Magnetar

Full definition

In the zoology of compact objects, the magnetar is the extreme. Take a neutron star — already the pinnacle of known stable density — and multiply its magnetic field by a thousand. You get a magnetar: a 20 km sphere wrapped in a magnetic field of 10¹⁴ to 10¹⁵ gauss. For comparison, a fridge magnet sits at 0.01 gauss, Earth's field at 0.5 gauss, and the strongest field ever produced in a laboratory on Earth reached 2 × 10⁶ gauss for microseconds. A magnetar's field is ten billion times stronger than that.

This field is so strong it distorts the quantum vacuum and polarizes the light passing through. At 1,000 km from a magnetar, the magnetic force on the atoms in your body would shatter chemical bonds and stretch molecules into field-aligned filaments — biochemistry would simply cease. It is an inaccessible laboratory of fundamental physics.

Magnetars form, like other neutron stars, from the collapse of a massive star in a supernova. But for reasons still debated — probably a dynamo amplification mechanism in the first seconds of their life — about 10 % of neutron stars develop an extreme magnetic field. That field cannot remain stable for long: it decays over ~10⁴-10⁵ years, releasing energy as violent bursts in X-rays and gamma rays.

This mechanism explains the Soft Gamma Repeaters (SGRs) — gamma-ray sources that emit in fits and starts — and Anomalous X-ray Pulsars (AXPs) — X-ray pulsars radiating more than their rotation can account for. Duncan and Thompson unified the two phenomena in 1992 under a single idea: an extreme magnetic field.

Numbers and orders of magnitude

Magnetic field: 10¹⁴-10¹⁵ gauss (10¹⁰-10¹¹ tesla). That is 10¹⁵ times more intense than Earth's field.

Rotation period: 2 to 12 s, longer than classical pulsars. They spin more slowly because their powerful field brakes their rotation quickly.

Spin-down rate: 10⁻¹¹ s/s, extremely fast — an order of magnitude above normal pulsars.

Quiescent luminosity: 10²⁶-10²⁸ W in X-rays.

Flares: sporadic outbursts. Ordinary flares 10³⁴-10³⁶ J. Super-flares ('giant flares') reach 10⁴⁶ J in 0.1 second — more energy than the Sun produces in 100,000 years released in a tenth of a second.

Historic event. On December 27, 2004, SGR 1806-20 (50,000 ly away in Sagittarius) produced a super-flare that saturated every gamma-ray satellite in orbit and briefly ionized Earth's upper atmosphere. It is the most energetic event of its kind ever recorded.

Known count: ~30 magnetars in the Milky Way and Magellanic Clouds (McGill catalog 2025). Models predict a significant fraction of neutron stars pass through a magnetar phase, but their short active lifetime (10⁴-10⁵ years) makes them rare at any given moment.

Types and famous examples

Soft Gamma Repeaters (SGRs). Magnetars primarily observed through their bursts of soft gamma rays. Archetype: SGR 1806-20, responsible for the December 27, 2004 super-flare. SGR 1935+2154 is the most studied since the discovery of its link to a fast radio burst (see below).

Anomalous X-ray Pulsars (AXPs). Magnetars mostly seen as 'anomalous' X-ray pulsars because their X-ray luminosity far exceeds the available rotational energy. Example: 1E 1048.1-5937. The SGR vs AXP distinction is today seen as a temporary state rather than a strict separation: the same magnetar can switch between behaviors.

Swift J1818.0-1607. The youngest known magnetar, ~240 years old (its supernova went off just before our era), discovered in 2020. Period 1.36 s — the fastest of all magnetars.

XTE J1810-197. First confirmed 'radio-magnetar', occasionally emitting radio like a classical pulsar.

Link to FRBs. On April 28, 2020, the CHIME (Canada) and STARE2 (USA) radio telescopes detected a millisecond radio burst (FRB 200428) from SGR 1935+2154, a galactic magnetar 30,000 ly away in Vulpecula. For the first time, an FRB was clearly associated with a known magnetar. This observation shifted the theory: a fraction of extragalactic FRBs (the radio puzzle of the 2010s) likely originate from magnetars.

Magnetars and super-luminous supernovae. Several exceptionally bright supernovae (SLSNe) may be powered by a newborn magnetar whose rotational and magnetic energy boost the ejecta's luminosity.

How do we observe them?

Magnetars are multi-wavelength objects by nature: they radiate mostly in X-rays and gamma rays, sometimes in radio and optical.

Gamma rays. SGRs are historically discovered by gamma-ray satellites: Vela in 1979 (first SGR, SGR 0525-66 in the LMC), then BATSE on Compton Gamma Ray Observatory (1991-2000), RHESSI, INTEGRAL (ESA, 2002-), Fermi-GBM (NASA, 2008-), Swift/BAT (2004-). These satellites alert the community in near real-time when a magnetar becomes active.

X-rays. Chandra (NASA, 1999), XMM-Newton (ESA, 1999), NuSTAR (2012), NICER (ISS, 2017). They track quiescent luminosity and longer 'outbursts' (weeks to years).

Radio. Parkes, VLA, CHIME (Canada, 2018 — the instrument that detected FRB 200428), MeerKAT (South Africa, 2018). Radio-emitting magnetars are rare but fascinating.

Optical/IR. Hubble and VLT have imaged a few magnetars at optical counterpart (very faint: magnitude > 24).

Gravitational waves (hope). Super-flares could produce potentially detectable gravitational waves. No confirmation for the 2004 event, but the next generation of instruments (Cosmic Explorer, Einstein Telescope) may detect galactic flares.

What about amateur astronomy? No magnetar is observable optically with amateur equipment — the brightest ones are fainter than magnitude 22. However, several lie inside accessible supernova remnants. SGR 0526-66 sits in the N49 remnant (Large Magellanic Cloud, southern sky), and SGR 1806-20 is in a Sagittarius region rich in observable objects you can locate with our sky map tool.

Not to be confused with

The neighborhood of magnetized compact objects is crowded.

Classical neutron star. A magnetar IS a neutron star — but only ~10 % of neutron stars become magnetars. The distinction is field strength (10¹⁴-10¹⁵ G vs 10⁸-10¹³ G for ordinary neutron stars).

Classical pulsar. A pulsar draws its radiated energy from the star's rotation, which gradually slows. A magnetar draws its energy from the decay of its magnetic field — letting it radiate far more than rotation alone would allow. Some magnetars are also radio pulsars (radio-magnetars), but that is a minority.

Black hole. A black hole has no intrinsic magnetic field (the 'no-hair' theorem). If it shows magnetic behavior, it is through the surrounding accretion disk.

Long gamma-ray burst (long GRB). Energetic explosion caused by a very massive star collapsing into a black hole with a relativistic jet. Magnetar super-flares resemble short GRBs in their initial phase but are far less energetic (10⁴⁶ J vs 10⁴⁷-10⁵¹ J) and entirely different in origin.

Fast Radio Burst (FRB). Some FRBs come from magnetars (FRB 200428 = SGR 1935+2154), but not all FRBs are likely magnetar-driven. Repeating FRB populations suggest several mechanisms sharing one name.