Very Long Baseline Interferometry Measured Proper Motion and Parallax of the $\gamma$-ray Millisecond Pulsar PSR J0218+4232

Abstract

PSR J0218$+$4232 is a millisecond pulsar (MSP) with a flux density $\sim$ 0.9 mJy at 1.4 GHz. It is very bright in the high-energy X-ray and $\gamma$-ray domains. We conducted an astrometric program using the European VLBI Network (EVN) at 1.6 GHz to measure its proper motion and parallax. A model-independent distance would also help constrain its $\gamma$-ray luminosity. We achieved a detection of signal-to-noise ratio S/N > 37 for the weak pulsar in all five epochs. Using an extragalactic radio source lying 20 arcmin away from the pulsar, we estimate the pulsar's proper motion to be ${\mu }_{\alpha }\cos \delta =5.35\pm 0.05$ mas yr${}^{-1}$ and ${\mu }_{\delta }=-3.74\pm 0.12$ mas yr${}^{-1}$, and a parallax of $\pi =0.16\pm 0.09$ mas. The very long baseline interferometry (VLBI) proper motion has significantly improved upon the estimates from long-term pulsar timing observations. The VLBI parallax provides the first model-independent distance constraints: $d=6.3_{-2.3}^{+8.0}$ kpc, with a corresponding $3\sigma$ lower-limit of $d=2.3$ kpc. This is the first pulsar trigonometric parallax measurement based solely on EVN observations. Using the derived distance, we believe that PSR J0218$+$4232 is the most energetic $\gamma$-ray MSP known to date. The luminosity based on even our 3$\sigma$ lower-limit distance is high enough to pose challenges to the conventional outer gap and slot gap models.

5 pages, 2 figures, 2 tables; published in the Astrophysical Journal Letters on 2014 Feb. 10

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