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Neutron star cooling implications and magnetic field of the Vela Junior central compact object from all XMM-Newton and Chandra spectra

Wynn C. G. Ho, Esther Simkhayeva, Alexander Y. Potekhin

TL;DR

This study performs a uniform, multi-instrument analysis of all high-quality X-ray spectra of the Vela Junior CCO from 2001–2010, using XMM-Newton and Chandra data to probe the neutron star's surface emission, magnetic field, and cooling state. Through joint fits with NS atmosphere models (nsx, nsmaxg) and multi-component thermal scenarios, the authors find a predominantly thermal spectrum with a hot, compact spot (kT^∞ ≈ 0.40 keV) atop a cooler surface (kT^∞ ≈ (0.66–0.88) MK), and potential absorption features near ~0.6 keV and ~0.9 keV that point to a surface magnetic field of ~3×10^10 G. The inferred luminosities at a distance of 1.4 kpc imply L^∞_hot ≈ 5×10^32 erg s^-1 and L^∞_cool ≈ (1.3–4.0)×10^32 erg s^-1, with a bolometric L^∞ ≈ (5±1)×10^32 erg s^-1 dominated by the hot spot. The results suggest that Vela Junior is colder than many young neutron stars, consistent with a relatively high mass that activates fast neutrino cooling, potentially via direct Urca processes for EOSs like BSk24 above a threshold around M_th ≈ 1.59 M_⊙. By demonstrating that atmosphere-based modelling with absorption features provides a robust interpretation of the data, the paper strengthens Vela Junior's role as a key laboratory for dense matter and NS cooling physics.

Abstract

The central compact object (CCO) in the Vela Junior supernova remnant is a young neutron star whose relatively low X-ray flux and small distance suggest it has a mass high enough to activate fast neutrino cooling processes. Here we analyse all XMM-Newton MOS and pn and Chandra ACIS-S spectra of the Vela Junior CCO, with observations taking place over the 9 years from 2001 to 2010. We find that the best-fit flux and spectral model parameters do not vary significantly when treating each observation independently, and therefore we fit all the spectra simultaneously using various spectral models to characterize the predominantly thermal emission from the neutron star surface. Our results indicate the Vela Junior CCO has an atmosphere composed of hydrogen, a hot spot temperature (unredshifted) of 3.5x10^6 K, and a colder surface temperature of (6.6-8.8)x10^5 K. Possible absorption lines at ~0.6 keV and 0.9 keV provide evidence for the first-time of an average surface magnetic field B~3x10^10 G for this CCO, which is similar to the magnetic field of other CCOs. At the accurate new Vela Junior distance of 1.4 kpc, the observed luminosity that is dominated by the hot spot is ~5x10^32 erg s^-1. The luminosity from the rest of the colder surface is (1.3-4.0)x10^32 erg s^-1. The cool luminosity and temperature imply the Vela Junior CCO is indeed colder than many other young neutron stars and probably has a high mass that triggered fast neutrino cooling.

Neutron star cooling implications and magnetic field of the Vela Junior central compact object from all XMM-Newton and Chandra spectra

TL;DR

This study performs a uniform, multi-instrument analysis of all high-quality X-ray spectra of the Vela Junior CCO from 2001–2010, using XMM-Newton and Chandra data to probe the neutron star's surface emission, magnetic field, and cooling state. Through joint fits with NS atmosphere models (nsx, nsmaxg) and multi-component thermal scenarios, the authors find a predominantly thermal spectrum with a hot, compact spot (kT^∞ ≈ 0.40 keV) atop a cooler surface (kT^∞ ≈ (0.66–0.88) MK), and potential absorption features near ~0.6 keV and ~0.9 keV that point to a surface magnetic field of ~3×10^10 G. The inferred luminosities at a distance of 1.4 kpc imply L^∞_hot ≈ 5×10^32 erg s^-1 and L^∞_cool ≈ (1.3–4.0)×10^32 erg s^-1, with a bolometric L^∞ ≈ (5±1)×10^32 erg s^-1 dominated by the hot spot. The results suggest that Vela Junior is colder than many young neutron stars, consistent with a relatively high mass that activates fast neutrino cooling, potentially via direct Urca processes for EOSs like BSk24 above a threshold around M_th ≈ 1.59 M_⊙. By demonstrating that atmosphere-based modelling with absorption features provides a robust interpretation of the data, the paper strengthens Vela Junior's role as a key laboratory for dense matter and NS cooling physics.

