Pulsar Studies of Tiny-Scale Structure in

the Neutral ISMJoel Weisberg, Carleton College,

Northfield, MN

and

Snezana 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 of

Tiny-Scale Structure in the Neutral ISM

1.Introduction and Context

2.Pulsar - ISM Spectroscopic Techniques

3.Results:

• Pulsar HI Studies and Comparison with Interferometric Results

• Pulsar OH Studies

Introduction and Context:Principal observational techniques for studying small-scale neutral

structure 1. VLBI mapping of HI absorption in front of extended continuum sources. [Brogan review]2. Optical interstellar lines in double and cluster stars (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 the interstellar

medium

pulsarIntervening (along the path)

cloud

observer

• Use the pulsar pulse to study the intervening ISM:-The pulsar signal can be absorbed by intervening gas-The pulsar signal can stimulate maser emission in the intervening gas

Pulsar spectroscopy of the interstellar

medium

pulsarIntervening (along the path)

cloud

observer

• Pulsar HI Absorption

PSR Cool HI Cloudlet Observer Absorption Spectra Ipsr

21 cm

• Pulsar HI Absorption:

• Multiepoch observing:

PSR at epoch 1 Cool HI Cloudlet Observer PSR at epoch 2 Absorption Spectra Epoch 1 Epoch 2 Ipsr Ipsr

21 cm 21 cm

[Clifton et al (1988)]

(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

I/Io

(also called the pulsar absorption spectrum, or the pulsar pulse spectrum)

Results from Pulsar - ISM Spectroscopic

TechniqueA. 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 a pulsar.•Multi-epoch observations (in progress)

Results from Pulsar - ISM Spectroscopic

TechniqueA. 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 a pulsar.•Multi-epoch observations (in progress)

The first multi-epoch pulsar HI

comparisonsA.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 AU scale.

Frail et al (1994) Multi-epoch PSR HI Spectra from Arecibo: Three epochs;

t = (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 variations with 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 tiny structures.

•Correlation of equiva-lent width changes(EW) with EW. (See Figure.)

€

[EW = τ dυ∫ ]

EW

logEW)

Interferometer and Frail et al. PSR results stimulated extensive theoretical work.

•Heiles (1997): A geometrical model (asymmetric filaments or sheets modeled as cylinders or disks) can solve the overpressure problem.

•Deshpande (2000): Observed fluctuations are the extrapolated tail of the observed CNM power-law structure distribution.

•Gwinn (2001): Velocity gradient in a cloud, coupled with scintillation variations, leads to apparent .

See their talks for details!

HI emission

PSR B0736-40 absorption

B0736-40 absorption noise envelopes: (1,2, uncertainties (lines)

Multi-epoch PSR HI Abs. Spectra.

Johnston et al (2003, Parkes).

T = Tsys / Sqrt( B tint)

Two-session absorption differences (dots): t=1.9 yr.No significant variations in this case! Only one significant variation detected among all their measurements.

4 epochs: 2000.6 2000.9 2001.72001.9

Our new Arecibo Experiment

Stanimirovic, Weisberg, & Carleton students

B0540+23

B0823+26

B1737+13B1133+16

B2016+28

B1929+10

PSRB

l b d (kpc)

Vtransv

(AU/yr)

0540+23

184 -3.3

3.5 80

0823+26

197 32 0.4 40

1133+16

242 69 0.4 130

1737+13

37 22 4.7 140

1929+10

47 -3.9

0.3 40

2016+28

68 -4.0

1.0 7

Line of Sight Parameters

Four epochs for each PSR: 2000.6, 2000.9, 2001.7, & 2001.9

t ~ (0.2 - 1.3) yr; l ~ (1 - 200) AU

Two-session absorption differences:

Occasional “something”

Mostly “tight nothing”

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

± 2

In the case of B1929+10: “really something”

•Significant variations found at similar velocities (5 & 10 km/sec) in most comparisons.

•Four features at Δτ = 0.015-0.036; scales 6-45 AU.

•The closest PSR in our sample, with high scattering caused by the Local Bubble.

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

± 2

What’s going on with B1929+10 ? l.o.s.

PSR at~330pc*

Lallement et al. (2003)

TSAS at 5 km/sec:

T~30K from TSAS linewidth.

N~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 (Heiles 1997).

Integrated absorption variations:

€

[EW = τ dυ∫ ]

Our two-session equivalent-width variations (EW) versus time separation t

EW

t

Comparison of our new equivalent width variation data (EW) with Frail et al. (1994) :

Our new workFrail, Weisberg, et al (1994)

Our new work

EW

EW

Log(EW)

EW

Log(EW)

Multi-epoch HI measurements of B0329+54 with 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 2 upper limits τ < 0.03.

Bottom line: A few recent pulsar detections of TSAS; plus

lots of non-detectionsOur new work (Arecibo): 9 detections + 21 non-detections (some limits are <0.02).

Johnston et al. 2003, (Parkes):1 detection (t~9 yr) + many non-detections (a few limits as stringent as <0.02).

Minter et al. 2005 (GBT): B0329+54, 18 epochs plus subepochs, ~150 non-detections (<0.03).

Cold neutral HI clouds on scales 10-2 to 102 AUs are not very common in the ISM. They may not be a general property of the ISM, and could be related to some local phenomena.

