Speaker
Description
The Galactic transient X-ray binary GRS J1739–278 was simultaneously monitored by the IXPE, NuSTAR, Swift and Insight-HXMT satellites during its recent 'failed' outburst in 2025. We have performed a broadband spectral evolution study of the source using Swift XRT, IXPE and NuSTAR data in the 1–50 keV band with different sets of phenomenological and physical models. The spectral fitting shows an increase in thermal flux and photon index, suggesting a transition from hard to intermediate spectral states. Spectral fitting with the physical relxill model provides an estimate of the dimensionless black hole spin parameter a ∼ 0.99 and an inclination angle of i ∼ 54◦. The evolution of color-color or hardness-intensity diagrams also indicates the outburst as a failed one. Prior to the main outburst, multiple short-duration low-intensity flares are seen. A Low-Frequency QPO of 0.32 mHz has been found in the 2-4 keV band for one of the HXMT data during the declining phase of the outburst. We also present a detailed spectro-polarimetric study of the source using data from IXPE and NSTAR satellites covering the observation period from September 29 to October 01, 2025. The polarimetric analysis in the 2–8 keV band using a model-independent method reveals a significant detection of a polarization degree (PD) of (2.47 ± 0.47)% at a polarization angle (PA) of 66◦ ± 5.5◦ (significant at ∼ 5σ). The spectral model-dependent polarization analysis using the polconst model yields consistent values of PD and PA. Using the model-independent method, we also observe an energy-dependent increasing trend in both PD and PA. In the 6–8 keV band, a maximum PD of (13.4 ± 3.8)% is detected at a PA of 78.6◦ ± 8.1◦ (significant at > 3σ).
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