Single Pulse Studies of Pulsar Emission MechanismsJoanna 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

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

3Kijak & 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 regionand strong gravity as well “Spark” breakdowns, in particular

patterns, may produce the radiation

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

van Leeuwen et al 2002, A&A, 387,.169

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 (P3) drift up to about pulse 130

And, a restarted, slower drift “mode”.

Pulsars exhibit a rich variety of emission phenomena—e.g.,

• 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

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

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

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!!

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.

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/τ)]

• Characteristic time τ 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.

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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

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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??

Rankin & Wright 2007 MNRAS, submitted

• 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

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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.

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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 fluctuationfeature

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.

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

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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

Pulsar CT (P1) R&S (P1) Means Reference B0943+10 ~37 10.9 FS Deshpande & Rankin (2001)

B0809+74 >55 1.9 FS van Leeuwen et al. (2003)

B0834+06 ~30 11.9 FS Rankin & Wright (2007a)

J1819+1305 57.9 3.7 Null period’y Rankin & Wright (2007b)

B1857–26 147 5.6 FS Mitra & Rankin (2007)

B0826–34 >10 1.4 FS Gupta et al. (2004)

B1133+16 32±3 10.0 Null period’y Herfindal & Rankin (2007a)

B0301+19 ~57 4.7 Null period’y Herfindal & Rankin (2007b)

B0525+21 22 5.8 Null period’y Herfindal & Rankin (2007b)

B0751+32 51 4.0 Null period’y Herfindal & Rankin (2007b)

J1649+2533 ~27 4.9 Null period’y Herfindal & Rankin (2007b)

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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

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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

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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

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 +/–

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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

• 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