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Information from the European Southern
Observatory

ESO Press Release 19/00

11 September 2000

For immediate release

The Mystery of the Lonely Neutron
Star

The VLT Reveals Bowshock Nebula around
RX J1856.5-3754

Deep inside the Milky Way, an old and lonely neutron star
plows its way through interstellar space. Known as RX J1856.5-3754 , it measures only ~ 20 km
across. Although it is unusually hot for its age, about
700,000 °C, earlier observations did not reveal any activity
at all, contrary to all other neutron stars known so far.

In order to better understand this extreme type of object,
a detailed study of RX J1856.5-3754 was
undertaken by Marten van Kerkwijk (Institute of
Astronomy of the University of Utrecht, The Netherlands) and
Shri Kulkarni (California Institute of Technology,
Pasadena, California, USA).

To the astronomers' delight and surprise, images and
spectra obtained with the ESO Very Large Telescope (VLT) now
show a small nearby cone-shaped ("bowshock") nebula. It shines
in the light from hydrogen atoms and is obviously a product of
some kind of interaction with this strange star.

Neutron stars - remnants of supernova
explosions

Neutron stars are among the most extreme
objects in the Universe. They are formed when a massive star
dies in a "supernova explosion" . During
this dramatic event, the core of the star suddenly collapses
under its own weight and the outer parts are violently ejected
into surrounding space.

One of the best known examples is the Crab
Nebula in the constellation Taurus (The Bull). It is the
gaseous remnant of a star that exploded in the year 1054 and
also left behind a pulsar , i.e., a rotating neutron
star [1].

A supernova explosion is a very complex event that is still
not well understood. Nor is the structure of a neutron star
known in any detail. It depends on the extreme properties of
matter that has been compressed to incredibly high densities,
far beyond the reach of physics experiments on Earth [2].

The ultimate fate of a neutron star is also unclear. From
the observed rates of supernova explosions in other galaxies,
it appears that several hundred million neutron stars must
have formed in our own galaxy, the Milky Way. However, most of
these are now invisible, having since long cooled down and
become completely inactive while fading out of sight.

An unsual neutron star - RX J1856.5-3754

Some years ago, the X-ray source RX
J1856.5-3754 was found by the German ROSAT X-ray satellite observatory. Later
observations with the Hubble Space Telescope
(cf. STScI-PR97-32
) detected extremely faint optical emission from this source
and conclusively proved that it is an isolated
neutron star [3].

There is no sign of the associated supernova remnant and it
must therefore be at least 100,000 years "old". Most
interestingly, and unlike younger isolated neutron stars or
neutron stars in binary stellar systems, RX
J1856.5-3754 does not show any sign of activity
whatsoever, such as variability or pulsations.

As a unique member of its class, RX
J1856.5-3754 quickly became the centre of great interest
among astronomers. It apparently presented the first, very
welcome opportunity to perform detailed studies of the
structure of a neutron star, without the disturbing influence
of ill-understood activity.

One particular question arose immediately. The emission of
X-rays indicates a very high temperature of RX
J1856.5-3754 . However, from the moment of their violent
birth, neutron stars are thought to lose energy and to cool
down continuously. But then, how can an old neutron star like
this one be so hot?

One possible explanation is that some interstellar
material, gas and/or dust grains, is being captured by its
strong gravitational field. Such particles would fall freely
towards the surface of the neutron star and arrive there with
about half the speed of light. Since the kinetic energy
of these particles is proportionate to the second power of
the velocity, even small amounts of matter would deposit much
energy upon impact, thereby heating the neutron star.

The spectrum of RX J1856.5-3754

The new VLT study by van Kerkwijk and Kulkarni
of RX J1856.5-3754 was first aimed at
taking optical spectra, in order to study its structure. The
astronomers hoped to find in its spectrum some "signatures",
i.e., emission or absorption lines and/or bands, that might
provide information about the physical conditions on its
surface.

