A low mass and radius neutron star candidate in XTE J1810-189?

Abstract

Photosphere radius expansion (PRE) bursts provide a crucial tool for constraining the mass and radius of neutron stars. In this study, we analyze time-resolved spectroscopic data from XTE J1810-189 in 2008, which exhibit evidence of a PRE event. We report here the possibility of a small-size and low-mass neutron star in XTE J1810-189 with use of the advantage of the direct cooling tail method. We obtained three sets of results, which can be broadly divided into high metal abundance (20 $\rm{Z}_{\odot}$ and 40 $\rm{Z}_{\odot}$), low metal abundance and hydrogen-rich (pure hydrogen, $\rm{Z}_{\odot}$, 0.3 $\rm{Z}_{\odot}$, 0.1 $\rm{Z}_{\odot}$, 0.01 $\rm{Z}_{\odot}$), and pure helium. In the high-metallicity scenario, the inferred neutron star mass is $<1.3\,M_{\odot}$ with a radius $<8\,\rm{km}$. In the low-metallicity, hydrogen-rich case, the mass ranges from 0.3 to 2.1 $M_{\odot}$ with radii of 7-13 km. For a pure-helium composition, we find two mass solutions: $1.08_{-0.22}^{+1.32}M_{\odot}$ (with $R>14\,\rm{km}$) and $2.5-2.9\,M_{\odot}$ (above the highest observed neutron star masses). Additionally, we applied the touchdown method combined with an MCMC analysis, the results are consistent with those from the direct cooling tail method, but with a broader range. Our analysis of the time-resolved spectrum of burst suggests a high-metallicity atmosphere, but new observations are required to confirm this result.

Figures (8)

• Figure 1: Persistent emission spectrum of XTE J1810-189 (ObsID 93433-01-06-02). Red represents the tbabs$\times$(powerlaw+gau) model, and blue represents the tbabs$\times$(bbodyrad+powerlaw+gau) model. Solid lines, dash-dot line, dashed line, and dotted line correspond to the complete model, Gaussian, Powerlaw, and blackbody, respectively. The residuals related to the above two fitting models for each spectrum are also shown (bottom panel).
• Figure 2: Time-resolved spectra of PRE burst in XTE J1810-189 (ObsID 93433-01-06-02). The red dashed line labels the touchdown moment. The bold data points correspond to the fitting parameters at the touchdown moment.
• Figure 3: Fitting between the cooling tail data and the theoretical curve of the PRE burst. The black dots marked with error bars represent the observed $K$ - $F_{\text{BB}}$. The upper figure shows the results of Group 1, where the red dashed line represents $F$ = 0.4 $F_{\text{td}}$, and the data points on the left are not considered during the fitting process. The lower figure represents the results of Group 2, where the red dashed line represents $F$ = 0.5 $F_{\text{td}}$.
• Figure 4: Constraints on $M$, $R$, and $D$ obtained from direct cooling. The color mapping is consistent with Figure \ref{['fig:fit']} for models with different atmospheric composition. The curves represent the 99$\%$ confidence levels boundaries, with color blocks indicating the Group 1 and solid lines indicating to the Group 2. The gray dashed line represents the causality condition ($R_c = 1.45 R_S$), with the non-physical region above it shaded in gray.
• Figure 5: Posterior distribution of Group 1 for the mass, radius and distance parameters from the MCMC simulations, the contour lines from inside to outside represent confidence intervals of 68%, 90%, and 99%. The top tag displays the median, with its error corresponding to the 68% confidence level. The outermost points correspond to the marginal distribution of the parameters. Along the diagonal are the marginalized posteriors. The maximum likelihood estimates for the posteriors are indicated at the intersection of the two red lines.