Using Pulsars to probe the interstellar medium
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Transcript of Using Pulsars to probe the interstellar medium
Using Pulsars to probe the interstellar medium
Barney Rickett, University of California San Diego
Department of Electrical & Computer Engineering
Presentation at PAC 2012 - KIAA/PKU October 2012
Probing the Galaxy with Pulsarsbig picture
• FAST sensitivity and sky coverage => More pulsars and DMs (DM = ∫ ne dl)
• Pulsar HI absorption measurements => new pulsar distances
• More pulsars, DMs & distances
=> Better model for electron distribution in Galaxy
=> Better model for 3D distribution of pulsars
• Pulsar Rotation Measures + better electron model
=> Better model for Galactic Magnetic Fields
DM sin(b) versus Latitude bassume stratified disk => electron density ne(z)
DM = ∫ Lp ne dl = ∫ Zp ne(z) dz/sinb
DM sinb = ∫ Zp ne(z)dz
• if Zp < Hne DM => Lp ne(0) Lp
• if Zp > Hne DM sinb => DM90 = ∫ ∞ ne(z)dz ~ ne(0) Hne
ne(z)
z
HneLp
Zp
*
*
DM versus Latitude bDM = ∫ ne dl = ∫ ne(z)dz/sinb < DM90 /sinb
20/sinb
DM sin(b) versus Latitude
DM sinb = ∫ Zp ne(z)dz < DM90
Delay spectrum (Jenet et al. 2010) PSR B1937+21
Possibility to determine the temperature and number of cool HI clouds.
emission
absorption
delay
H- Galactic Distribution
SMC LMC
I ~ ∫ ne2 dl = EM (cm-6 pc)
Large Ican be a lower bound to EM (due to saturation)
Cygnus Region
Pulsar DMs + Galaxy HFAST: ZA< 40deg AO: ZA< 20deg
Psrs & high DM in LMC & SMC
Zoom Pulsar DMs + Galaxy HAO: ZA< 20deg FAST: ZA< 40deg
Zoom2 Pulsar DMs + Galaxy H
Probing the Galaxy with PulsarsModelling the electrons in the Galaxy:
Taylor & Cordes 1993; Cordes & Lazio 2001-2006
Questions:
1. Are pulsars concentrated in spiral arms?At 100 km/sec a psr moves 1kpc in 107 years
2. Is the concentration of psrs near 40deg longitude:
a spiral arm? or due to AO observational (sensitivity) bias
3. What is ne between the spiral arms?
4. Are there more pulsars hidden by scattering in the Cygnus region?
5. Better estimates of the perpendicular distribution of pulsars & electron density
Spiral structure
42 deg
Cordes & Lazio 2006
Cordes Lazio Ne model (~2006)
Need to comparedistributions of Plasma and Pulsars
Neutron star distribution as history of star formation ?
FAST simulationSmits et al.
Probing the Galaxy with Pulsarssmall-scale picture
• Small-scale structure in the ISM scatters radiowaves• Refractive index deviation 2
• Scattering is typically consistent with Kolmogorov turbulence over scales from 1000 km -> 100 AU (Armstrong et al. 1995)
• But turbulence level is very inhomogeneous i.e. “patchy” see the H maps
• Turbulence is often anisotropic• Probe by pulsar scattering:
DM variationPulse Broadening time & ISS bandwidth
Scattering hides pulsars (esp. MSPs)
Scintillation Arcs (Stinebring et al.)
ISS GeometryFrom Radio Galaxy Quasar or AGN
2000pc
Ramachandran et al 2006:
Structure function of Dispersion Measure PSR B1937+21Ramachandran et al. 2006
Slope = 1.66
The solid line gives the best fit line with power law index =1.66 ±0.04 consistent with Kolmogorov = 5/3
zo
scattering layer
Temporal broadening
Scattered Pulse shape:
P(t) = ∫ 0
2π
B[=√(2ct/zeff)] d
zeff = (zo+zp)(zp/zo)
Pulse Broadening timescatt = zeff 2 /2c f -4.4
zp
pulsar
Scattered Image Brightness = B(
Pulse Broadening vs frequency
Scattered pulse shape for PSR J1644-45 observed at 660 MHz at ParkesRickett, Johnston and Tomlinson, 2004
Detailed shape is a diagnostic of scattering at high wavenumbers (ie due to very small scales)
Conclude linner ~ 75 km
Allowing for anisotropy makes this a lower limit
This requires very high signal to noise ratio (ie FAST)
log e[
P(t
)]
Kolmogorov:Inner scale < 10km
Inner scale > 1000km
scatt versus DM
The uniform Kolmogorov model
predicts:scatt DM2.2
But the observations show a much steeper dependence on DM. They imply that at larger distances through the electron layer, there is an increasing chance of encountering regions of high density and high turbulence.
This result is built in to the Galactic electron model of Cordes & Lazio (2003) as a high level of patchy turbulence in the inner GalaxyNote that scatt responds to a column of density-variance (related to emission measure). Since we expect ne ~ne , scatt picks out the highest densities along a line of sight.
“Secondary Spectrum” (S2) with three scintillation arcs
PSR B1133+16 at Arecibo
(Stinebring et al.)
d
td
Primary Dynamic Spectrum
Scintillation Arcs
scatt = 1/(2π d)
The Puzzle of the “Arc-lets”
Hill, Stinebring et al. (2005) showed this example of the arcs observed for pulsar B0834+06. In addition to the main forward arc (following the dotted curve) there are “reverse arclets”. Those labelled a-d are particularly striking.
