1. Single Pulse Studies of Pulsar Emission Mechanisms Joanna Rankin Physics & Astronomy Department, University of Vermont Frontiers of Astronomy with the World’s Largest Radio Telescope Collaborators: Avinash Deshpande, Yashwant Gupta, Jeff Herfindal, Joeri van Leeuwen, Dipanjan Mitra, Stephen Redman, Ben Stappers, Svetlana Suleymanova, Patrick Weltevrede, Geoffrey Wright

2. Pulsar “action” has long been regarded as originating from a polar cap accelerator— a so-called “inner gap” (R&S 1972)—and extensive observational evidence is largely compatible with this view. Evidence grows, however, that this cannot be the whole story— X- and  -ray emission appears emitted from an “outer gap” Certain radio observations suggest high altitude emission or shadowing Subbeam “carousel” solutions and theory efforts argue that the accelerator “gap” extends high above the star Very difficult to understand dynamic pulsar emission properties in terms only of polar-cap physics The Problem of Pulsar Radiation Astronomy Dept., Indiana Univ. City-sized stars with the Sun’s mass Radio beams from magnetic poles sweep across the Earth like a lighthouse Magnetic fields that pull surrounding material into rotation up to lightspeed

3. 3 Kijak & Gil 2002 A&A, 392, 189 The Pulsar Polar Cap: One of the Most “Exotic” Regions in the Cosmos! Magnetic and electric fields, typically a million-million times stronger than on Earth, Sun or Jupiter in a city-block sized region and strong gravity as well “Spark” breakdowns, in particular patterns, may produce the radiation

4. van Leeuwen et al 2002, A&A, 387,.169 Longitu de Subpulse drifting, nulling & mode changing B0809+74 Then about ten “null” pulses Subpulse drift shows that the emission is highly ordered over surprising long time scales, and Nulls have seemed to indicate that the emission can stop and start within a single stellar rotation! Recent research suggests that all three effects are produced by subbeam “carousels” All three phenomena can be seen in this pair of 500-pulse sequences. Note the usual 11-period (P 3 ) drift up to about pulse 130 And, a restarted, slower drift “mode”. Pulsars exhibit a rich variety of emission phenomena—e.g.,

5. Sensitive pulse-sequence (PS) observations of weaker pulsars Deep PS observations of stronger pulsars Sensitive and accurate PS polarimetry Correlated X-ray/radio PS observations Census of pulsar emission phenomena Key Arecibo Capabilities for Pulsar Emission Research

6. Sensitive Observations of Weak Pulsars Prime example is B0943+10 This “normal” pulsar has taught more than any other in recent years Most all useful observations of its individual pulses require Arecibo sensitivity Notice the prominent drifting subpulses —and that the weaker ones are just above the noise level This pulsar drifts so regularly that we could confirm that the drift is produced by a subbeam “carousel” Deshpande & Rankin 1999, Astrophysical Journal, 534,.1008

7. The Rotating Subbeam “Carousel” of B0943+10 20 subbeams within the B-mode emission cone Rotates in about 37 stellar-rotation periods or 41 seconds Spin axis at top, magnetic axis at center Carousel rotation through sightline produces drifting! Deshpande & Rankin, 2001 MNRAS, 322, 438 Magnetic axis + Many (most?) pulsars probably have similar subbeam carousel systems … B0943+10’s other (Q) mode shows no drift. Let us look at a typical transition from the Q to B behaviour. Sightline traverse

8. Q-to-B-Mode Transitions in B0943+10 Six rise-to-set Arecibo 327-MHz observations in 2003 3 Q-to-B-mode transitions, 3 B-mode days Rankin &Suleymanova 2006 A&A, 453, 679. March 10 observation is shown in terms of 480-pulse averages First 5 are broad, single, non-drifting Q mode B mode begins at pulse 2540 and is clearly brighter Note behavior of unusual second profile component which starts strong and dies away in several hours!!

9. Q-mode Circulation Time was determined for the first time Fluctuation feature represents a carousel rotation time of 36.4 +/- 0.9 periods B-mode Circulation Times can be measured as usual from the drift-modulation feature, which is a first-order alias of the true frequency. Rankin &Suleymanova 2006 A&A, 453, 679.

10. Q-to-B-Mode Transition Recovery Three observed transitions all behave similarly, both in circulation time and profile form. Q-to-B Days B Days Fitted along curve Strongly suggests exponential recovery ∝ [1-exp(-t/  )] Characteristic time  is then some 54 mins! Or 2950 rotations; Or 80 carousel circulation times. Could this behavior be caused by surface temperature changes? Rankin &Suleymanova 2006 A&A, 453, 679.

11. 11 Polar-cap Map Movies Polar-cap maps can be made for a single carousel circular time...... and overlapping ones combined into movies of the polar emission patterns. Here is one showing how B0943+10’s beamlets behave across a Q- to B-mode transition This is not a “model”. Rather it is a way of displaying the actual observed pulse sequence in the frame of the rotating carousel 11

12. 12 Another Weak Pulsar J1819+1305 Discovered twice, the second time at Arecibo by Navarro et al. (2003) —who reported that it exhibited “periodic nulls” —a hitherto unknown effect! Note the weakness The polarization is just measured Nulls are prominent, but hardly periodic Whatever can be happening??

13. Rankin & Wright 2007 MNRAS, submitted J1819+1305’s “periodic nulls” apparently reflect a carousel Indeed, J1819+1305’s nulls do not appear very regular.... but every observation shows the same 57-period fluctuation feature! Analysis shows that this long period feature probably reflects a carousel circulation time.... and the nulls reflect an only partially filled carousel beam pattern... that rotates though our sightline, with a subbeam pattern like this....... producing the roughly periodic nulls

14. 14 New Insights From Bright Pulsars Recent studies of two of the four original Cambridge pulsars have also provide major insights. In this situation Arecibo observations have provided an enormous signal-to-noise ratio, so weak features can be interpreted confidently. 14

15. Rankin & Wright 2007 MNRAS, 379, 507 ✴ Highly non-random: 3X alternating nulls seen at left; cases of 4X elsewhere! B0834+06’s full and partial nulls fall on the weak phase of its even-odd modulation pattern  2.17-period fluctuation feature Large S/N and the power- distribution is continuous between nulls and pulses ✴ Nulls and pulses cannot be completely distinguished, even at very high sensitivity  Analyses show that these faux nulls result from “empty” sightline passes through a subbeam carousel with a regularly-spaced beamlet pattern.

16. Herfindal & Rankin 2007 MNRAS, 380, 430 Periodic nulls in B1133+16 (aka CP 1133) also ✴ Very bright and shows has ~15% nulls ✴ It exhibits no known subpulse drift,... but shows giant pulses, and... a weak 30-pulse periodicity. ✴ If this Pulse Modulation is Quenched,... by filling pulses with the profile,... ✴ The 30-period modulation remains... ✴... showing it is carried by the nulls! ✴ Similar effect found in other pulsars ✴ Also, nulls non-random in many pulsars Indicative of sparse, irregular carousel patterns

17. 17 What Do These Periodicities Mean?? If interpreted as subbeam carousel Circulations Times driven by ExB above the polar cap, as predicted by the Ruderman & Sutherland theory, all are longer or much longer than R&S PulsarCT (P 1 )R&S (P 1 )MeansReference B0943+10~3710.9FS Deshpande & Rankin (2001) B0809+74>551.9FS van Leeuwen et al. (2003) B0834+06~3011.9FS Rankin & Wright (2007a) J1819+130557.93.7 Null period’yRankin & Wright (2007b) B1857–261475.6FS Mitra & Rankin (2007) B0826–34>101.4FS Gupta et al. (2004) B1133+1632±310.0 Null period’yHerfindal & Rankin (2007a) B0301+19~574.7 Null period’yHerfindal & Rankin (2007b) B0525+21225.8 Null period’yHerfindal & Rankin (2007b) B0751+32514.0 Null period’yHerfindal & Rankin (2007b) J1649+2533~274.9 Null period’yHerfindal & Rankin (2007b)

18. 18 Polarimetry Provides critical information about the emission geometry and potentially the physical mechanisms of emission and/or propagation Is absolutely dependent upon sensitivity because the PA remains undefined unless the S/N > 1 The Orthogonal Polarization Modes (OPMs) are a key property of pulsar radiation and fundamental to understanding its physical origins Most such single-pulse polarimetric studies ever conducted are based on Arecibo observations 18

19. Srostlik & Rankin, 2005 MNRAS, 362, 1121. We also see many instances of polarized emission in pulses so weak that they would surely qualify as nulls, and having appropriate polarization for that longitude position in the pulse sequence. Two such pulses, #1997 and 2012, can be seen at the left, but other examples can be seen in any similar 100-pulse display of this pulsar B1237+25. Conclusion: pulsar emission “sputters” over a large dynamic range  

20. 20 ✴ Polarimetry has provided most of our knowledge about the geometry of pulsar emission... ✴... however, this knowledge has been relative, not absolute, because we have usually not known the PA orientation wrt the pulsar magnetic field ✴ A major development is Johnston et al’s (2006) result that the rotation axes and proper-motion directions remain aligned for many pulsars... ✴... an effect that I believe may be much more general than these authors found ✴ Thus a particular OPM can now be identified as the X or O propagation mode, and these then related physically to the magnetic geometry in the emission and/or propagation regions Rankin 2007 ApJ, 664, 443. Connecting Polarimetry Directly to the Physics The angle  between the proper- motion direction and fiducial polarization angle for 47 pulsars

21. Also, carousel-beam polarization has much to teach us. Here is one such typical map. Coutours are total power; colors are the intensities of the two polarization modes. As expected from average profile studies, the modal ‘beamlets’ are displaced in both magnetic azimuth and radius from the axis. Rankin, Ramachandran, van Leeuwen & Suleymanova 2006 A&A, 455, 215. This polarization complexity of subbeam carousel systems seems a general feature of pulsar radiation. OPMs here +/–

22. 22 Xray-Radio Observations About 40 pulsars are seen in X-rays The Crab (pictured) is most famous Both magnetospheric and surface emission is observed Already two radio-X-ray correlations have been demonstrated Satellite sensitivities are expected to go up dramatically in the near future Many more opportunities to correlate radio & X-ray emission (we are now working on two more) Nearby pulsars discovered in X-rays will require sensitive radio follow-up

23. Recent progress in understanding pulsar emission stems from more accurate geometry, providing more connection to theory. This will continue and accelerate in the coming years. Many weaker pulsars, key to pulsar emission research (known and new), can only be studied usefully using Arecibo. Polarimetry provides fundamental insight into both the geometry and physics of pulsar emission. Its absolute signal-to- noise requirement makes Arecibo incomparable to other instruments. Progress in X-ray observations of pulsars will provide rich new insights about the emission and prompt sensitive new work at radio frequencies. Arecibo’s ability to make very deep observations of stronger pulsars will continue to be productive as new questions arise. Summary