Modeling surface radiation of rotating neutron stars with Monk-NS

Abstract

Neutron stars serve as unique laboratories for studying ultra-dense nuclear matter. The equation of state of neutron star matter can be effectively constrained by their masses and radii. Particular attention has been paid to rapidly rotating neutron stars, where strong relativistic effects leave imprints on their electromagnetic emission. To model the emission of rotating neutron stars in more realistic situations, especially when their surface emission is further re-processed by a scattering medium, we develop Monk-NS, a customized version of the general relativistic Monte-Carlo radiative transfer code Monk. We validate the code through a series of benchmarking tests, including computing the energy spectrum, pulse profile, and polarisation of rotating neutron stars, and comparing the results with those of the established codes in the X-ray timing community, yielding consistent outcomes. As an example to demonstrate Monk-NS's capabilities, we apply it to investigate various models proposed to explain the low pulsation amplitude of neutron star low-mass X-ray binaries. Our findings indicate that the dependence of the X-ray polarisation degree on the observer's inclination can serve as a key factor in distinguishing these models. We also find that complex hotspot morphologies yield polarisation properties different from those of circular hotspots.

Figures (16)

• Figure 1: The energy spectra of a thermally-emitting, non-rotating neutron star, with $M_{\rm NS}=1.4~\rm M_\odot$, $R_{\rm eq}=12~\rm km$, $T_{\rm NS}=0.35~\rm keV$, and $D=200~\rm pc$. The results obtained with the mc and o2e schemes are plotted in different colors and linestyles, as indicated in the plot. The flux density at $1~\rm keV$ calculated by bogdanov_constraining_2019 is $17.2279~\rm counts~s^{-1}~cm^{-2}~keV^{-1}$, which is marked by a black cross.
• Figure 2: A comparison of the pulse profile predictions between Monk-NS (dashed blue and dotted orange for the o2e and mc schemes, respectively) and bogdanov_constraining_2019 (black). The parameters used in tests SD1a through SD1f can be found in Table \ref{['tab:sd1']}.
• Figure 3: A comparison of the pulse profile predictions between Monk-NS (dashed blue and dotted orange for the o2e and mc schemes, respectively) and bogdanov_constraining_2019 (black), for tests OS1a--OS1f. The parameters can be found in Table \ref{['tab:sd1']}.
• Figure 4: A comparison of the pulse profile predictions between Monk-NS (dashed blue and dotted orange for the o2e and mc schemes, respectively) and bogdanov_constraining_2019 (black), for tests OS1g--OS1j. The parameters can be found in Table \ref{['tab:sd1']}.
• Figure 5: Comparison of the pulse profiles computed by Monk-NS (dashed blue and dotted orange for the o2e and mc schemes, respectively) with choudhury_exploring_2024 (black), assuming oblate neutron stars with hotspots of complex geometries (see Table \ref{['tab:c24']} for parameters).