New photometry and astrometry of the isolated neutron star RX J0720-3125 using recent VLT/FORS observations

TL;DR

This study presents new optical photometry and astrometry of the isolated neutron star RX J0720-3125 using VLT/FORS observations, providing updated position, proper motion, and spectral energy distribution data.

Contribution

First detection of RX J0720-3125 in V Bessel filter, with new astrometric position and proper motion measurements from deep VLT/FORS images.

Findings

Visual magnitude V = 26.81 ± 0.09 mag.

Proper motion of 105.1 ± 7.4 mas/yr towards NW.

Optical flux exceeds X-ray expectations but aligns with other optical data.

Abstract

Since the first optical detection of RXJ0720.4-3125 various observations have been performed to determine astrometric and photometric data. We present the first detection of the isolated neutron star in the V Bessel filter to study the spectral energy distribution and derive a new astrometric position. At ESO Paranal we obtained very deep images with FORS 1 (three hours exposure time) of RXJ0720.4-3125 in V Bessel filter in January 2008. We derive the visual magnitude by standard star aperture photometry.Using sophisticated resampling software we correct the images for field distortions. Then we derive an updated position and proper motion value by comparing its position with FORS 1 observations of December 2000. We calculate a visual magnitude of V = 26.81 +- 0.09mag, which is seven times in excess of what is expected from X-ray data, but consistent with the extant U, B and R data. Over about a seven year epoch difference we measured a proper motion of mu = 105.1 +- 7.4mas/yr towards theta = 296.951 deg +- 0.0063 deg (NW), consistent with previous data.