Probing the accretion geometry of the atoll source 4U 1702-429 in different spectral states with NICER, NuSTAR, and AstroSat
arxiv(2024)
Abstract
We perform a comprehensive spectral study of a carefully selected sample
(total exposure ∼ 50.5 ks) of NICER observations of the atoll neutron star
low-mass X-ray binary 4U 1702-429. Our sample encompasses nearly all classical
spectral states found within the NICER dataset. We require two thermal emission
components, originating from the accretion disc and the boundary layer, to
describe the soft state spectra in the energy band 0.3-10.0 keV. In contrast,
in our model, only the disc component directly contributes to the
intermediate/hard state. Additionally, we use a thermally Comptonized component
(or a power-law with pegged normalization) to represent the hard coronal
emission in the soft and intermediate/hard state spectra. The boundary layer
emerges as the principal source providing soft seed photons for Comptonization
across all spectral states. In contrast to a previously held assertion
regarding this source, our analyses reveal a decrease in the inner disc
temperature coupled with the retreat of the inner disc from the neutron star
surface as the source evolves from the soft to the intermediate/hard state. The
reflection features are either absent or weak (∼ 3σ-4σ) in all
these observations. Further investigation using broad-band NuSTAR (3.0-50.0
keV) and AstroSat spectra (1.3-25.0 keV) shows a slightly stronger iron
emission line (∼ 5.8σ) in the NuSTAR spectra. However, this feature
is not significantly detected in the AstroSat observation. The AstroSat data
suggests a highly ionized disc, explaining the absence of reflection features.
In the case of NuSTAR, a truncated disc is likely responsible for the weak
reflection features.
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