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

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.

💡 Research Summary

The paper presents new deep V‑band observations of the isolated neutron star RX J0720‑3125 obtained with the FORS1 instrument on the ESO Very Large Telescope (VLT) in January 2008. The authors acquired a total exposure time of three hours (12 × 900 s exposures) under good seeing conditions (≤ 0.8″) and processed the data with a combination of the standard ESO pipeline and advanced astrometric tools (SCAMP for global astrometric solutions and SWarp for image resampling). This workflow allowed them to correct for field distortions down to a few tens of milliarcseconds, which is essential for precise position measurements of such a faint target (V ≈ 27 mag).

Photometric calibration was performed using Landolt standard stars observed on the same nights. Aperture photometry (1.5 × FWHM radius) was carried out with IRAF/DAOPHOT, and colour terms as well as atmospheric extinction coefficients were derived from the standard star observations. The resulting V‑band magnitude of the neutron star is V = 26.81 ± 0.09 mag. This value is roughly seven times brighter than the extrapolation from the X‑ray black‑body spectrum (which would predict V ≈ 29 mag), but it aligns well with the previously measured U, B, and R magnitudes, confirming a consistent spectral energy distribution (SED) across the optical band. The excess optical flux suggests that a simple pure‑thermal black‑body model is insufficient; additional non‑thermal components (e.g., magnetospheric emission, a thin hydrogen atmosphere, or cyclotron/synchrotron processes) may contribute.

For astrometry, the authors compared the 2008 VLT/FORS1 image with archival FORS1 data taken in December 2000. After applying the same distortion corrections and aligning the two epochs using ~30 common field stars, they measured the positional shift of RX J0720‑3125 over a 7.0‑year baseline. The derived proper motion is μ = 105.1 ± 7.4 mas yr⁻¹ directed toward a position angle θ = 296.951° ± 0.0063° (north‑west). This result is fully consistent with earlier measurements obtained with HST and other VLT observations (μ ≈ 107 mas yr⁻¹). The precise proper motion, together with the distance estimate of ~300 pc, refines the neutron star’s transverse velocity and helps constrain its kinematic age and trajectory through the Galaxy.

The discussion emphasizes that the V‑band detection fills a gap in the multi‑wavelength coverage of RX J0720‑3125, enabling more robust modeling of its atmosphere and magnetospheric environment. The authors argue that the observed optical excess cannot be explained by a single black‑body component and that future work should combine the new V‑band point with ultraviolet data from HST and high‑resolution X‑ray spectra from XMM‑Newton or NICER to construct a comprehensive SED. Such an SED would allow testing of atmosphere models that include magnetic fields, condensed surfaces, and possible non‑thermal emission mechanisms.

In conclusion, the study demonstrates that deep, well‑calibrated optical imaging with modern large telescopes can achieve both high‑precision photometry and astrometry for extremely faint isolated neutron stars. The new V‑band magnitude and updated proper motion for RX J0720‑3125 provide valuable constraints for theoretical models of neutron‑star cooling, surface composition, and magnetospheric activity, and they set the stage for future coordinated multi‑wavelength campaigns aimed at unraveling the physics of these enigmatic objects.