UVOIR spectrum, X-ray emission, and proper motion of the isolated neutron star RX J2143.0+0654

TL;DR

RX J2143.0+0654 exhibits a multiwavelength spectrum best described by a thermal Rayleigh-Jeans tail in the UV plus a nonthermal power-law component in the NIR/optical, while contemporaneous X-ray data require a two-temperature blackbody model with absorption features near $0.74$ keV and potentially $0.4$ keV. A joint UVOIR+X-ray fit constrains the hot surface to $kT_{\rm hot}\approx 105$--$106$ eV with $R_{\rm hot}/d_{260}\approx1.5$ km, and the cold surface to $kT_{\rm cold}\approx40$--$50$ eV with $R_{\rm cold}/d_{260}\approx5.5$--$7$ km, indicating substantial nonuniform heating and possible condensed-surface emission. The UVOIR spectrum is well described by $f_\nu^{\rm mod}=[f_0(\nu/\nu_0)^\alpha+(R_{\rm uv}^2/d^2)\pi B_\nu(T_{\rm uv})]10^{-0.4A_\nu}$ with $\alpha\approx-0.8$ and a Rayleigh-Jeans UV tail, while an additional faint extended NIR component suggests a surrounding nebula or disk-like structure. A measured proper motion of $\mu\approx5.8$ mas yr$^{-1}$ at $d\approx260$ pc yields a transverse velocity $v_\perp\approx7$ km s$^{-1}$ and supports a relatively recent birth scenario, potentially linked to the $\beta$ Pic–Cap stellar association. Overall, the results place RX J2143.0+0654 among XTINSs with mixed thermal and nonthermal UVOIR emission, offering insights into condensed-surface physics, internal heating, and the late-stage evolution of isolated neutron stars.

Abstract

We observed the isolated neutron star RX J2143.0+0654 with the Hubble Space Telescope (HST) in the UVOIR wavelength range (0.14-1.7 $μ$m). The UV part is consistent with a Rayleigh-Jeans tail of a thermal spectrum, $f_ν\propto ν^2$, while a power-law spectrum, $f_ν\propto ν^α$ with $α\sim -0.8$, dominates in the NIR-optical. A joint fit of the UVOIR and contemporaneous X-ray spectra with a two-component blackbody with possible absorption features + power-law optical spectrum yields the following temperature and apparent radius of the colder component (which gives the main contribution in the UV): $kT_{\rm cold}\approx 45$ eV, $R_{\rm cold}\approx 6 d_{260}$ km, where $d_{260}$ is the distance in units of 260 pc. The temperature and radius of the hotter component, $kT_{\rm hot}\approx 106$ eV and $R_{\rm hot} \approx 1.5d_{260}$ km, the parameters of an absorption feature at 0.74 keV, and the properties of X-ray pulsations, are the same as found in previous X-ray observations. In the NIR images the neutron star is possibly surrounded by extended emission with a characteristic size of $\sim 2''$ and flux densities of about 1.7 and 0.9 $μ$Jy at 1.54 and 1.15 $μ$m, respectively. Comparison with a previous HST observation in the optical 14 years ago shows a proper motion $μ\approx 6$ mas yr$^{-1}$, which corresponds to a small transverse velocity of $7d_{260}$ km s$^{-1}$. It is consistent with the hypothesis that the neutron star was born in the vicinity of the solar system about 0.5 Myr ago.

Figures (9)

• Figure 1: Images of the J2143 field, observed in five HST filters; north is up, east to the left. The position of the pulsar is indicated by blue marks. Except for the bottom right, the images are of the same angular sizes, $\approx16"\times 11"$. To illustrate the enhanced emission near the location of the pulsar, the bottom right image shows a zoom-out of the WFC3/IR F160W image in the middle-bottom panel, with the red box marking the same sky area. The F475W (top right) image also shows the (dark-cyan) proper motion vector (see Section \ref{['sec:propmot']}).
• Figure 2: Dependence of the extinction coefficient $A_V$ on distance in the direction to J2143. The blue line corresponds to data from Vergely2022, obtained via the G-Tomo tool Lallement2022 that is available at ESA Datalabs. The red line shows the model by Edenhofer2024. Shaded regions mark the $3\sigma$ uncertainty ranges.
• Figure 3: UVOIR spectrum of J2143. The data points with vertical error bars show the measured mean flux densities $\langle f_\nu\rangle$ with the statistical $1\sigma$ errors; they are plotted at pivot wavelengths. The shaded areas show the filter throughputs (arbitrarily scaled). The solid curve shows the best fit for the PL+BB model (see Equation \ref{['eq:PL+BB_model']}) for $A_V=0.12$ at the fixed $R_{10}/d_{260} = 1$. The dashed curves show the PL and BB components. The dotted curve shows the dereddened best-fit spectrum.
• Figure 4: Confidence contours for the UVOIR spectral fit with the PL+BB model (see Figure \ref{['fig:uvoir_spectrum']}) at 68.3% and 99.7% confidence levels (for two parameters of interest) in the $\alpha$-$kT$ plane for $R_{10}/d_{260} =1$ at the extinction values shown in the legend.
• Figure 5: Pulsations of J2143 with frequency $\nu=0.1060619$ Hz in the 0.15--2 keV energy range, obtained from the EPIC pn data. The red dashed lines display the $k\leq 3$ Fourier harmonics. The solid blue line shows their sum; its 68% uncertainty is indicated by the shaded area. The horizontal dotted line at 4053 counts/bin shows the constant component of the phase-folded light curve. The zero phase corresponds to the epoch MJD 60443.6296296 (TDB). The histogram demonstrates the more traditional binned profile, with 16 bins.