1. Magnetars
Jared Filseth

2. What is a Magnetar? Type of neutron star
Magnetic field strength ~1014 or 1015 G (other types of NS between G)
Strongest magnets in the universe (as far as we know)
Slower rotational periods than similar pulsars (typically on the order of seconds)
Short lifetime (~105 yrs)
Fanciful depiction of neutron star
Source:

3. History of Magnetar Discovery
Short, intense “gamma ray burst” (now known not to be a true GRB) observed in 1979 (2 more “giant” bursts observed since then)
Object that emitted it known as SGR (Soft Gamma Repeater)
Evident that it was coming from a neutron star
Strong magnetic field hypothesis originally made in 1980, but predicted field strength several orders of magnitude too low
Soviet Venera 12 space probe gamma ray count reading on March 5, 1979 at 10:51am Source:

4. History of Magnetar Discovery
AXPs (Anomalous X-ray Pulsars) originally observed separately in 1990s – pulsars with anomalously high x-ray luminosity
Eventually, AXPs discovered to emit gamma ray flares
Now, SGRs and AXPs generally agreed to be the same thing (magnetars)
Magnetar hypothesis nicely explains both AXP and SGR properties
Another major SGR flare, as seen by Stanford’s Ulysses gamma-ray detectors Source:

5. Magnetars are generally believed to originate in supernovae
Magnetar Properties
Magnetar hypothesis: energy for SGR bursts and high x-ray emissions comes from decay of extremely strong magnetic field
“Spin-down”: angular velocity of magnetar decreases over time
B-field decays over time
Energy from B-field decay powers SGR bursts and AXP emissions, and all other magnetar properties
Magnetars are generally believed to originate in supernovae
Source:

6. Evidence for Magnetars
Rotational energy loss alone (from spin-down) cannot explain AXP
SGR : based on time elapsed since supernova, SGR spin-down rate was calculated, which allowed for calculation of magnetic field strength: ~8*1014 G (Kouveliotou 1998)
High magnetic field (at least 1014 G) decreases scattering opacity, raising Eddington limit, which explains the super-Eddington luminosities of some bursts
Magnetic field changes provide energy needed to power sudden large SGR bursts, for magnetic fields of at least 1014 G
While accreting magnetars may exist, discovered SGRs have no stellar companions, meaning accretion cannot be an energy source

7. Magnetar Formation
Exact process by which magnetars form, as well as what happens after the field reduces sufficiently, is still largely unknown
Duncan and Thompson (1992) propose theory of magnetic dynamo formation
Key parameter of their theory is Rossby number Ro, which is the ratio of the angular period of the star to the convective overturn time
Magnetic dynamo
Source:

8. Duncan and Thompson Hypothesis (1992)
For Rossby number of order unity, initial orbital period must be ~1ms – faster than the fastest observed NS pulsars
With Rossby number of order unity, efficient magnetic dynamos can theoretically form during neutron star birth, resulting in magnetic field up to 3*1017 G in ideal case; more than enough for magnetars
Magnetars are evolution of extremely rapidly rotating cores in massive stars
No evidence for highly energetic supernova ejects associated with extremely rapid star core rotation
Other hypotheses (e.g. fossil fields), but equally or more questionable

9. Magneto-Thermal Evolution
Complicated to simulate, first reasonably comprehensive model was produced in 2013 (Vigano et al)
Magnetic field evolves according to Hall Induction Equation:
First term on RHS is Ohmic dissipation term (i.e. energy loss), 2nd term is Hall effect term (which builds up a toroidal component to magnetic field, speeding up dissipation).
Highly density and temperature dependent, so these factors need to be simultaneously modeled
Also depends on thermal and electrical conductivity, specific heat, and neutrino emissivities, as well as magnetic geometry (e.g. crust vs. core)

10. Magneto-Thermal Evolution
Source: Vigano et al 2013
Simulated time-evolution of field strength and luminosity of a magnetar with M=1.4Msolar, Bdipole,initial = 1014 G for different models
Spin-down rate (i.e. time derivative of period, unitless) is , dependent on period and magnetic field strength due to dipole losses in vacuum

11. Persistent Magnetar Emission
Magnetar has “liquid” core and “solid” (crystallized) crust
Charged plasma has electric current j = (c/4π)∇ × B
This creates a force on the plasma F = j × B/c
On the surface, this results in enormous stresses on the crust, resulting in crust slippage, causing a “twisted” magnetosphere
Current travels along twisted field lines, filling magnetosphere with highly energized charged particles
Left: untwisted dipole field; right: twisted dipole field Source: Turolla et al. (2015)

