XMM-Newton's view of pulsar J1826-1256
Credit: ESA/XMM-Newton/J. Li, DESY, Germany
Based on a new theoretical model, a team
of scientists explored the rich data archive of ESA's XMM-Newton and
NASA's Chandra space observatories to find pulsating X-ray emission from
three sources. The discovery, relying on previous gamma-ray
observations of the pulsars, provides a novel tool to investigate the
mysterious mechanisms of pulsar emission, which will be important to
understand these fascinating objects and use them for space navigation
in the future.
Lighthouses of the Universe, pulsars are
fast-rotating neutron stars that emit beams of radiation. As pulsars
rotate and the beams alternatively point towards and away from Earth,
the source oscillates between brighter and dimmer states, resulting in a
signal that appears to 'pulse' every few milliseconds to seconds, with a
regularity rivalling even atomic clocks.
Pulsars are the incredibly dense, extremely magnetic, relics of
massive stars, and are amongst the most extreme objects in the Universe.
Understanding how particles behave in such a strong magnetic field is
fundamental to understanding how matter and magnetic fields interact
more generally.
Originally detected through their radio emission, pulsars are now
known to also emit other types of radiation, though typically in smaller
amounts. Some of this emission is standard thermal radiation – the type
that everything with a temperature above absolute zero emits. Pulsars
release thermal radiation when they accrete matter, for example from
another star.
But pulsars also emit non-thermal radiation, as is often produced in
the most extreme cosmic environments. In pulsars, non-thermal radiation
can be created via two processes: synchrotron emission and curvature
emission. Both processes involve charged particles being accelerated
along magnetic field lines, causing them to radiate light that can vary
in wavelength from radio waves to gamma-rays.
Non-thermal X-rays result mostly from synchrotron emission, while
gamma-rays may come from so-called synchro-curvature emission – a
combination of the two mechanisms. It is relatively easy to find pulsars
that radiate gamma-rays – NASA's Fermi Gamma-Ray Space Telescope has
detected more than 200 of them over the past decade, thanks to its
ability to scan the whole sky. But only around 20 have been found to
pulse in non-thermal X-rays.
"Unlike gamma-ray detecting survey instruments, X-ray telescopes
must be told exactly where to point, so we need to provide them with
some sort of guidance," says Diego Torres, from the Institute of Space Sciences in Barcelona, Spain.
Aware that there should be many pulsars emitting previously
undetected non-thermal X-rays, Torres developed a model that combined
synchrotron and curvature radiation to predict whether pulsars detected
in gamma-rays could also be expected to appear in X-rays.
"Scientific models describe phenomena that can't be experienced directly," explains Torres.
"This model in particular helps explain the emission processes in
pulsars and can be used to predict the X-ray emission that we should
observe, based on the known gamma-ray emission.
"
The model describes the gamma-ray emission of pulsars detected by
Fermi – specifically, the brightness observed at different wavelengths –
and combines this information with three parameters that determine the
pulsar emission. This allows a prediction of their brightness at other
wavelengths, for instance in X-rays.
Torres partnered with a team of scientists, led by Jian Li from the
Deutsches Elektronen Synchrotron in Zeuthen near Berlin, Germany, to
select three known gamma-ray emitting pulsars that they expected, based
on the model, to also shine brightly in X-rays. They dug into the data
archives of ESA's XMM-Newton and NASA's Chandra X-ray observatories to
search for evidence of non-thermal X-ray emission from each of them.
"Not only did we detect X-ray pulsations from all three of the
pulsars, but we also found that the spectrum of X-rays was almost the
same as predicted by the model," explains Li.
"This means that the model very accurately describes the emission processes within a pulsar."
Non-thermal X-ray emission from three pulsars
Credit: Adapted from J. Li et al. (2018)
In particular, XMM-Newton data showed
clear X-ray emission from PSR J1826-1256 – a radio quiet gamma-ray
pulsar with a period of 110.2 milliseconds. The spectrum of light
received from this pulsar was very close to that predicted by the model.
X-ray emission from the other two pulsars, which both rotate slightly
more quickly, was revealed using Chandra data.
This discovery already represents a significant increase in the total
number of pulsars known to emit non-thermal X-rays. The team expects
that many more will be discovered over the next few years as the model
can be used to work out where exactly to look for them.
Finding more X-ray pulsars is important for revealing their global
properties, including population characteristics. A better understanding
of pulsars is also essential for potentially taking advantage of their
accurate timing signals for future space navigation endeavours.
The result is a step towards understanding the relationships between the emission by pulsars in different parts of the electromagnetic spectrum, enabling a robust way to predict the brightness of a pulsar at any given wavelength. This will help us better comprehend the interaction between particles and magnetic fields in pulsars and beyond.
"This model can make accurate predictions of pulsar X-ray
emission, and it can also predict the emission at other wavelengths, for
example visible and ultraviolet," Torres continues.
"In the future, we hope to find new pulsars leading to a better understanding of their global properties."
The study highlights the benefits of XMM-Newton's vast data archive
to make new discoveries and showcases the impressive abilities of the
mission to detect relatively dim sources. The team is also looking
forward to using the next generation of X-ray space telescopes,
including ESA's future Athena mission, to find even more pulsars
emitting non-thermal X-rays.
"As the flagship of European X-ray astronomy, XMM-Newton is
detecting more X-ray sources than any previous satellite. It is amazing
to see that it is helping to solve so many cosmic mysteries," concludes Norbert Schartel, XMM-Newton Project Scientist at ESA.
Notes for Editors
"Theoretically motivated search and detection of non-thermal pulsations from PSRs J1747-2958, J2021+3651, and J1826-1256" by Li et al. is published in Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/aae92bThe prepint is available on the arXiv/astro-ph server (arXiv:1811.08339).
For more information, please contact:
Jian Li
Deutsches Elektronen Synchrotron DESY
Zeuthen, Germany
Email: jian.li@desy.de
Diego Torres
Institute of Space Sciences (ICE, CSIC)
Institut d'Estudis Espacials de Catalunya (IEEC)
Institució Catalana de Recerca i Estudis Avanc¸ats (ICREA)
Barcelona, Spain
Email: dtorres@ice.csic.es
Norbert Schartel
XMM-Newton Project Scientist
European Space Agency
Email: norbert.schartel@esa.int
Source: ESA/XMM-Newton
