Probing the Interstellar Medium on Solar System Size Scales. Alex S. Hill 2004 April 27

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1 Probing the Interstellar Medium on Solar System Size Scales Alex S. Hill 2004 April 27

2 Outline The interstellar medium (ISM) Pulsar scintillation Secondary spectrum Imaging the ISM 2004 January Observations Physical structures in the ISM

3 The Interstellar Medium (ISM) Diffuse gas and dust between stars Gas 90% atomic hydrogen, 10% He Ionized component with electron number densities of 0.03 cm 3 Warm ISM: Temperature T 10 4 K Distribution of matter is clumpy Components in pressure balance Structure and turbulence on a range of size scales from m

4 Extreme Scattering Events (ESEs) Dramatic changes in the intensity of quasars, first seen in 1987 by Fiedler et al. (Nature, 326, 675) Caused by overdensities in the ISM, which act as diverging lenses Clegg, Fey, & Lazio (1998, ApJ, 496, 253)

5 Pulsars First discovered in 1967 by Jocelyn Bell Neutron stars Diameter 10 km; mass 1.4M Rotate rapidly (period 1 s) Emit a narrow cone of radiation in the radio, like a lighthouse Compact, coherent source of radiation, with unchanging intensity For our purposes, the pulsar is a probe of the interstellar medium, like an X-ray source in an X-ray diffractometer

6 Pulsar Scintillation Electron density inhomogeneities in the ionized interstellar medium scatter passing radio waves Multi-path propagation Interference Twinkling θ scatt θ o θ D s D D s

7 Thin Screen Model Scattering dominated by a thin screen in the ISM Origin of the screen is unknown Screen is most likely an isolated region of turbulence, perhaps a supernova shock front or the boundary between two regions of the ISM

8 Arecibo Observatory 305 m radio telescope in Arecibo, PR Most sensitive radio telescope in the world We achieve dynamic ranges of 1000 : 1 Excellent spectral resolution

9 Observing Pulsar Scintillation Single spectrum Pulsar velocity creates dynamic spectrum

10 Secondary Spectrum Squared modulus of the two dimensional Fourier transform of the dynamic spectrum P (f ν, f t ) = S(ν, t) 2 Conjugate frequency (f ν ) represents periodicities in frequency; conjugate time (f t ) represents periodicities in time Units of f ν : cycles/mhz, or µs Differential time delay Units of f t : cycles/s, or Hz Differential transverse Doppler shift (W. Coles, UCSD)

11 Scintillation Arcs Parabolic signature in secondary spectrum discovered by Dan Stinebring and Oberlin students (2001, ApJ, 549, L97) Delay scales as f ν θ 2 Doppler shift scales as f t θ Curvature of arc proportional to distance to scattering screen Sharply defined arc indicates that the scattering is dominated by a single screen

12 Variety of Secondary Spectra

13 Change in character of secondary

14 Arclets

15 What Images Cause Arclets?

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34 Arclet motion Uniform motion over the course of the month Motion primarily due to pulsar velocity V arclets = 130 ± 2 µas day 1 V eff, = 140 ± 9 µas day 1

35 Plasma Lenses Physical scattering entities at fixed positions in the screen, scanned by the moving pulsar Scattering entities are regions with an excess electron density, which act as refractive lenses From the strength of the lens, we can estimate its physical parameters

36 Lens Parameters Electron number density n e 200 cm 3 (compared to background: 0.03 cm 3 Size scale a 0.1 AU Mass M l n e a 3 m p M = g (Earth: M ) Similar to lenses thought to cause Extreme Scattering Events

37 Extreme Scattering Events Clegg, Fey, & Lazio (1998, ApJ, 496, 253)

38 12)34!30:,2!30),6.5(!"!#$%& *+ '(!) % '(!)!,-)(./(.012)34!5(0$1& 7,56)2!8.(82,!),63(. 546)325 2!!(. 546)325),63(. 546)325 1(7,56)2!80.(82,! %9 546)3250):2=() 2$%9& 5,..():,!12!80'28;3056./(04',!80%904<2) Clegg, Fey, & Lazio (1998, ApJ, 496, 253)

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40 ESEs and Pulsar Scintillation Quasar monitoring programs of 400 sources over 11 years have identified 10 ESEs Pulsar scintillation observations of 5 sources over one month have identified at least 1, and maybe 4, of the same structures Scintillation covers much broader area around the pulsar (the entire halo) than quasar observations, so the detection probability is much higher

41 Conclusions Pulsar scintillation allows single-dish interferometric imaging of the ISM Detection of arclets implies the presence of discrete regions of electron overdensities in the screen Physical origin of the thin screen and the discrete lens structures remains unknown

42 The Cutting Edge of Our Ignorance What causes lens structures? How long do they take to dissipate? How many of them are there? How much mass is locked up inside them? Why are they in the thin screen? What is the thin screen?

43 Acknowledgments Curtis Asplund Dan Berwick Wendy Everett Natalie Hinkel Max Rudolph Dan Stinebring

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