Ongoing Pulsar VLBI Projects

Few pulsar parallaxes have been measured until now, because pulsars are weak and their spectra favor observations at low frequencies (e.g. < 1.7 GHz) where the ionosphere is problematic. However, the pace of parallax measurements has increased dramatically with the ability to gate the VLBA correlator and the development of new techniques for correcting ionospheric phase perturbations. The current state-of-the-art is summarized here, and includes published results for B0950+08 (Brisken et al. 2000) and B0919+06 (Chatterjee et al. 2001).

Although pulsars usually have a steep negative spectral index (flux ~ f^-alpha, with 0 < alpha < 2.5), observations at higher frequencies benefit from the narrower beam, lower system temperatures and, in particular, reduced ionospheric effects. The astrometric precision, which depends on both the resolution and the S/N ratio, is maximized by observations at 1.4-1.7 GHz for most (but not all) pulsars. In prior and ongoing projects (e.g. BB126, BB136, BC115, BC116) we have developed a set of techniques for these observations:

Wide-band Ionospheric Calibration solves for the differential ionosphere between the calibrator and target, using the visibility data itself over a wide spanned bandwidth. This technique was used to determine the parallax of B0950+08 and 8 other pulsars: see the results here.
In-Beam Calibration uses a compact faint extra-galactic source in the same primary beam (~30 arcmin) as the target pulsar, reducing the target-calibrator separation from a few degrees to a few arcminutes, and eliminating time interpolation. The parallax to B0919+06 was determined using this technique: see the results here.
Higher Frequency Observations at 5 GHz have comparable (and small) ionospheric and tropospheric phases, leading to successful astrometry with phase-referenced observations. However, the number of pulsars that can be observed at 5 GHz is limited when using the VLBA alone.
GPS Corrections may be usable in order to reduce the effects of the differential ionosphere between the phase reference source and the target. This technique may be used in combination with the others, or in cases where a nearby calibrator (< 2 degrees) exists for the target.

We have demonstrated the feasibility of providing the calibration required for sub-mas astrometry using these techniques. Ongoing projects with the VLBA are continuing with the source samples used in these pilot studies.

Additionally, we have projects in the queue that will use Arecibo (AO) and the Green Bank Telescope (GBT) in conjunction with the VLBA. In these, we will explore how to best utilize the large boosts in sensitivity on VLBA-GBT or VLBA-AO baselines for astrometry, while tackling the different slew rates of these antennas when nodding between sources and calibrators. First fringes have been obtained between the VLBA and both AO and the GBT, and our observations with the VLBA+Arecibo have been successful.

(Also see: fringes between Arecibo and the VLBA for PSR J1740+1000,
seen without any processing, in ungated data and gated data.
Note the improvement in S/N from using the pulsar gate.)

Proposed work here does not initially request the use of AO, the GBT or the phased VLA, but on successful completion of the first phases of our project, we plan to integrate AO, the GBT and possibly the phased VLA with the VLBA in order to observe fainter pulsars that provide scientifically interesting targets.

More details:
Scientific Case
Results from Ongoing Projects
Technical Details
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Shami Chatterjee
Contact Information
Last modified: 14 Feb 2002