The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales (1701.01099v1)

Abstract: The millisecond-duration radio flashes known as Fast Radio Bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (~ 100mas precision) of FRB121102 using the VLA has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here we report very-long-baseline radio interferometric observations using the European VLBI Network and the 305-m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of < 40pc (< 12mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (~ 2-4mas at 1.7GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be < 0.7pc (< 0.2mas angular extent at 5.0GHz) and a lower limit for the brightness temperature of T_b > 5 x 10^7K. Together, these observations provide strong evidence for a direct physical link between FRB121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB121102 that best match the observed data.

• The paper achieves precise VLBI localization of FRB 121102, establishing a connection between bursts and a persistent radio source within a sub-40 parsec range.
• It employs very-long-baseline interferometry with the European VLBI Network and Arecibo to measure angular separations under 12 mas at a 95% confidence level.
• The findings support theories involving a young neutron star or a low-luminosity AGN, enhancing our understanding of repeating fast radio bursts.

Overview of FRB 121102's Characteristics Observed at Milliarcsecond Angular Scales

The research outlined in the paper focuses on the repeating Fast Radio Burst (FRB) known as FRB 121102, captured at unprecedented milliarcsecond (mas) angular resolution. Conducted through very-long-baseline radio interferometric observations, this paper employs the European VLBI Network and the Arecibo telescope to achieve highly precise localization of FRB 121102, tying its position both to its burst emissions and a persistent radio source. The paper presents an angular separation of less than 40 parsecs between the two, reinforcing a strong contextual link between these components and the potential implications for the origin of such repeating bursts.

Key Findings and Numerical Results

The analysis reveals several important findings about the burst characteristics and the persistent source, supported by quantitative measurements:

• Localization Precision: The measurements consolidate the location of bursts and the persistent source within a projected linear separation of less than 40 parsecs, with an angular distance under 12 mas at a 95% confidence level. This is accomplished primarily through fine angular resolution and extensive interferometry, achieving angular broadening results consistent with terrestrial models for scattering due to the Milky Way.
• Size and Temperature Constraints: The persistent source exhibits a projected size less than 0.7 parsecs, with a brightness temperature exceeding $5 \times 10^7$ K, indicating a compact and energetic nature.
• Persistent Radio Source Luminosity: The persistent source shows a consistent radio emission with a luminosity in the order of $10^{38}$ erg/s across different sessions, indicating stability over the observation period. The radio spectrum approximately follows a spectral index of $\alpha = -0.27 \pm 0.24$ between 1.7 GHz and 5.0 GHz.
• Proper Motion Constraint: Over the span of observations, proper motion considerations suggest a movement constraint of less than several mas per year, reinforcing its extragalactic identification.

Theoretical and Practical Implications

The results have profound implications for understanding FRB origins. The closeness of the source of bursts and the persistent emission argues strongly for a shared physical mechanism or origin:

• Young Neutron Star Hypothesis: An emerging view aligns these observations with a young neutron star or magnetar energizing a surrounding nebula, possibly a direct product of a supernova remnant. Such a setting could satisfy both the intense, repeat nature of the bursts and the steady emission qualities.
• Low-Luminosity AGN Interpretation: Another scenario interprets the persistent emission as emanating from an active galactic nucleus (AGN). Yet, unlike standard AGNs, the attributes suggest a low-luminosity AGN, potentially involving unique accretion properties or even jet interactions, given the radio-to-X-ray luminosity ratios.

Speculations on Future Developments

Further studies could refine our understanding of the FRB and its cosmic environment. Investigating the temporal evolution, energy spectrum, and environmental interactions allow extending observational baselines and employing multi-wavelength approaches to scrutinize candidate models. The methodological rigor applied in localizing FRB 121102's emission could become benchmark protocols for unraveling other enigmatic astronomical features across vast cosmological distances.

In summary, the paper significantly augments our understanding of FRB 121102, suggesting coherent physical interplay between the repeating bursts and persistent radio emissions, likely embedded within or powered by highly energetic, youthful cosmic bodies or phenomena. The ramifications bear relevance across broader astrophysical inquiry fields, offering new paradigms for interpreting transient events at the universal scale.