NICER view of XTE J1701−462: A super-Eddington Outburst

XTE J1701-462, a transient neutron star low-mass X-ray binary (NS-LMXB), was first discovered during a 2006 outburst by the Rossi X-ray Timing Explorer. This peculiar source exhibits characteristics of all NS-LMXB sub-classes, potentially serving as a key to understanding their distinctions. The 2006 outburst lasted approximately 560 days, with transitions through Cygnus-like and Sco-like Z-source patterns, banana branches featuring type I X-ray bursts, and an island state before quiescence. In September 2022, XTE J1701-462 underwent a second, shorter outburst lasting ~190 days. We analyzed its spectral evolution using 62 observational epochs from the Neutron Star Interior Composition Explorer (NICER). The 0.3–12.0 keV light curve displayed a Fast-Rise Exponential-Decay (FRED) profile, similar to its 2006 outburst. Hardness-intensity diagrams (HIDs) revealed a ν-shaped pattern during the early stages, evolving into a C-shaped trajectory and isolated clumps in the decay phase, distinct from the Z-shaped tracks of the 2006 event, likely due to differences in duration and energy band selection. Spectral modeling combined an absorbed (tbfeo) blackbody (bbodyrad), a power-law for Comptonization, and a Gaussian component for Fe emission lines. The blackbody radius evolved from ~15.59 km at onset to a peak of ~53.65 km at maximum flux, contracting to ~0.81 km by quiescence, consistent with hotspot emission on the neutron star surface. Anti-correlation between blackbody radius and temperature supports this interpretation. Flux analysis revealed FRED-like trends in total, blackbody, and power-law fluxes, with blackbody flux dominating the rise phase and power-law flux prevailing in decay. The source reached super-Eddington luminosities up to 1.76FEdd​, consistent with its 2006 behavior. These findings offer valuable insights into the spectral evolution of XTE J1701-462, enhancing our understanding of NS-LMXB outbursts and will be presented at the conference.