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arXiv · 2411.10968

Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER

Suchismito Chattopadhyay·Ranjeev Misra·Soma Mandal·Akash Garg·Sanjay K Pandey
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Abstract

We present the broadband spectral and timing properties of the atoll source 4U 1702-429 using two observations of AstroSat with the second one having simultaneous NICER data. For both observations, the spectra can be represented by a Comptonizing medium with a black body seed photon source which can be identified with the surface of the neutron star. A disk emission along with a distant reflection is also required for both spectra. For the first observation, the coronal temperature ($\sim 7$ keV) is smaller than the second ($\sim 13$ keV), and the disk is truncated at a larger radius, $\sim 150$ km, compared to the second, $\sim 25$ km, for an assumed distance of 7 kpc. A kHz QPO at $\sim 800$ Hz is detected in the first and is absent in the second observation. Modeling the energy-dependent r.m.s and time lag of the kHz QPO reveals a corona size of $\leq$ 30 km. A similar model can explain the energy dependence of the broadband noise at $\sim 10$ Hz for the second observation. The results suggest that kHz QPOs are associated with a compact corona surrounding the neutron star and may occur when the disk is truncated at large distances. We emphasize the need for more wide-band observations of the source to confirm these results.

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Suchismito Chattopadhyay, Ranjeev Misra, Soma Mandal, Akash Garg, Sanjay K Pandey. 2024-11-17. Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER. https://doi.org/10.3847/1538-4357%2Fad9332

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