Subscribe free to our newsletters via your
. 24/7 Space News .

Baffling pulsar leaves astronomers in the dark
by Staff Writers
Paris (ESA) Jan 28, 2013

Artist's impression of a pulsar in radio-bright mode. Credit: ESA/ATG medialab.

New observations of a highly variable pulsar using ESA's XMM-Newton are perplexing astronomers. Monitoring this pulsar simultaneously in X-rays and radio waves, astronomers have revealed that this source, whose radio emission is known to 'switch on and off' periodically, exhibits the same behaviour, but in reverse, when observed at X-ray wavelengths.

It is the first time that a switching X-ray emission has been detected from a pulsar, and the properties of this emission are unexpectedly puzzling. As no current model is able to explain this switching behaviour, which occurs within only a few seconds, these observations have reopened the debate about the physical mechanisms powering the emission from pulsars

Few classes of astronomical objects are as baffling as pulsars - which were discovered as flickering sources of radio waves and soon after interpreted as rapidly rotating and strongly magnetised neutron stars. Even though about 2000 pulsars have been found since the first was discovered in 1967, a detailed understanding of the mechanisms that power them still eludes astronomers.

"There is a general agreement about the origin of the radio emission from pulsars: it is caused by highly energetic electrons, positrons and ions moving along the field lines of the pulsar's magnetic field, and we see it pulsate because the rotation and magnetic axes are misaligned," explains Wim Hermsen from SRON, the Netherlands Institute for Space Research in Utrecht, The Netherlands.

"How exactly the particles are stripped off the neutron star's surface and accelerated to such high energy, however, is still largely unclear," he adds.

Hermsen led a new study based on observations of the pulsar known as PSR B0943+10, which were performed simultaneously in X-rays, with ESA's XMM-Newton, and in radio waves.

By probing the emission from the pulsar at different wavelengths, the study had been designed to discern which of various possible physical processes take place in the vicinity of the magnetic poles of pulsars. Instead of narrowing down the possible mechanisms suggested by theory, however, the results of Hermsen's observing campaign challenge all existing models for pulsar emission, reopening the question of how these intriguing sources are powered.

"Many pulsars have a rather erratic behaviour: in the space of a few seconds, their emission becomes weaker or even disappears for a while, just to go back to the previous level after some hours," says Hermsen.

"We do not know what causes such a switch, but the fact that the pulsar keeps memory of its previous state and goes back to it suggests that it must be something fundamental."

Recent studies indicate that the switch between what are usually referred to as 'radio-bright' and 'radio-quiet' states is correlated to the pulsar's dynamics. As pulsars rotate, their spinning period slows down gradually, and in some cases the slow-down process has been observed to accelerate and slow down again, in conjunction with the pulsar switching between radio-bright and quiet states.

The existence of correlated variations in both the rotation and emission suggest a connection between a pulsar's immediate vicinity and, on a grander scale, its co-rotating magnetosphere, which may extend up to about 50 000 km for objects like PSR B0943+10. In order for the radio emission to vary so radically on the short timescales observed, the pulsar's global environment must undergo a very rapid - and reversible - transformation.

"Since the switch between a pulsar's bright and quiet states links phenomena that occur on local and global scales, a thorough understanding of this process could clarify several aspects of pulsar physics. Unfortunately, we have not yet been able to explain it," says Hermsen.

Hermsen and his colleagues planned to search for an analogous pattern at a different wavelength - in X-rays - to investigate what causes this switching behaviour. They chose as their subject PSR B0943+10, a pulsar that is well known for its switching behaviour at radio wavelengths and for its X-ray emission, which is brighter than might be expected for its age.

"Young pulsars shine brightly in X-rays because the surface of the neutron star is still very hot. But PSR B0943+10 is five million years old, which is relatively old for a pulsar: the neutron star's surface has cooled down by then," explains Hermsen.

Astronomers know of only a handful of old pulsars that shine in X-rays and believe that this emission comes from the magnetic poles - the sites on the neutron star's surface where the acceleration of charged particles is triggered. "We think that, from the polar caps, accelerated particles either move outwards to the magnetosphere, where they produce radio emission, or inwards, bombarding the polar caps and creating X-ray emitting hot-spots," Hermsen adds.

There are two main models that describe these processes, depending on whether the electric and magnetic fields at play allow charged particles to escape freely from the neutron star's surface. In both cases, it is believed that the emission of X-rays follows that of radio waves, but the emission that is observed in each scenario is characterised by different temporal and spectral characteristics. Monitoring the pulsar in X-rays and radio waves at the same time, the astronomers hoped to be able to discern between the two models.

Obtaining observing time on the requested telescopes turned out to be a rather lengthy procedure. "We needed very long observations, to be sure that we would record the pulsar switching back and forth between bright and quiet states several times," says Hermsen, "So we asked for a total of 36 hours of observation with XMM-Newton. This is quite a lot of time, and it took us five years before our proposal was accepted."

The observations were performed in late 2011. The X-ray monitoring performed with XMM-Newton was accompanied by simultaneous observations at radio waves from the Giant Metrewave Radio Telescope (GMRT) in India and the recently inaugurated Low Frequency Array (LOFAR) in the Netherlands, which was used during its commissioning phase, while testing its science operations.

