interstellar medium mitigation techniques in pulsar timing ... · multipath broadening of pulses...

$: Interstellar Medium Mitigation Techniques in Pulsar Timing ... · Multipath broadening of pulses from diffraction, from interstellar density structure of scale size ~106 – 108 cm,$
Interstellar Medium Mitigation

Techniques in Pulsar Timing Arrays

Lina Levin Preston

West Virginia University

LISA symposium – 21 May 2014

•!Dispersion

•! Characteristics

•! Correction Methods

•! ISM models

•!Scattering

•! Characteristics

•! Magnitudes

•! Correction Methods

•!Summary

Overview


Efforts in all areas of ISM and

DM mitigation are underway in EPTA, PPTA and NANOGrav

Examples in this talk are from

NANOGrav data

Pulsar radio emission interacts with the ISM

mainly in two ways:

- Dispersion

From the overall integrated column density of free electrons

- Scattering

Due to spatial inhomogeneities in the ISM

The Interstellar Medium


Dispersion Measure


The broad band radio emission from a pulsar

interacts with the ISM plasma

Group velocity

Plasma frequency

The electron density in the Galactic plane

Dispersion Measure definition

Time delay of the wave due to the DM

Dispersion Measure


Lorimer & Kramer (2005)

Dispersion Measure


Pulsar distance can be inferred from its DM

Higher DM does not necessarily mean larger distance

Models of the free electrons in the Galaxy are used to predict distance, most

comprehensive model to date is the NE2001-model (Cordes & Lazio 2002)

Scattering


-! Mainly directly observed in high

DM pulsars

!! not included in PTAs

-! Indirectly through dynamic

spectra

- Scattering proportional to v-4

Lorimer & Kramer (2005)

Pulsar spectrum


Maron et al. (2000)

Typical ordinary pulsar spectrum

Average pulsar spectral index: -1.4

Scattering proportional to v-4

=> PTA pulsars are chosen to be

observed at frequencies where they

are bright, but not too scattered

To accurately determine the DM, PTAs observe pulsars in the array

at multiple frequency bands, at a maximum of a few days apart


GASP

64 MHz

GUPPI

800 MHz

GUPPI

200 MHz

GASP

64 MHz

Dispersion Measure Correction


The pulsar, the interstellar plasma and the earth

all have different velocities => DM value

measured for a pulsar varies with time

Demorest et al. in prep

J1600-3053

Pulsar radio emission interacts with the ISM

mainly in two ways:

- Dispersion

From the overall integrated column density of free electrons

- Scattering

Due to spatial inhomogeneities in the ISM

The Interstellar Medium


The Thin Screen Model

Lorimer & Kramer 2005

!

"#d$

1

2%&d

!

"d#D$

d

2

2c

!

"td #c

2$%Veff

D

2c& d

Pulse broadening time Scintillation bandwidth Scintillation timescale

Veff


Scattering


Multi-path propagation effects are produced due to

spatial inhomogeneities in the ISM

Most important manifestations for pulsars:

- Refractive Scintillation

- Diffractive Scintillation

- Pulse Broadening

Multi-path scattering


Refractive scintillation

Diffractive scintillation

Dispersion

Measure

Cordes & Shannon 2010




Dispersion

Measure

!

"tPBF

!

"t#PBF

Multipath broadening of pulses from diffraction, from interstellar

density structure of scale size ~106 – 108 cm,

changes slowly with time, on time scales of weeks to months

Diffractive fluctuations of the PBF,

associated with the finite number of scintles in an observation

which is statistically independent between epochs

Diffractive Scintillation

Diffractive Scintillation


- Scale size ~106 - 108 cm

- Measured through scintles in

dynamic spectra

- Limited in time because interference

pattern at observer has a characteristic length scale which becomes a characteristic

time scale due to the relative motion of the

system.

- Limited in frequency because coherent rays have extra scattering delays of ~

which gives rise to a diffractive bandwidth

!

"#d$

1

2%&d

!

"d




Dispersion

Measure

!

"tAOA

!

"tAOA ,SSBC

Geometrical delay from angle of arrival fluctuations,

Inhomogeneities at refractive scales (1010 – 1012 cm) cause flux

density variations and image distortions.

Associated with transforming measured TOAs to the Solar

System Barycenter. AOA fluctuations changes the effective

position of the source, which introduces an error that would have

a period of 1 year, if it was not time dependent on its own.

!

"t#PBF ,RISSRefractive modulations of the PBF, observationally hard to

distinguish from the PBF error, but conceptually different.

Connected with the expansion and contraction of the ray bundle

by refractive scintillation




Dispersion

Measure

!

"tDM ,#

Frequency dependent variations of the dispersion measure,

Multi-path scattering causes the volume of the ISM used to

determine the DM to be frequency dependent, and gives rise to

a timing delay proportional to v -23/6.

Multi-path scattering




Dispersion

Measure

Cordes & Shannon 2010

Scattering delay measurements

•! Dynamic spectra analysis of wideband G/PUPPI data - Intensity of the on-pulse over frequency band and observing time

•! Create 2D Autocorrelation function and fit

with Gaussians - !"d = half-width at half-maximum

- !td = half-width at 1/e

•! Convert !"d into scattering delays


!

"#d$

1

2%&d


Scattering Delay Variations

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Levn et al. in prep

LISA symposium – 21 May 2014 Levin et al. in prep

Scattering Delay Variations


-! For most pulsars the scattering delay variations

are << TOA uncertainty

-! PTA pulsars are chosen to have small values of

scattering (through small DM)

Summary

•! Time-variable Dispersion Measure can be exactly corrected for in

the timing procedure

•! Interstellar density fluctuations cause delays in the pulse time of

arrival, but for most PTA pulsars, scattering delays are much smaller

than median TOA error at L-band

•! While scattering is not a limiting factor for most MSPs today, ability

to correct for scattering delays may become increasingly important

with new wide-band receivers and higher timing accuracy. In

addition, it may allow adding more distant pulsars to the array in the

future.


interstellar medium mitigation techniques in pulsar timing ... · multipath broadening of pulses...

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