A state change in the low-mass X-ray binary XSS J12270-4859 (1402.0765v1)

Abstract: Millisecond radio pulsars acquire their rapid rotation rates through mass and angular momentum transfer in a low-mass X-ray binary system. Recent studies of PSR J1824-2452I and PSR J1023+0038 have observationally demonstrated this link, and they have also shown that such systems can repeatedly transition back-and-forth between the radio millisecond pulsar and low-mass X-ray binary states. This also suggests that a fraction of such systems are not newly born radio millisecond pulsars but are rather suspended in a back-and-forth state switching phase, perhaps for giga-years. XSS J12270-4859 has been previously suggested to be a low-mass X-ray binary, and until recently the only such system to be seen at MeV-GeV energies. We present radio, optical and X-ray observations that offer compelling evidence that XSS J12270-4859 is a low-mass X-ray binary which transitioned to a radio millisecond pulsar state between 2012 November 14 and 2012 December 21. Though radio pulsations remain to be detected, we use optical and X-ray photometry/spectroscopy to show that the system has undergone a sudden dimming and no longer shows evidence for an accretion disk. The optical observations constrain the orbital period to 6.913+-0.002 hr.

A Comprehensive Analysis of State Changes in the Low-Mass X-ray Binary XSS J12270−4859

The paper under discussion presents a detailed observational paper of the low-mass X-ray binary XSS J12270−4859 and its transition from an accretion-dominated state to a rotation-powered millisecond pulsar (MSP) state. Such transitions provide critical insights into the evolutionary link between low-mass X-ray binaries (LMXBs) and millisecond pulsars, broadening our understanding of these complex astrophysical systems.

Summary of Observations and Evidence

Through coordinated optical, X-ray, and radio observations covering several epochs, the authors provide compelling evidence for the state transition of XSS J12270−4859. The key findings include:

1. Optical and X-ray Observations: A significant dimming was observed in both optical and X-ray wavelengths, correlating with the disappearance of features associated with an accretion disk. Optical data revealed a sudden decrease in brightness by 1.5 to 2 magnitudes, while X-ray flux dropped by an order of magnitude, consistent with a reduction in accretion activity.
2. Spectral Analysis: The optical spectrum post-transition showed absorption lines indicative of a late-G/early-K type companion, suggesting irradiation effects and validating the cessation of accretion disk dominance.
3. Radio Observations: A previously detected continuum radio source associated with XSS J12270−4859 was observed to have significantly decreased by a factor of at least six, indicating a change in the radio emission mechanism coinciding with the state transition.
4. Pulsation Searches: Despite extensive searches, radio pulsations were not detected. However, this non-detection might be due to geometric factors or insufficient sensitivity rather than the absence of a pulsar.

Implications and Theoretical Context

The transition of XSS J12270−4859 provides important data for the paper of transitional MSPs, systems that vacillate between accreting and pulsar states. These findings lend support to the hypothesis that MSPs acquire their rapid spin through sustained accretion from an LMXB phase before transitioning to be observable as radio pulsars. The high mass ratio observed in XSS J12270−4859 also opens new avenues for understanding the dynamics in similar binaries.

Additionally, the detection of gamma-ray variability in LMXBs like XSS and similar systems introduces new questions about the high-energy processes occurring during these state transitions, potentially related to changes in the magnetosphere or shock regions within the systems.

Future Prospects

Future work should focus on enhancing the sensitivity of radio pulsation searches and extending multi-wavelength observational campaigns to capture similar state transitions. In particular, identifying the lag between X-ray and radio/MSP states can provide constraints on the mechanisms that regulate these transitions, enriching evolutionary models of neutron stars in binary systems. Long-term monitoring will be essential for capturing rare transitional events, and investigating possible gamma-ray emissions during accretion will further clarify the role of LMXBs in the high-energy astrophysical landscape.

In conclusion, the state change of XSS J12270−4859 from an LMXB to an MSP-like state marks a significant contribution to our understanding of binary evolution and highlights the need for continuous multi-wavelength surveillance of these dynamic systems. The observational techniques and findings presented provide a robust framework that will undoubtedly stimulate further research into the dynamic lives of LMXBs and their transformation into MSPs.