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Interstellar Scintillation

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Latest results for the fast scintillator, PKS 1257-326. Brief overview of last week's workshop ... 1984: 'flicker' - timescales 2-10 days at 2.7 GHz (Heeschen) ... – PowerPoint PPT presentation

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Title: Interstellar Scintillation


1
Interstellar Scintillation micro-arcsecond
scale structure in AGN
  • Hayley Bignall (JIVE)
  • J-P Macquart (Kapteyn Institute, Groningen)
  • Dave Jauncey, Jim Lovell, Tasso Tzioumis (ATNF)
  • Lucyna Kedziora-Chudczer (U. Sydney)

2
Outline
  • IDV and ISS some background
  • Latest results for the fast scintillator,
    PKS 1257-326
  • Brief overview of last weeks workshop in
    Dwingeloo Interstellar scintillation of
    Extragalactic Radio Sources

3
Discovery of IDV
  • 1984 flicker - timescales 2-10 days at 2.7 GHz
    (Heeschen)
  • "Intraday variability" (IDV Witzel et al. 1986,
    Heeschen et al. 1987)
  • Debate over origin intrinsic vs scintillation

4
(1966)
5
Regimes of scintillation
STRONG SCATTERING
WEAK SCATTERING
Fresnel scale rF(lL/2p)1/2
tref
Log time scale
tweak
tdiff
Log frequency
6
PKS 0405-385 the first intra-hour variable
  • Kedziora-Chudczer et al. (1997)

8.6 GHz
4.8 GHz
2.3 GHz
1.4 GHz
7
PKS 0405-385 ISS model
RMS fractional variations as a function of
frequency
8
PKS 0405-385
  • Rapid variability interpreted as interstellar
    scintillation (ISS) by Kedziora-Chudczer et al.
    (1997)
  • Still a high brightness temperature (D1000
    required) using standard model of Galactic
    electron density (TC93)
  • Model of Rickett et al. (2002) implies nearby
    screen (3-30 pc) Tb reduced to 2x1013 K

9
PKS 1257-326 the third intra-hour variable
  • The second well-studied IHV?! is J18193845
    (Dennett-Thorpe de Bruyn, 2000), discovered
    with WSRT
  • PKS 1257-326 was discovered serendipitously in
    2000, in ATCA data at 4.8 and 8.6 GHz
  • ID quasar at z1.256 (Perlman et al. 1998),
    B18.7
  • X-ray flux 2x10-13 erg cm-2 s-1 (0.1-2 keV)
  • Flat radio spectrum ltSgt 0.3 Jy

10
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11
Two-station time delay
  • Simultaneous VLA/ATCA observations
  • Performed in 2002 May, 2003 Jan, March
  • 2 consecutive days, 2 frequencies
  • 4.86 GHz, 8.46 GHz, 100 MHz bandwidth
  • Only 2.8 h common visibility low elevations
  • Secondary calibrator PKS 1255-316 1o separation,
    non-IDV (? 1), 2 Jy
  • Arcsecond-scale structure subtracted

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14
Time delay, May 13-14 2002
VLA
ATCA
May 13, 4.9 GHz
May 13, 8.5 GHz
Flux density (0.2-0.4 Jy)
May 14, 4.9 GHz
May 14, 8.5 GHz
Time (4 hour range)
15
Time delay, Jan 10 2003
Jan 10, 4.9 GHz
Jan 10, 8.5 GHz
Flux density (0.2-0.4 Jy)
VLA
ATCA
Time (4 hour range)
16
Time delay, Mar 6-7 2003
VLA
ATCA
Mar 6, 4.9 GHz
Mar 6, 8.5 GHz
Flux density (0.2-0.4 Jy)
Mar 7, 4.9 GHz
Mar 7, 8.5 GHz
Time (4 hour range)
17
Fitting to a constant time delay (May data)
c2
Time delay (0-1000 s)
18
Fitting to a constant time delay (Jan data)
c2
Time delay (0-1000 s)
19
Fitting to a constant time delay (March data)
c2
Time delay (0-1000 s)
20
Flux density difference, before and after
applying time shift (March data)
21
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22
4.8 GHz
8.6 GHz
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25
Fitting time delay annual cycle
  • Free parameters are
  • Velocity of scattering screen (vRA, vdec)
  • Characteristic length scale s0
  • Axial ratio and position angle of anisotropy, R,
    b
  • Data ? J-Ps fitting code combined c2
    minimization ? fits!
  • (work in progress)

26
S01.5x105 km vRA27 km s-1 Vdec6 km
s-1 R10 B-40o
27
S0105 km vRA27 km s-1 Vdec6 km s-1 R5.6 B-40o
28
LSR
29
Implications
  • If rF105 km at 4.8 GHz (l6.25 cm), then screen
    distance L30 pc
  • For best fit, source cant be bigger than 100x40
    mas ? Tb 3 x 1012 K

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33
PKS 1257-326
  • Combined time delay annual cycle is a neat
    method of constraining scintillation parameters
  • Still much more to get out of the data! e.g
  • intrinsic variations and frequency dependence
  • polarization (cf Rickett et al. 2002)
  • Behaviour of PKS 1257-326 is remarkably similar
    to J18193845!
  • What makes these sources special?

34
  • A small workshop held at ASTRON/JIVE, Dwingeloo,
    the Netherlands, 5-7 April 2004
  • SOC de Bruyn, Jauncey, Bignall, Macquart,
    Gurvits
  • 25 participants, most gave talks
  • Plenty of discussion time

35
Summary of ISS workshop
  • Majority of presentations were on radio IDV/ISS
    in AGN observations and theory/modelling.
  • Several presentations on VLBI results
  • A few on the ISM, scattering screens
  • Two talks on scintillation of GRBs
  • All presentations will soon appear online at
    http//www.jive.nl/meetings/iss_workshop
  • Some points of discussion

36
ISS workshop some of the topics discussed
  • Lovell MASIV Survey 700 sources observed, only
    one J1819! Are 1257 and J1819 special sources,
    or on a special line-of-sight (or both?!)
  • Walker IDV scattering screens clues from
    pulsar spectroscopy
  • Scintillation arc analysis (Stinebring et al.)
  • Anisotropic scattering, thin screens
  • Fast scintillators dont need special scatterers,
    just local examples of a population which
    pervades the Galactic disk
  • Other talks related to ISM/screens Cordes (the
    Galaxy is grossly undersampled) Fuhrmann

37
ISS workshop some of the topics discussed
  • Macquart diffractive scintillation in
    J18193845 modelling implies minimum Tb 1015 K
  • Rickett scintillation arc analysis finds
    different limits on source angular size
  • Conflicting results need to be reconciled

J18193845 dynamic spectrum
38
ISS workshop some of the topics discussed
  • The (very few!) fast scintillators allow fairly
    detailed modelling (Earth Orbit Synthesis), but
    to do the same with the slower scintillators
    requires much more dedicated observing
  • Jauncey monitoring programs like Ceduna, GAVRT
  • Future instruments? The ATA, other SKA
    demonstrators.

39
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