A repeating fast radio burst source in the outskirts of a quiescent galaxy (2410.23374v1)

Abstract: We report the discovery of the repeating fast radio burst source FRB 20240209A using the CHIME/FRB telescope. We have detected 22 bursts from this repeater between February and July 2024, six of which were also recorded at the Outrigger station KKO. The 66-km long CHIME-KKO baseline can provide single-pulse FRB localizations along one dimension with $2^{\prime\prime}$ accuracy. The high declination of $\sim$86 degrees for this repeater allowed its detection with a rotating range of baseline vectors, enabling the combined localization region size to be constrained to $1^{\prime\prime}\times2^{\prime\prime}$. We present deep Gemini observations that, combined with the FRB localization, enabled a robust association of FRB 20240209A to the outskirts of a luminous galaxy (P(O|x) = 0.99; $L \approx 5.3 \times 10^{10}\,L_{\odot}$). FRB 20240209A has a projected physical offset of $40 \pm 5$ kpc from the center of its host galaxy, making it the FRB with the largest host galaxy offset to date. When normalized by the host galaxy size, the offset of FRB 20240209A is comparable to that of FRB 20200120E, the only FRB source known to originate in a globular cluster. We consider several explanations for the large offset, including a progenitor that was kicked from the host galaxy or in situ formation in a low-luminosity satellite galaxy of the putative host, but find the most plausible scenario to be a globular cluster origin. This, coupled with the quiescent, elliptical nature of the host as demonstrated in our companion paper, provide strong evidence for a delayed formation channel for the progenitor of the FRB source.

• The paper achieves precise localization of FRB 20240209A within a 1″×2″ region using CHIME and VLBI, detecting 22 bursts over five months.
• It documents narrowband bursts with downward-drifting frequency structures, highlighting diverse emission mechanisms in a non-star-forming environment.
• The study suggests a globular cluster progenitor with a 40 ± 5 kpc offset, implying a delayed formation channel for this class of repeating FRBs.

Analysis of the Repeating Fast Radio Burst in a Quiescent Galaxy

The paper led by Shah et al. focuses on the discovery and analysis of the repeating fast radio burst (FRB) source, FRB 20240209A, positioned in the outskirts of a quiescent galaxy. This research combines high-precision localization techniques with spectral observations to deduce the properties and potential origins of the FRB.

Key Findings and Methodologies

The identification of FRB 20240209A was conducted using the CHIME/FRB instrument, which detected 22 bursts from this source within a span of approximately five months. A notable feature of this detection was the precise localization achieved through the use of Very Long Baseline Interferometry (VLBI) between CHIME and one of its outrigger stations, KKO. This approach enabled the team to localize the FRB within a $1^{\prime\prime}\times2^{\prime\prime}$ region, a significant enhancement over previous localization capabilities, providing a robust association with the host galaxy.

Spectral analysis of the detected bursts revealed narrowband characteristics with wide temporal distributions, consistent with other known repeating FRBs. The bursts displayed varying frequency-time structures, including downward-drifting patterns—a common feature observed in FRB emission.

The host galaxy, identified as a massive elliptical galaxy at a redshift of z=0.1384, is characterized by a lack of star formation, further providing clues about the progenitor system. The FRB was found to have a projected physical offset of 40 ± 5 kpc from the galaxy center, which is unprecedented in terms of FRB host offsets. This significant offset challenges the conventional understanding that FRB sources are typically associated with regions of active star formation or close to central stellar environments.

Implications and Theoretical Considerations

The paper explores several hypotheses regarding the progenitor of FRB 20240209A. The most compelling scenario suggests an origin in a globular cluster, given the FRB’s large offset and the properties of its host galaxy. This association with a globular cluster implies a possibly delayed formation channel, different from the formation channels involving young, massive stars typically associated with FRBs thought to arise in active star-formation regions.

The findings underscore the diversity of environments in which FRBs are detected, suggesting that multiple formation and emission mechanisms might be at play. This FRB source, with its unusual host association, could indicate a substantial delay in the progenitor's appearance after its initial formation, possibly due to dynamic interactions in dense stellar environments like globular clusters.

Additionally, the absence of a prominent persistent radio source at the FRB location, a distinctive characteristic for some other well-studied repeaters, provides further distinction to this FRB, challenging straightforward classification based on existing observations.

Future Directions and Observations

The paper suggests that further VLBI measurements and multi-wavelength observations will be crucial in confirming the nature of the host environment and refining the understanding of the progenitor systems for FRBs located at large galactocentric distances. With ongoing and future advancements in FRB detection and localization techniques, including the deployment of additional CHIME/FRB outriggers, the prospect for sub-arcsecond localizations will significantly expand, bolstering efforts to directly associate FRBs with their host galaxies' structural features.

The broader astrophysical community is anticipated to benefit from these findings, as they add valuable insights into the population statistics of FRBs and their diverse host environments, thereby enriching the theoretical models that seek to explain the origins and mechanisms behind these enigmatic cosmic phenomena.