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Pulsar Studies of Tiny-Scale Structure in the Neutral ISM

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:

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Pulsar Studies of Tiny-Scale Structure in the Neutral ISM

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  1. Pulsar Studies of Tiny-Scale Structure in the Neutral ISM Joel Weisberg, Carleton College, Northfield, MNandSnezana 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

  2. Pulsar Studies of Tiny-Scale Structure in the Neutral ISM • Introduction and Context • Pulsar - ISM Spectroscopic Techniques • Results: • Pulsar HI Studies and Comparison with Interferometric Results • Pulsar OH Studies

  3. 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

  4. Pulsar spectroscopy of the interstellar medium 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). observer Intervening (along the path) cloud pulsar

  5. Pulsar spectroscopy of the interstellar medium • 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 observer Intervening (along the path) cloud pulsar

  6. Pulsar HI Absorption

  7. Pulsar HI Absorption: • Multiepoch observing:

  8. Pulsar spectroscopy procedure: Create a set (n=2,…) of spectra across the pulsar period Pulse profile dimension Neutral hydrogen (HI) spectral lines Pulsar pulse Pulsar Longitude l (Spectral dimension) [Clifton et al (1988)]

  9. Creation of the Pulsar (PSR) Spectrum (also called the pulsar absorption spectrum, or the pulsar pulse spectrum) T (K) PSR-on spectrum PSR-off spectrum PSR spectrum = PSR-on - PSR-off PSR-offo Optical depth t I/Io

  10. Results from Pulsar - ISM Spectroscopic 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 a pulsar. • Multi-epoch observations (in progress)

  11. Results from Pulsar - ISM Spectroscopic 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 a pulsar. • Multi-epoch observations (in progress)

  12. The first multi-epoch pulsar HI comparisons • 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.

  13. Frail et al (1994) Multi-epoch PSR HI Spectra from Arecibo: Three epochs; t = (0.7-1.7) yr PSR B0540+23 PSR B0823+26 PSR B1133+16 AverageAbsorption Two-sessiondifferences AverageAbsorption Two-sessiondifferences PSR B1737+13 PSR B1929+10 PSR B2016+28

  14. Frail, Weisberg et al. (1994) found: EW logEW) • 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.)

  15. 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!

  16. HI emission B0736-40 absorption noise envelopes: (1,2,3)suncertainties (lines) PSR B0736-40 absorption Two-session absorptiondifferences (dots): t=1.9 yr. No significant variations in this case! Only one significant variation detected among all their measurements. Multi-epoch PSR HI Abs. Spectra. Johnston et al (2003, Parkes). DT = Tsys/ Sqrt( B tint)

  17. B0540+23 B0823+26 B1737+13 B1133+16 B2016+28 B1929+10 Our new Arecibo ExperimentStanimirovic, Weisberg, & Carleton students Line of Sight Parameters 4 epochs: 2000.6 2000.9 2001.7 2001.9 Four epochs for each PSR: 2000.6, 2000.9, 2001.7, & 2001.9 t ~ (0.2 - 1.3) yr; l ~ (1 - 200) AU

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

  19. In the case of B1929+10: “really something” • Significant variations found at similar velocities (5 & 10 km/sec) in most comparisons. • Four features atΔτ= 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

  20. What’s going on with B1929+10 ? l.o.s. PSR at ~330pc * 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). Lallement et al. (2003)

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

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

  23. HI emission PSR HI absorption Two-session absorp. diff., random ±1σ (envelope), and syst. (ghost fit est.): (1-yr baseline) Multi-epoch HI measurements of B0329+54 with the GBT (Minter, et al 2005, and poster at this meeting) • 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.

  24. 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. 2005 (GBT):B0329+54, 18 epochs plus subepochs, ~150 non-detections (<0.03).  Cold neutral HIclouds on scales 10-2 to 102 AUsare not very common in the ISM. They may not be a general property of the ISM, and could be related to some local phenomena.

  25. Optical depth variations (and limits) versus size (VLBA & PSR) 3C138

  26. Deshpande (2000): extrapolation of the power spectrum from larger scales TSAS is a tail of much larger hierarchy in the ISM τ Power spectrum of  with 3D Slope ~ 2.75, as seen in Cas A. power peak rms Extrapolated! spatial freq 10AU 102 AU scale • 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.

  27. Optical depth variations (and limits) versus Size, with Desh power law extrapolation 3C138 Deshpande theory • No obvious trend ofτwith spatial scales. • ---> may indicate that inner scale and hence the turbulent dumping scale is >100 AU.

  28. Adding further complexity: “low-N(HI) clouds” (Stanimirovic talk) 3C138 Deshpande theory Low-N clouds: Size=800-4000 AU =~10-3 to ~10-2

  29. Results from Pulsar - ISM Spectroscopic 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 a pulsar. • Multi-epoch observations (in progress)

  30. First successful detection of OH absorption against a pulsar PSR B1849+00 from Arecibo (Stanimirovic et al. 2003) PSR spectrum: Absorption againstpulsar’s continuum emission ONLY - obtained in same fashion as PSR HI spectra. AND PSR-off spectrum: C C C C C C Absorption againstSNR G33.6+0.1 continuum emissionONLY PSR B1849+00 Why are absorption spectra along the same l-o-s so different?

  31. Second successful detection of OH absorption against a pulsar The optical depth t 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 B1641-45 from Parkes (Weisberg et al 2005) 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 C C C C C C C C PSR-off spectra PSR spectra

  32. psr Why is the optical depth t of spectral lines in pulsar-offmuch less than in pulsar spectra? --the pulsar-off spectrum samples the medium throughout the several arcmin telescope beam cloud observer cloud

  33. psr Why is the optical depth t of spectral lines in pulsar-offmuch less than in pulsar spectra? --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 cloud observer cloud

  34. psr Why is the optical depth t of spectral lines in pulsar-offmuch less than in pulsar spectra? --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 cloud observer cloud

  35. psr Why is the optical depth t of spectral lines in pulsar-offmuch less than in pulsar spectra? --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 These observations confirm other measurementsindicating that the molecular medium is signi-ficantly more clumped than HI. cloud observer cloud

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

  37. 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. T Optical Depth

  38. 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.

  39. Introduction and Context: • Definition of “tiny-scale neutral structure” • 101-2 AU structures in, e.g., HI or OH.

  40. 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?

  41. PSR spectrum = PSR-on - PSR-off PSR-offo Optical depth t I/Io Vel Creation of the Pulsar (PSR) Spectrum is difficult in the presence of wildly varying pulsar pulse strength! C C C C C C C C T (K) during strong pulse during weak pulse PSR-on spectra HI PSR-off spectrum

  42. Brogan et al. (2005): 3C138 results. Very different from pulsar findings! VLBA observations • Large variations, >0.1. • Scale ~25 AU, • t~ a few yrs. • Plane-of-sky covering • factor ~10%.

  43. 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.

  44. Two possible explanations for shallower and broader OH absorption of SNR than PSR. PSR B1849+00 1. PSR is absorbed by cold molecular cloud beyond SNR MOLECULARCLOUD PSR SIGNAL OH ABSORPTION OCCURS HERE C C C C C C 2. PSR is absorbed by a clump inside same molecular cloud as SNR

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