Over the last year, the greatest development in pulsar research at the
Arecibo Observatory has been the start of the main ALFA pulsar
survey. These surveys will greatly increase the rate of detection of
millisecond pulsars (MSPs) and compact binary pulsars relative to the
Parkes Multibeam survey (Freire et al. 2006, see the powerpoint presentation here),
i.e., they will greatly increase the number (and hopefully the quality)
of natural physics laboratories suitable for experiments in gravitation
and nuclear physics. This is mostly due to this survey's high frequency
and time resolution. A description of the observing system and of the
discoveries of the preliminary survey has now been published (Cordes et at. 2006).
Mass - mass plot for the PSR J1906+0746 binary system. The hatched
region is excluded by knowledge of the mass function. The diagonal
straight lines limit the region allowed by the measurement of the rate
of advance of periastron (7.57 +/- 0.03 degrees per year, which implies
a total mass for the system of 2.61 +/- 0.02 solar masses). The grey
band indicates the range of precisely measured neutron star masses to
date. At these two extremes, we have inclinations of about 42 and 51
degrees, and companion masses between 1.17 and 1.36 solar
masses.
A great example of the new, compact binary systems that ALFA can find
is PSR J1906+0746. This is a 144-ms pulsar in a binary system
with an orbital period just under 4 hours and a massive companion.
These characteristics, together with the eccentricity of the system,
make it the second most relativistic system ever
found, i.e., the system with the second largest measured rate of
advance of periastron (see Figure 1 and Lorimer et al. 2006
for details). Timing of this system over the next few years will
yield a precise measurement of the pulsar and companion masses, and
in ~5 years or so a measurement of the relativistic decay for this
system, i.e., we will have a good test of general relativity. This will
be particularly interesting if the companion is a white dwarf star; in
that case we will be able to put stringent limits on the emission of
dipolar gravitational radiation, predicted to occur by some alternative
theories of gravitation. If the companion is a neutron star, then it is
likely to be a recycled pulsar, and PSR J1906+0746 is a double pulsar.
In that case, the recycled pulsar is eluding detection, despite the
deep companion surveys we have done at Arecibo. This is possible if the
radiation beam of the companion is presently not pointing at the Earth
at any rotational phase.
One particularity of this system is that it is about 1000 times
younger than the double pulsar, i.e., it is by far the youngest
binary system known. Another one is that a posteriori data
processing showed
that the pulsar is in an observation of the Parkes Multi-beam survey
made in 1998. A comparison with recent data shows significant evolution
of the pulse profile (see below), indicating fast geodetic precession,
as predicted by general relativity.
Integrated
pulse profiles of PSR J1906+0746 showing 360 degrees of rotational
phase. The upper panel shows the detection at 1.374 GHz from the 35-min
of the Parkes Multi-Beam Survey data taken on 1998 August 3. The lower
panel shows a 35-min observation with the same observing system taken
on 2005 September 4. The lower panel shows the difference profile
(i.e. 1998 minus 2005 data) after scaling both profiles to the area of
the main pulse. The dashed horizontal lines show +/- 3 standard
deviations computed from the off-pulse noise region. The limiting
instrumental time resolution of both these profiles is 2.1 ms.
