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TOPIC: Anomalous X-ray Pulsars


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RE: Anomalous X-ray Pulsars
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Title: AXPs/SGRs: Magnetars or Quark-stars?
spectra for a population of stars of 0.8 to 2 solar masses and orbital periods of less then 20 days
Authors: Renxin Xu (PKU)

The magnetar model and a solid quark star model for anomalous X-ray pulsars/soft gamma-ray repeaters (AXPs/SGRs) are discussed. Different pulsar-like stars, whose actions manifest diversely, are speculated to be due to both their nature (e.g., mass and strain) and their nurture (ambience) in the solid quark star scenario. Relevant arguments made by the author's group, including a debate on solid cold quark matter, are briefly summarised too.

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Title: High frequency oscillations during magnetar flares
Authors: Anna L. Watts (MPA Garching), Tod E. Strohmayer (NASA GSFC)

The recent discovery of high frequency oscillations during giant flares from the Soft Gamma Repeaters SGR 1806-20 and SGR 1900+14 may be the first direct detection of vibrations in a neutron star crust. If this interpretation is correct it offers a novel means of testing the neutron star equation of state, crustal breaking strain, and magnetic field configuration. We review the observational data on the magnetar oscillations, including new timing analysis of the SGR 1806-20 giant flare using data from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Rossi X-ray Timing Explorer (RXTE). We discuss the implications for the study of neutron star structure and crust thickness, and outline areas for future investigation.

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XTEJ1810-197
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Astronomers using CSIRO's Parkes telescope in eastern Australia have detected radio "heartbeats" from a star that was not expected to have them. A US-Australian research team found that a "magnetar" -- a kind of star with the strongest magnetic fields known in the Universe -- is giving off extraordinary radio pulses, which links this rare type of star with the much more common "radio pulsars."

The findings will be published in the journal Nature on 24 August, and are also being presented at the International Astronomical Union General Assembly taking place in Prague (14-25 August).
The research team, led by Dr Fernando Camilo of Columbia University in New York, includes staff of the CSIRO Australia Telescope National Facility and the US National Radio Astronomy Observatory. The discovery observations were made on 17 March 2006 by CSIRO scientist John Sarkissian. Further observations at Parkes were made by the Observatory's officer-in-charge, John Reynolds.

"We hoped to detect a radio pulse if we were lucky. But we were genuinely surprised at how strong it actually was" - John Sarkissian.

Dr. Reynolds says the unexpected strength of the pulsar puts it in a category of its own.

"The pulsar was so strong we could easily see and hear individual pulses of emission at the discovery frequency, which is rare enough. But we were stunned to find that as we tuned to higher and higher frequencies the single pulses kept booming in" - Dr John Reynolds.

The object in question is a neutron star – a small star made of extremely dense "neutron matter" – called XTE J1810-197. It lies about 10,000 light-years away in the constellation Sagittarius. The Parkes observations found it to be emitting radio pulses at every turn of the star, or every 5.54 seconds. These pulses have now been confirmed and studied with other telescopes in Australia, the USA and Europe. Radio pulsars are neutron stars that put out regular pulses of radio waves. In almost all cases these pulses are easiest to detect at low frequencies (long radio wavelengths), and get fainter and much harder to detect at higher frequencies (short wavelengths).

"But this object is extraordinary. Its brightness is essentially the same over a factor of 100 in frequency. For wavelengths less than about a centimetre, it is brighter than every other known neutron star" - Dr Fernando Camilo.

XTE J1810-197 was discovered in 2003 as an X-ray source and is one of a handful of unusual objects called "anomalous X-ray pulsars" or AXPs: slowly rotating neutron stars with bright and variable pulsing X-ray emission.
Debate raged for many years over the nature of AXPs. They are now thought to be magnetars, of which only a dozen are known in our Galaxy – very young neutron stars with magnetic fields a hundred million million times stronger than Earth's (10exp14 gauss, as compared with the Earth's 0.5 gauss).
Radio pulsars are another, much more common, type of neutron star. More than 1700 are known. Their magnetic fields, while strong by terrestrial standards, are typically about 100 times weaker than those of magnetars. Radio pulsars also generally spin much faster than magnetars.
Because the physical conditions in the "atmosphere" of magnetars are very different from those in normal pulsars, it was not clear whether magnetars should emit radio waves.

"Clearly we've found that you can get radio emission from a magnetar, but whether any models for it are correct in detail remains to be seen. In any case, this discovery connects the rare magnetars to the much more common radio pulsars, and helps put some order and understanding into the zoo of neutron stars"- Dr Fernando Camilo.

But much is still unexplained.

"The brightness of the radio emission detected from XTE J1810-197 varies day-by-day in a way that is inconsistent with what we know about ordinary pulsars" - Scott Ransom, Co-author, NRAO.

