FRB 121102 Bursts Show Complex Time-Frequency Structure (1811.10748v1)

Abstract: FRB 121102 is the only known repeating fast radio burst source. Here we analyze a wide-frequency-range (1-8 GHz) sample of high-signal-to-noise, coherently dedispersed bursts detected using the Arecibo and Green Bank telescopes. These bursts reveal complex time-frequency structures that include sub-bursts with finite bandwidths. The frequency-dependent burst structure complicates the determination of a dispersion measure (DM); we argue that it is appropriate to use a DM metric that maximizes frequency-averaged pulse structure, as opposed to peak signal-to-noise, and find DM = 560.57 +/- 0.07 pc/cc at MJD 57644. After correcting for dispersive delay, we find that the sub-bursts have characteristic frequencies that typically drift lower at later times in the total burst envelope. In the 1.1-1.7 GHz band, the ~ 0.5-1-ms sub-bursts have typical bandwidths ranging from 100-400 MHz, and a characteristic drift rate of ~ 200 MHz/ms towards lower frequencies. At higher radio frequencies, the sub-burst bandwidths and drift rate are larger, on average. While these features could be intrinsic to the burst emission mechanism, they could also be imparted by propagation effects in the medium local to the source. Comparison of the burst DMs with previous values in the literature suggests an increase of Delta(DM) ~ 1-3 pc/cc in 4 years, though this could be a stochastic variation as opposed to a secular trend. This implies changes in the local medium or an additional source of frequency-dependent delay. Overall, the results are consistent with previously proposed scenarios in which FRB 121102 is embedded in a dense nebula.

Analysis of Time-Frequency Structures in FRB 121102 Bursts

The paper, authored by Hessels et al., presents a detailed examination of the time-frequency structures in radio bursts from the recurring Fast Radio Burst (FRB) source known as FRB 121102. Historically recognized as the sole repeating FRB, FRB 121102 offers a unique opportunity to explore the complexities of FRB emission mechanisms and propagation phenomena. This research utilizes observations from Arecibo and Green Bank Telescopes over a 1–8 GHz frequency range, unearthing intricate structures within the burst emissions that contribute to our understanding of extragalactic radio phenomena.

Key Observational Findings

The Arecibo and Green Bank Telescopes provided coherently dedispersed burst data, adding an unprecedented level of detail to the analysis. The paper indicates that FRB 121102 bursts possess elaborate time-frequency structures, which include sub-bursts of finite bandwidth. For instance, in the 1.1-1.7 GHz band, these sub-bursts range in bandwidth from 100-400 MHz and exhibit a downward frequency drift at approximately 200 MHz/ms. The drift is larger at higher frequencies, suggesting a dynamic emission or propagation mechanism.

A rigorous dispersion measure (DM) analysis using a frequency-averaged pulse structure metric yields a value of 560.57 ± 0.07 pc cm$^-3$ at MJD 57644. The paper notes a DM increase of around 1-3 pc cm$^-3$ over four years, hinting at a change in the local medium surrounding the source.

Interpretational Framework and Hypotheses

The intricate nature of the emission structure raises questions about whether these features are intrinsic to the emission mechanism or affected by local propagation effects. The drifting frequency of the sub-bursts across different times might imply an intrinsic emission mechanism similar to those seen in the emission mechanisms of the Sun or Jupiter. Meanwhile, the propagation influence through plasma lensing effects is another plausible explanation, suggesting that the bursts might traverse a medium causing refraction and diffraction, thereby imprinting the observed spectral features.

Implications for Astrophysical Understanding

This characterization of burst morphology advances our interpretations of compact astrophysical objects, suggesting parallels with young neutron stars potentially located in dense nebulae. The prevailing hypothesis argues for an extreme magneto-ionic environment, potentially indicative of a young and rapidly rotating neutron star. The observed behavior might parallel the high-frequency interpulses of the Crab pulsar, suggesting a common basis of emission processes with known astrophysical analogues.

Prospects for Future Research

The complexity and variability detected in FRB 121102 call for further high-sensitivity and broad-spectrum observations. These studies could refine the observed frequency drift rates and their variation with observing frequencies, eventually leading to an improved model of the source's local environment. Ongoing monitoring is critical, complemented by technological advancements allowing for improved temporal and spectral resolution to discern between intrinsic and extrinsic properties of the observed structures.

Cross-referencing the details observed in FRB 121102 with other FRBs, especially those detected at low frequencies, could provide insights into whether similar mechanisms might underlie diverse FRB phenomena, or whether the repetition is a unique property of FRB 121102. Such comparative analysis may also allow for the exploration of potential periodicity in burst arrival times and further elucidate the connection between DM variations and local environmental changes.