While the chances for this were admittedly rather slim, a
detection of such spectral features would be a real
break-through in the study of neutron stars. If present in the
spectrum, they could for instance be used to measure directly
the immense strength of the gravitational field
on the surface, expected to be about 10 12 times stronger
than that on the surface of the Earth. Moreover, it might be
possible to determine the gravitational
redshift , a relativistic effect whereby the light quanta
(photons) that are emitted from the surface lose about 20% of
their energy as they escape from the neutron star. Their
wavelength is consequently red-shifted by that amount.

The spectral observations were difficult, first of all
because of the extreme faintness of RX
J1856.5-3754 . But even though an excellent spectrum was
obtained with the multi-mode FORS1 instrument at VLT ANTU, it
was indeed quite featureless and no spectral features were
seen.

Surprises from RX J1856.5-3754

Nevertheless, as it often happens in astronomy, these
observations did bring surprises. The first was that the
neutron star had obviously moved on the sky since the HST had
observed it in 1997. From positional measurements and the
assumed distance, approx. 200 light-years, RX
J1856.5-3754 was found to be moving with a velocity of
about 100 km/s [4].
However, at such a high speed, it is hard to imagine how it
would be able to catch much interstellar matter, whose infall
might heat the surface as described above. The puzzle was
deepening!

Another surprise was that the spectra showed very faint
emission from the neighbourhood of the neutron star. The
measured wavelengths identified these emission lines as
H-alpha and H-beta , two of the so-called Balmer
lines that originate in hydrogen atoms. Most likely, the
strong radiation from the very hot surface of the neutron star
is ionizing hydrogen atoms (separating them in a proton and an
electron) in the surroundings, a process that also takes place
near very hot, normal stars. The observed emission is then
produced when, at a later time, the protons and electrons
again (re)combine into hydrogen atoms.

Interestingly, a simple estimate of the hydrogen density
near the neutron star that is needed to produce the observed
glow indicates the presence of about one hundred hydrogen
atoms per cubic centimetre. This is no less than one hundred
times the usual density in the interstellar medium. So maybe
the surface of RX J1856.5-3754 could still
be heated by infalling hydrogen atoms?

VLT images of the RX J1856.5-3754 region

With the inferred hydrogen density near the neutron star,
about one thousand years on the average will elapse between
the moment of ionization by the passing neutron star and the
subsequent re-unification of a proton with an electron to form
a hydrogen atom.

During this time, however, the fast-moving neutron star
will have covered a substantial distance. For this reason, it
is expected that much of the hydrogen emission will not be
seen very close to the neutron star, but rather along its
"recent" trajectory in space.

ESO PR Photo 23a/00

[Preview
- JPEG: 400 x 474 pix - 192k]
[Normal
- JPEG: 800 x 948 pix - 622k]
[Full-Res
- JPEG: 1975 x 2340 pix - 2.2Mb]

ESO PR Photo 23b/00

[Preview
- JPEG: 400 x 472 pix - 184k]
[Normal
- JPEG: 800 x 944 pix - 424k]

Caption : False-colour
composite photo of the sky field with the lonely neutron
star RX J1856.5-3754 and the related
cone-shaped nebula. It is based on a series of exposures
obtained with the multi-mode FORS2 instrument at VLT
KUEYEN through three different optical filters: R (29
exposures of 136 sec each; ~1.1 hrs total; here rendered
as green); H-alpha (19; 1020 sec; ~5.5 hrs; red); and B
(10; 138 sec; ~0.4 hrs; blue). The seeing was good to
excellent during the exposures (0.66 arcsec on average).
The trails of some moving objects, most likely asteroids
in the solar system, are seen in the field with
intermittent blue, green and red colours. The large
field ( PR Photo 23a/00 ) measures
6.6 x 6.7 arcmin 2 , with 0.2 arcsec/pixel. For clarity,
a smaller area around the neutron star and the cone
("bowshock") nebula has been enlarged in PR Photo 23b/00 . The object is at the
centre of the circle and the neutron star is indicated
with an arrow; the field measures 80 x 80 arcsec 2.
North is to the lower right and East is upper right. The
motion of the neutron star as seen on the sky (see the
text) is towards East, exactly in the direction
indicated by the nebula.

