The Distance to a Magnetar
XTE J1810−197 (J1810) was the first magnetar identified to emit radio pulses, and has been extensively studied during a radio-bright phase in 2003–2008. Magnetars are slowly rotating neutron stars with the largest magnetic fields measured in the Universe (> 1014 Gauss). The magnetar J1810 is estimated to be relatively nearby compared to other Galactic magnetars, and provides a useful prototype for the physics of high magnetic fields, magnetar velocities, and the plausible connection to extragalactic fast radio bursts.
Upon the rebrightening of the magnetar at radio wavelengths in late 2018, Ding et al. resumed an astrometric campaign on J1810 with the VLBA, and sampled 14 new positions of J1810 over 1.3 yr. The phase calibration for the new observations was performed with two-phase calibrators that are quasi-colinear on the sky with J1810, enabling substantial improvement of the resultant astrometric precision. Combining their new observations with two archival observations from 2006, they refined the proper motion and reference position of the magnetar and have measured its annual geometric parallax, the first such measurement for a magnetar.
The parallax of 0.40 ± 0.05 milli-arcseconds corresponds to a most probable distance 2.5+0.4−0.3 kpc for J1810. Their new astrometric results confirm an unremarkable transverse peculiar velocity of ≈200 km/sec for J1810, which is only at the average level among the pulsar population. The magnetar proper motion vector points back to the central region of a supernova remnant (SNR) at a compatible distance at ≈70 kyr ago, but a direct association is disfavored by the estimated SNR age of ∼3 kyr
Figure caption: Astrometric observations at 5.7 GHz with the VLBA, showing the proper motion relative to the phase reference source, and the annual parallax after subtraction of the proper motion, of XTE J1810.
Publication: Hoa Ding (Swinburne University of Technology) et al., A Magnetar Parallax, Monthly Notices of the Royal Astronomical Society, 498, 3736 (November 2020).
NRAO Press Release: VLBA Makes First Direct Distance Measurement to Magnetar
