Archive for ‘Neutron stars and pulsars’

November 21, 2011

Neutron star research points to different classes of supernovae

Classifying things is the starting point for almost all scientific fields – from flowers to fundamental particles. Once one has classes the next step is to find subclasses, and then sub-subclasses. Finding correlations between different classification schemes, then, often leads to significant understandings.

Neutron stars are not stars in the normal sense. They are remnants composed entirely of neutrons left after a star larger than the Sun, but not too large, explodes as a supernova at the end of its life as a star. There are different types of both neutron stars and supernovae.

Consider neutron stars first. When a star whose inner core is more massive than the Chandrasekhar limit – about 1.4 M (M denoting the mass of the Sun) – exhausts its nuclear fuel it collapses as a supernova because it is too heavy to support itself through degeneracy pressure. If the material remaining after the explosion is less than about 3 M (the Tolman–Oppenheimer–Volkoff limit) the remnant is a neutron star. Otherwise the result is a black hole. Typically, the total mass of the progenitor of a neutron star is in the range of 5 to 15 M.

Since a neutron star can no longer release energy from thermonuclear reactions, it may radiate very little electromagnetic energy. Consequently it may be rather difficult to detect, like a black hole, unless it’s a member of a binary system, so that there are visible gravitational effects on the companion.

However, some neutron stars may have energy sources that allow them to emit electromagnetic radiation at wavelengths all the way from radio to X-rays or even gamma rays. If the neutron star also has a strong magnetic field, such emissions may be observable in periodic pulses occurring at frequencies from a thousandth of a second on up. In this case the neutron star is a pulsar.

There are different types of pulsars too, depending on the energy source.

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November 4, 2011

Record-breaking gamma-ray emissions from a millisecond pulsar

Breaking old records for some statistic or another generally makes for catchy headlines, at least. If nothing else, the fact that the statistic is being tracked (whether or not by Guinness) suggests it’s a matter of more than passing interest. Now astrophysicists have come up with a new find that breaks three records at the same time. This one is a pulsar, designated J1823−3021A, located in the globular cluster NGC 6624. It’s a special subtype of pulsars – millisecond pulsars, which rotate from 100 to 1000 times per second (rotational period from 1 to 10 milliseconds).

Research just published about J1823−3021A estimates that, among millisecond pulsars, it has the strongest radiation in the gamma-ray part of the electromagnetic spectrum (about 8×1034 ergs/second), has the highest magnetic field strength, and is probably the youngest known.

Pulsars are rapidly rotating neutron stars that radiate most of their energy in narrow jets. The jets are aligned with the poles of the object’s very strong magnetic field. As with the Earth, this magnetic axis need not be parallel to the object’s rotation axis, so that we observe the strongest emissions in sharp peaks, like the beams of a lighthouse.

Neutron stars are the remnants left of core collapse supernovae whose progenitor stars were not massive enough to have left only a black hole after collapse – generally stars of less than 10 solar masses. Typically a neutron star starts life with a rapid rate of spin, which decreases gradually with time as the kinetic energy of rotation is lost through electromagnetic radiation. However, if the neutron star has a binary companion it may accrete matter from the companion, thus gaining angular momentum and increasing its rate of spin up to a rate of more than 100 rps.

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October 18, 2011

Crab nebula’s neutron star is pulsing with gamma rays

The Crab Nebula is a pretty strange crustacean. It’s a supernova remnant, from a supernova whose detonation was seen on Earth in 1054 CE. The explosion left behind a rapidly spinning neutron star and a large quantity of ejected matter that’s still expanding away from the blast site – and putting on an impressive show in optical and many other wavelengths.

Last year and earlier this year there were reports, based on detection by satellite observatories, of occasional very high-energy gamma-ray flares, cause unknown, having energies of at least 1015 eV. These are the highest energy particles detected in a discrete source, and the mechanism of their acceleration is unclear. (See here, here.)

And now very high-energy gamma-ray pulses – less energetic than the flares, but still with energies up to 400 GeV – have shown up, associated with the central pulsar itself, which spins and pulses about 30 times per second. Normally, energetic X-ray and gamma-ray photons are generated from high-speed charged particles in a strong magnetic field, by the process of synchrotron radiation (also known as “curvature radiation”). But that appears to be ruled out in this case, since the energy limit on gamma rays that could be produced by this process is about 100 GeV.

Crab nebula’s neutron star is pulsing with gamma rays

The new Science paper packages up 107 hours of Crab Nebula observations spread over the course of four years. When the data was analyzed, a clear pattern of pulses became apparent at energies above 120GeV, and the timing of the pulses lined up nicely with observations at lower energies made using the Fermi space telescope. The object there is pulsing at much higher energies than any previously detected.

The findings have a number of implications. For starters, they clearly demonstrate that the predictions of exponential decay in pulse energy, those based on observations of other pulsars, aren’t happening here. Instead, the fade-off at higher energies follows what’s called a “broken power law,” with a far more gradual tailing off. The results also allow the authors to estimate the distance from the neutron star to the source of the gamma rays: between 10 and 40 stellar radii from the surface.

If the process responsible for the gamma rays isn’t synchrotron radiation, what is it? The research paper suggests inverse Compton scattering, which involves high-energy particles transferring some of their energy to lower energy photons. But there’s as yet no evidence to confirm the nature of the process. It’s not even known whether a single process accounts for most of the gamma-ray flux, or instead different processes are responsible for the lower and higher energy portions.

Whatever is happening in the Crab Nebula, it’s of more interest than simply a curiosity of that object. There could be some connection with the still unknown mechanism in which ultra-high-energy cosmic rays are produced. There might even be new phenomena, such as a minute dependence of the speed of the highest-energy photons on their energy.

Further reading:

Crab pulsar beams most energetic gamma rays ever detected from a pulsar

Crab Pulsar emits light at highest energies ever detected in a pulsar system, scientists report

Astrophysicists spot pulsed radiation from Crab Nebula that wasn’t supposed to be there

Crab pulsar dazzles astronomers with its gamma-ray beams

Crab pulsar beams most energetic gamma rays ever detected from a pulsar

Star packs big gamma-ray jolt, researchers discover

Detection of Pulsed Gamma Rays Above 100 GeV from the Crab Pulsar