pulsar studies of tiny-scale structure in the neutral ismsstanimi/tiny/sins_talks1/weisberg.pdf ·...
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Pulsar Studies ofTiny-Scale Structure in
the Neutral ISMJoel Weisberg, Carleton College, Northfield, MN
andSnezana Stanimirovic, U. California, Berkeley
With many thanks to these collaborators through the years:
Dale Frail, Jim Cordes, Stuart Anderson, Rick Jenet, Simon Johnston,Baerbel Koribalski; and Katie Devine and other Carleton students
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Pulsar Studies ofTiny-Scale Structurein the Neutral ISM
1. Introduction and Context
2. Pulsar - ISM Spectroscopic Techniques
3. Results:
• Pulsar HI Studies and Comparison withInterferometric Results
• Pulsar OH Studies
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Introduction and Context:Principal observational techniques
for studying small-scale neutralstructure
1. VLBI mapping of HI absorption in front ofextended continuum sources. [Brogan review]2. Optical interstellar lines in double and clusterstars (various atomic and molecular species)[Lauroesch review]3. HI and OH spectroscopy along the path to PSRs
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Pulsars are especially useful for probing the ISM:• Pulsars are tiny background sources.• Pulsar signals switch on and off.• Pulsars are high velocity objects (102-3 km/sec).
Pulsar spectroscopy of theinterstellar medium
pulsarIntervening (along the path)
cloud
observer
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• Use the pulsar pulse to study the interveningISM:-The pulsar signal can be absorbed byintervening gas-The pulsar signal can stimulate maseremission in the intervening gas
Pulsar spectroscopy of theinterstellar medium
pulsarIntervening (along the path)
cloud
observer
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• Pulsar HI Absorption
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• Pulsar HI Absorption:
• Multiepoch observing:
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[Clifton et al (1988)]
λ (Spectral dimension)
Pulsar Longitude
Pulseprofiledimension
Neutral hydrogen(HI) spectrallines
Pulsar pulse
Pulsar spectroscopy procedure:Create a set (n=2,…) of spectra across the pulsar period
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Creation of the Pulsar (PSR) Spectrum
T (K)
PSR-on spectrum
PSR-off spectrum
PSR spectrum =PSR-on - PSR-off
PSR-offo
Optical
depth
τI/Io
(also called the pulsar absorption spectrum, or the pulsar pulse spectrum)
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Results from Pulsar - ISMSpectroscopic Technique
A. HI measurements.•Kinematic distance and <ne> determinations.•Multi-epoch observations.
B. OH measurements:•Optical depth versus angular size along same l.o.s.•Discovery of a pulsed maser stimulated by apulsar.•Multi-epoch observations (in progress)
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Results from Pulsar - ISMSpectroscopic Technique
A. HI measurements.•Kinematic distance and <ne> determinations.•Multi-epoch observations.
B. OH measurements:•Optical depth versus angular size along same l.o.s.•Discovery of a pulsed maser stimulated by apulsar.•Multi-epoch observations (in progress)
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The first multi-epoch pulsarHI comparisons
A. Clifton et al (1988):• The HI absorption spectrum of PSR B1821+05
changed significantly between ~1981 and 1988.
B. Deshpande et al (1992):• Between ~1976 and 1981, HI absorption toward
B1557-50 did change and B1154-52 did not.Positive result suggested structure on 1000 AUscale.
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Frail et al (1994) Multi-epoch PSR HI Spectra fromArecibo: Three epochs; Δt = (0.7-1.7) yr
AverageAbsorption
Two-sessiondifferences
AverageAbsorption
Two-sessiondifferences
PSR B0540+23 PSR B0823+26 PSR B1133+16
PSR B1737+13 PSR B1929+10 PSR B2016+28
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Frail, Weisberg et al. (1994) found:
• Pervasive variationswith Δτ ~ 0.01-0.1;N ~(1019-5 X 1020) cm-2.
• Scales: (5-100) AU.• Fraction: (10-15)% of
cold HI is in the tinystructures.
• Correlation of equiva-lent width changesΔ(EW) with EW. (SeeFigure.)
!
[EW = " d#$ ]
ΔEW
log(EW)
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Interferometer and Frail et al. PSR resultsstimulated extensive theoretical work.
• Heiles (1997): A geometrical model (asymmetric filamentsor sheets modeled as cylinders or disks) can solve theoverpressure problem.
