The primary manifestation of this precession is expected to be a slow change in the shape of the pulse profile, as different regions of the pulse emission beam move into the observable region.
Evidence for long-term profile shape changes is in fact seen in PSRs B1913+16 and B1534+12. For PSR B1913+16, profile shape changes were first reported in the 1980s , with a clear change in the relative heights of the two profile peaks over several years (Figure 12). No similar changes were found in the polarization of the pulsar . Interestingly, although a simple picture of a cone-shaped beam might lead to an expectation of a change in the separation of the peaks with time, no evidence for this was seen until the late 1990s, at the Effelsberg 100-m telescope , by which point the two peaks had begun to move closer together at a rather fast rate. Kramer  used this changing peak separation, along with the predicted precession rate and a simple conal model of the pulse beam, to estimate a spin-orbit misalignment angle of about 22° and to predict that the pulsar will disappear from view in about 2025 (see Figure 13), in good agreement with an earlier prediction by Istomin  made before the peak separation began to change. Recent results from Arecibo  confirm the gist of Kramer’s results, with a misalignment angle of about 21°. Both sets of authors find there are four degenerate solutions that can fit the profile separation data; two can be discarded as they predict an unreasonably large misalignment angle of 180° – 22° = 158° , and a third is eliminated because it predicts the wrong direction of the position angle swing under the Rotating Vector Model . The main area of dispute is the actual shape of the emission region; while Weisberg and Taylor find an hourglass-shaped beam (see Figure 14), Kramer maintains that a nearly circular cone plus an offset core is adequate (see Figure 15). In any event, it is clear that the interpretation of the profile changes requires some kind of model of the beam shape. Kramer [82, 83] lets the rate of precession vary as another free parameter in the pulse-shape fit, and finds a value of 1.2° ± 0.2°. This is consistent with the GR prediction but still depends on the beam-shape model and is therefore not a true test of the precession rate.
PSR B1534+12, despite the disadvantages of a more recent discovery and a much longer precession period, also provides clear evidence of long-term profile shape changes. These were first noticed at 1400 MHz by Arzoumanian [7, 10] and have become more obvious at this frequency and at 430 MHz in the post-upgrade period at Arecibo . The principal effect is a change in the low-level emission near to the main pulse (Figure 16), though related changes in polarization are now also seen. As this pulsar shows polarized emission through most of its pulse period, it should be possible to form a better picture of the overall geometry than for PSR B1913+16; this may make it easier to derive an accurate model of the pulse beam shape.
As for other tests of GR, the pulsar–white-dwarf binary PSR J1141–6545 promises interesting results. As noted by the discoverers , the region of sky containing this pulsar had been observed at the same frequency in an earlier survey , but the pulsar was not seen, even though it is now very strong. It is possible that interference corrupted that original survey pointing, or that a software error prevented its detection, but it is also plausible that the observed pulsar beam is evolving so rapidly that the visible beam precessed into view during the 1990s. Clearly, careful monitoring of this pulsar’s profile is in order.
© Max Planck Society and the author(s)