| Name: Sachindra Naik |
| Affiliation: Physical Research Laboratory |
| Conference ID: ASI2026_11 |
| Title: Photospheric Radius Expansion and Burst-Driven X-ray Reflection in NS LMXB 4U 1702–429 |
| Abstract Type: Oral |
| Abstract Category: Stars, Interstellar Medium, and Astrochemistry in Milky Way |
| Author(s) and Co-Author(s) with Affiliation: Sachindra Naik(Physical Research Laboratory, Ahmedabad - 380009, India), Manoj Mandal(Physical Research Laboratory, Ahmedabad - 380009, India), Gaurava K. Jaisawal(DTU Space, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark) |
| Abstract: Thermonuclear X-ray bursts arise from the unstable burning of material accreted from a low-mass companion onto the surface of a neutron star. Such bursts are commonly observed in low-mass X-ray binaries hosting weakly magnetized neutron stars (∼10⁷–10⁹ G). In some cases, the burst luminosity reaches the Eddington limit, causing the neutron-star photosphere to expand and producing a photospheric radius expansion (PRE) event. We present a comprehensive study of thermonuclear bursts from the neutron-star low-mass X-ray binary 4U 1702–429 using NICER and XMM-Newton observations. The NICER burst shows clear evidence of a PRE event along with a distinct feature in the burst profile. The burst exhibits strong energy dependence, with the hardness ratio varying significantly during the PRE phase. During the expansion, the photospheric radius reaches a maximum of ∼25 km, while the temperature drops to a minimum of ∼1.4 keV. Time-resolved spectroscopy of the NICER burst, modeled using the variable persistent emission approach, suggests that the observed soft excess may originate from enhanced mass accretion onto the neutron star. Alternatively, disc reflection during the burst can also account for the soft excess emission. We perform time-resolved spectral analysis of three thermonuclear bursts detected with XMM-Newton, which are well described by an absorbed blackbody model and show no signatures of PRE. Additionally, we analyze a 2025 NuSTAR observation of 4U 1702–429, revealing a broad Fe Kα line at ∼6.4 keV and a Compton hump near 20 keV, indicative of X-ray reflection in the persistent emission. Reflection modeling yields an inner disc radius of ∼24 km and an inclination of ∼39°, impliing a neutron-star polar magnetic field strength of ∼5.1 × 10⁸ G assuming disc truncation at the magnetospheric boundary. |
