ALFA Pulsar Surveys Find Youngest Binary Pulsar Ever

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 1906+0746

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.

Pulse profile change for PSR J1906+0746

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.