Pulsars can be used to study the strong nuclear force.
The density of matter at the center of neutron stars is one or two
orders of magnitude larger than the density of atomic nuclei. The
behavior (i.e., the exact relation between pressure and density, known
as equation of state, or EOS) of "cold" matter under such conditions is
not known. Understanding the behavior of
matter at very high densities is one of the priorities for
research in astrophysics outlined in the report of the National
Academies entitled From
Quarks to the Cosmos: Eleven Science Questions for the New Century
(Board on Physics and Astronomy, 2003, National Academies Press).
Most massive pulsar ever!
One way to learn about the EOS
is by measuring the masses of recycled pulsars, which have accreted
matter for a long period of time. Very "soft" EOSs predict lower
pressures for a given density, i.e., highly compressible matter. This
results in very small, compact stars with very high gravitational
fields,
these are very close to forming black holes. Most of these EOSs
predict upper limits for the mass of a neutron star of about 1.6 solar
masses (above that limit, the star implodes and forms a black
hole). If one can find a more massive star, we can exclude such "soft"
EOSs.
Constraints on
pulsar and secondary masses from the general relativistic timing model.
Confidence limits of 68% and 95% are shown. The shaded region in the
lower left is disallowed by the Keplerian mass function. Dashed
lines show constraints from the orbital decay alone. A dotted line
indicates an inclination of 60 degrees.
A recent Arecibo experiment has important implications for this study. Nice
et al. 2005 have recently published the results of the
long-term timing of PSR J0751+1807, a 3.48-ms pulsar in a tight binary
system with a white dwarf companion. The measurement of the Shapiro
delay and orbital decay of this system indicate that its mass is 2.1
+/- 0.2 solar masses (see figure above). If more precise measurements
can confirm this high value, then many model EOSs for dense matter can
be ruled out. In this case, matter at the cores of neutron stars is
highly incompressible.
Fastest pulsar ever!
Another way of studying the behavior of super-dense matter is to find
fast pulsars. This excludes "stiff" EOSs, which predict that matter is highly
incompressible. That would produce very large stars that can not
withstand large spin frequencies without breaking apart. Until
recently, the fastest known pulsar was PSR B1937+21, the first
millisecond pulsar (MSP) to be discovered. This object rotates 642
times per second, its discovery in 1982 at the Arecibo Observatory
excluded some very stiff EOSs.
Pulse profile for PSR J1748-2446ad, repeated for
clariy, for the best detection of this pulsar. The apparent interpulse
has high statistical significance, particularly in plots where the
best observations are co-added.
Paulo Freire, an Arecibo Observatory Staff member, is part of an
international team that is using the S-band receiver of the Green Bank
Telescope to search for millisecond pulsars in globular clusters. So
far, a total of 50 MSPs have been found by this project (Ransom et al. 2006), 30 of these in the globular cluster Terzan 5 (for the first 21 discoveries in that cluster, see Ransom et al. 2005, see also the list
of pulsars in globular clusters which includes these 50 new
objects). One of the recent discoveries, Terzan 5 ad, has a spin
frequency of 716 Hz, i.e., it is now the most rapidly spinning
pulsar known (Hessels
et al. 2006). This is not yet fast enough to introduce significant
constraints on the EOS, but it constrains models of neutron stars
crusts and models of the emission
of gravitational waves by the star's rotation. It also opens up the
possibility that faster pulsars might be found in the future.