Abstract

The central compact object (CCO) in the Vela Junior supernova remnant is a young neutron star whose relatively low X-ray flux and small distance suggest it has a mass high enough to activate fast neutrino cooling processes. Here we analyse all XMM-Newton MOS and pn and Chandra ACIS-S spectra of the Vela Junior CCO, with observations taking place over the 9 years from 2001 to 2010. We find that the best-fit flux and spectral model parameters do not vary significantly when treating each observation independently, and therefore we fit all the spectra simultaneously using various spectral models to characterize the predominantly thermal emission from the neutron star surface. Our results indicate the Vela Junior CCO has an atmosphere composed of hydrogen, a hot spot temperature (unredshifted) of 3.5x10^6 K, and a colder surface temperature of (6.6-8.8)x10^5 K. Possible absorption lines at ~0.6 keV and 0.9 keV provide evidence for the first-time of an average surface magnetic field B~3x10^10 G for this CCO, which is similar to the magnetic field of other CCOs. At the accurate new Vela Junior distance of 1.4 kpc, the observed luminosity that is dominated by the hot spot is ~5x10^32 erg s^-1. The luminosity from the rest of the colder surface is (1.3-4.0)x10^32 erg s^-1. The cool luminosity and temperature imply the Vela Junior CCO is indeed colder than many other young neutron stars and probably has a high mass that triggered fast neutrino cooling.
Paper Structure (7 sections, 1 equation, 7 figures, 7 tables)

This paper contains 7 sections, 1 equation, 7 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Data analysis
  3. Chandra data
  4. XMM-Newton data
  5. Spectral modelling
  6. Results
  7. Discussion

Figures (7)

  • Figure 1: Spectra of Vela Junior from Chandra ACIS-S and XMM-Newton MOS1+MOS2 and pn data. Top panel shows data with 1$\sigma$ errors (crosses) and spectral model made up of a single blackbody. Bottom panel shows $\chi^2=\hbox{(data-model)/error}$.
  • Figure 2: Interstellar absorption $N_{\rm H}$ and blackbody temperatures $kT^\infty$ and $kT_2^\infty$ and emission radii $R^\infty$ and $R_2^\infty$ (relative to distance $d$) for a bbodyrad+bbodyrad (BB+BB) model fit to ACIS-S (circles), MOS (crosses), and pn (squares) spectra. Errors are 1$\sigma$. Some values for the ACIS-S and MOS 2001 fits are not shown because they lie outside displayed ranges due to these data being of lower quality (see text for details). Horizontal solid lines show results for a simultaneous fit to all the data, and the shaded regions encompass the 1$\sigma$ error.
  • Figure 3: Interstellar absorption $N_{\rm H}$, blackbody temperature $kT^\infty$ and emission radius $R^\infty$ (relative to distance $d$), and power law index and normalization for a bbodyrad+powerlaw (BB+PL) model fit to ACIS-S (circles), MOS (crosses), and pn (squares) spectra. Errors are 1$\sigma$. Some values for the ACIS-S and MOS 2001 fits are not shown because they lie outside displayed ranges due to these data being of lower quality (see text for details). Horizontal solid lines show results for a simultaneous fit to all the data, and the shaded regions encompass the 1$\sigma$ error.
  • Figure 4: Spectra of Vela Junior from Chandra ACIS-S and XMM-Newton MOS1+MOS2 and pn data. Top panel shows data with 1$\sigma$ errors (crosses) and spectral model (nsx+PL) made up of a partially ionized non-magnetic hydrogen atmosphere component (dotted lines) and a power law component (dashed lines). Bottom panel shows $\chi^2=\hbox{(data-model)/error}$.
  • Figure 5: Spectra of Vela Junior from Chandra ACIS-S and XMM-Newton MOS1+MOS2 and pn data. Top panels show data with 1$\sigma$ errors (crosses) and spectral model made up of two partially ionized non-magnetic (nsx+nsx; left) and magnetic (nsmaxg+nsmaxg; right) hydrogen atmosphere components (dashed and dotted lines for cool and hot components, respectively). For the models on the right, the magnetic fields are $B=3.16\times10^{10}\hbox{G}$ and $7\times10^{11}\hbox{G}$ for the cool and hot components, respectively. Bottom panels show $\chi^2=\hbox{(data-model)/error}$.
  • ...and 2 more figures