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13th of October 2022
The eccentric millisecond pulsar PSR J0955−6150
by Serylak et al.
PSR J0955−6150 was discovered several years ago in a Parkes survey of unidentified Fermi-LAT gamma-ray sources. The pulsar has a spin period of 1.99 ms, and is believed to be a "recycled" millisecond pulsar. Like most such pulsars, it is in a binary system, in this case with an orbital period of 24.6 days. The companion has a relatively low mass and is presumably a helium white dwarf star (He WD). The unusual feature of this system is its orbital eccentricity of 0.12, which is much larger than the eccentricities of most similar systems. Measuring the masses of the stars in this system is important for testing the different hypotheses for the formation of such systems.

Serylak et al. have carried out timing observations of this pulsar with the Parkes radio telescope, Murriyang, and the MeerKAT radio telescope. The observations reveal a strong frequency evolution of this millisecond pulsar’s intensity, with a flux density spectral index of −3.1. The improved sensitivity of MeerKAT has resulted in a > 10-fold improvement in the timing precision obtained compared to the older Parkes observations. This, combined with the 8-year timing baseline, has allowed precise measurements of a very small proper motion and three orbital "post-Keplerian" parameters, namely the rate of advance of periastron, and Shapiro delay parameters. Assuming general relativity, they obtain 1.7 solar masses for the mass of the pulsar and 0.25 solar masses for the mass of the companion. While the pulsar mass is consistent with expectations, the companion mass is significantly smaller than expected by several formation hypotheses proposed, which are therefore unlikely to be correct and can be ruled out. The figure above shows total intensity profiles made from observations made with MeerKAT (left plot) and Parkes (right plot) radio telescopes. The MeerKAT profile was made from 32.6 hours of observations centred at 1284 MHz with a bandwidth of 856 MHz. The Parkes profile was made from 14.9 hours of observations with the two lower sub-bands of the UWL receiver, with a centre frequency of 1536 MHz and a bandwidth of 1664 MHz. The bottom panels show the dynamic spectra, with the frequency evolution of the pulse profile and the steep spectral index clearly visible.

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