VLBI astrometry of PSR J2222-0137: a pulsar distance measured to 0.4% accuracy

The binary pulsar J2222-0137 is an enigmatic system containing a partially recycled millisecond pulsar and a companion of unknown nature. Whilst the low eccentricity of the system favors a white dwarf companion, an unusual double neutron star system is also a possibility, and optical observations will be able to distinguish between these possibilities. In order to allow the absolute luminosity (or upper limit) of the companion object to be properly calibrated, we undertook astrometric observations with the Very Long Baseline Array to constrain the system distance via a measurement of annual geometric parallax. With these observations, we measure the parallax of the J2222-0137 system to be 3.742 +0.013 -0.016 milliarcseconds, yielding a distance of 267.3 +1.2 -0.9 pc, and measure the transverse velocity to be 57.1 +0.3 -0.2 km/s. Fixing these parameters in the pulsar timing model made it possible to obtain a measurement of Shapiro delay and hence the system inclination, which shows that the system is nearly edge-on (sin i = 0.9985 +/- 0.0005). Furthermore, we were able to detect the orbital motion of J2222-0137 in our VLBI observations and measure the longitude of ascending node. The VLBI astrometry yields the most accurate distance obtained for a radio pulsar to date, and is furthermore the most accurate parallax for any radio source obtained at low radio frequencies (below ~5 GHz, where the ionosphere dominates the error budget). Using the astrometric results, we show the companion to J2222-0137 will be easily detectable in deep optical observations if it is a white dwarf. Finally, we discuss the implications of this measurement for future ultra-high-precision astrometry, in particular in support of pulsar timing arrays.

Evidence for chaotic behaviour in pulsar spin-down rates

We present evidence for chaotic dynamics within the spin-down rates of 17 pulsars originally presented by Lyne et al. Using techniques that allow us to re-sample the original measurements without losing structural information, we have searched for evidence of a strange attractor in the time series of frequency derivatives for each of the 17 pulsars. We demonstrate the effectiveness of our methods by applying them to a component of the Lorenz and Rossler attractors that were sampled with similar cadence to the pulsar time series. Our measurements of correlation dimension and Lyapunov exponent show that the underlying behaviour appears to be driven by a strange attractor with approximately three governing non-linear differential equations. This is particularly apparent in the case of PSR B1828$-$11 where a correlation dimension of 2.06pm0.03 and a Lyapunov exponent of $(4.0pm0.3)times10^{-4}$ inverse days were measured. These results provide an additional diagnostic for testing future models of this behaviour.

Radio and X-ray observations of the intermittent pulsar J1832+0029

We report on radio and X-ray observations of PSR 1832+0029, a 533-ms radio pulsar discovered in the Parkes Multibeam Pulsar Survey. From radio observations taken with the Parkes, Lovell and Arecibo telescopes, we show that this pulsar exhibits two spindown states akin to PSRs B1931+24 reported by Kramer et al. and J1841-0500 reported by Camilo et al. Unlike PSR B1931+24, which switches between on and off states on a 30--40 day time-scale, PSR 1832+0029 is similar to PSR J1841-0500 in that it spends a much longer period of time in the off-state. So far, we have fully sampled two off-states. The first one lasted between 560 and 640 days and the second one lasted between 810 and 835 days. From our radio timing observations, the ratio of on/off spindown rates is $1.77 pm 0.03$. Chandra observations carried out during both the on- and off-states of this pulsar failed to detect any emission. Our results challenge but do not rule out models involving accretion onto the neutron star from a low-mass stellar companion. In spite of the small number of intermittent pulsars currently known, difficulties in discovering them and in quantifying their behavior imply that their total population could be substantial.

Upper Limits on Pulsed Radio Emission from the 6.85 s X-ray Pulsar XTE J0103-728 in the Small Magellanic Cloud

X-ray pulsations with a 6.85 s period were recently detected in the SMC and were subsequently identified as originating from the Be/X-ray binary system XTE J0103-728. The recent localization of the source of the X-ray emission has made a targeted search for radio pulsations from this source possible. The detection of pulsed radio emission from XTE J0103-728 would make it only the second system after PSR B1259-63 that is both a Be/X-ray binary and a radio pulsar. We observed XTE J0103-728 in Feb 2008 with the Parkes 64-m radio telescope soon after the identification of the source of X-ray pulsations was reported in order to search for corresponding radio pulsations. We used a continuous 6.4 hour observation with a 256 MHz bandwidth centered at 1390 MHz using the center beam of the Parkes multibeam receiver. In the subsequent data analysis, which included a folding search, a Fourier search, a fast-folding algorithm search, and a single-pulse search, no pulsed signals were found for trial dispersion measures (DMs) between 0 and 800 pc cm^-3. This DM range easily encompasses the expected values for sources in the SMC. We place an upper limit of ~45 mJy kpc^2 on the luminosity of periodic radio emission from XTE J0103-728 at the epoch of our observation, and we compare this limit to a range of luminosities measured for PSR B1259-63, the only Be/X-ray binary currently known to emit radio pulses. We also compare our limit to the radio luminosities of neutron stars having similarly long spin periods to XTE J0103-728. Since the radio pulses from PSR B1259-63 are eclipsed and undetectable during the portion of the orbit near periastron, repeated additional radio search observations of XTE J0103-728 may be valuable if it is undergoing similar eclipsing and if such observations are able to sample the orbital phase of this system well.