FAST and the neutron star zoo Patrick Weltevrede Jodrell Bank Centre of Astrophysics University of Manchester

~2500 neutron stars known, the overwhelming majority detected in radio [1]. L-band is the radio-band of choice in many pulsar experiments (spectral index in range -1 … -3, with -1.8 being the average; Maron et al. 2000; Bates et al ) There are several “classes” of radio emitting neutron stars. Discussion roughly from high to low magnetic fields in P- Pdot diagram. [1]

Believed to be very young Decay of intense magnetic field results in X-ray and soft gamma-ray burst Of the 23 confirmed magnetars [2], only 4 detected in radio ( Camilo et al. 2006, 2007a; Levin et al. 2010; Eatough et al ) Radio properties very different from “normal” pulsars Radio emitting magnetars [2]

Radio emitting magnetars Torne et al (PSR J ) Magnetar PSR J is only 3’’ away from Sgr A* Found in radio after a Swift X-ray flare and NuSTAR detection of pulsations ( Eatough et al ) Highest known DM=1780 cm -3 pc and RM= rad/m 2 for any NS Highly scattered at L-band, but detected up to 225 GHz! Erratic emission & profile changes Serylak et al XTE J Effelsberg

Radio emitting magnetars Levin et al Chandra X-ray image PSR J found in radio during X-ray quiescence. FAST potentially could find similar objects (galactic centre/plane survey?). Other science questions Is there weak persistent radio emission for currently non-radio detected magnetars? If detected, how is it related to the outburst? Does the polarization give clues about differences in the magnetic field? Where do the ultra-strong magnetic fields come from? What fraction of pulsars born as magnetars? How many are really radio quiet?

Pulsars with very high inferred magnetic field strengths (close or above “quantum critical” strength=4.4  G). Some overlap with magnetars. Thought to be rotation powered. High-B pulsars

PSR J – Profile changes related to a glitch? Radio variability associated with large spin-down glitch? ( Weltevrede et al. 2011; Keith et al 2013 gives possible other example ). Anomalous glitch recovery ( Antonopoulou et al ). How unique is this event? Does great sensitivity of FAST allow smaller changes to be detected? Weltevrede et al Parkes

Timing of young and high-B pulsars allow the measurement of the direction of motion in the P-Pdot diagram. PSR J moves toward magnetars! ( Espinoza et al ). High-B pulsars Braking indices as listed in Espinoza et al Arrows show evolution after 20,000 yrs (2,000 yrs for J , left most pulsar with arrow)

Rotating RAdio Transients “Class” of neutron stars discovered by McLaughlin et al Often defined to be: Pulsars easier/only detectable via single pulse searches rather than periodicity searches. Bursts are very infrequent (once every few minutes/hours) RRATs

Problem: RRATs are NOT defined by intrinsic properties of the neutron star. Classification depends on: Size of your telescope Length of your observations Distance of the object Observing frequency. Example: PSR B should be classified as a RRAT if it would be further away. RRATs Weltevrede et al. 2006

There are 103 known RRATs [3], many without known P & Pdot. Periods larger, or is it an observational bias? Difficult to detect, hence large total population incompatible with SN rate ( Keane et al ). Are different NS classes evolutionary connected? Sensitivity of FAST allows a better comparison with the “normal” pulsar population.RRATs [3]

More and more variability is being discovered in the region in the P-Pdot diagram occupied by “normal” pulsars. On long timescales (~years) this variability is shown to be linked to the spin-down. Intermittent pulsars switch completely off ( Kramer et al ). Others show subtle profile switches ( Lyne et al ). “Normal” pulsars

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?

Intermittent pulsars PSRs B , B and B ( Kramer et al. 2006; Camilo et al. 2012; Lorimer et al ). Radio emission “on” → large spin-down (change with factor 1.5 … 2.5). Magnetospheric switching between a vacuum state and a force-free state ( Kramer et al. 2006; Li et al ). Radio emission is a probe of drastic changes in the magnetosphere. Are changes in geometry caused by changes in the current distribution responsible ( Timokin 2010 )? Kramer et al. 2006; Young et al. 2012

Link with “nulling”? Normal pulsars are known to “null”, up to ~90% of the time ( e.g. Wang et al ). Are these shorter timescale events related to intermittent pulsars? Or the RRATs? If a pulsar is off, is it really off? Can be hard to distinguish ( e.g. Esamdin et al ). If not, in what way is the emission different? Back-end with single-pulse recording capabilities essential. PSR J Hessels et al 2008Wang, Manchester & Johnston, MNRAS, 2007

LOFAR observation of PSR B LOFAR observation (143 MHz) of PSR B : “ Nulls” are very weak emission > 100X weaker as the normal emission ( Sobey et al ). Weak emission more erratic? Possibly the pulsar does completely switches off, or is it?

Pulsar observations with LOFAR Science includes: Spectral properties, profile evolution, ISM effects. Benefits from: large field of view, relative brightness of pulsars at low frequencies, easier RFI excision. More than 250 pulsars detected so far, including 50 MSPs. Discovered 20 pulsars so far. Pilia et al. 2015

Timing noise: “random” variations of the spin-down rate. Generalisation of the intermittent pulsar scenario: Magnetospheric state change → change in braking torque → simultaneous change in spin- down and emission pattern. These pulsars show switches in the spin-down rate (can be <1%) on a timescale of year(s) and profile shapes ( Lyne et al ). 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! Profile changes can be very small, hence difficult to detect. S/N limited. The combination of regular timing with a smaller dish and high-precision profiles with a big dish might be very powerful ( Perera et al. in prep. ) Lyne et al Timing noise = magnetospheric changes

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. Related to equally drastic magnetospheric changes compared to the intermittent pulsars? 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 ). For this type of work instantaneous sensitivity is crucial, so FAST would be perfect.

Conclusions ● Radio emission is variable at all known timescales (sub- second to at leas years). ● At all these timescales there is evidence that the radio emission is a probe of global (magnetosphere wide) phenomena. ● Radio emission gives direct insight in the dynamics of the most energetic processes in pulsar magnetospheres. ● Polarization capabilities are crucial. ● FAST will be a very powerful instrument for various types of pulsar science. ● The great instantaneous sensitivity will be crucial to understand the link between strong and weak states of various neutron-star classes. ● This will help understanding the underlying physics, but also the relation between different classes of neutron stars.