FRB-periodicity: mild pulsars in tight O/B-star binaries

Abstract: Periodicities observed in two Fast Radio Burst (FRB) sources (16 days in FRB 180916.J0158+65 and 160 days in FRB 121102) are consistent with that of tight, stellar mass binary systems. In the case of FRB 180916.J0158+65 the primary is an early OB-type star with mass loss rate $\dot{M} \sim 10^{-8}- 10^{-7} M_\odot$ yr$^{-1}$, and the secondary a neutron star. The observed periodicity is not intrinsic to the FRB's source, but is due to the orbital phase-dependent modulation of the absorption conditions in the massive star's wind. The observed relatively narrow FRB activity window implies that the primary's wind dynamically dominates that of the pulsar, $\eta = L_{sd}/(\dot{M} v_w c) \leq 1$, where $L_{sd} $ is pulsar spin-down, $\dot{M}$ is the primary's wind mass loss rate and $v_w$ is its velocity. The condition $\eta \leq 1$ requires mildly powerful pulsar with $L_{sd} \lesssim 10^{37}$ erg $s^{-1}$. The observations are consistent with magnetically-powered radio emission originating in the magnetospheres of strongly magnetized neutron stars, the classical magnetars.

• The paper proposes that FRB periodicity arises from the orbital motion in tight O/B-star binaries, where wind interactions create periodic transparency windows.
• It uses numerical estimates to show that moderate neutron star spin-down power (L_sd ≲ 10^37 erg s⁻¹) establishes a conical cavity affecting FRB visibility.
• The model predicts observable dispersion measure variations, frequency-dependent activity windows, and significant rotation measure effects, guiding future FRB observations.

An Analysis of FRB Periodicity due to Stellar Mass Binary Systems

The paper "FRB-periodicity: mild pulsars in tight O/B-star binaries" presents an intriguing investigation into the periodicities observed in Fast Radio Burst (FRB) sources, specifically the 16-day cycle of FRB 180916.J0158+65 and the 160-day cycle of FRB 121102. The authors propose that these periodicities can be explained by the orbital motion of tight, stellar-mass binary systems where a neutron star orbits a more massive early OB-type star. This analysis offers a fresh perspective on FRB mechanisms, challenging the intrinsic periodicity assumption of the bursts themselves.

Overview of the Model

The authors postulate that the periodicity observed in FRB 180916.J0158+65 arises from the orbital motion within a binary system rather than from the bursts themselves. This hypothesis is grounded on the modulation of the absorption conditions due to the interaction of the neutron star's wind with the wind of an early-type O/B star, whose mass loss rate falls between $\sim 10^{-8} \mbox{ to } 10^{-7} M_\odot$ per year. This results in predictable transparency windows for FRB emissions where the neutron star's magnetic activity becomes observable as it aligns optimally through the low-density cavity formed in the OB star's wind.

Numerical Estimates and Constraints

The researchers derive constraints for the system’s dynamics through numerical estimates. They assert that the wind interactions lead to a conical cavity that dynamically dominates and causes the observed periodicity. Crucially, the parameter $\eta = L_{sd}/(\dot{M} v_w c) \leq 1$ demands a moderate spin-down power for the pulsar ($L_{sd} \lesssim 10^{37} \mbox{ erg s}^{-1}$), thereby dismissing scenarios in which the pulsar’s wind is dominant, like those observed in the Black Widow systems.

Implications and Predictions

The paper predicts several observational outcomes resultant from the proposed model:

1. Dispersion Measure (DM) Variations: Slight DM variations are anticipated within the activity window due to line-of-sight changes through differing plasma densities in the OB star's wind.
2. Frequency-Dependent Activity Windows: The transparency window for FRBs is expected to broaden as the observation frequency increases, attributed to the frequency-dependent nature of free-free absorption.
3. Rotation Measure (RM): The model anticipates substantial RM effects due to interactions with the primary's magnetized wind, which remains a critical measure to observe.

Theoretical and Practical Implications

This research underscores the relevance of binary dynamics in understanding FRB emissions, suggesting that binary systems are a viable environment for these events. The indication that some FRBs may not be isolated occurrences but linked to massive binary systems could redefine observational strategies, pointing telescopes towards young and massive star formation regions intensively.

Theoretically, the findings bolster the argument for magnetospheric-origin FRBs, supporting the notion of magnetic emissions generated under the influence of a neutron star. This furthers the discourse on neutron star behavior, influencing models on magnetic fields, wind interactions, and emission mechanisms.

Conclusion and Speculations

While this analysis introduces compelling insights on the role of binary systems in FRB emissions, it does not claim that binarity is a universal characteristic inherent to all FRBs; other FRBs can feasibly reside in isolated environments. Future exploration should focus on verifying the predictions regarding RM and DM fluctuations, enhancing the understanding of FRB progenitors, and potentially identifying other similar binary systems. This study lays the groundwork for subsequent investigations into understanding the complex relationship between massive star systems and their fast radio emissions.