• Deshpande (2000): Observed fluctuations are theextrapolated tail of the observed CNM power-law structuredistribution.
• Gwinn (2001): Velocity gradient in a cloud, coupled withscintillation variations, leads to apparent Δτ.
See their talks for details!
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HI emission
PSR B0736-40 absorption
B0736-40 absorption noise envelopes: ±(1,2,3)σ uncertainties (lines)
Multi-epoch PSR HI Abs. Spectra.Johnston et al (2003, Parkes).
ΔT = Tsys / Sqrt( B tint)
Two-session absorption differences (dots): Δt=1.9 yr.No significant variations in this case! Only onesignificant variation detected among all theirmeasurements.
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4 epochs:2000.62000.92001.72001.9
Our new Arecibo ExperimentStanimirovic, Weisberg, & Carleton students
B0540+23
B0823+26
B1737+13B1133+16
B2016+28
B1929+10 71.0-4.0682016+28
400.3-3.9471929+10
1404.722371737+13
1300.4692421133+16
400.4321970823+26
803.5-3.31840540+23
Vtransv
(AU/yr)d
(kpc)blPSR
B
Line of Sight Parameters
Four epochs for each PSR: 2000.6, 2000.9, 2001.7, & 2001.9Δt ~ (0.2 - 1.3) yr; Δl ~ (1 - 200) AU
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Two-session absorption differences:
Occasional “something”Mostly “tight nothing”
(I/Io)Sess X - (I/Io)Sess Y
± 2σ
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In the case of B1929+10:“really something”
• Significantvariations found atsimilar velocities (5& 10 km/sec) inmost comparisons.
• Four features atΔτ = 0.015-0.036;scales 6-45 AU.
• The closest PSR inour sample, withhigh scatteringcaused by the LocalBubble.
(I/Io)Sess X - (I/Io)Sess Y
± 2σ
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What’s going onwith B1929+10 ?
l.o.s.
PSR at~330pc*
Lallement et al. (2003)
TSAS at 5 km/sec:
T~30K from TSAS linewidth.
ΔN~1018 cm-2, L=30 AU, -> n~104 cm-3.
-> P = nT ~ 3x105 cm-3 K (approx 100x PCNM).
Geometrical factor of ~100 is needed (Heiles1997).
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Integrated absorption variations:
!
[EW = " d#$ ]
Our two-session equivalent-width variations (ΔEW) versus time separation Δt
ΔEW
Δt
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Comparison of our new equivalent widthvariation data (ΔEW) with Frail et al. (1994) :
Our new workFrail, Weisberg, et al (1994)
Our new work
ΔEW
ΔEW
Log(EW)
ΔEW
Log(EW)
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Multi-epoch HI measurements of B0329+54with the GBT (Minter, et al 2005, and poster at
this meeting)
HI emission
PSR HI absorption
Two-session absorp. diff., random ±1σ (envelope), and syst. (ghost fit est.):
(1-yr baseline)
•vtrans ~ 20 AU / yr.•Up to 20-hour continuous sessions. •Eighteen separate sessions over 1.3 years.•No significant variations found on scales of (0.0025 - 12.5) AU -- with typical 2σ upper limits τ < 0.03.
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Bottom line: A few recent pulsar detections ofTSAS; plus lots of non-detections
Our new work (Arecibo):9 detections + 21 non-detections (some limits are Δτ<0.02).
Johnston et al. 2003, (Parkes):1 detection (Δt~9 yr) + many non-detections (a few limits asstringent as Δτ<0.02).
Minter et al. 2005 (GBT):B0329+54, 18 epochs plus subepochs,~150 non-detections (Δτ<0.03).
Cold neutral HI clouds on scales 10-2 to 102
AUs are not very common in the ISM. Theymay not be a general property of the ISM,and could be related to some localphenomena.
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Optical depth variations (and limits)versus size (VLBA & PSR)
3C138
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Deshpande (2000): extrapolation of thepower spectrum from larger scales
10AU
Power spectrum of τwith 3D Slope ~ 2.75, as seen in Cas A.
rms
peak
Larger variations expected on longer time- and distance-scales. Optical depth fluctuations of 0.2-0.4 on scales of 50-100 AU
easily reproduced.
Δτ
102 AU scale
TSAS is a tail of much larger hierarchy in the ISM
spatial freq
power
Extrapolated!
