Cycle of pulsed gamma rays from the
Vela pulsar.
Pulsars are highly magnetized, rotating
neutron stars that emit a beam of
electromagnetic radiation. The
observed periods of their pulses range from 1.4
milliseconds to 8.5 seconds. The radiation can
only be observed when the beam of emission is pointing towards the
Earth. This is called the lighthouse effect and gives rise to the
pulsed nature that gives pulsars their name. Because neutron stars
are very dense objects, the rotation period and thus the interval
between observed pulses is very regular. For some pulsars, the
regularity of pulsation is as precise as an
atomic clock. A few pulsars are known to have
planets orbiting them, as in the case of
PSR B1257+12.
Werner Becker of the Max Planck Institute for Extraterrestrial
Physics said in 2006, "The theory of how pulsars emit their
radiation is still in its infancy, even after nearly forty years of
work."
Discovery
The first pulsar was observed on November 28, 1967 by
Jocelyn Bell Burnell and
Antony Hewish. Initially baffled as to the
seemingly unnatural regularity of its emissions, they dubbed their
discovery
LGM-1, for "
little green men" (a name for intelligent
beings of extraterrestrial
origin). The hypothesis that pulsars were beacons from
extraterrestrial civilizations was never serious, but some
discussed the far-reaching implications if it turned out to be
true. Their pulsar was later dubbed
CP
1919, and is now known by a number of designators
including
PSR 1919+21,
PSR
B1919+21 and
PSR J1921+2153.
Although CP 1919 emits in
radio
wavelengths, pulsars have, subsequently, been found to emit in
visible light,
X-ray, and/or
gamma ray wavelengths.
The word "pulsar" is a contraction of "pulsating star", and first
appeared in print in 1968:
The suggestion that pulsars were rotating neutron stars was put
forth independently by
Thomas Gold and
Franco Pacini in 1968, and was soon
proven beyond reasonable doubt by the discovery of a pulsar with a
very short (33-
millisecond) pulse period
in the
Crab nebula.
In 1974, Antony Hewish became the first astronomer to be awarded
the
Nobel Prize in physics.
Considerable controversy is associated with the fact that Professor
Hewish was awarded the prize while Bell, who made the initial
discovery while she was his Ph.D student, was not.
Subsequent history
In 1974,
Joseph Hooton Taylor,
Jr. and
Russell Hulse discovered
the first time pulsar in a
binary
system,
PSR B1913+16. This pulsar
orbits another neutron star with an orbital period of just eight
hours.
Einstein's theory of
general relativity predicts that this
system should emit strong
gravitational radiation, causing the
orbit to continually contract as it loses
orbital energy. Observations of the pulsar
soon confirmed this prediction, providing the first ever evidence
of the existence of gravitational waves. As of 2004, observations
of this pulsar continue to agree with general relativity. In 1993,
the Nobel Prize in Physics was awarded to Taylor and Hulse for the
discovery of this pulsar.
In 1982,
Don Backer led a group which
discovered
PSR B1937+21, a pulsar with
a rotation period of just 1.6 milliseconds. Observations soon
revealed that its magnetic field was much weaker than ordinary
pulsars, while further discoveries cemented the idea that a new
class of object, the "
millisecond
pulsars" (MSPs) had been found. MSPs are believed to be the end
product of
X-ray binaries. Owing to
their extraordinarily rapid and stable rotation, MSPs can be used
by
astronomers as clocks rivaling the
stability of the best
atomic clocks on
Earth. Factors affecting the arrival time of pulses at the Earth by
more than a few hundred
nanoseconds can
be easily detected and used to make precise measurements. Physical
parameters accessible through pulsar timing include the 3D position
of the pulsar, its
proper motion, the
electron content of the
interstellar medium along the
propagation path, the orbital parameters of any binary companion,
the pulsar rotation period and its evolution with time. (These are
computed from the raw timing data by
Tempo, a computer program specialized for
this task.) After these factors have been taken into account,
deviations between the observed arrival times and predictions made
using these parameters can be found and attributed to one of three
possibilities: intrinsic variations in the spin period of the
pulsar, errors in the realization of
Terrestrial Time against which arrival
times were measured, or the presence of background gravitational
waves. Scientists are currently attempting to resolve these
possibilities by comparing the deviations seen amongst several
different pulsars, forming what is known as a
Pulsar Timing Array. With luck, these
efforts may lead to a
time scale a factor
of ten or better than currently available, and the first ever
direct detection of gravitational waves.