While XTE J1810-197 was born a few thousand years ago, it became visible only in early 2003, when it produced a bright outburst of X-rays. Archival X-ray data from the previous 24 years shows no such strong emission.
Following the 2003 outburst, the Very Large Array telescope in the USA detected radio emission from the source in January 2004. The Parkes observations showed that this emission was, in fact, pulsed.
Archived Parkes observations from the late 1990s don't reveal any radio sources in the vicinity of the magnetar. The radio emission was probably turned on by the X-ray outburst of 2003.
The X-ray brightness of the magnetar is decreasing rapidly, and within the next year it should fade to pre-2003 levels. The same will probably happen to the radio emission.

"[I]We have no idea whether this will happen in six months or 50 years"- Dr Fernando Camilo.

Source CSIRO Australia

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Neutron Stars
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A controversial new analysis proposes that neutron stars can get a second "lease on life" by passing through clouds of stellar gas, and begin to glow at X-ray wavelengths when gas falls onto them.

These solitary stars may account for hundreds of mysterious X-ray sources near our galaxy's centre that had previously been attributed to pairs of stars.

"There has been a lot of debate about the nature of these sources" - Shuang Nan Zhang of Tsinghua University in Beijing, China.

He and colleagues propose that this can account for most of the 800 unidentified X-ray emitting objects near the galaxy's centre that were detected by NASA's Chandra X-ray Observatory in 2001.

The research will be published in an upcoming issue of the Chinese Journal of Astronomy.

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Anomalous X-ray Pulsars
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Anomalous X-ray pulsars
Tiny stellar 'corpses' have been caught blasting surprisingly powerful X-rays and gamma rays across our galaxy by ESA’s gamma-ray observatory Integral.

This discovery links these objects to the most magnetically active bodies in the Universe and forces scientists to reconsider just how dead such stellar corpses really are.
Known as anomalous X-ray pulsars (AXPs), the stellar corpses were first spotted pulsing low-energy X-rays into space during the 1970s by the Uhuru X-ray satellite. AXPs are extremely rare with only seven known to exist. The X-rays were first thought to be produced by matter falling from a companion star onto the AXP.
An alternative was that each AXP is the spinning core of a dead star, known as a neutron star, sweeping beams of energy through space like a cosmic lighthouse. When these beams cross Earth’s line of sight, the AXP blinks on and off.

However, this scenario required the AXP’s magnetic field to be a thousand million times stronger than the strongest steady magnetic field achievable in a laboratory on Earth. Nevertheless, the Integral observations show that the magnetic solution is correct.
The newly detected emission, known to astronomers as a ‘hard tail’, of high-energy (‘hard’) X-rays and gamma rays also comes in the form of regular pulses every 6–12 seconds depending upon which AXP is observed.
Discovered in three of the four AXPs studied, the hard tails have a distinctive energy signature that forces astronomers to consider that they are produced by super-strong magnetic fields.

"The amount of energy in the hard tail is ten to almost one thousand times more than can be explained by a kind of magnetic friction between the spinning AXP and surrounding space" - Wim Hermsen of SRON, the Netherlands Institute for Space Research, Utrecht, who together with SRON colleagues made the observations. This leaves so-called 'magnetic field decay' as the only viable alternative.

Neutron stars with super-strong magnetic fields are dubbed ‘magnetars’. Created from the core of a gigantic star that has exploded at the end of its life, each magnetar is only around 15 kilometres in diameter yet contains more than one and a half times the mass of the Sun.
Magnetars are also responsible for the ‘soft gamma-ray repeaters’ (SGRs), which explosively release massive quantities of energy when catastrophic reorganisations of their magnetic fields spontaneously take place. The big difference between an SGR and an AXP is that the process is continuous rather than explosive in an AXP and less energetic.

"Somehow these objects are tapping the enormous magnetic energy contained beneath their surfaces and funnelling it into space" - Wim Hermsen .

Exactly how that happens is the focus of future work. It is possible that SGRs, of which five are known, turn into AXPs once they have exploded enough of their energy into space.
All known AXPs except one are clustered towards the plane of our galaxy, the Milky Way, indicating that they are the result of recent stellar explosions; some are even wreathed in the exploded gaseous remnants of their former stars.
The other known AXP is in a satellite galaxy of the Milky Way. The hard tails were discovered by Integral serendipitously, thanks to its unique wide-field camera, the Imager on-Board Integral Satellite (IBIS).

"This is one of the things you hope for when you run an observatory like Integral" - Christoph Winkler, ESA’s Integral project scientist.

As the AXPs prove, the stellar afterlife is more alive than astronomers once thought.

Source

Integral Mission

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