High Energy Astrophysics 5 JAN, 2015

The extremely long period X-ray source in a young supernova remnant: a Thorne-Zytkow Object descendant?

The extremely long period X-ray source in a young supernova remnant: a Thorne-Zytkow Object descendant?

The origin of the 6.67 hr period X-ray source, 1E161348-5055, in the young supernova remnant RCW 103 is puzzling. We propose that it may be the descendant of a Thorne-Zytkow Object (TZO). A TZO may at its formation have a rapidly spinning neutron star as a core, and a slowly rotating envelope. We found that the core could be braked quickly to an extremely long spin period by the coupling between its magnetic field and the envelope, and that the envelope could be disrupted by some powerful bursts or exhausted via stellar wind. If the envelope is disrupted after the core has spun down, the core will become an extremely long-period compact object, with a slow proper motion speed, surrounded by a supernova-remnant-like shell. These features all agree with the observations of 1E161348-5055. TZOs are expected to have produced extraordinary high abundances of lithium and rapid proton process elements that would remain in the remnants and could be used to test this scenario.

💡 Research Summary

The paper tackles the long‑standing puzzle of the 6.67 hour X‑ray pulsar 1E 161348‑5055 (hereafter 1E 1613) located in the very young supernova remnant (SNR) RCW 103. Conventional explanations—such as a magnetar with a fallback disk, a binary system, or an ultra‑slow rotating neutron star formed directly after the supernova—fail to reproduce the combination of observed properties: an exceptionally long spin period, large X‑ray luminosity variations, a lack of any detectable companion, a very low proper motion (≈10 km s⁻¹), and the presence of a shell that resembles a supernova remnant rather than a typical pulsar wind nebula.

The authors propose that 1E 1613 is the descendant of a Thorne‑Żytkow Object (TZO), a hybrid star consisting of a neutron‑star core embedded in the envelope of a massive red supergiant. In the formation scenario, the neutron star initially spins rapidly (millisecond periods) and carries a strong dipolar magnetic field (∼10¹³ G). The surrounding envelope, inherited from the red supergiant, rotates much more slowly (periods of tens of days) and is highly conductive. The magnetic field threads the ionised envelope, establishing a magnetic‑plasma coupling that exerts a torque on the core. Using magnetohydrodynamic (MHD) torque estimates and angular‑momentum balance equations, the authors show that the core can be spun down from millisecond to hour‑scale periods within a few thousand seconds. The key point is that the torque scales with B²R⁶/ (GM) and the large moment of inertia of the envelope acts as an efficient angular‑momentum sink.

After the core has been braked, the envelope must be removed for the system to appear as an isolated compact object. Two removal channels are discussed. (1) A “core‑burst” triggered by the exotic nuclear reactions that TZO interiors are predicted to host (rapid proton‑capture processes, lithium production, etc.) can release enough energy to eject the envelope in a violent outburst, analogous to a low‑energy supernova. (2) Continuous, strong stellar winds driven by the high luminosity of the envelope can gradually strip it away over ∼10⁴ yr. In either case, once the envelope disappears, the neutron‑star core remains with a very long spin period, a strong magnetic field, and a low space velocity because it never received a large natal kick—the kick was absorbed by the massive envelope during the original supernova. Consequently, the core resides inside the pre‑existing SNR shell, exactly as observed for 1E 1613.

The model naturally explains several otherwise puzzling aspects of 1E 1613:

  • Extremely long period – magnetic coupling provides a rapid spin‑down mechanism that does not rely on a long‑lived fallback disk.
  • Low proper motion – the core inherits the bulk motion of the massive envelope, which is expected to be modest (∼10 km s⁻¹).
  • SNR‑like shell – the surrounding nebula is the original supernova remnant, not a pulsar wind nebula, because the neutron star’s wind is weak after spin‑down.
  • X‑ray variability – residual accretion of fallback material or sporadic magnetic reconnection events can produce the observed flares without requiring a binary companion.

A crucial, testable prediction of the TZO‑descendant hypothesis is the chemical imprint left by the exotic nucleosynthesis that occurs inside a TZO. The authors argue that the ejecta (or the remaining shell) should be enriched in lithium and rapid‑proton‑capture (rp‑process) isotopes such as ⁷Li, ⁹Be, ¹⁰B, ¹¹B, and ¹³C. High‑resolution X‑ray spectroscopy (e.g., with Chandra or XMM‑Newton) and optical/infrared spectroscopy of the nebular material could search for anomalous line strengths or abundance ratios. Detection of such signatures would provide strong support for the TZO origin, while their absence would favor alternative models.

In summary, the paper presents a self‑consistent evolutionary pathway from a Thorne‑Żytkow Object to an ultra‑slow rotating neutron star that matches the observed properties of 1E 1613. By coupling magnetic spin‑down with envelope removal, the authors resolve the period, motion, and environmental issues that have challenged previous explanations. The proposed nucleosynthetic fingerprints offer a concrete observational test, making the hypothesis both physically plausible and falsifiable.