In June 2006, the
astronomer John Middleditch and his
team at LANL announced
the first prediction of pulsar
glitches with observational data from the Rossi X-ray Timing
Explorer. They used observations of the pulsar
PSR J0537-6910.
In 1992,
Aleksander Wolszczan
discovered the first
extrasolar
planets around
PSR B1257+12. This
discovery presented important evidence concerning the widespread
existence of planets outside the
solar
system, although it is very unlikely that any
life form could survive in the environment of
intense radiation near a pulsar.
Theory
There is general agreement that what we observe as a pulse is what
happens when a beam of radiation points in our direction, once for
every rotation of the neutron star. The origin of the beam is
related to the misalignment of the rotation axis and the axis of
the
magnetic field of the star. The
beam is emitted from the poles of the neutron star's magnetic
field, which may be offset from the rotational poles by a wide
angle. The source of the power of the beam is the
rotational energy of the neutron star.
This rotation slows down over time as
electromagnetic power is emitted. When a
pulsar's spin period slows down sufficiently, the radio pulsar
mechanism is believed to turn off (the so-called "death line"). As
this seems to take place after ~10-100 million years, but neutron
stars have been formed throughout the ~13.6 billion year age of the
universe, more than 99% of neutron stars are thought to no longer
be pulsars.
Millisecond pulsars are thought
to have been spun up to high rotational speed (periods down to 1.4
milliseconds) by matter falling in that had been pulled off from a
companion star. As this matter lands on the neutron star, it is
thought to "bury" the magnetic field of the neutron star (although
the details are unclear), leaving millisecond pulsars with magnetic
fields 1000-10,000 times weaker than average pulsars. This low
magnetic field is less effective at slowing the pulsar's rotation,
so millisecond pulsars live for billions of years, making them the
oldest known pulsars. Millisecond pulsars are seen in globular
clusters, which stopped forming neutron stars billions of years
ago.
Of interest to the study of the state of the matter in a
neutronstars are the
glitches observed in the rotation
velocityof the neutron star. This velocity is decreasing slowly but
steadily, except by sudden variations. One model put forward to
explain these glitches is that they are the result of "starquakes"
that adjust the crust of the neutron star. Models where the glitch
is due to a decoupling of the possibly
superconducting interior of the star have
also been advanced. In both cases, the star's
moment of inertia changes, but its
angular momentum doesn't, resulting
in a change in rotation rate.
In 2003, observations of the
Crab
nebula's pulsar electromagnetic signal revealed "sub-pulses"
within the main signal with durations of only nanoseconds. It is
thought that these nanosecond pulses are emitted by regions on the
pulsar's surface 60 cm in diameter or smaller, making them the
smallest structures outside the solar system to be measured.
Categories
Three distinct classes of pulsars are currently known to
astronomers, according to the source of the power
of the electromagnetic radiation:
The
Fermi Space
Telescope has uncovered a subclass of rotationally-powered
pulsars that emit only
gamma rays. There
have been only about twelve gamma-ray pulsars identified out of
about 1800 known pulsars.
Although all three classes of objects are neutron stars, their
observable behavior and the underlying physics are quite different.
There are, however, connections. For example,
X-ray pulsars are probably old
rotationally-powered pulsars that have already lost most of their
power, and have only become visible again after their
binary companion had expanded and began
transferring matter on to the neutron star. The process of
accretion can in turn transfer enough
angular momentum to the neutron star to
"recycle" it as a rotation-powered
millisecond pulsar.
