1. Long-Term Timing of Globular Cluster Pulsars
Sergio Smith
National Radio Astronomy Observatory
Green Bank, West Virginia
Summer 2016
Long-Term Timing of Globular Cluster Pulsars

2. Research objectives Learn to do pulsar astronomy Time some pulsars
How to Time a Pulsar
Time some pulsars
Update the timing model for these pulsars
Study timing models
Find and study interesting physics

3. So…what’s a pulsar? Neutron Star Strong magnetic fields
Bill Saxton, RAO/AUI/NSF
So…what’s a pulsar?
Neutron Star
Supernova Remnant
Strong magnetic fields
Up to Gauss
Produces radio emission from magnetic poles
Rotating Rapidly
Up to 700 times per second
Trove of questions…
…but also answers!

4. What is pulsar timing? Careful tracking of pulsar rotations
When do we EXPECT pulses to arrive?
Monitoring changes in pulse arrivals over time
Learning more about pulsars and their environment

5. What’s a timing model? “Set of parameters”
List of numbers
Describes several properties of the pulsar
Rotational period, orbital information, proper motion…
Leads to “derived parameters”
Pulsar mass, companion mass, relativistic effects…

6. Times of Arrival (TOAs)
What’s a timing model?
More Science!
TEMPO2
Timing Model
Pulse
Times of Arrival (TOAs)
Some Science

7. Getting TOAs – Observation Scheme
Observations using the Robert C. Byrd Green Bank Telescope (GBT)
L-band frequencies (1500 MHz center, with 800 MHz bandwidth)
Observations over ~5 years
~30 Observation dates
Lots of data processing

8. Getting TOAs – RFI zapping
Removing Radio Frequency Interference (RFI) from data

9. Getting TOAs – RFI zapping
Removing Radio Frequency Interference (RFI) from data

10. Getting TOAs – Calibration
Calibrate for absolute flux as well as polarization
Compares data to intensity of known object
Allows us to measure brightness of pulsar
Useful for studying energy dissipation, changes in flux over time, etc.
Assumes orthogonal polarizations
Measures circular as well as linear polarization

11. Getting TOAs – Derotation
Removes Faraday rotation from signal
Rotation in linear polarization polarization
Caused by interstellar medium (ISM)

12. Timing results – Residuals
Difference between expected TOA and measured TOA
Post-Fit Residuals for NGC 6544A

13. Timing Results – Parameters

14. Timing Results – Dispersion Measure
Effect of ISM
Smearing of signal in time
Frequency dependent

15. Timing Results – Dispersion measure
Jumps in data suggested DM was possibly changing over time
Calculated DM for each day

16. Timing Results – Dispersion measureB
Fluctuations in dispersion measure over observation span

17. Timing Results – Jerk Effect of neighboring stars on rate of rotation
Mean Field and Nearest Neighbor contributions
Seen in second period (or frequency) derivative
Fitted in timing model
F2A≈10-26 s-3
F2B≈10-25 s-3

18. Mean field contribution
Timing Results – Jerk
Full equation for second period derivative
Mean field contribution
F2mf≈10-27 s-3
(too small)
Large nearest neighbor contribution?

19. Summary Revisited values of certain timing parameters
Close agreement with published results
Dispersion measure as possible cause for residual jumps
Possible measurement of jerk
Nearest neighbor contribution?
Tests of general relativity?
Shapiro delay

20. Acknowledgements Dr. Ryan Lynch (advisor)
National Radio Astronomy Observatory
Green Bank staff members
National Astronomy Consortium
Attendees and participants
Thank you!