CXC: What Lies Beneath? Magnetar Enigma Deepens

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CXC: What Lies Beneath? Magnetar Enigma Deepens

Post by bystander » Fri Oct 15, 2010 8:08 am

SGR 0418+5729: What Lies Beneath? Magnetar Enigma Deepens
Chandra X-ray Observatory | 14 Oct 2010
Observations with NASA's Chandra, Swift, and Rossi X-ray observatories, Fermi Gamma-ray Space Telescope, and ESA's XMM-Newton have revealed that a slowly rotating neutron star with an ordinary surface magnetic field is giving off bursts of X-rays and gamma rays. This discovery may indicate the presence of an internal magnetic field much more intense than the surface magnetic field, with implications for how the most powerful magnets in the cosmos evolve.

The neutron star, SGR 0418+5729, was discovered on June 5, 2009 when the Fermi Gamma-ray Space Telescope detected bursts of gamma-rays from this object. Follow-up observations four days later with the Rossi X-Ray Timing Explorer (RXTE) showed that, in addition to sporadic X-ray bursts, the neutron star exhibits persistent X-ray emission with regular pulsations that indicate that the star has a rotational period of 9.1 seconds. RXTE was able to monitor this activity for about 100 days. This behavior is similar to a class of neutron stars called magnetars, which have strong to extreme magnetic fields 20 to 1000 times above the average of the galactic radio pulsars.

As neutron stars rotate, the radiation of low frequency electromagnetic waves or winds of high-energy particles carry energy away from the star, causing the rotation rate of the star to gradually decrease. Careful monitoring of SGR 0418 was possible because Chandra and XMM-Newton were able to measure its pulsation period even though it faded by a factor of 10 after the initial detection. What sets SGR 0418 apart from other magnetars is that careful monitoring over a span of 490 days has revealed no detectable decrease in its rotation rate.

The lack of rotational slowing implies that the radiation of low frequency waves must be weak, and hence the surface magnetic field must be much weaker than normal. But this raises another question: where does the energy come from to power bursts and the persistent X-ray emission from the source?

The generally accepted answer for magnetars is that the energy to power the X- and gamma-ray emission comes from an internal magnetic field that has been twisted and amplified in the turbulent interior of the neutron star, as depicted in the illustration above. Theoretical studies indicate that if the internal field becomes about ten or more times stronger than the surface field, the decay or untwisting of the field can lead to the production of steady and bursting X-ray emission through the heating of the neutron star crust or the acceleration of particles.

A crucial question is how large an imbalance can be maintained between the surface and interior fields. SGR 0418 represents an important test case. The observations already imply an imbalance of between 50 and 100. If further observations by Chandra push the surface magnetic field limit lower, then theorists may have to dig deeper for an explanation of this enigmatic object.
Mysterious pulsar with hidden powers discovered
University College London | via EurekAlert | 14 Oct 2010
Dramatic flares and bursts of energy - activity previously thought reserved for only the strongest magnetized pulsars - has been observed emanating from a weakly magnetised, slowly rotating pulsar. The international team of astrophysicists who made the discovery believe that the source of the pulsar's power may be hidden deep within its surface.

Pulsars, or neutron stars, are the collapsed remains of massive stars. Although they are on average only about 30km in diameter, they have hugely powerful surface magnetic fields, billions of times that of our Sun.

The most extreme kind of pulsars have a surface magnetic field 50-1000 times stronger than normal and emit powerful flares of gamma rays and X-rays. Named magnetars (which stands for "magnetic stars") by astronomers, their huge magnetic fields are thought to be the ultimate source of power for the bursts of gamma rays.

Theoretical studies indicate that in magnetars the internal field is actually stronger than the surface field, a property which can deform the crust and propagate outwards. The decay of the magnetic field leads to the production of steady and bursting X-ray emission through the heating of the neutron star crust or the acceleration of particles.

Now, research published today in Science, suggests that the same power source can also work for weaker, non-magnetar, pulsars. The observations, which were made by NASA's Chandra and Swift X-ray observatories of the neutron star SGR 0418, may indicate the presence of a huge internal magnetic field in these seemingly less powerful pulsars, which is not matched by their surface magnetic field.

"We have now discovered bursts and flares, i.e. magnetar-like activity, from a new pulsar whose magnetic field is very low," said Dr Silvia Zane, from UCL's (University College London) Mullard Space Science Laboratory, and an author of the research.

Pulsars are highly magnetized, and as they rotate winds of high-energy particles carry energy away from the star, causing the rotation rate of the star to gradually decrease. What sets SGR 0418 apart from similar neutron stars is that, unlike those stars that are observed to be gradually rotating more slowly, careful monitoring of SGR 0418 over a span of 490 days has revealed no evidence that its rotation is decreasing.

"It is the very first time this has been observed and the discovery poses the question of where the powering mechanism is in this case. At this point, we are also interested in how many of the other normal, low field neutron stars that populate the galaxy can at some point wake up and manifest themselves as a flaring source," said Dr Zane.

A crucial question is how large an imbalance can be maintained between the surface and interior magnetic fields. SGR 0418 represents an important test case.

"If further observations by Chandra and other satellites push the surface magnetic field limit lower, then theorists may have to dig deeper for an explanation of this enigmatic object," said Dr Nanda Rea, Institut de Ciencies de l'Espai (ICE-CSIC, IEEC) in Barcelona, who led the discovery.
Are most pulsars really magnetars in disguise?
ESA Science and Technology | XMM-Newton | 14 Oct 2010
Image
Artist's impression of a magnetar (Credit: ESA)
Astronomers using XMM-Newton and other world-class X-ray telescopes have probed a curious source, which emits flares and bursts just like a magnetar but lacks the extremely high external magnetic field typical of these objects. The detection of this source, which could be powered by a strong, internal magnetic field hidden to observations, may mean that many 'ordinary' pulsars are dormant magnetars waiting to erupt.

Massive stars remain objects of curiosity even well after their demise. Ending their lives in dramatic fashion, as supernova explosions, they leave a very dense and compact remnant behind - a neutron star or a black hole, depending on the mass of the star. These remnants, characterised by intense gravitational fields, are the source of some extremely energetic events and give rise to a variety of interesting phenomena which can be observed throughout the entire electromagnetic spectrum.

Neutron stars, in particular, derive from the collapse of stars originally as massive as 8 to 25 times the mass of the Sun and they harbour magnetic fields a million times stronger than the strongest ones ever produced on Earth. Spinning neutron stars can be observed as pulsating sources -hence the name, pulsars- with exceptionally short periods, ranging from about one thousandth of a second to ten seconds. Powerful beams of electromagnetic radiation are created by jets of energetic particles that stream out above the magnetic poles of the star; the 'blinking' effect arises because the pulsar's magnetic dipole is not always aligned with its axis of rotation. Young pulsars rotate extremely fast but release rotational energy and slow down as they age: older pulsars have thus longer periods than younger ones. By measuring the rate at which a pulsar spins down it is possible to estimate the intensity of its surface dipolar magnetic field.

Magnetars are a special class of pulsars that stand out from the crowd because of their striking characteristics: they have long rotations periods, occasionally undergo episodes of extremely enhanced emission (about 10–100 times the usual value) and produce intense, short bursts of X-rays and gamma-rays.
A Low-Magnetic-Field Soft Gamma Repeater - N Rea et al Even ‘Weakling’ Magnetars are Strong and Powerful
Universe Today | Chandra | 14 Oct 2010

Discovery Puts New Spin on Universe's Most Powerful Magnets
Space.com | Science | 14 Oct 2010