Naming
Initially pulsars were named with letters of the discovering
observatory followed by their
right
ascension (e.g. CP 1919). As more pulsars were discovered, the
letter code became unwieldy and so the convention was then
superseded by the letters PSR (Pulsating Source of Radio) followed
by the pulsar's right ascension and degrees of
declination (e.g. PSR 0531+21) and sometimes
declination to a tenth of a degree (e.g. PSR 1913+167). Pulsars
that are very close together sometimes have letters appended (e.g.
PSR 0021-72C and PSR 0021-72D).
The modern convention is to prefix the older numbers with a B (e.g.
PSR B1919+21) with the B meaning the coordinates are for the 1950.0
epoch. All new pulsars have a J indicating 2000.0 coordinates and
also have declination including minutes (e.g. PSR J1921+2153).
Pulsars that were discovered before 1993 tend to retain their B
names rather than use their J names (e.g. PSR J1921+2153 is more
commonly known as PSR B1919+21). Recently discovered pulsars only
have a J name (e.g.
PSR J0437-4715).
All pulsars have a J name that provides more precise coordinates of
its location in the sky.
Miscellaneous facts
- The magnetic axis of pulsars determines the direction of its
jets (the lighthouse spewing out the north and south poles of the
magnetic axis of rotation), and their magnetic axis is not
necessarily the same as their spin axis - just as Earth's magnetic
north pole is not the same as its true (spin) north pole. That's
why pulsars don't just "sit there" and beam at the same point in
their own celestial sphere (if
their outer spin axis coincided with their magnetic spin axis). If
this happened, they would not pulse... there would just be
detectable sources of radiation (when their jets pointed straight
at us), or not, but no pulsing.
- Although 8.5 seconds is the slowest observed pulsar period to
date, note that as pulsars slow, their power output decreases.
Conceivably there are much slower ones, below current levels of
detection. On the other hand, the fastest that they can spin (e.g.
1.4 msec) seems to be dependent on the speed at which a pulsar can
rotate without neutronium breaking up. In
summary, young pulsars are fast and energetic; old ones are slow
and weak, with the exception of millisecond pulsars, which are old
but have been "recycled" to very short periods.
- It is currently not known if original star mass (pre supernova)
or current neutron star mass is related to pulse period.
Applications
The study of pulsars has resulted in many applications in physics
and astronomy. Striking examples include the confirmation of the
existence of
gravitational
radiation as predicted by
general
relativity and the first detection of an extrasolar planetary
system.
The discovery of pulsars allowed astronomers to study an object
never observed before, the
neutron
star. This kind of object is the only place where the behavior
of matter at
nuclear density can be
observed (though not directly). Also, millisecond pulsars have
allowed a test of
general
relativity in conditions of an intense gravitational
field.
As probes of the interstellar medium
The radiation from pulsars passes through the
interstellar medium (ISM) before
reaching Earth. Free
electrons in the warm
(8000 K), ionized component of the ISM and
H
II regions affect the radiation in two primary ways. The
resulting changes to the pulsar's radiation provide an important
probe of the ISM itself.
Due to the
dispersive nature of
the interstellar
plasma,
lower-frequency radio waves travel through the medium slower than
higher-frequency radio waves. The resulting delay in the arrival of
pulses at a range of frequencies is directly measurable as the
dispersion measure of the pulsar. The dispersion measure
is the total
column density of free
electrons between the observer and the pulsar,
- \mathrm{DM} = \int_0^D n_e(s) ds,
where D is the distance from the pulsar to the observer and n_e is
the electron density of the ISM. The dispersion measure is used to
construct models of the free electron distribution in the
Milky Way Galaxy.
