Pulsar Studies of Tiny-Scale Structure in the Neutral ISM Joel 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 ( km/sec). Pulsar spectroscopy of the interstellar medium pulsar Intervening (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 pulsar Intervening (along the path) cloud observer

Pulsar HI Absorption

Pulsar HI Absorption: Multiepoch observing:

[Clifton et al (1988)] (Spectral dimension) Pulsar Longitude Pulse profile dimension Neutral hydrogen (HI) spectral lines 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-off o Optical depth  I/I o (also called the pulsar absorption spectrum, or the pulsar pulse spectrum)

Results from Pulsar - ISM Spectroscopic Technique A. HI measurements. Kinematic distance and 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 Technique A. HI measurements. Kinematic distance and 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 comparisons A.Clifton et al (1988): The HI absorption spectrum of PSR B changed significantly between ~1981 and B. Deshpande et al (1992): Between ~1976 and 1981, HI absorption toward B did change and B did not. Positive result suggested structure on 1000 AU scale.

Frail et al (1994) Multi-epoch PSR HI Spectra from Arecibo: Three epochs;  t = ( ) yr Average Absorption Two-session differences Average Absorption Two-session differences PSR B PSR B PSR B PSR B PSR B PSR B

Frail, Weisberg et al. (1994) found: Pervasive variations with Δτ ~ ; N ~( X ) 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 log  EW)

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 B absorption B absorption noise envelopes:  (1,2,  uncertainties (lines) Multi-epoch PSR HI Abs. Spectra. Johnston et al (2003, Parkes).  T = T sys / Sqrt( B t int ) 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: Our new Arecibo Experiment Stanimirovic, Weisberg, & Carleton students B B B B B B PSR B lbd (kpc) V transv (AU/yr) Line of Sight Parameters Four epochs for each PSR: , , , &  t ~ ( ) yr;  l ~ ( ) AU

Two-session absorption differences: Occasional “something” Mostly “tight nothing” (I/Io) Sess X - (I/Io) Sess Y ± 2 

In the case of B : “really something” Significant variations found at similar velocities (5 & 10 km/sec) in most comparisons. Four features at Δτ = ; 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 B ? l.o.s. PSR at ~330pc * Lallement et al. (2003) TSAS at 5 km/sec: T~30K from TSAS linewidth.  N~10 18 cm -2, L=30 AU, -> n~10 4 cm -3. -> P = nT ~ 3x10 5 cm -3 K (approx 100x P CNM ). Geometrical factor of ~100 is needed (Heiles 1997).

Integrated absorption variations: Our two-session equivalent-width variations (  EW) versus time separation  t  EW tt

Comparison of our new equivalent width variation data (  EW) with Frail et al. (1994) : Our new work Frail, Weisberg, et al (1994) Our new work  EW Log(EW)  EW Log(EW)

Multi-epoch HI measurements of B 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) v trans ~ 20 AU / yr. Up to 20-hour continuous sessions. Eighteen separate sessions over 1.3 years. No significant variations found on scales of ( ) AU -- with typical 2  upper limits τ < 0.03.

Bottom line: A few recent pulsar detections of TSAS; plus lots of non-detections Our 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 (GBT) : B , 18 epochs plus subepochs, ~150 non-detections (  <0.03).  Cold neutral HI clouds on scales to 10 2 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 on scales of AU easily reproduced. τ τ 10 2 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= AU  =~10 -3 to ~10 -2

Results from Pulsar - ISM Spectroscopic Technique A. HI measurements. Kinematic distance and 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. AND PSR-off spectrum: Why are absorption spectra along the same l-o-s so different? PSR B Absorption against SNR G continuum emission ONLY First successful detection of OH absorption against a pulsar PSR B from Arecibo (Stanimirovic et al. 2003) CCCCCCCCCCCC

Second successful detection of OH absorption against a pulsar PSR B 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 spectra C 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

--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

--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

--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

--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 These observations confirm other measurements indicating 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 B

T Optical Depth Pulsed maser OH 1720 MHz maser stimulated by PSR B pulses This maser turns on only during the pulsar pulse, for ~14 milliseconds during 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 which need 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” AU structures in, e.g., HI or OH.

Introduction and Context: Expected size of structures in cold neutral medium (CNM) CNM: P thermal ~ 2000 K cm -3 ; T ~ 70 K Hence by ideal gas law, n CNM = P / T = 30 cm -3. Observed CNM column densities N ~ 5 x cm -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 ( AU) structure exist in this medium?

Creation of the Pulsar (PSR) Spectrum T (K) PSR-on spectra PSR-off spectrum I/I o PSR spectrum = PSR-on - PSR-off PSR-off o Optical depth  Vel HI during weak pulse during strong pulse is difficult in the presence of wildly varying pulsar pulse strength! CCCCCCCCCCCCCCCC

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 B MOLECULAR CLOUD 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 CCCCCCCCCCCC

v HI = - 60 km/s v recomb = km/s v recomb = - 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!

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)

v HI = - 60 km/s v recomb = km/s v recomb = - 45 km/s

4.6 kpc3.3 kpc6.7 kpc Schematic map of the B line of sight