Optical depth variations (and limits) versus size

(VLBA & PSR)

3C138

Deshpande (2000): extrapolation of the power

spectrum from larger scales

10AU

Power spectrum of with 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.

τ

102 AU scale

TSAS is a tail of much larger hierarchy in the ISM

spatial freq

power

Extrapolated!

Optical depth variations (and limits) versus Size, with Desh

power law extrapolation

3C138

Deshpande theory

•No obvious trend of τ 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 AU=~10-3 to ~10-2

Results from Pulsar - ISM Spectroscopic

TechniqueA. 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 a pulsar.•Multi-epoch observations (in progress)

PSR spectrum:Absorption against

pulsar’s continuum emission ONLY - obtained in same fashion as PSR HI spectra.

ANDPSR-off spectrum:

Why are absorption spectra along the same l-o-s so different?

PSR B1849+00

Absorption

against SNR G33.6+0.1 continuum emission ONLY

First successful detection of

OH absorption against a pulsarPSR B1849+00 from Arecibo (Stanimirovic et al. 2003)

C

C

C

C

C

C

Second successful detection of OH absorption against a pulsarPSR B1641-45 from Parkes (Weisberg et al 2005)

The optical depth of spectral lines in pulsar-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 spectra

PSR-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 the several arcmin telescope beam

Why is the optical depth of spectral lines in

pulsar-off much less than in pulsar spectra?

observer

psr

cloud

cloud

--the pulsar-off spectrum samples the medium throughout the several arcmin telescope beam

--pulsars are so small that their signal samples a tiny column through the medium

Why is the optical depth of spectral lines in

pulsar-off much less than in pulsar spectra?

observer

psr

cloud

cloud

--the pulsar-off spectrum samples the medium throughout the several arcmin telescope beam

--pulsars are so small that their signal samples a tiny 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 of spectral lines in

pulsar-off much less than in pulsar spectra?

observer

psr

cloud

cloud

--the pulsar-off spectrum samples the medium throughout the several arcmin telescope beam

--pulsars are so small that their signal samples a tiny 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 of spectral lines in

pulsar-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 maser

An 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 interstellar medium.•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 should be 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 that molecular medium is more patchy/clumpy than is HI.–A pulsed interstellar maser, stimulated by a pulsar, at 1720 MHz, turns on and off on 14 millisecond timescales -- the first direct detection of astrophysical stimulated emission.–Additional measurements are in progress, including multi-epoch observations of OH as a complementary approach to studying small-scale structure.

Introduction and Context:

•Definition of “tiny-scale neutral structure”

•101-2 AU structures in, e.g., HI or OH.

Introduction and Context:

Expected size of structures in cold

neutral medium (CNM)• CNM: Pthermal ~ 2000 K cm-3; T ~ 70 K

• Hence by ideal gas law, nCNM = P / T = 30 cm-

3.• Observed CNM column densities N ~ 5 x 1019cm-2

• So the typical scale length in CNM, l ~ N/n ~ 1 pc.

• Therefore little structure would be expected on scales much smaller than 1 pc.

• The puzzle: How could tiny-scale (101-2 AU) structure exist in this medium?

Creation of the Pulsar (PSR) Spectrum

T (K)

PSR-on spectra

PSR-off spectrum

I/Io

PSR spectrum =PSR-on - PSR-off

PSR-offo

Optical

depth

Vel

HI

during weak pulse

during strong pulse

is difficult in the presence of wildly varying pulsar pulse strength!

C

C

C

C

C

C

C

C

Brogan et al. (2005): 3C138 results. Very different from pulsar findings!

Large variations, >0.1. Scale ~25 AU, t~ a few yrs. Plane-of-sky covering factor ~10%.

VLBA observations

Brogan et al sample their 3C138 map at numerous pairs of

locations, all with l = 25 AU

Simulation shows that most observed two-session PSR variations would be in the lowest bin if l~ 25 AU.But current PSR measurements have upper limits of ~ 0.03 on many scales, and yet still usually fail to see variations.

Two possible explanations for shallower and broader OH

absorption of SNR than PSR.PSR B1849+00

MOLECULARCLOUD

PSR SIGNAL OH ABSORPTION OCCURS HERE

1. PSR is absorbed by cold molecular cloud beyond SNR

2. PSR is absorbed by a clump inside same molecular cloud as SNR

C

C

C

C

C

C

vHI = - 60 km/s

vrecomb = - 120 km/s

vrecomb = - 45 km/s

20 cm continuum map of region

from Compact Array

(McClure-Griffiths et al)

Derived side view

The 1720 and 1612 MHz spectra are mirror images!

The 1720 and 1612 MHz spectra are mirror images!

1720 masers are pumped by 119 photons (to start with)

1612 absorption lines are also “pumped” by 119 photons (to start with) (“stimulated absorption”)

20 cm continuum map of region from Compact Array

(McClure-Griffiths et al)

20 cm continuum map of region from Compact Array

(McClure-Griffiths et al)

vHI = - 60 km/s

vrecomb = - 120 km/s

vrecomb = - 45 km/s

4.6 kpc 3.3 kpc6.7 kpc

Schematic map of the B1641-45 line of sight