Photospheric radius expansion thermonuclear burst and X-ray reflection from the neutron star X-ray binary 4U 1702-429

TL;DR

This study analyzes thermonuclear bursts from neutron star binary 4U 1702-429, revealing photospheric radius expansion, spectral features, and disk reflection, providing insights into neutron star properties and accretion processes.

Contribution

It presents the first detailed spectral analysis of PRE and non-PRE bursts from 4U 1702-429 using NICER, XMM-Newton, and NuSTAR data, including neutron star radius and magnetic field estimates.

Findings

Photospheric radius expanded to ~23 km during PRE

Disk reflection features detected with NuSTAR

Magnetic field strength estimated at 5.1×10^8 G

Abstract

We perform a comprehensive study of thermonuclear bursts from the neutron star low-mass X-ray binary 4U 1702-429 detected with NICER and XMM-Newton. The thermonuclear burst detected with NICER shows clear evidence of a photospheric radius expansion (PRE) event and a distinct feature in the burst profile. The burst profiles demonstrate significant energy dependence, with the hardness ratio varying notably during the PRE phase. The radius of the neutron star photosphere expanded to a maximum of $23.1_{-3.2}^{+3.8}$ km while its temperature reached a minimum of 1.4 keV. The time-resolved burst spectra can be modeled using variable persistent emission method, indicating that the soft excess may arise from enhanced mass accretion onto the neutron star, potentially due to the Poynting-Robertson drag. Alternatively, the disk reflection model can be used to explain the soft excess emission during a burst. The time-resolved spectral study is performed for three thermonuclear bursts detected with XMM-Newton. The XMM-Newton time-resolved burst spectra can be modeled using an absorbed blackbody model, without any signatures of the PRE. We conduct a detailed spectral analysis of the 2025 NuSTAR observation of 4U 1702-429, revealing a broad iron line at 6.4 keV and a Compton hump around 20 keV, indicating X-ray reflection features. The disk reflection model relxill provides an inner disk radius of 12 $R_g$ and an inclination angle of $\sim39^{\circ}$. The magnetic field strength at the pole of the neutron star is estimated to be 5.1 $\times10^8$ G, assuming that the accretion disk is truncated at magnetosphere boundary.