| Name: SAYANTAN BHATTACHARYA |
| Affiliation: Tata Institute of Fundamental Research |
| Conference ID: ASI2026_371 |
| Title: On the Origin of Long Outbursts in Short-Period Neutron Star Low-Mass X-ray Binaries |
| Abstract Type: Poster |
| Abstract Category: High Energy Phenomena, Fundamental Physics and Astronomy |
| Author(s) and Co-Author(s) with Affiliation: Sayantan Bhattacharya(Tata Institute of Fundamental Research, Mumbai - 400005, India), Sudip Bhattacharyya(Tata Institute of Fundamental Research, Mumbai - 400005, India) |
| Abstract: Neutron star low-mass X-ray binaries (NS LMXBs) with short orbital periods (~hours) typically alternate between outburst and quiescent phases, making them powerful probes of transient accretion processes. These outbursts are commonly explained by the thermal–viscous instability in the accretion disc. However, a small subset of transient systems displays unusually long-lasting outbursts that are difficult to reconcile with this standard framework. One notable example is EXO 0748–676, which remained active for at least 23 years before entering quiescence and later went into another outburst after a gap of 16 years. In this work, we examine whether such extended outbursts can be produced by the conventional disc instability mechanism or whether additional physics is required. We perform a systematic comparison of NS LMXBs with long and short outbursts, restricting our sample to systems with short orbital periods. By analyzing long-term X-ray light curves, we determine outburst durations, estimate mass accretion rates, and infer accretion disc masses. We find that sources with long outbursts are distinctly separated from those with short outbursts across multiple parameter spaces involving accretion rate, disc mass, and outburst duration. This clear separation suggests that the thermal–viscous instability can account for short outbursts but is insufficient to explain the long-duration events. We further discuss the challenges faced by both disc-based and donor-star-driven models in reproducing the properties of long outbursts. Our results place important constraints on the nature of accretion in transient neutron star and black hole X-ray binaries. (https://arxiv.org/abs/2504.07621) |