Additionally,
turbulence in the
interstellar gas causes density inhomogeneities in the ISM which
cause
scattering of the radio waves from
the pulsar. The resulting
scintillation of the radio
waves—the same effect as the twinkling of a star in
visible light due to density variations in the
Earth's atmosphere—can be used to reconstruct information about the
small scale variations in the ISM. Due to the high velocity (up to
several hundred km/s) of many pulsars, a single pulsar scans the
ISM rapidly, which results in changing scintillation patterns over
timescales of a few minutes.
Significant pulsars
Gamma-ray pulsars detected by the
Fermi Gamma-ray Space Telescope.
- The first radio pulsar CP 1919 (now
known as PSR 1919+21), with a pulse
period of 1.337 seconds and a pulse width of 0.04 second, was
discovered in 1967. A drawing of this pulsar's radio waves was used
as the cover of British rock band Joy
Division's debut album, Unknown Pleasures.
- The first binary pulsar, PSR 1913+16, whose orbit is decaying at the
exact rate predicted due to the emission of gravitational radiation by general relativity
- The first millisecond pulsar, PSR
B1937+21
- The brightest millisecond pulsar, PSR
J0437-4715
- The first X-ray pulsar, Cen X-3
- The first accreting millisecond X-ray pulsar, SAX J1808.4-3658
- The first pulsar with planets, PSR
B1257+12
- The first double pulsar binary system, PSR J0737−3039
- The longest period pulsar, PSR
J2144-3933
- The most stable pulsar in period, PSR
J0437-4715
- The magnetar SGR 1806-20 produced
the largest burst of power in the Galaxy ever experimentally
recorded on 27 December 2004
- PSR B1931+24 "... appears as a normal pulsar for about a week
and then 'switches off' for about one month before emitting pulses
again. [..] this pulsar slows down more rapidly when the pulsar is
on than when it is off. [.. the] braking mechanism must be related
to the radio emission and the processes creating it and the
additional slow-down can be explained by the pulsar wind leaving the pulsar's magnetosphere
and carrying away rotational energy.
- PSR J0108-1431, the closest known
pulsar to the Earth. It lies in the direction of the constellation
Cetus, at a distance of about
85 parsecs (280 light
years). Nevertheless, it was not discovered until 1993 due to
its extremely low luminosity. It was discovered by the Danish
astronomer Thomas Tauris. in collaboration with a team of
Australian and European astronomers using the Parkes 64-meter radio
telescope. The pulsar is 1000 times weaker than an average radio
pulsar and thus this pulsar may represent the tip of an iceberg of
a population of more than half a million such dim pulsars crowding
our Milky Way.
See also
Notes
- Young, M.D.; Manchester, R.N.; Johnston, S. " A Radio Pulsar with an 8.5-Second Period that
Challenges Emission Models." Nature, Volume 400, 26
August 1999 (pages 848-849).
- D.N. Matsakis, J.H. Taylor and T.M. Eubanks. " A Statistic for Describing Pulsar and Clock
Stabilities." Astronomy and Astrophysics, Volume 326,
October 1997 (pages 924-928).
- Press Release: Old Pulsars Still Have New Tricks to Teach Us.
European Space Agency, 26 July 2006.
- Pranab Ghosh, Rotation and accretion powered pulsars.
World Scientific, 2007, p.2.
- M. S. Longair, Our evolving universe. CUP Archive,
1996, p.72.
- M. S. Longair, High energy astrophysics, Volume 2.
Cambridge University Press, 1994, p.99.
- Sturrock, Peter A. The UFO Enigma: A New Review of the
Physical Evidence. Warner Books, 1999 (page 154).
- Courtland, Rachel. " Pulsar Detected by Gamma Waves Only." New
Scientist, 17 October 2008.
- Atkinson, Nancy. " Fermi Telescope Makes First Big Discovery: Gamma
Ray Pulsar." Universe Today, 17 October 2008.
