the highest time-resolution measurements in radio astronomy: the crab pulsar giant pulses
DESCRIPTION
The Highest Time-Resolution Measurements in Radio Astronomy: The Crab Pulsar Giant Pulses. Tim Hankins New Mexico Tech and NRAO, Socorro, NM Extreme Astrophysics in an Ever-Changing Universe 16-20 June, 2014. Acknowledgments. Jim Cordes Jared Crossley Tracey Delaney Jean Eilek - PowerPoint PPT PresentationTRANSCRIPT
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The Highest Time-Resolution Measurements in Radio
Astronomy: The Crab Pulsar Giant Pulses
Tim HankinsNew Mexico Tech and NRAO, Socorro, NM
Extreme Astrophysics in an Ever-Changing Universe
16-20 June, 2014
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AcknowledgmentsJim Cordes Jared Crossley Tracey Delaney Jean Eilek Glenn JonesJeff Kern Mark McKinnonDavid Moffett Jim SheckardJim Weatherall Staffs of NRAO and NAIC
Cornell UniversityNew Mexico Tech, NRAO
New Mexico Tech, WV WesleyanNew Mexico Tech, NRAO
Cal Tech, NRAONew Mexico Tech, NRAONew Mexico Tech, NRAO
New Mexico Tech, Furman UniversityNew Mexico Tech
New Mexico Tech, FAA
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Science objectives• What is the pulsar radio emission
mechanism?
• How does a relativistic magnetized pair plasma radiate at equivalent brightness temperatures of 1036 1042 K?
• Can we understand Crab Nebula pulsar? Does the Crab fit the canonical pulsar model?
Or is it unique?
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Summary
Time resolution down to 0.2 nanoseconds achieved using a large-memory digital oscilloscope and coherent dedispersion
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Scientific Method:Form Hypothesis:
Emission is a form Shot noise: Cordes, 1976
Make predictions: Shot noise cause: Collapsing solitons in
turbulent plasma: Weatherall, 1998
Test by experiment: High-time resolution observations
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Collapsing soliton prediction I
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Prediction II
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How to get high time resolution:
Coherent dedispersion required.
Sample receiver voltage at Nyquist rate. Pass signal through a filter with the inverse dispersion characteristic of the Interstellar Medium. Use square-law detectors to obtain intensity. (Polarization slightly more complex.)
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Coherent dedispersion
• Emitted signal: s(t) S(w)• Dispersive ISM: H(w) = exp[ik(w)z] h(t)• Received signal: s(t)*h(t) S(w) H(w)• Dedispersion processing: S(w)H(w)•H(w)–1 s(t)
» and 10,000 lines of code
: Fourier Transform * : Convolution
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What Can You Do With It?
Diagnostic for emission mechanism studies:Found nanostructure predicted by Weatherall
Propagation studies:Precision DM determination
Discoveries:Echoes of Crab “giant” pulsesCrab Interpulse spectral bands
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Crab “Megapulse” at 9.25 GHz
0.4 ns
2.2 Mega-Jansky pulseDuration: 0.4 nanoseconds
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Not all pulses are so short Typical Main Pulse, 9 GHz
0 1 2 3 4 5 6Time (Microseconds)
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Giant Pulse Widths
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Dispersion Measure Determination Methods:
Time delay between two frequenciesMust account for pulse shape change & Scattering broadening
Split receiver passbandCross-correlate micro-, nanostructure
Adjust dispersion removal filter to Maximize pulse intensity varianceMinimize equivalent width
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Two-frequency Cross-correlation
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Split-band Cross-correlation
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Adjust Dispersion Measure
DM = 56.739780
DM = 56.739780
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Adjust Dispersion Measure
DM = 56.735001
DM = 56.735001
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Modulation Spectra
Main Pulses Interpulses
10-3
10-3
10-3
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Unexpected Discoveries
Giant Pulse Echoes
Dynamic Spectra: Interpulse Bands
Main pulse DM ≠ Interpulse DM
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Giant pulse Echoes
1435.1 MHz Echo1435.1 MHz 1435.1 MHz 1435.1 MHz
4885.1 MHz 4885.1 MHz 4885.1 MHz
Echo
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Dynamic Spectra
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Intensity and spectrum of a Main pulse
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Intensity and spectrum of an Interpulse
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Main pulse:Wideband
Interpulse:Banded
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Some definitions:
BandSeparation
BandWidth
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Interpulse band bandwidths
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Interpulse Band FrequenciesAt 30 GHz: Q ≈ Band Separation = 1.8 GHz = 36 Band Width 0.05 GHz
Band SeparationSky Freq
Band Separation
= 0.058
Least-squaresfit slope =
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Band Center Frequency Memory
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Main pulse/Interpulse
Dispersion
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Main pulse
Interpulse
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Main Pulse
Interpulse
Dispersioncorrected
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DM vs. Flux
Interpulses
Main pulses
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DM vs. Time
InterpulsesMain pulses−Jodrell Bank DM
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Summary
Fast sampling: versatility in processing allows detailed emission studies.
“The more you look, the more you see.”
The Crab pulsar: Continues to “amaze and mystify”
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Future
Dedispersion Processing: My old, 8-core Mac:
5 GHz data bandwidth: 4000x real time.
(2 ms data in 8 seconds) [with lots of diagnostic overhead]
Add n GPUs (Graphics Processor Units): Processing time reasonable.
Moore’s Law:
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Coherent Dedispersion History: Bandwidth vs. Date
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Giant Main Pulses are Wideband
From Glenn Jones at the GAVRT Telescope
Time (Microseconds) 5 10 15 20 25 30 5 15 25
3
4
5
6
7
8
9
1
0Fr
eque
ncy
(GH
z)
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The End
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Giant pulse Echoes
20 40 60 Time (microseconds) 100 120 140
1435.1 MHz Echo