New observations of Crab Nebula and pulsar reveal polarised emissions

The view from the gondola at 40 km altitude during a Crab
observation. Credit: SSC

New observations of polarised X-rays from the Crab Nebula and
Pulsar, published today in Scientific Reports, may
help explain sudden flares in the Crab’s X-ray intensity, as
well as provide new data for modeling – and understanding –
the nebula.

Since it was first observed little more than a thousand years
ago, the Crab Nebula has been studied by generations of
astronomers. Yet new observations by researchers in Sweden show
this “cosmic lighthouse” has yet to give up all of its secrets.

The researchers’ observations of polarised X-rays from the Crab
Nebula and Pulsar, published today in Scientific
Reports
, may help explain sudden flares in the Crab’s X-ray
intensity, as well as provide for modeling – and understanding – the nebula.

The polarisation of Crab X-rays reveals how and where they are
produced in the extreme environment of the nebula, says Mark
Pearce, Professor of Physics at KTH Royal Institute of
Technology and lead author of the study.

“Our measurements indicate that the X-rays come from an
organized region in the vicinity of the at the centre of the nebula,” Pearce says.
“Electrons gyrating around magnetic field lines in this region
produce the X-rays. The measurements are made in an unexplored
energy range, so they provide new information which will help
to solve the puzzle of how high energy radiation is generated.”

The PoGO+ gondola developed by SSC containing the X-ray
telescope and pointing system developed by DST Control. The PoGO+
flight team is lined up in the foreground. From left to right:
Jan-Erik Strömberg (DST Control), Nagomi Uchida (Hiroshima Uni.),
Christian Lockowandt (SSC), H.-G. Florén (Stockholm Uni.), Mark
Pearce (KTH), Victor Mikhalev (KTH), Hiromitsu Takahashi
(Hiroshima Uni.), Maxime Chauvin (KTH), Mette Friis (KTH),
Takafumi Kawano (Hiroshima Uni.), Mózsi Kiss (KTH),
Theodor-Adrian Stana (KTH). Credit: Mark Pearce, KTH

In 1054 CE, Chinese astronomers recorded the appearance of a
new bright star on the sky – an event we now refer to as a
supernova, or exploding star. The aftermath of this cataclysmic
event was a rapidly rotating neutron star: the Crab pulsar,
barely 15 km in diameter but with a mass equal to our solar
system’s Sun, surrounded by an expanding nebula of particles
and radiation.

Neutron stars are a kind of ultra-dense zombie sun that forms
when a star exhausts its fuel and collapses upon itself due to
the force of its own gravity. If they were any denser, they’d
be black holes.

Pearce says that while detecting the Crab’s X-rays is routine
business for research satellites, examining the polarisation of
these emissions—that is, the plane in which the radiation waves
oscillate—is new territory.

“Neutron stars are fascinating objects,” Pearce says. “The Crab
pulsar rotates around an axis 30 times per second producing
flashes of X-rays – a sort of cosmic lighthouse. The X-rays
arise from the acceleration of electrons in intense magnetic
fields (10 trillion times stronger than the earth’s magnetic
field), up to energies typically a hundred times higher than
obtainable at the LHC accelerator.”

In the paper published in Scientific Reports on August
10th 2017, new light is shed on the pulsar through novel
measurements conducted by a balloon-borne telescope, PoGO+
(“PoGO plus”), flown at the top of the atmosphere in summer
2016.

New observations of Crab Nebula and pulsar reveal polarised emissions

The flight path of the PoGO+ balloon. Credit: SSC

Just like visible light or radio waves, X-rays are
electromagnetic and they can be polarised, or in other words,
the electric field can oscillate in a specific plane. Usually,
the polarisation cannot be measured by X-ray telescopes, so
researchers miss out on some of the information carried by
these X-ray messengers, Pearce says. The PoGO+ mission was
developed specifically to measure the polarisation of X-rays
from the Crab and other celestial bodies, with the aim of
opening a new observational window on these objects.

Since X-rays are readily absorbed by the earth’s atmosphere,
observations need to take place high in the stratosphere. In
the early hours of July 12 2016, an enormous, 1.1
million-cubic-metre helium balloon carrying a specially built
telescope was released from the SSC Esrange Space Centre, near
Kiruna in northern Sweden, to do just that.

The PoGO measurements are the first-made in the so-called “hard
X-ray” band, covering the energy range 20-160 keV, and provide
new data for Crab modeling. Results from the PoGO missions are
the first from a dedicated X-ray polarimetry mission in more
than 40 years. PoGO+ reveals that a relatively high fraction,
21 percent, of Crab X-rays are polarised even though
observations encompassed both the pulsar and topologically
complex nebula.

Pearce says that this indicates the X-rays originate from a
compact region with a well-ordered magnetic field. “The angle
of the polarisation plane is aligned to the rotation axis of
the pulsar, as expected for electrons which generate X-rays
through synchrotron processes while trapped in toroidal
trajectories around the pulsar,” he says. “By accurately
determining the arrival time of X-rays, PoGO+ was able to
distinguish between X-rays which originate from the nebula and
pulsar.”

The overall emission was found to be dominated by the nebula.
Comparing the measured nebula polarisation angle with that
measured at optical wavelengths also indicates that the
emission site is associated with the torus – a donut shaped
luminous structure in the inner part of the . Pearce says that the lower polarisation
angle seen for the pulsar is in line with results at optical
wavelengths – an important confirmation that these more
straight-forward measurements are a reasonable proxy for X-ray
models.The PoGO+ polarisation results are compatible with those
obtained in 2013 from the PoGOLite Pathfinder.

The consistency between these results may help to elucidate the
cause of sudden increases in the Crab X-ray intensity which
were recently observed. Such flares were unexpected for an
object which was long considered to be a celestial standard
candle for X-rays.

Explore further:

Observatories combine to crack open the Crab Nebula

More information: M. Chauvin et al. Shedding new light
on the Crab with polarized X-rays, Scientific Reports
(2017). DOI: 10.1038/s41598-017-07390-7

Journal reference: Scientific
Reports

Provided by:
KTH Royal Institute of Technology

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