- NASA'S Fermi Telescope Unveils a Dozen New Pulsars
http://www.nasa.gov/mission_pages/GLAST/news/dozen_pulsars.html
- Cosmos Online - New Kind of pulsar discovered
(http://www.cosmosmagazine.com/news/2260/new-kind-pulsar-discovered)
- Lyne, Andrew G.; Graham-Smith, Francis. Pulsar
Astronomy. Cambridge University Press, 1998.
- Ferriere, K. " The Interstellar Environment of Our Galaxy."
Reviews of Modern Physics, Volume 73, Issue 4, 2001 (pages
1031–1066).
- Taylor, J. H.; Cordes, J. M. " Pulsar Distances and the Galactic Distribution of Free
Electrons." Astrophysical Journal, Volume 411, 1993
(page 674).
- Rickett, Barney J. " Radio Propagation Through the Turbulent Interstellar
Plasma." Annual Review of Astronomy and Astrophysics,
Volume 28, 1990 (page 561).
- Rickett, Barney J.; Lyne, Andrew G.; Gupta, Yashwant. "
Interstellar Fringes from Pulsar B0834+06."
Monthly Notices of the Royal Astronomical Society, Volume
287, 1997 (page 739).
- Hewish, A. et al. " Observation of a Rapidly Pulsating Radio
Source." Nature, Volume 217, 1968 (pages
709-713).
- " Galactic Magnetar Throws Giant Flare."
Astronomy Picture of the Day, 21 February 2005.
- " Part-Time Pulsar Yields New Insight Into Inner Workings of
Cosmic Clocks." Particle Physics and Astronomy Research
Council, 3 March 2006.
- Tauris, T. M. et al. " Discovery of PSR J0108-1431: The Closest Known
Neutron Star?" Astrophysical Journal, Volume 428, 1994
(page L53).
- Crowsell, K. " Science: Dim Pulsars May Crowd Our Galaxy."
New Scientist, Number 1930, 18 June 2008. (page 16).
- " Closest Pulsar?" Sky & Telescope,
October 1994 (page 14).
- Champion, David J. et al. " An Eccentric Binary Millisecond Pulsar in the Galactic
Plane." Science, 6 June 2008 Volume 320, Number 5881
(pages 1309-1312).
References and further reading
- Lorimer, Duncan R. " Binary
and Millisecond Pulsars at the New Millennium." Living
Reviews, Relativity 4, 2001.
- Lorimer, Duncan R.; Kramer, M. "Handbook of Pulsar Astronomy."
Cambridge Observing Handbooks for Research Astronomers,
2004.
- Stairs, Ingrid H. " Testing
General Relativity with Pulsar Timing." Living
Reviews, Relativity 6, 2003.
- Sturrock, Peter A. The UFO Enigma: A New Review of the
Physical Evidence. Warner Books, 1999 (ISBN
0-446-52565-0).
External links
- " Pinning Down a Pulsar’s Age". Science
News.
- " Astronomical whirling dervishes hide their age
well". Astronomy Now.
- Animation of a Pulsar. Einstein.com,
17 January 2008.
- " The Discovery of Pulsars." BBC, 23 December
2002.
- " A
Pulsar Discovery: First Optical Pulsar." Moments of
Discovery, American Institute of Physics, 2007 (Includes audio
and teachers guides).
- Discovery of Pulsars: Interview with Jocelyn
Bell-Burnell. Jodcast, June 2007 ( Low Quality Version).
- Listing for PULS CP 1919 (The First Pulsar),
Simbad Database
- Australia National Telescope Facility: Pulsar Catalogue
- Johnston, William Robert. " List of Pulsars in Binary Systems." Johnston
Archive, 22 March 2005.
- Staff Writers. " Scientists Can Predict Pulsar Starquakes."
Space Daily, 7 June 2006.
- Staff Writers. " XMM-Newton Makes New Discoveries About Old
Pulsars." Space Daily, 27 July 2006.
- Than, Ker. " Hot New Idea: How Dead Stars Go Cold."
Space.com, 27 July 2006.
- "New Kind of Pulsar Discovered"[98468]. Cosmos Online.