Speaker
Description
We present a comprehensive broadband spectral and timing study of the bright atoll neutron star low-mass X-ray binary GX~9+9 using two simultaneous NICER, NuSTAR, and MAXI observations obtained in 2019 and 2022. During both epochs, the source resides in the lower banana branch of the color–color diagram, consistent with a soft spectral state. The broadband spectra in the $0.7-40$ keV band are adequately described by a combination of multicolor disk emission, thermal Comptonization of blackbody seed photons originating from the neutron star surface, and disk reflection. We employ both phenomenological and physical models, including blurred reflection models \texttt{relxillCp} and \texttt{relxillNs}, and find that both frameworks adequately describe the reflection features, including a prominent narrow Fe K emission line present in both observations. However, the reflection hump is found to be relatively stronger in the 2022 epoch. Our spectral analysis reveals clear evidence of spectral evolution between the two observations. The spectrum obtained from the 2022 observation shows a steeper Comptonized component, characterized by a lower optical depth, along with an increase in both the inner disk temperature and the seed blackbody temperature. Despite the soft spectral state, reflection modeling indicates a substantially truncated inner accretion disk in both epochs, while thermal Comptonization strongly dominates the overall source flux. Independent constraints from disk continuum modeling also support this truncated geometry, with a modest inward shift of the disk in 2022. The inferred reflection fraction in terms of the solid angle subtended by the reflector appears to be consistent with the truncated disk scenario. Timing analysis shows that the variability is dominated by the Comptonized component, while the disk remains relatively stable. We detect significant soft (negative) lags at high frequencies, which may be associated with thermal reprocessing of hard X-rays in a truncated accretion disk. The lag–energy spectra exhibit structured features, with deviations around the Fe K band, indicative of Fe K reverberation.
| Participate the oral/poster presentation award competition | No |
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