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Optical depth variations (and limits) versusSize, with Desh power law extrapolation
3C138
Deshpande theory
•No obvious trend of Δτ with spatial scales.
---> may indicate that inner scale and hence the turbulent dumping scale is >100 AU.
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Adding further complexity: “low-N(HI) clouds” (Stanimirovic talk)
3C138
Deshpande theory
Low-N clouds:Size=800-4000 AUΔτ=~10-3 to ~10-2
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Results from Pulsar - ISMSpectroscopic Technique
A. HI measurements.•Kinematic distance and <ne> determinations.•Multi-epoch observations.
B. OH measurements:•Background source angular size comparisons.•Discovery of a pulsed maser stimulated by apulsar.•Multi-epoch observations (in progress)
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PSR spectrum:Absorption against
pulsar’scontinuum emissionONLY - obtained in samefashion as PSR HI spectra.
ANDPSR-off spectrum:
Why are absorption spectra along the same l-o-s so different?
PSR B1849+00Absorption against
SNR G33.6+0.1continuum emissionONLY
First successful detection ofOH absorption against a pulsar
PSR B1849+00 from Arecibo (Stanimirovic et al. 2003)
C
C
C
C
C
C
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Second successful detection of OH absorptionagainst a pulsar
PSR B1641-45 from Parkes (Weisberg et al 2005)
The optical depth τ of spectral lines inpulsar-off (left side) is again much less than in pulsar (right side) spectra!
(All spectra are plotted here with the same optical depth scales):
PSR spectraPSR-off spectraC
C Each of these four 18-cm OH
PSR spectra was obtained by
differencing PSR-on and PSR-off
spectra, exactly as is done at HI.
C
C
C
C
C
C
C
C
C
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--the pulsar-off spectrum samples the medium throughout theseveral arcmin telescope beam
Why is the optical depth τ of spectral lines inpulsar-off much less than in pulsar spectra?
observer
psr
cloud
cloud
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--the pulsar-off spectrum samples the medium throughout theseveral arcmin telescope beam
--pulsars are so small that their signal samples atiny column through the medium
Why is the optical depth τ of spectral lines inpulsar-off much less than in pulsar spectra?
observer
psr
cloud
cloud
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--the pulsar-off spectrum samples the medium throughout theseveral arcmin telescope beam
--pulsars are so small that their signal samples atiny column through the medium
--Patchy, clumpy clouds only cover only a fraction of the telescope beam, but all of the pulsar column
Why is the optical depth τ of spectral lines inpulsar-off much less than in pulsar spectra?
observer
psr
cloud
cloud
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--the pulsar-off spectrum samples the medium throughout theseveral arcmin telescope beam
--pulsars are so small that their signal samples atiny column through the medium
--Patchy, clumpy clouds only cover only a fraction of the telescope beam, but all of the pulsar column
Why is the optical depth τ of spectral lines inpulsar-off much less than in pulsar spectra?
observer
psr
cloud
cloud
These observations confirm other measurementsindicating that the molecular medium is signi-ficantly more clumped than HI.
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The first pulsed interstellar maserAn OH 1720 MHz interstellar maser is stimulated by pulses from PSR B1641-45
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T
OpticalDepth
Pulsed maser
OH 1720 MHz maser stimulated by PSR B1641-45 pulses•This maser turns on only during the pulsar pulse, for ~14 millisecondsduring each pulse period (455 milliseconds).
•These are the fastest variations ever observed in an interstellar maser,by many orders of magnitude.
•This is the first direct astronomical observation of a maser in action:---we see it turn on when the pulsar pulse stimulates the maser, and---we see it turn off when the pulsar pulse disappears.
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Conclusions and Future Work•Pulsar spectrometry is a very useful and unique probe of the interstellarmedium.•HI pulsar multiepoch measurements provide constraints on TSAS whichneed to be reconciled with interferometer measurements and with theory.
-- Delicate measurements are becoming more reliable and additional ones shouldbe made along different lines of sight and different time baselines.
•Our new OH pulsar spectra have yielded a number of interesting results:–Much deeper absorption in pulsar spectra than in pulsar-off, indicates thatmolecular medium is more patchy/clumpy than is HI.–A pulsed interstellar maser, stimulated by a pulsar, at 1720 MHz, turns on and offon 14 millisecond timescales -- the first direct detection of astrophysical stimulatedemission.–Additional measurements are in progress, including multi-epoch observations ofOH as a complementary approach to studying small-scale structure.