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

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

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

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

• 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

• Pulsar HI Absorption

• Pulsar HI Absorption:

• Multiepoch observing:

[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

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)

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)

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)

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.

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

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)

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!

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.

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

Two-session absorption differences:

Occasional “something”Mostly “tight nothing”

(I/Io)Sess X - (I/Io)Sess Y

± 2σ

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

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).

Integrated absorption variations:

!

[EW = " d#$ ]

Our two-session equivalent-width variations (ΔEW) versus time separation Δt

ΔEW

Δt

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)

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.

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.

Optical depth variations (and limits)versus size (VLBA & PSR)

3C138

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!

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.

Adding further complexity: “low-N(HI) clouds” (Stanimirovic talk)

3C138

Deshpande theory

Low-N clouds:Size=800-4000 AUΔτ=~10-3 to ~10-2

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)

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

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

--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

--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

--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

--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.

The first pulsed interstellar maserAn OH 1720 MHz interstellar maser is stimulated by pulses from PSR B1641-45

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.

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.