Observational constraints on theories of pulsations Can we expect to learn something fundamental about neutron stars from their variable radio emission? Patrick Weltevrede

Is radio emission just a probe of high-order effects? Energy output in radio is tiny compared to total available Ė. Do radio waves therefore tell us only about the high order effects in pulsar magnetospheres?

PSR B The intermittent pulsar Radio emission “on” → large spin-down. Change in spin-down (factor 1.5) caused by a change in the pulsar wind. Implied charge density change: ρ GJ Radio emission is a probe of drastic changes in the magnetosphere. Kramer et al. 2006

PSR B The intermittent pulsar Young et al analysed 13 years of data of PSR B Spin-down rates in the on/off mode appear to be stable over time: truly bimodal. Young et al. 2012

PSR B The intermittent pulsar On-state durations ~7 days Off –state durations ~25 days Switch timescale highly variable. On/off transitions are very quick: few rotational periods or less (while magnetosphere is being rearranged). Young et al. 2012

More intermittent pulsars: PSR B PSR B was off for 580 days (Camilo et al. 2012). Spin-down rate changes with a factor 2.5 Camilo et al. 2012

More intermittent pulsars PSR B Lorimer et al PSR B was off for ~600 and 820 days (Lorimer et al. 2012). Spin-down rate changes with a factor 1.8

Magnetosphere modelling of intermittent pulsars All cases: spin-down rate changes imply changes of the order of the GJ charge density in the open zone. Confirmed by numerical modelling (Li et al. 2012): a range in magnetic inclination angles can explain the observed spin-down rates. Li et al Ratio between on/off spin-down rate Magnetic inclination angle [degr.]

PSR B : no spin-down changes? This pulsar shows both very short-timescale switching “nulling” and long term switches “intermittent pulsar”. Spin-down rate changes with < 6% during off-phase. Why does spin-down change less compared with B ? Note: we can only measure long-duration changes in the spin-down. ON durations OFF durations Young et al. 2012

Timing noise: “random” variations of the spin-down rate. Generalisation of the intermittent pulsar scenario: Magnetospheric state change → change in currents → simultaneous change in spin- down and emission pattern. Emission change is not necessarily imply switching off. Timing noise = magnetospheric changes Lyne et al. 2010

These pulsars show switches in the spin-down rate on a timescale of year(s). Spin-down rate changes can be very small (<1%). These switches are accompanied by changes in profile shapes (Lyne et al. 2010). Timing noise = magnetospheric changes Lyne et al. 2010

Spin-down rate larger when core component of the radio profile is brighter. Measuring profile shapes provides a way to correct timing noise! Accuracy that can be reached will depend on the total amount of on- source time. Lyne et al Timing noise = magnetospheric changes

PSR B – Link between magnetosphere and NS interior? More recent data suggest that the correlation between the profile shape and spin-down rate became stronger after a glitch. Link between NS interior and magnetosphere? Coincidence? Keith et al glitch

PSR J – Profile changes related to a glitch? Weltevrede et al. 2011: Evidence for a radio profile change after a large glitch. Anomalous glitch recovery – See talk Ali Alpar.

PSR B – Simultaneous radio and X-ray changes Radio pulse bright: X-ray light-curve is unpulsed & non-thermal. Radio pulse weak: the X-ray light-curve shows an additional pulsed thermal component. Radio change = global magnetospheric change. Challenge for theories: see posters George Melikidze and talk Joeri van Leeuwen. Hermsen et al. 2013

PSR B – Spin-down changes at short timescales? Gil et al → Pulse phase → Subintegration number (10 pulses) Half a rotation Lyne et al PSR B : Long timescale spin-down changes related to changes we can see on a minute timescale? Are short-timescale mode changes physically the same as the long-timescale changes? What about the even shorter timescale (sub second) variability we observe in the radio emission? Are pulsar magnetospheres constantly readjusting? Radio observations can show us these changes on these very short timescales.

Subpulse modulation PSR 328 MHz (WSRT) Radio emission is variable (shape & amplitude) on a sub-period timescale: subpulse modulation. Can be visualised in a pulse-stack. Are there equally drastic magnetospheric compared to the intermittent pulsars on these timescales? Does the magnetosphere ever settle down?

Subpulse phase modulation PSR 328 MHz (WSRT) For some pulsars drifting subpulses are very clear: magnetosphere can be dynamic, but organized at the same time. Drifting subpulses detectable in a large fraction of pulsars (e.g. Weltevrede et al. 2006, 2007). They are clearer in older pulsars (e.g. Rankin 1986).

Carousel model: drifting subpulses caused by beamlets rotating about the magnetic axis PSR 328 MHz (WSRT) Ruderman & Sutherland 1975 Gil, Melikidze & Zang 2006 Carousel model?

Profile shape Modulation power Edwards et al ● Some problems with the model: phase steps ● Looks like interference of two out of phase patterns: magnetospheric double imaging? ● See talk Tom Hassall. Carousel model? Subpulse phase = phase offset of pattern as function of pulse longitude. Here the overall slope is subtracted. PSR B

Poster Paul Brook ● Asteroids in a fallback disk could potentially modulate the observed radio emission (e.g. Cordes & Shannon 2008; Luo & Melrose 2007). ● Can potentially explain the long timescale variability. ● Used to explain variability seen on a years timescale of PSR J Fallback disk?

Interpole communication? For PSR B a MP profile component switches off when IP switches on! MP & IP are not independent of each other. (Somewhat) more conventional interpretation: they are produced on two opposite magnetic poles, but then it implies the magnetospheric changes must be global. Are they produced on same pole (e.g. Gil 1985, Dyks et al. 2005, Petrova 2008)?

Interpole communication? PSRs B , B and B (Latham et al. 2012, Weltevrede 2007, Weltevrede et al. 2012) have quasi-periodic drifting subpulses at MP and IP, which remain at a constant phase delay over many years. How do poles communicate with each other? Again points towards the magnetosphere being a global system and variable on a second timescale. Weltevrede et al. 2007Weltevrede et al. 2012

Summary ● Radio emission is variable at a timescale of less than a second up to years (at least). ● At all these timescales there is evidence that the radio emission is a probe of global (magnetosphere wide) phenomena. ● Although the radio emission is highly insignificant in terms of the total energetics of neutron stars, it is a good tracer of the dynamics of the most energetic processes in pulsar magnetospheres. ● How to construct a global magnetospheric model than can explain the dynamics we see at all these timescales? ● Does a steady state solution exist?