In order to test these ideas, additional observing time was
granted on the VLT to obtain very "deep", direct images that
would attempt to map the hydrogen glow. They were carried out
by ESO staff astronomers at Paranal in "service mode".
Exposures lasting more than five hours in total were taken
through a narrow optical filter that isolates the H-alpha
hydrogen emission. In addition, shorter exposures were taken
through B(lue) and R(ed) filters. The exposures have been
combined into the false-colour PR Photos
23a-b/00 .

Legions of stars are seen in the photos. This is partly
because of the extraordinary light sensitivity of the VLT, and
partly because a star-forming region is located in this
direction. Stars like our Sun appear whitish, relatively cool
stars emit little blue light and appear more reddish, while
hot stars appear blue.

The photos clearly show a lot of diffuse light, especially
in the lower left area. This is most likely starlight
reflected off interstellar dust grains.

The cone-shaped nebula near RX
J1856.5-3754

A small area, just a little above and to the right of the
centre of PR Photo 23a/00 , has been
enlarged in PR Photo 23b/00 . It shows a small, cone-shaped nebula never seen before -
this is the emission from hydrogen atoms near the neutron star
RX J1856.5-3754 . The star itself is the
very faint, blue object very close to the top of the cone.

The shape of the cone is like that of a "bowshock" from a
ship, plowing through water. Similarly shaped cones have been
found around fast-moving radio pulsars and massive stars, cf.
e.g., ESO
PR 01/97. However, for those objects, the bowshock forms
because of a strong outflow of particles from the star or the
pulsar (a "stellar wind"), that collides with the interstellar
matter.

Because of this analogy, one may think that a "wind" also
blows from RX J1856.5-3754 . However, for
this a new hypothesis would have to be invoked. An
alternative, perhaps more plausible possibility is that when
the surrounding hydrogen atoms are ionized, the resulting
electrons and protons acquire substantial velocities, heating
the interstellar gas near the passing neutron star. The heated
gas expands and pushes aside the surrounding cooler gas. In
the end, this process may lead to a geometrical shape similar
to that caused by a stellar wind.

Whither RX J1856.5-3754?

At present, it is still uncertain whether the observed
density of the surrounding interstellar matter is sufficient
to heat RX J1856.5-3754 to the observed
temperature.

However, it is possible that sometimes in the past the
neutron star managed to collect more matter during its travel
through interstellar space, was heated, and is now slowly
cooling down. In another million years or so, it will become
undetectable, until it happens to pass through another dense
interstellar region. And so on...

Notes

[1]: Images of the Crab
Nebula and its pulsar from VLT KUEYEN and FORS2 are
available in ESO
PR 17/99.

[2]: In fact, a neutron star is like one
big atom with a diameter of 10-20 kilometres, and weighing
about as much as the Sun. The mean density is an unimaginable
10 15 g/cm 3. Thus, a pinhead of neutron star material (1
millimetre across) weighs almost 1 million tons, or about as
much as the largest oil carrier ever built, fully loaded.

[3]: The apparent visual magnitude of RX J1856.5-3754 is 25.6, or nearly 100 million
times fainter than what can be perceived with the unaided eye
in a dark sky.

[4]: The motion of RX
J1856.5-3754 was also found by Frederick M. Walter
(Stony Brook, New York, USA), who also determined the
distance, cf. the corresponding research article that is now available on
the web.

Contact

Dr. Marten van Kerkwijk
Institute of Astronomy

University of Utrecht
The Netherlands

Tel.:
+31-30-253-5203
email: [email-masked]

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ESO Education & Public Relations Department

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Last Update: 18.04.07 ©ESO

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