# First measurement of the total gravitational quadrupole moment of a black widow companion

Abstract : We present the first measurement of the gravitational quadrupole moment of the companion star of a spider pulsar, namely the black widow PSR J2051$-$0827. To this end we have re-analysed radio timing data using a new model which is able to account for periastron precession caused by tidal and centrifugal deformations of the star as well as by general relativity. The model allows for a time-varying component of the quadrupole moment, thus self-consistently accounting for the ill-understood orbital period variations observed in these systems. Our analysis results in the first detection of orbital precession in a spider system at $\dot{\omega} = -68.6_{-0.5}^{+0.9}$ deg/yr and the most accurate determination of orbital eccentricity for PSR J2051$-$0827 with $e = (4.2 \pm 0.1) \times 10^{-5}$. We show that the variable quadrupole component is about 100 times smaller than the average quadrupole moment $\bar{Q} = -2.2_{-1}^{+0.6} \times 10^{41} \si{kg.m^2}$ . We discuss how accurate modelling of high precision optical light curves of the companion star will allow its apsidal motion constant to be derived from our results.
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Submitted on : Thursday, April 2, 2020 - 12:33:19 PM
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Guillaume Voisin, C J Clark, R P Breton, V. Dhillon, M R Kennedy, et al.. First measurement of the total gravitational quadrupole moment of a black widow companion. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, In press, 494 (3), pp.4448-4453. ⟨10.1093/mnras/staa953⟩. ⟨hal-02427662v2⟩

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