Evidence for Atomic Absorption Features in the High Resolution X-ray Spectrum of the Neutron Star in Puppis A

Abstract

We present evidence for atomic absorption lines in the high-resolution 4-30 A X-ray spectrum of the neutron star RX J0822-4300 in the supernova remnant Puppis A. Comparison with model atmosphere calculations shows that features in the observed spectrum can be uniquely associated with redshifted and pressure-broadened transitions in highly ionized oxygen and neon. We also spectroscopically confirm the previously estimated strength of the surface magnetic dipole field; we detect both the linear and the quadratic Zeeman effect. We derive values for both the gravitational redshift and the acceleration of gravity at the stellar surface, yielding the first purely spectroscopic estimates for the radius and mass of a neutron star.

Figures (9)

• Figure 1: Model atmosphere spectrum for a hot neutron star with an oxygen-neon dominated atmosphere. The effective temperature is $T_{\rm eff} = 4.6 \times 10^6$ K, $\log g = 14.6$, $B = 2.9 \times 10^{10}$ Gauss. The most prominent atomic features have been labeled. The wavelength scale is the rest frame of the neutron star (no redshift). A nomogram for the corresponding photon energies is displayed at the top; a simple conversion is $E [{\rm keV}] = 12.3984/\lambda[{\mathrm\AA}]$.
• Figure 2: As \ref{['fig:2']}, but for a higher temperature, $T_{\rm eff} = 5.9 \times 10^6$ K, and slightly different composition; note the S XV and XVI edges at 3.5 Å. The wavelength scale is the rest frame of the neutron star (no redshift).
• Figure 3: Model atmosphere spectrum at $T_{\rm eff} = 5.3 \times 10^6$ K, $\log g = 14.6$, $B = 2.9 \times 10^{10}$ Gauss, plotted on a linear flux scale. Note that this figure shows the photon flux, which is more relevant for a comparison to an observed photon count spectrum. The wavelength scale is the rest frame of the neutron star (no redshift).
• Figure 4: Observed spectrum of RX J$0822-4300$, background-subtracted, binned in 0.1 Å bins, as a function of wavelength (black histogram). The expected Poisson fluctuations in the spectrum are shown as the red curve. The blue curve is a model composed of two blackbodies at $kT_{\rm BB} = 4.69 \times 10^6$ K and $2.53 \times 10^6$ K (as seen by a distant observer), in the ratio 1.00:0.12, absorbed by $N_{\rm H} = 5.8 \times 10^{21}$ cm$^{-2}$. The wavelength scale is Chandra's wavelength scale (so models have been redshifted).
• Figure 5: Observed spectrum of RX J$0822-4300$, background-subtracted, binned in 0.2 Å bins, as a function of wavelength (black histogram). The blue curve is a model composed of two blackbodies at $kT_{\rm BB} = 4.69 \times 10^6$ K and $2.53 \times 10^6$ K (as seen by a distant observer), in the ratio 1.00:0.12, absorbed by $N_{\rm H} = 5.8 \times 10^{21}$ cm$^{-2}$. The lower panel shows the residuals to the 2-blackbody model, expressed as the ratio (data$-$model)/data. We suppressed the parts of the residual spectrum entirely dominated by noise fluctuations, outside the range $3.7-21$ Å. The mismatch between 3.7 and 7.0 Å stands out. A depression between 15 and 17 Å appears, as well as a positive feature between $\sim 17$ and $20$ Å. The 2-blackbody model has $\chi^2 = 209$ for 85 bins of 0.2 Å, in the $4-21$ Å range. The red histogram is a model atmosphere spectrum, with $\chi^2 = 107$ for 85 bins of 0.2 Å over the same range; residuals to this model are shown in the lowest panel, analogous to the residuals to the 2-blackbody model.