"The X-ray emission of pulsar PSR B0943+10 beautifully mirrors the switches that are seen at radio wavelengths but, to our surprise, the correlation between these two emissions appears to be inverse: when the source is at its brightest in radio waves, it reaches its faintest in X-rays, and vice versa," says Hermsen.

The XMM-Newton data also show that the source pulsates in X-rays only during the X-ray-bright phase - which corresponds to the quiet state at radio wavelengths. During this phase, the X-ray emission appears to be the sum of two components: a pulsating component consisting of thermal X-rays, which is seen to switch off during the X-ray-quiet phase, and a persistent one consisting of non-thermal X-rays. Neither of the leading models for pulsar emission predicts such behaviour.

"The data collected during our monitoring campaign are truly challenging our understanding of pulsars, since no current model is able to explain them," comments Hermsen. "In the second half of 2013, we plan to repeat the same study for another pulsar, PSR B1822-09, which exhibits similar radio emission properties, but is characterised by a different geometrical configuration. This will allow us to study these extreme objects under different viewing angles," he adds.

In the meantime, these observations will keep theoretical astrophysicists busy investigating possible physical mechanisms that could cause the sudden and drastic changes to the pulsar's entire magnetosphere and result in such a curious emission.

"The unpredictable behaviour of this pulsar, revealed using the great sensitivity of the telescopes on board XMM-Newton, may require a radically new approach to study the fundamental processes that power these fascinating objects," comments Norbert Schartel, XMM-Newton Project Scientist at ESA.

The study presented here is based on X-ray observations of pulsar PSR B0943+10 performed with ESA's XMM-Newton between 4 November and 4 December 2011. These observations consisted of six observations in the energy range between 0.2 and 10 keV, each lasting six hours. The X-ray data were gathered at the same time as observations at radio wavelengths performed with the Indian Giant Metrewave Radio Telescope (GMRT) at 320 MHz and the international Low Frequency Array (LOFAR) at 140 MHz.

The research was led by Wim Hermsen (SRON Netherlands Institute for Space Research and Astronomical Institute 'Anton Pannekoek', University of Amsterdam), Lucien Kuiper and Jelle de Plaa (SRON Netherlands Institute for Space Research), Jason Hessels and Joeri van Leeuwen (ASTRON and Astronomical Institute 'Anton Pannekoek', University of Amsterdam), Dipanjan Mitra and Rahul Basu (NCFRA-TIFR, Pune, India), Joanna Rankin (University of Vermont, Burlington, USA), Ben Stappers (University of Manchester, UK), and Geoffrey Wright (University of Sussex, UK). The Pulsar Working Group and the Builders Group from the LOFAR-telescope, which was at the time in the commissioning phase, supported the observations. Related publications: W. Hermsen, et al., "Synchronous X-ray and Radio Mode Switches: a Rapid Global Transformation of the Pulsar Magnetosphere", 2013, Science, 339, 6118, 436-439, DOI:10.1126/science.1230960


Related Links
XMM-Newton at ESA
Stellar Chemistry, The Universe And All Within It

Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks DiggDigg RedditReddit GoogleGoogle

Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News

Betelgeuse braces for a collision
Paris (ESA) Jan 25, 2013
Multiple arcs are revealed around Betelgeuse, the nearest red supergiant star to Earth, in this new image from ESA's Herschel space observatory. The star and its arc-shaped shields could collide with an intriguing dusty 'wall' in 5000 years. Betelgeuse rides on the shoulder of the constellation Orion the Hunter. It can easily be seen with the naked eye in the northern hemisphere winter nig ... read more

US, Europe team up for moon fly-by

Russia to Launch Lunar Mission in 2015

US, Europe team up for moon fly-by

Mission would drag asteroid to the moon

Is there life on Mars?

Opportunity At Work At Whitewater Lake

Thawing Dry Ice Drives Groovy Action On Mars

Mars Rover Curiosity Uses Arm Camera at Night

How to predict the future of technology

Iran Manufacturing Hi-Tech Spacesuits

TDRS-K Offers Upgrade to Vital Communications Net

An Astronaut's Guide

Reshuffle for Tiangong

China to launch 20 spacecrafts in 2013

Mr Xi in Space

China plans manned space launch in 2013: state media

NASA to Send Inflatable Pod to International Space Station

ISS to get inflatable module

ESA workhorse to power NASA's Orion spacecraft

Competition Hopes To Fine Tune ISS Solar Array Shadowing

First Ariane 5 For 2013 Ready For Loading

Azerspace And Africasat-1a "fit" for Ariane 5 launch

NASA Selects Experimental Commercial Suborbital Flight Payloads

Payload elements come together in Starsem's wrap-up Soyuz mission from Baikonur Cosmodrome for Globalstar

New Evidence Indicates Auroras Occur Outside Our Solar System

Glitch has space telescope shut down

Earth-size planets common in galaxy

NASA's Hubble Reveals Rogue Planetary Orbit For Fomalhaut B

Supercomputer sets computing record

New information on binding gold particles over metal oxide surfaces

Researchers Create Method for More Sensitive Electrochemical Sensors

Phoenix Rising: New Video Shows Advances in Satellite Repurposing Program

The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement