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 authors present new V‑band photometry and an updated astrometric solution for the isolated neutron star RX J0720.4‑3125. Using the ESO Very Large Telescope (VLT) equipped with the FOcal Reducer/low dispersion Spectrograph (FORS1), they obtained deep imaging in January 2008: thirteen 900‑second exposures (total integration ≈3 h) with a pixel scale of 0.125″ covering a 4.2′ × 4.2′ field. After standard bias subtraction and flat‑fielding, the images were processed with SCAMP and SWarp to correct for field distortions (fitted with a fifth‑order polynomial) and to co‑add the two CCD chips into a single, distortion‑free mosaic.

Photometric calibration was performed in two ways. On the first night, Landolt standard field SA 98 was observed. Two standard stars (98‑5556 and 98‑5557) provided a zero‑point of c = −21.0832 ± 0.0017 mag and a first‑order extinction coefficient of k = 0.1562 ± 0.0058 mag. The authors identified faint companions (c1, c2) near the standards and corrected their contribution, thereby minimizing systematic errors. Using a 7‑pixel radius aperture (≈0.9″) on the neutron star, they measured a signal‑to‑noise ratio of ~6 and derived V = 26.88 ± 0.15 mag after accounting for the airmass variation (Y = 1.028 ± 0.019).

On the second night (January 13), no standard stars were taken, but three field stars with previously published V magnitudes (Motch & Haberl 1998) served as secondary calibrators. A 5‑pixel aperture was adopted for the neutron star because its point‑spread function fell below background at that radius. The resulting magnitude, V = 26.81 ± 0.09 mag, is fully consistent with the first‑night result but with a smaller statistical error, and it is adopted for the remainder of the analysis.

For proper motion, the authors compared the 2008 V‑band image with the B‑band FORS1 image from Motch et al. (2003) taken in December 2000, providing an 8‑year baseline. Source catalogs from both epochs were matched to the 2MASS reference frame, and SCAMP’s distortion solutions were applied to obtain precise world‑coordinate system (WCS) alignment. Positional shifts for all common sources were computed, and a Kolmogorov–Smirnov two‑sample test was used to verify that the distribution of shifts follows a Rayleigh distribution, as expected for random measurement errors. Outliers beyond 2σ were iteratively removed, and the remaining sample was used to derive the neutron star’s motion: μ = 105.1 ± 7.4 mas yr⁻¹ at a position angle θ = 296.951° ± 0.0063° (north‑west). This agrees with previous measurements from HST and earlier VLT data (≈107 mas yr⁻¹, θ≈297°).

The authors place their V‑band flux into the broader spectral energy distribution (SED). When extrapolating the X‑ray blackbody model (derived from XMM‑Newton EPIC‑pn spectra) into the optical, the predicted V magnitude is about 1 mag fainter than observed, confirming the well‑known “optical excess” of RX J0720.4‑3125. The excess persists across the U, B, V, and R bands and is roughly a factor of seven higher than the simple thermal model predicts. The paper discusses possible origins: non‑uniform surface temperature, magnetized atmosphere effects, or additional non‑thermal emission components (e.g., magnetospheric processes). The authors also note that the X‑ray temperature of the source varies on year‑long timescales (86.5–94.6 eV), which may be linked to the optical behaviour, but the exact mechanism remains unresolved.

In summary, this work provides the first direct V‑band detection of RX J0720.4‑3125, delivers a high‑precision proper motion measurement consistent with earlier studies, and reinforces the presence of a significant optical excess relative to X‑ray thermal emission. The results contribute valuable constraints for models of isolated neutron star emission and underline the need for continued multi‑wavelength monitoring and high‑resolution spectroscopy to unravel the physical processes governing the optical/UV output of these objects.