Transcript Measuring Dispersion in Signals from the Crab Pulsar

Measuring Dispersion in
Signals from the Crab Pulsar
Jared Crossley
National Radio Astronomy Observatory
Tim Hankins & Jean Eilek
New Mexico Tech
FORS Team, 8.2 m VLT, ESO
FORS Team, 8.2 m VLT, ESO
Pulsar Basics
• Pulsars are magnetized
neutron stars that rotate
rapidly
• Magnetic field is a
dipole (north and south
pole)
• Light is emitted in a
beam from the
magnetic poles
• 1800+ pulsars have
been found since 1968
Imagine the Universe! at NASA/GSFC
Michael Kramer (University of Manchester)
The Crab Pulsar is Unique
• Only 6000 light years away
• Only 956 years old
• 2 pulses per rotation: “main pulse” and
“interpulse”
• Occasional very bright pulses -- over 1
million times brighter than average
Very Bright
pulses
We can observe high-timeresolution single pulses
Dispersion
• Dispersion = velocity of light depends on frequency
• Radio wave propagation through ionized charges undergoes
dispersion
• For cold plasma, lower frequencies propagate slower
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• Measured using the time of arrival difference between
pulses at two frequencies
Dispersion
• Dispersion is important because:
• It must be properly removed to see pulse structure in its
original form
• Tells us about the medium between pulsar and Earth
• Previous studies have measured dispersion for pulse
ensembles, averaged over minutes to hours of observation.
My research is a study of dispersion in single pulses, which
occur on microsecond time scales.
We can now see how dispersion changes over very short
times.
Observations
Observed 9 days using Very
Large Array,1993 and 1999
Observed 20 days using
Arecibo Radio Telescope,
2002 - 2007
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•We record data using customized “back-end”
instrumentation for high time resolution measurements
•Only the brightest pulses are recorded
•Recorded pulses at observing frequencies 1 to 10 GHz
Measuring
Dispersion
1. Remove dispersion
using avg-profile DM
2. Cross-correlate pulses
3. Measure the CCFpeak offset from zerolag
4. Offset
true <DM
Offset ==>
typically
1 µs
Bright-pulse DM follows the same long-time-scale trend
as average profile DM
•Main pulse DM is closer to the avg-profile DM
•Interpulse DM is larger and more scattered
Suggests interpulse has additional, variable dispersion
Main pulse
Interpulse
•DM scatter is larger than single pulse uncertainty
•Interpulse DM scatter is larger than main pulse scatter
•No systematic variation with time or pulsar phase
Location:
The pulsar magnetosphere - the region very close to the
star - is the only place where variations occur this rapidly!
Main pulse
Interpulse
Interpulse DM has a weak tendency to increase with
frequency ==> suggests non-cold-plasma dispersion
Measure Alternative Dispersion Law
•Two dispersion sources:
•Assume magnetosphere dispersion is power law:
x = 2 for cold plasma
•Measure x using interpulse data:
• Scatter in single-pulse DM data produces wide range
of x.
Compare with Magnetosphere Model
#1
•A strong radio wave ==> relativistic plasma motion ==>
change in dispersion law
•Index of refraction (Wu & Chian, 1995) convert to DM:
B depends on magnetospheric conditions
•My data shows no correlation between DM and flux
•Correlation may be hidden by DM variability from
some other phenomena
•I measure an upper limit on B to constrain
magnetospheric conditions.
Compare with Magnetosphere Model
#2
•Strong magnetic field ==> change in particle motion
==> change dispersion law
•Index of refraction (Lyutikov & Parikh, 2000) ==> DMmag
•Result: DMmag < 0 for all radio frequencies
•My data shows the opposite: DMmag = DMIP > 0
This dispersion model does not apply to my data.
Dispersion Conclusions
Main Pulse
Interpulse
Less variable; consistent with DM larger and more variable
average profile DM
than main pulse
No dependence on
observing frequency
DM increases slightly with
increasing frequency
•Additional, variable interpulse dispersion, likely from
magnetosphere
•Compare interpulse DM with mag-sphere dispersion
models:
–Strong radio waves:
I find no correlation between DM and flux
–Strong magnetic field:
Predicts less DM, but I see more DM
The Big Picture 1
Time scale info shows
•Variability in microbursts
•Small delay echoes
•Unexpected dispersion
variability
Frequency info shows
•IP dispersion increases with
frequency (new dispersion
law!)
•Microburst have finite
bandwidth, < 4 GHz
The Big Picture 2
• Variability shows that something changes on short scales.
• This something cannot be in the interstellar medium ==>
something is changing in the star
• Differences between main pulse and interpulse ==> variability
does not affect all emission
– It may be localized within the magnetosphere
Next Steps
• Additional observations
– Good spectral coverage
• Further constrain microburst bandwidth
• Confirm or refute magnetospheric dispersion
– Extend microburst study to interpulses
– Better quantify the microburst flux-width upper limit
• Archival data may reveal additional pulse echo events
• New theory is needed to explain
– New information from microburst study
– Magnetospheric dispersion