[Paper Review] A neutron star progenitor for FRBs? Insights from polarisation measurements

Vikram Ravi, P. D. Lasky|arXiv (Cornell University)|Jan 22, 2016

Pulsars and Gravitational Waves Research3 citations

TL;DR

This paper investigates whether FRB 110523 originates from a rotating, magnetized neutron star by analyzing its polarisation position angle (PA) variation. Using the rotating vector model (RVM), it finds that if the progenitor is slowly rotating, emission likely originates near a magnetic pole; if rapidly rotating, emission geometry cannot be constrained. The PA variation strongly supports a neutron star origin with pulsar-like emission mechanisms.

ABSTRACT

Fast Radio Bursts (FRBs) are intense, millisecond-duration broadband radio transients, the emission mechanisms of which are not understood. Masui et al. recently presented Green Bank Telescope observations of FRB 110523, which displayed temporal variation of the linear polarisation position angle (PA). This effect is commonly seen in radio pulsars and is attributed to a changing projected magnetic field orientation in the emission region as the star rotates. If a neutron star is the progenitor of this FRB, and the emission mechanism is pulsar-like, we show that the progenitor is either rapidly rotating, or the emission originates from a region of complex magnetic field geometry. The observed PA variation could also be caused by propagation effects within a neutron-star magnetosphere, or by spatially varying magnetic fields if the progenitor lies in a dense, highly magnetised environment. Although we urge caution in generalising results from FRB 110523 to the broader FRB population, our analysis serves as a guide to interpreting future polarisation measurements of FRBs, and presents another means of elucidating the origins of these enigmatic ephemera.

Motivation & Objective

• To assess whether the observed polarisation position angle (PA) variation in FRB 110523 is consistent with a rotating, magnetized neutron star progenitor.
• To determine constraints on the magnetic and rotation axis geometry of the progenitor using the rotating vector model (RVM).
• To evaluate whether the observed PA variation is intrinsic or affected by propagation effects such as scattering and Faraday rotation.
• To explore the implications of these findings for understanding the origin of fast radio bursts (FRBs) and their potential link to pulsars or magnetars.
• To guide future polarisation measurements of FRBs by establishing a framework for interpreting PA variations in neutron star models.

Proposed method

• The rotating vector model (RVM) is applied to the observed PA variation of FRB 110523 to infer the orientation of the magnetic and rotation axes of a potential neutron star progenitor.
• The analysis accounts for observed scattering and Faraday rotation by modeling the pulse profile and polarisation evolution through a turbulent, magnetized plasma environment.
• Two scenarios are considered: intrinsic PA variation of 20° over 2 ms (as reported) and 40° over 2 ms (estimated intrinsic variation after correcting for scattering).
• The RVM equation is used to compute the expected PA evolution as a function of rotation phase, magnetic obliquity α, and line-of-sight inclination β.
• The effects of dispersion smearing and coherent dedispersion are evaluated to assess whether the intrinsic pulse width is resolved.
• The model compares the observed PA evolution to predictions from pulsar-like emission, considering both aligned and misaligned rotator configurations.

Experimental results

Research questions

• RQ1 Is the observed polarisation position angle (PA) variation in FRB 110523 consistent with emission from a rotating, magnetized neutron star?
• RQ2 What constraints can be placed on the magnetic obliquity α and line-of-sight inclination β if the progenitor is a slowly-rotating neutron star?
• RQ3 How does scattering and Faraday rotation affect the interpretation of the observed PA variation, and what is the likely intrinsic PA variation?
• RQ4 Can the RVM, successful for pulsars, be applied to FRBs to infer emission geometry and progenitor properties?
• RQ5 To what extent can the results from FRB 110523 be generalized to the broader FRB population?

Key findings

• The observed PA variation of ~20° over 2 ms in FRB 110523 is consistent with the rotating vector model (RVM), supporting a neutron star progenitor.
• If the progenitor is slowly rotating (P > 0.1 s), the emission must originate from within β < 20° of the magnetic pole, and β < 10° after correcting for scattering effects.
• For rapidly rotating neutron stars (P < 0.1 s), the emission geometry cannot be constrained using the RVM due to the rapid rotation phase evolution.
• The intrinsic PA variation is estimated to be ~40° over 2 ms, twice the observed variation, after accounting for scattering broadening.
• The results suggest that FRB 110523 is consistent with pulsar-like emission from a neutron star, but the origin of other FRBs remains uncertain.
• Propagation effects such as spatially varying magnetic fields or magnetospheric effects could also explain the PA variation, introducing uncertainty in RVM interpretation.