12. Nonthermal X-rays Extremely fast particles flow into magnetosphere
Large number of particles form in the blue regions due to pair production from collision with keV-energy photons
High energy (hard x-ray) photons ejected, but the resulting cloud of slowed particles in the “radiative” (red in diagram) zone up- scatters low energy photons, resulting in an abundance of very soft x-rays
Additional soft x-rays produced from particles slamming back into crust
Thus, twist energy is dissipated into x-rays, resulting in high x-ray luminosity (characteristic of AXPs)
Particle emission along a twisted field line
Source: Beloborodov 2013
X-ray Radiation spectrum at 4 different angles from dipole axis
Source: Kaspi 2017

13. SGR Flares Overview
Categories of flare: short burst (E < 1041 erg), intermediate bursts (1041 erg < E < 1043 erg), and giant flares (1044 erg < E < 1046 erg)
Short bursts: last 0.1-1s, seen in SGRs and AXPs
Intermediate bursts: last 1-40s, higher peak energy than short bursts (up to 1043 erg/s), seen in SGRs and AXPs
Giant flares: Initial spike of energy output up to 1047 erg/s lasting ~0.1s, followed by long pulsating tail lasting several hundred seconds, resonant with neutron star rotational frequency
Giant flares quite rare; only 3 have ever been observed (1979, 1998, 2004), and only in SGRs

14. Starquakes Rapid rearrangement of magnetic field causes bursts
Starquakes likely responsible for this rearrangement – plastic deformation of crust as a result of critical values of strain due to magnetic fields (actual rupture is impossible)
Predicted characteristic burst energy:
σmax is breaking strain of crust – up to 0.1. d is depth, Rc is crust thickness, R is star radius (~10km), l is rupture length
Corresponding local field strength matches up to theorized magnetar fields:

15. Alternatives to S tarquakes
Magnetic field evolves into unstable configuration within core
However, core is superconducting, meaning unstable core configurations may be difficult to achieve
Spontaneous plasma reconnection in magnetosphere as a stress relief mechanism
Could be a combination of all 3 of these mechanisms (which may help explain the high diversity of SGR flare properties)
All of these mechanisms fundamentally result in similar outcomes: producing rapid changes to magnetic field

16. Emission Processes
Rapid changes to magnetic field accelerate electrons, resulting in pair creation and gamma ray emission
Low proportion of baryonic material in magnetosphere (otherwise, scattering would occur, resulting in softening of gamma rays)
Radio afterglow: also seen in flares
Plasma cooling after running into existing material in outer magnetosphere (possibly trapped by magnetic field) may produce radio waves
How is gamma ray emission visible despite baryonic plasma? Various theories (“fireball model”)

17. Classification of Magnetars
Many classes of Isolated Neutron Star (INS), unclear how they all relate to magnetars
Magnetars tend to have longer and faster- increasing periods as a result of magnetic fields
RRaTs rotationally powered
CCOs are low-magnetic-field supernova remnant neutron stars
XINS are neutron stars with dim x-ray emissions
All of these have properties different from standard RPPs (radio powered pulsars)
Various INS’s categorized by period and period derivative
Source: Turolla et al. (2015)

18. Possible Evolution Pathways of Magnetars
Evolutionary links between different INS types likely, since otherwise galaxy core- collapse supernova rate will not be high enough to explain the combined birthrates of all INS types
Has been suggested due to period-period derivative relationship and high magnetic field strength that XINS may be mature magnetars
Period-spin down rate plot of various INS’s, along with evolution pathways in bold lines (dashed lines = constant magnetic field strength)
Source: Turolla et al. 2015

19. Conclusions
Magnetars, while not having been directly observed, are extremely likely to exist, and explain a whole host of observed phenomena among SGRs and AXPs
Active, open area of research. Many unanswered questions, crude models needing improvement.
“Low-B” and “transient” magnetars also observed
Gravitational wave observations may provide useful insights into magnetar burst properties – candidate for LIGO?

20. Sources (not including image sources)
Kaspi and Beloborodov, Magnetars. 2017
Turolla et al. Magnetars: the physics behind observations. A review
Igoshev et al. How to make a mature accreting magnetar. 2017
Duncan and Thompson, Formation of very strongly magnetized neutron stars: Implications for gamma-ray bursts. 1992
Thomson and Duncan, Neutron star dynamos and the origins of pulsar magnetism. 1993
Vigano et al. Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models. 2013