Aris Karastergiou - Lorentz Center · Aris Karastergiou. pulsar science searching / timing emission...
Transcript of Aris Karastergiou - Lorentz Center · Aris Karastergiou. pulsar science searching / timing emission...
Aris Karastergiou
pulsar science
searching / timing
emission mechanismbeam structure
polarizationsingle pulses ISM studies
scintillationFaraday Rotation
dispersion/scatteringIGM studies
binary systemsgravity
ionosphere
weak sources raw sensitivity; phased arrays
all-sky distribution, transient wide fov, multiple beams
very fast modulations high (μs) temporal resolution; pulsar backends; huge data rates
ISM and IGM high resolution in frequency, polarization; dedispersion capabilities
3D beam structures broad and continuous frequency coverage
interesting polarization high polarization purity, accurate calibration
raw sensitivity
• pulsars are mJy sources even at 100 MHz
• long integration not an option for most exciting science - individual bursts and pulses, interstellar scintillation, extragalactic sources etc...
where does 100 MHz
emission come from?
lightclosed
openfieldlines
cylinderfieldlines
rotation axis
magnetic axis
radi
o pu
lses
starneutron
radio beam
single pulses• statistics of drifting subpulses: a
key to the emission mechanism
• multifrequency drifting properties
• modulation properties of core and cone components
• irregular drifting
• correlation with pulsar age
• millisecond pulsarsWeltevrede et al.nulling, giant pulses,
microstructure...
phased aperture arrays• extended low frequency dipole arrays - very long
baselines
• beam-forming at station and instrument level
achieve/monitor coherence of dipole/station addition
track & measure beam shape at different sky positions
monitor and correct for ionospheric effects
multiple beams - clock/frequency stability errors
characterization and techniques to avoid RFI
multi-level quasi on-line calibration
...and instrument level.beamforming at station...
How much of each array can be maintained in phase?
wide field-of-view -- multiple beams• powerful surveys - pulsars brightest at ~100 MHz
Flux density spectra of normal vs millisecond pulsars
For pulsar spectra that can satisfactorily fit a single power law, the range of observed spectral indices is broad: 0 >∼ ξ >∼ −4, with a mean value of −1.8±0.2 (Maron et al. 2000)
AA reliable flux calibration
Malofeev et al.
wide field-of-view -- multiple beamsRRATs: How many are there?
What exactly are they? Is their emission mechanism the same as pulsars? Spectrum?
McLaughlin et al.
highest possible time resolution to catch “spiky”
emission - take care in digital beamforming - flexible baseband recorders
wide field-of-view -- multiple beams
At lower frequencies (~ 400 MHz) where many pulsar surveys were conducted, although the steep spectral index of the source implies an even higher flux density, the predicted scattering time (~2 s) would make the bursts difficult to detect over the radiometer noise. At frequencies near 100 MHz, where low-frequency arrays currently under construction will operate, the predicted scattering time would be on the order of several minutes, and hence would be undetectable.
Lorimer et al.
x
x 0.
1s
Bhat et al.
terrestrial interferencepersistent directions/frequencies
Keane et al.
wide field-of-view -- multiple beams
N beams is equivalent to N telescopes:
- N arrays to maintain in phase- calibration highly direction-dependent, N x effort- N data streams to process and store- N pulsar backends
On-line versus Off-line processing
emission from other strange objects: AXPs, magnetars...
the beam shape
Karastergiou & Johnston 2007profile survey to draw statistical
conclusions of the pulsar radio beam
the beam shape
α
β
geometry info from polarization
lightclosed
openfieldlines
cylinderfieldlines
rotation axis
magnetic axis
radi
o pu
lses
starneutron
radio beam
polarization calibration• unlike dishes, AAs don’t move to point; polarization
calibration highly direction dependent;
• continuous calibration information
• using well understood calibrators inside field-of-view
• very large number of receiving elements, difficult to solve for the instrument response
Hamaker et al.
Important role for simulations in developing calibration techniques - SKADS project using MeqTrees package
(Smirnov, Carozzi, Smits, Levrier)
• polarization usually highest at low frequencies (Suleymanova et al., Mitra et al.)
• high polarization purity can be used for improved timing (van Straten et al.)
• polarization of all types of pulsars as diagnostic for the radio emission mechanism
• polarization to measure RMs, Galactic magnetic
• polarization of all types of pulsars as diagnostic for the radio emission mechanism
• polarization in single pulses changes between two, simultaneously observed frequencies
• continuous or sudden change?
• only the brightest pulsars can be observed in this mode with current telescopes at multiple frequencies (~10). 2 have been published.
Petrova, for theoretical explanations....
multi-frequency observations� pulsar profiles are generally
narrower at higher observing frequencies, up to ~1GHz
� the radio emission mechanism is expected to have some frequency dependence (Melrose 2000)
� Radius-to-frequency mapping; height of emission determined by local plasma density (Ruderman and Sutherland 1975)
Current telescopes sampling discrete frequencies
Low frequency arraysLOFAR, LWA, MWA...
Higher frequency arraysATA, meerKAT, ASKAP?Single dish telescopes
multi-frequency observations• for simultaneous observations: all
instruments should develop protocols for this mode of operation
• standards for pulsar data output: psrfits?
• software for processing: PSRCHIVE?
• synergies with new high-frequency instruments
• polarization usually highest at low frequencies
• high polarization purity can be used for improved timing (van Straten et al.)
• polarization of all types of pulsars as diagnostic for the radio emission mechanism
• polarization to measure RMs, Galactic magnetic field (Noutsos et al.)
• polarization to measure RMs, Galactic magnetic field (Noutsos et al.)
At low frequencies, PA changes faster with
frequency; fine frequency resolution,
high sensitivity
sensitivitypolarization
flexible backends
ISM
• Scattering variability: τ larger at low frequencies; easier to measure?
Hemberger & Stinebring
ISM• Pulsar dynamic spectra
• Holographic imaging can be used to determine the influence of multi-path propagation on pulse arrival time measurements and thus to correct for these propagation delays.
needs narrow frequency channels at low frequency
the last slide
• Raw Sensitivity
• Wide FoV - multiple beams
• Polarization
• Multi-frequency capabilities
• phased arrays; monitoring; ionosphere
• high time and frequency resolution backends, calibration
• new calibration techniques
• pulsar observing/data standards
we want... we need to achieve...