They observed them over 25 days and found that they moved in Delay and Doppler, precisely as expected for the known pulsar proper motion.
dela
y
(µ
sec)
Doppler Frequency fD (mHz)
The Puzzle of the “Arc-lets” 2
The left plot shows the angular position of the structures (in mas) responsible for each reverse arclet, mapped from the Doppler frequency fD . The lines have the slope
expected for the known pulsar proper motion.
The right plot shows how fD varies with observing frequency.
Remarkably this shows that the spatial location of the scatterers is independent of frequency. They DO NOT show the expected shift due to the dispersive nature of plasma refraction.
Predicted for plasma refraction
334 MHz321 MHz
Dop
pler
Fre
quen
cy f
D (m
Hz)
VLBI of Scintillation Arcs (Brisken et al 2010)
Note reverse arclets and one group at delay of 1 msec
Scattered Brightness from B0834+06
Scattered image reconstructed by mapping from the secondary spectrum. The phase provides orientation in RA/Dec
(J-J Gao PhD UCSD)
RA (mas)
d
ec (
ma
s)
Note the faint offset scattering responsible for the “1 msec” arclet
Walker’s decomposition of Hill/Stinebring observations of B0834+06 327 MHz Arecibo Imaged by Gao assuming Vpsr
Blue line shows the axis derived from VLBI by Brisken, Gao et al.
Scattered Brightness is Anisotropic, Asymmetrical & Intermittent
Doppler Frequency fD (mHz)
What do arcs tell us?New tool for study of ISM Thin screen model is often remarkably successful => ISM is patchy
Examples of thin arcs and multiple “reverse arclets” require: a) Highly anisotropic scattering b) very patchy distribution of “turbulence” Intense turbulent regions ~10 AU dominate in a path of 108 AU !
Together these upset the assumptions of isotropy and uniformity in a turbulent & ionized ISM. Instead we have anisotropy and intermittency in the turbulence.
It leaves us with fascinating puzzles:What are the astro-physical sites that cause peaks in the scattering?What is the cause of the 1-D fine structure ? What role for magnetic field?What consequences for MSP timing ?
New ideas from Cyclo-Stationary spectral analysis New facilities GBT, EVLA, LOFAR, FAST
SummaryThe sensitivity of the FAST telescope will explore the ISM on the large scale:1. Spatial distribution of Pulsars
• Inside and outside of spiral arms• More associations with supernova remnants
2. New distance measurements by sensitive HI absorption• Delay spectrum as a new probe of HI
3. New DMs improve the modelling of plasma in the Galaxy (Ne2020?) • What ionizes the ISM?
• Influence of HII regions and supernova remnants
4. New Rotation Measures improve knowledge of the Galactic Magnetic Field• RM from pulsars, extra-galactic sources and diffuse synchrotron emission
Scattered pulse shapes and secondary spectra will explore the ISM on the small scale: 1. Monitoring the non-uniform ISM for corrections to pulsar timing
• DM variation of MSPs for timing correction
2. Particular discrete regions of scattering• What is their physical origin?• What is their density in interstellar space?
3. Study of turbulence in the interstellar plasma
Planck map
PSR B1737+13 mjd 53857 Arecibo 320 MHz Stinebring
In the 1-D scattering we find secondary spectrum: S2(,fD) B(/AfD+AfD) x B(/AfD-AfD) / |fD|in terms of the 1-D brightness function B() and a scaling constant AHence from observations of S2 one can fit the observations S2 to a 1-D model and so estimate B()
PSR B1737+13 mjd 53857 Arecibo 320 MHz Stinebring
PSR B1737+13 mjd 53857 1700 MHz 1-dim Scattered Power
Scattered Brightness is Anisotropic, Asymmetrical & Intermittent
I = |E1 + E2|2 if E1 and E2 are coherent at frequency :
= |E1|2 + |E2|2 + 2E1E2cos()
where = 2π(1t1-2t2)+01-02
t1 = t+t1 , 1 = 1
= 2π[(t1- t2)+(1- 2)t .. + ..O(t) + 01-02]
θ
θ1
2
z
where
t1 = z12/2c is the relative time delay
1 = (V.1)/c is the relative Doppler frequency
V
scattering screen
ft
fx
x
2DFT
S2
Relative to a center time t0 and frequency 0, the interference term is:
Cos[2πf(0) + 2πft(t-t0)+0]
f= t1- t2 = [12- 1
2] (z/2c)
ft = 1- 2 = (1x- 2x)V/ -0
t-t0
S1
Secondary spectrum theory 1
scattering screenArc Equations
= t1- t2 = [12- 2
2] (z/2c)
fD = 1- 2 = (1x- 2x)V/With 2 fixed there is a quadratic relation between and fD which depends
on 1y2. If in addition 1x and 1y lie on a straightline (ie 1-D scattered
brightness) the relation is a parabola through the origin => reverse arclet
1
2
z
V
b
In that case the visibility phase on baseline b corresponds to the mean position of the two angles => π (1+ 2). b/Apex of parabola is where 2=0, hence visibility phase at an apex gives astrometric
measure of 1.
B1737+13 10-weeks of 1-D models
Proper motion ~30 mas/yr
Psr distance 4.8 kpc (DM)
Angle units ~ mas
Proper motion predicts 0.5 mas per week
No coherent shifts seen
Some decorrelation even over half-hour
Perpendicular Geometry
Anisotropic & intermittent - spaghetti-like filaments in SN remnants
Separate offset feature also needed
Parallel Geometry
Isotropic scatterers clumped linearly
Other clumps too far from line of sight
Individual scattering centers
Background of distributed turbulence
Alternative Geometries for Arcs
Part of Cygnus Loop Supernova Remnant age 5-10 Kyr
Bright shell:EM ~ 100 pc cm-3
L ~ 1 pc=> max ne ~ 10 cm-3
~4 pc
Density of Galactic plane pulsars vs longitude
H intensity ± 5 deg latitude