The binary pulsar PSR J1811-1736: evidence of a low amplitude supernova kick

Aims.The binary pulsar PSR J1811-1736 has been identified, since its discovery, as a member of a double neutron star system. Observations of such binary pulsars allow the measurement of general relativistic effects, which in turn lead to information about the orbiting objects, to their binary evolution and, in a few cases, to tests of theories of gravity. Methods: Regular timing observations have since January 1999 been carried out with three of the largest European radio telescopes involved in pulsar research. Pulse times of arrival were determined by convolving the observed profiles with standard templates, and were fitted to a model that takes into account general relativistic effects in binary systems. The prospects of continued observations were studied with simulated timing data. Pulse scattering times were measured using dedicated observations at 1.4 GHz and at 3.1 GHz, and the corresponding spectral index has also been determined. The possibility of detecting the yet unseen companion as a radio pulsar was investigated as a function of pulse period, observing frequency and flux density of the source. A study of the natal kick received by the younger neutron star at birth was performed considering the total energy and total angular momentum for a two body system. Results: We present an up to date and improved timing solution for the binary pulsar PSR J1811-1736. One post-Keplerian parameter, the relativistic periastron advance, is measured and leads to the determination of the total mass of this binary system. Measured and derived parameters strongly support the double neutron star scenario for this system. The pulse profile at 1.4 GHz is heavily broadened by interstellar scattering, limiting the timing precision achievable at this frequency. We show that a better precision can be obtained with observations at higher frequencies. This would allow one to measure a second post-Keplerian parameter within a few years. We find that interstellar scattering is unlikely to be the reason for the continued failure to detect radio pulsations from the companion of PSR J1811-1736. The probability distribution that we derive for the amplitude of the kick imparted on the companion neutron star at its birth indicates that the kick has been of low amplitude.