Looking at What We Cant See: Pulsar Radio Observations

1
Looking at What We Cant SeePulsar Radio
Observations

• ST 562 Radio Astronomy For Teachers
• By Cecilia Huang and Joleen Welborn

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The Tools

• SRT Single Radio Telescope
• One telescope detects and records radio waves at
  different
• frequencies. We took observations at
• 1420.4 MHz, the emission frequency of neutral
• hydrogen.
• 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.cassicorp.com
http//www.nrao.edu/epo/amateur/N2I2.pdf
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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.

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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 ? / by ?ecos d If
this matches the actual position of the sun, we
can conclude the equipment is working properly.
5
Choosing the Project

• Because of the fascinating nature of pulsars, we
  thought it would be interesting to observe one
  with one of these radio telescopes.
• We chose the pulsar in the Crab Nebula, PSR
  053121

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What is a Pulsar?

• Discovered in the 1960s 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
  electromagnetic radiation.

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Diagram of a Pulsar
Image from http//glast.gsfc.nasa.gov/public/scien
ce/pulsars.html
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X-Ray Image of the Pulsar in the Crab Nebula
Image from http//glast.gsfc.nasa.gov/public/scien
ce/pulsars.html
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What We Expected

• At the beginning of the course, we werent 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.

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Drift Scan of Crab Nebula using N2I2
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Second Reading, 30 minutes later
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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

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Calculating the Theoretical Fringe Period

• t ? / by ?ecos d
• t is the fringe period in seconds
• ? is the wavelength of the observation, in this
  case, 20 cm or 0.2 m
• by is the baseline distance between telescopes,
  24 m
• ?e is the equatorial rotation of Earth which is
  about 7.29 x 10-5 radians per second
• cos d 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.

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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.

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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.

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SRT Scan of PSR 0531-21
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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.

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YAY!!!

• Thats 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.

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Future Observations

• I dont 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 suns declination
  is not so close to the pulsar.
• Look at other known pulsars, such as PSR 032954

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References

• Danielles interferometer design paper
  http//www.nrao.edu/epo/amateur/N2I2.pdf
• Instructions on how to use the SRT
  http//www.astro.cf.ac.uk/observatory/radioman.htm
  l
• Pulsar explanations, diagrams, and images
  http//glast.gsfc.nasa.gov/public/science/pulsars.
  html
• Coordinate System of RA and Dec
  http//www.go-astronomy.com/a
  rticles/coordinate-system.htm
• Amateur Radio Astronomy Projects
  http//www.radiosky.com/rspplsr.html
• Messier Object Help http//longmontastro.org/albe
  rs/las/messier/mess_02_05.pdf
• Lyne, A.G. and Graham-Smith, F., Pulsar
  Astronomy Cambridge Astrophysics Series, 1990
  (ISBN0-521-32681-8)