Highest-frequency detection of FRB 121102 at 4-8 GHz using the Breakthrough Listen Digital Backend at the Green Bank Telescope (1804.04101v1)

Abstract: We report the first detections of the repeating fast radio burst source FRB 121102 above 5.2 GHz. Observations were performed using the 4$-$8 GHz receiver of the Robert C. Byrd Green Bank Telescope with the Breakthrough Listen digital backend. We present the spectral, temporal and polarization properties of 21 bursts detected within the first 60 minutes of a total 6-hour observations. These observations comprise the highest burst density yet reported in the literature, with 18 bursts being detected in the first 30 minutes. A few bursts clearly show temporal sub-structures with distinct spectral properties. These sub-structures superimpose to provide enhanced peak signal-to-noise ratio at higher trial dispersion measures. Broad features occur in $\sim 1$ GHz wide subbands that typically differ in peak frequency between bursts within the band. Finer-scale structures ($\sim 10-50$ MHz) within these bursts are consistent with that expected from Galactic diffractive interstellar scintillation. The bursts exhibit nearly 100% linear polarization, and a large average rotation measure of 9.359$\pm$0.012 $\times$ 10$^{\rm 4}$ rad m$^{\rm -2}$ (in the observer's frame). No circular polarization was found for any burst. We measure an approximately constant polarization position angle in the 13 brightest bursts. The peak flux densities of the reported bursts have average values (0.2$\pm$0.1 Jy), similar to those seen at lower frequencies ($<3$ GHz), while the average burst widths (0.64$\pm$0.46 ms) are relatively narrower.

• The paper reports the highest-frequency detections of FRB 121102, with 21 bursts observed, including a burst density spike in the first 30 minutes.
• It details subband spectral structures and nearly 100% linear polarization with a substantial rotation measure of 9.359×10⁴ rad m⁻².
• Utilizing a 4 GHz bandwidth at the Green Bank Telescope, the study emphasizes the need for wideband observations to capture intricate FRB emission characteristics.

Overview of FRB 121102 Detection at 4--8 GHz

The paper presented by Gajjar et al. explores the highest frequency detections to date of the fast radio burst (FRB) source FRB 121102. Utilizing the 4--8 GHz receiver at the Green Bank Telescope with the Breakthrough Listen Digital Backend, the research documents 21 bursts detected during a single observation session. These observations mark a significant contribution to our understanding of FRB 121102's emission characteristics at higher frequencies.

Key Findings

• Burst Detection and Frequency: The paper reports the detection of FRB 121102 bursts at frequencies beyond 5.2 GHz for the first time. A total of 21 bursts were identified within the first 60 minutes of a 6-hour observation period. Notably, 18 of these bursts occurred in the initial 30 minutes, indicating a high burst density.
• Spectral and Temporal Characteristics: The paper observed that certain bursts displayed temporal sub-structures with distinct spectral properties. These broader features occurred in approximately 1 GHz wide subbands and varied in peak frequency across bursts. Fine-scale structures on the order of 10-50 MHz were consistent with expectations from Galactic diffractive interstellar scintillation.
• Polarization and Rotation Measure: Almost all bursts exhibited near 100% linear polarization. The paper determined a substantial rotation measure (RM) in the observer's frame, quantified at 9.359 × 10^4 rad m^-2, with no circular polarization detected. This high RM suggests the bursts are propagating through a highly magnetized medium.
• Comparison with Lower Frequencies: The average peak flux densities of the detected bursts at higher frequencies are similar to those observed at lower frequencies (below 3 GHz), while the burst widths at higher frequencies were narrower.

Implications

The results underline how the large bandwidth of 4 GHz was critical to detecting these bursts. Narrower bandwidths would likely have missed many bursts due to the band-limited spectral structures they exhibit. The findings contribute to an emerging picture where FRB 121102 displays significant variability in spectral and temporal properties that can be better resolved at higher frequencies.

Theoretical and Practical Implications

The paper's insights into the spectral bandwidth and RM point towards a dynamic and complex environment around FRB 121102. The high RM values, particularly, suggest an extreme magneto-ionic setting, possibly indicative of proximity to a massive astrophysical object or within a dynamic magnetar wind. Future observations at various frequencies, combined with polarization studies, will be crucial in further elucidating the emission mechanisms and environmental conditions surrounding FRBs.

Future Directions

This research paves the way for enhancing burst search methodologies by incorporating spectral subband searches to capture band-limited and broadband bursts effectively. Furthermore, the possibility of detecting more nuanced spectro-temporal structures at similar or higher frequencies could offer additional clues about the intrinsic emission physics or propagation effects associated with FRB 121102 and potentially other similar FRBs.

In summary, the paper by Gajjar et al. contributes valuable data on the observational characteristics of FRB 121102 at higher frequencies, highlighting the intricate structures and environmental interactions that these enigmatic sources may experience.