of
pulsars, we thought it would be
interesting to observe one with one
• We chose the pulsar in the Crab
Nebula, PSR 0531+21
What is a Pulsar?
• Discovered in the 1960’s by Dr. Jocelyn
Bell who, as a grad-student, was
searching radio strip charts for something
new.
• Neutron star
• Very, very dense
• Spins really fast
• Emits high energy particles like x-rays
• Magnetic fields are intense
• “Pulses” over regular periods of time with
Diagram of a Pulsar
Image from http://glast.gsfc.nasa.gov/public/science/pulsars.html
X-Ray Image of the Pulsar in the
Crab Nebula
Image from http://glast.gsfc.nasa.gov/public/science/pulsars.html
What We Expected
At the beginning of the course, we weren’t quite sure what to
expect, so we performed the “shotgun approach” when
choosing our observations, hoping to find something that
would tell us a bit about pulsars.
We expected that:
• Drift Scans of known pulsars with SRT would
show obvious spikes at predictable or regular
times.
• N2I2 would show fringes with which we could run
calculations that would determine RA and Dec or
compare with theoretical fringe periods.
Drift Scan of Crab
Nebula using N2I2
Crab Nebula Pulsar N2I2 Drift Scan
3.2
3.15
3.1
Intensity
3.05
3
2.95
2.9
2.85
2.8
2.75
2.7
1
51
101
151
201
Tim e (sec)
251
301
351
minutes later
Crab Nebula #2
2.52
2.5
2.48
2.46
Intensity
2.44
2.42
2.4
2.38
2.36
2.34
2.32
1
51
101
151
201
Time
251
301
351
Calculations
• We determined the fringe period of
both graphs by dividing the average
number of fringes by the period of
time that went by.
• We found that not only were the
graphs very different, so were the
fringe periods.
Crab Scan I: 23.33 seconds per fringe
Crab Scan II: 15.22 seconds per fringe
Calculating the Theoretical
Fringe Period
t = λ / by ωecos δ
•
•
•
•
•
t is the fringe period in seconds
λ is the wavelength of the observation, in this case, 20 cm or 0.2
m
by is the baseline distance between telescopes, 24 m
ωe is the equatorial rotation of Earth which is about 7.29 x 10-5
cos δ is the cosine function of the declination angle
Using this calculation, the theoretical fringe period should be near
124.2 seconds per fringe. Unfortunately, neither of our observations
came anywhere near the theoretical.
Speculated Possibilities for
This Outcome
• The N2I2 has fairly accurately
detected this pulsar before. Perhaps
the N2I2 has lost some of its
sensitivity since the hail storm.
• Observation point too close to the
sun and we got a lobe.
• Pulsars are just REALLY difficult to
detect using interferometry.
What does the SRT tell us?
Our next observation was with the
SRT. We wanted to see if there were
going to be any regularly spaced
“pulses” from the Crab Nebula on the
graph.
SRT Scan of PSR 0531-21
Drift Scan of PSR 0531 -21
1040
1020
1000
Intensity
980
960
940
920
900
0
300
600
900
1200
1500
1800
Tim e Stam p
2100
2400
2700
3000
Pulse Frequency
• To get the pulse frequency, we counted the
peaks and divided the number over the amount
of time passed. We tried to be as discriminating
as possible,but it was rather difficult.
• # of “peaks” between 58 and 65.
• Time of observation ~ 19 minutes, or 1140
seconds.
• 58/1140, 65/1140 = 0.051, 0.057 seconds
between pulses, or 19.7,17.53 pulses per
second.
• Compare to the actual period pulse of the Crab
Nebula: which is 0.033 seconds or about 29
times per second.
YAY!!!
• That’s pretty darn close!
• However – We may have a better
number if we took a longer reading
and there was no lag in the data
stream between the AOC and the
VLA. PLUS, there may be a
sensitivity issue.
Future Observations
• I don’t think we should abandon the pulsar observation
with N2I2. I believe we can get close to the theoretical
fringe period by taking several more observations and
averaging them out somehow.
• Observe during a time when the sun’s declination is not so
close to the pulsar.
• Look at other known pulsars, such as PSR 0329+54
References:
•
•
•
•
•
•
•
Danielle’s interferometer design paper:
http://www.nrao.edu/epo/amateur/N2I2.pdf
Instructions on how to use the SRT: