Observing Vela With XDM The First Year
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Transcript of Observing Vela With XDM The First Year
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Observing Vela With XDMThe First Year
Sarah BuchnerKAT Bursary conference – Dec 2009
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Summary
• Pulsar timing• Timing Vela pulsar
– Looking for glitches – sudden spin-ups
• For last year I have observed Vela pulsar with XDM for 15 hours per day.
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Outline
• Introduction– What is a pulsar glitch?– Motivation for XDM observations
• Pulsar Timing 101
• Early results
• Challenges!
• Conclusion
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Observing Pulsars
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Pulsars as clocks
• Massive flywheel -> very good clock• Can unambiguously number pulses• Model rotation and compare observed arrival of
pulse with predicted arrival
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Pulsar Glitches
– Very good clock but …– Sudden increase in frequency
or “spin-up”– Frequency increases by few
parts per million– In energy terms
• earthquake of 17 on Richter scale
• surface of the earth moves by 15 m.
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HartRAO Glitch observations
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Questions
• How fast does the crust spin-up?
• What is the recovery?– Interior of neutron star
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XDM Glitch observations
• Hart 26m bearing failure
• Observe Vela while above the horizon (15 hours / day)
• Why?
• First prize: Catch a glitch “in the act”
• Second Prize: Parameterize the recovery
• Free gift: – Pulsar timing provides exacting test of polarization and timing
of XDM.– Soak test – continous observing
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XDM: First light
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Effect of ISM
• Observe over wide-bandwidth to increase sensitivity
• BUT
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Dispersive Smearing
DMB
s3.8Smear3
GHzMHz
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Dedispersion
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Dedispersion and folding
DBE Dedisperse Fold Find TOA
BarycentreFind residuals
clock DM P
Std Profile
Pls position
Timestamp
Obs x,y,z
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Arrival Times (TOA)
53075.871601404853076.606809902953076.631509316253076.635258953453076.639008589753076.745305551253076.772805320553076.814437082853076.8440091993
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1st order - frequency
‘early’
‘late’
Observed - Predicted
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2nd order
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2nd order
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Residuals
32.04 us
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12 m @ Parkes
50.66 us
Hobbs et al, 20090907.4847
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Residuals
32.04 us
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AsideWhat does a glitch look like?
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1000 days of Vela
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Challenges
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1000 days of Vela
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Offset relative to 26m transit
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Residuals
32.04 us
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Slopes
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Diurnal Slopes
100 us / day100 us / day
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No slopes in 26m data!!
• We do not see similar slopes in the HartRAO 26m data
• What causes the slopes?• What causes the offset?
– What is the difference between the two systems?– Does the ‘problem’ lies in the signal or our system?– Or in the wetware?
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HartRAO vs XDM
DBE Dedisperse Fold Find TOA
correlate
Barycentre Find residuals
GPS rudidium DMPobs Std Profile
Timestamp
Pls RA/Dec
XDMAlt/Az
Linear pol
WOPT Find TOA
Gaussian
Barycentre Find residuals
Hart 26mEquatorialCircular
polarization
H maser
XDMx,y,z
Hartx,y,z
model
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Clocks
• DBE– GPS disciplined rubidium
• HartRAO Timer– Hart hydrogen maser
• Feed 5MHz and 1 pps from H-maser into DBE
• Both systems now run off of the same clock
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Position offset
• Part of the formation of residuals involves transformation from observatory reference frame to solar system barycentre– Need to know position of observatory– Is this correct?
• 100 us per day is 30 km light travel• Can rule out
– Incorrect observatory position– Distortion of antenna– Cable expansion
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HartRAO vs XDM
DBE Dedisperse Fold Find TOA
correlate
Barycentre Find residuals
DMPobs Std Profile
Timestamp
Pls RA/Dec
XDMAlt/Az
Linear pol
WOPT Find TOA
Gaussian
Barycentre Find residuals
Hart 26mEquatorialCircular
polarization
H maser
XDMx,y,z
Hartx,y,z
model
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DBE or signal?
• Mix RF from XDM down to IF of 160 MHz and feed a 8 MHz band into Mark I timer.
• Now the same signal is being fed into both timers• Mark 1 timer narrow bandwidth & only one channel
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DBE or signal
• Same slope
• Problem lies in signal
• Polarisation issue!!
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Polarization
• XDM is alt-az mount with linear feeds
• Hart 26m is equatorial with circular feeds
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Effect of mount
sunsetsunrise
Polarization axes seem to shift in sky relative to feed
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Parallactic angle
• XDM compensates by rotating the feed in order to keep the polarization axis constant
• Is this being done correctly?• How would this effect the residuals?
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Parallactic angle
• Feed rotation and parallactic angle
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Pulse shape changes
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WARNING!!
• Sensitive KAT engineers should close their eyes
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Experimental RA
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Circular polarization
Convert from linear to circular using a hybrid
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Results
Mark I timerDBE
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Conclusion
There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know. Donald Rumsfeld
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Conclusion
• There are two effects– Polarization effect – corrected with circular polarization– Timing problem in the DBE
• We now obtained slope free residuals from XDM– Using circular polarization, narrow BW, Mark I timer
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Concluding remarks
• Pulsars are amazing!• Pulsar timing provides stringent testing
– Timing– polarization
• Highly recommend that pulsar timing forms part of single dish commisioning for KAT-7.
• meerKAT will be great pulsar instrument – galactic centre• Pulsar observing is more than plugging a timer into data
spigot• Capacity building of pulsar timing community
– First South African observations using multi channel pulsar timer with dedispersion
– Steep learning curve– International community
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Thanks
• meerKAT team especially Adriaan Pens-Hough• George Nicolson
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meerKAT science case
• Please see Roy if you would like to be involved• Meeting planned in January
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Questions?
Answers?
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Pointing Problems
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XDM prototype at HartRAO
Kitty
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KAT-7
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KAT -> meerKAT
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meerKAT
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Neutron Stars and Pulsars
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Observing Vela with XDM
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• Sudden increase in frequency or “spin-up”• Frequency increases by few parts per million• In energy terms equivalent to earthquake of 17
on Richter scale in which the surface of the earth moves by 15 m.
Pulsar Glitches
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Waiting for Vela to glitch
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Storm clouds over HartRAO
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• Sudden increase in frequency or “spin-up”• Frequency increases by few parts per million• In energy terms equivalent to earthquake of 17
on Richter scale in which the surface of the earth moves by 15 m.
Pulsar Glitches
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Timing Vela with XDM
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KAT
• Glitch detection strategies• Regular observations needed• Find glitching pulsars• Pre-cursors?
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Vela Pulsar Glitches
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Predicting glitches
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Parallactic angle
Parallactic angle is the angle between the object’s hour circle and its vertical circle
www.mmto.org/MMTpapers/pdfs/itm/itm04-1.pdf
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sunsetsunrise
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Pulsars and Neutron Stars
• Very dense • Rapidly spinning• High magnetic field• Cosmic lighthouse