Looking at What We Can’t See: Pulsar Radio Observations ST 562 Radio Astronomy For Teachers By: Cecilia Huang and Joleen Welborn

The Tools SRT: Single Radio Telescope One telescope detects and records radio waves at different frequencies. We took observations at MHz, the emission frequency of neutral hydrogen. N 2 I 2 : 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.

N 2 I 2 Drift Scan of Sun We took a reading using the N 2 I 2 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.

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 If this matches the actual position of the sun, we can conclude the equipment is working properly. 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 If this matches the actual position of the sun, we can conclude the equipment is working properly.

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

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 electromagnetic radiation.

Diagram of a Pulsar Image from

X-Ray Image of the Pulsar in the Crab Nebula Image from

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. N 2 I 2 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 N 2 I 2

Second Reading, 30 minutes later

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: seconds per fringe Crab Scan II: seconds per fringe

Calculating the Theoretical Fringe Period t = λ / b y ω e cos δ t is the fringe period in seconds λ is the wavelength of the observation, in this case, 20 cm or 0.2 m b y is the baseline distance between telescopes, 24 m ω e is the equatorial rotation of Earth which is about 7.29 x radians per second cos δ is the cosine function of the declination angle Using this calculation, the theoretical fringe period should be near seconds per fringe. Unfortunately, neither of our observations came anywhere near the theoretical.

Speculated Possibilities for This Outcome The N 2 I 2 has fairly accurately detected this pulsar before. Perhaps the N 2 I 2 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

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, seconds between pulses, or 19.7,17.53 pulses per second. Compare to the actual period pulse of the Crab Nebula: which is 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 N 2 I 2. 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

References: Danielle’s interferometer design paper: Instructions on how to use the SRT: Pulsar explanations, diagrams, and images: Coordinate System of RA and Dec: Amateur Radio Astronomy Projects: Messier Object Help: Lyne, A.G. and Graham-Smith, F., “Pulsar Astronomy”; Cambridge Astrophysics Series, 1990 (ISBN: )