'Deep impact' of pulsar around companion star (Forwarded)

ESA News
http://www.esa.int

28 February 2006

'Deep impact' of pulsar around companion star

Astronomers have witnessed a never-seen-before event in observations by
ESA's XMM-Newton spacecraft -- a collision between a pulsar and a ring of
gas around a neighbouring star.

The rare passage, which took the pulsar plunging into and through this
ring, illuminated the sky in gamma- and X-rays.

It has revealed a remarkable new insight into the origin and content of
'pulsar winds', which has been a long-standing mystery. The scientists
described the event as a natural but 'scaled-up' version of the well-known
Deep Impact satellite collision with Comet Tempel 1.

Their final analysis is based on a new observation from XMM-Newton and a
multitude of archived data which will lead to a better understanding of
what drives well-known 'pulsar nebulae', such as the colourful Crab and
Vela pulsars.

"Despite countless observations, the physics of pulsar winds have remained
an enigma," said lead author Masha Chernyakova, of the Integral Science
Data Centre, Versoix, Switzerland.

"Here we had the rare opportunity to see pulsar wind clashing with stellar
wind. It is analogous to smashing something open to see what's inside."

A pulsar is a fast-spinning core of a collapsed star that was once about
10 to 25 times more massive than our Sun. The dense core contains about a
solar mass compacted in a sphere about 20 kilometres across.

The pulsar in this observation, called PSR B1259-63, is a radio pulsar,
which means most of the time it emits only radio waves. The binary system
lies in the general direction of the Southern Cross about 5000 light-years
away.

Pulsar wind comprises material flung away from the pulsar. There is
ongoing debate about how energetic the winds are and whether these winds
consist of protons or electrons. What Chernyakova's team has found,
although surprising, ties in neatly with other recent observations.

The team observed PSR B1259-63 orbiting a 'Be' star named SS 2883, which
is bright and visible to amateur astronomers. 'Be' stars, so named because
of certain spectral characteristics, tend to be a few times more massive
than our Sun and rotate at astonishing speeds.

They rotate so fast that their equatorial region bulges and they become
flattened spheres. Gas is consistently flung off such a star and settles
into an equatorial ring around the star, with an appearance somewhat
similar to the planet Saturn and its rings.

The pulsar plunges into the Be star's ring twice during its 3.4-year
elliptical orbit; but the plunges are only a few months apart, just before
and after 'periastron', the point when the two objects in orbit are
closest to each other. It is during the plunges that X-rays and gamma rays
are emitted, and XMM-Newton detects the X-rays.

"For most of the 3.4-year orbit, both sources are relatively dim in X-rays
and it is not possible to identify characteristics in the pulsar wind,"
said co-author Andrii Neronov. "As the two objects draw closer together,
sparks begin to fly."

The new XMM-Newton data was collected nearly simultaneously with a HESS
observation. HESS, the High Energy Stereoscopic System, is a new
ground-based gamma-ray telescope in Namibia.

Announced last year, the HESS observation was puzzling in that the
gamma-ray emission fell to a minimum at periastron and had two maximums,
just before and after the periastron, the opposite of what scientists were
expecting.

The XMM-Newton observation supports the HESS observation by showing how
the maximums were generated by the double plunging into the Be star's
ring. By combining these two observations with radio observations from the
last periastron event, the scientists now have a complete picture of this
system.

Tracing the rise and fall of X-rays and gamma rays day after day as the
pulsar dug through the Be star's disk, the scientists could conclude that
the wind of electrons at an energy level of 10-100 MeV is responsible for
the observed X-ray light. (1 MeV represents one million electron volts.)

Although 10-100 MeV is energetic, this is about 1000 times less than the
expected energy level of 100 TeV. Even more puzzling is the multi-TeV
gamma-ray emission, which, although surely emanating from the 10-100 TeV
wind electrons, seems to be produced differently to how it was thought
before.

"The only fact that is crystal clear at the moment is that this is the
pulsar system to watch if we want to understand pulsar winds," said
Chernyakova.

"Never have we seen pulsar wind in such detail. We are continuing with
theoretical models now. We have some good explanation of the
radio-to-TeV-gamma-ray behaviour of this funny system, but it is still
'under construction.' "

Notes to editors:

A team led by Dr Masha Chernyakova of the Integral Science Data Centre,
Versoix, Switzerland, discusses these results in an article in the
scientific journal Monthly Notices of the Royal Astronomical Society.

Chernyakova's co-authors are Andrii Neronov (also ISDC, Versoix,
Switzerland), Alexander Lutovinov (IKI, Moscow, Russia), Jerome Rodriguez
(CEA Saclay, Gif sur Yvette, France), and Simon Johnston (ATNF, Epping,
Australia).

The paper is at
http://xxx.lanl.gov/abs/astro-ph/0601241

For more information:

Masha Chernyakova, Integral Science Data Centre, Versoix, Switzerland
Tel: +41 22 379 21 50 16
E-mail: Masha.Chernyakova @ obs.unige.ch

Norbert Schartel, ESA XMM-Newton Project Scientist, ESAC, Madrid
E-mail: norbert.schartel @ sciops.esa.int

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IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMK6HMVGJE_index_1.html]
Animation showing X-ray emission from pulsar and star's ring

The pulsar PSR B1259-63 plunges into the ring of Be star SS 2883 twice
during its 3.4-year elliptical orbit. The plunges are only a few months
apart, just before and after periastron, the point when the two objects in
orbit are closest to each other.

It is during the plunges that X-rays and gamma rays are emitted. The
XMM-Newton observatory detects X-rays as seen on the background image.

Credits: ESA

[Image 2:
http://www.esa.int/esaCP/SEMK6HMVGJE...html#subhead1]
Pulsar 1259-63 orbits a star (SS 2883) which is bright and visible to
amateur astronomers

Pulsar PSR B1259-63 is a radio pulsar, which means most of the time it
emits only radio waves. The binary system lies in the general direction of
the Southern Cross about 5000 light-years away.

PSR B1259-63 orbits a Be¹ star named SS 2883, which is bright and visible
to amateur astronomers, seen here in this image. Be¹ stars, so named
because of certain spectral characteristics, tend to be a few times more
massive than our Sun and rotate at astonishing speeds.

They rotate so fast that their equatorial region bulges and they become
flattened spheres, and some fling off gas which settles into an equatorial
ring around the star.

The pulsar plunges into the Be star¹s ring twice during its 3.4-year
elliptical orbit and it is during the plunges that X-rays and gamma rays
are emitted.

Credits: UK PPARC, Nederlandse Organisatie voor Wetenschappelijk
Onderzoek, Instituto de Astrofísica de Canarias

[Image 3:
http://www.esa.int/esaCP/SEMK6HMVGJE...html#subhead2]
Artist's impression of XMM-Newton

ESA's XMM-Newton is the most sensitive X-ray telescope ever built. Its
high-technology design uses over 170 wafer-thin cylindrical mirrors spread
over three telescopes.

Its orbit takes it almost a third of the way to the Moon, so that
astronomers can enjoy long, uninterrupted views of celestial objects.

This unique X-ray observatory was launched by Ariane 5 from the European
spaceport at Kourou in French Guiana on 10 December 1999. It derives its
name from its X-ray multi-mirror design and honours Sir Isaac Newton.

Credits: ESA