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
1012 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

ESO
PR Photo 23a/00

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[Full-Res – JPEG: 1975 x 2340 pix –
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ESO PR Photo 23b/00

ESO
PR Photo 23b/00

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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 arcmin2, 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
arcsec2. 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 1015
g/cm3. 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: M.H.vanKerkwijk@astro.uu.nl