Fast radio bursts (FRBs) are millisecond-duration radio transients of extragalactic origin. Their dispersive sweeps, caused by the interaction of the radio waves with the ionized interstellar and intergalactic medium, have been used to probe the distribution of baryons in the Universe. The assumption underlying this method is that the dispersion measure (DM) is primarily determined by the intergalactic medium (IGM). However, this assumption is challenged by the growing evidence suggesting that the host galaxy's contribution can be substantial. At least one FRB has shown evidence of an extreme local magneto-ionic environment and a compact persistent radio source (PRS). The accurate determination of FRB redshifts relies on a precise understanding of the DM contributions from various components, including the IGM, the Milky Way, the host galaxy, and the immediate circum-source environment. This study focuses on investigating a repeating FRB, FRB 20190520B, to better understand the relative contributions of these DM components and their implications for FRB physics and cosmology. The goal is to analyze its properties and determine its host galaxy and its potential association with a persistent radio source to provide constraints on models of FRB emission and propagation effects.

Previous research on FRBs has established the use of DM to estimate redshifts, under the assumption of IGM dominance. However, studies have revealed the significance of host galaxy DM contributions, with some cases exhibiting extreme magneto-ionic environments. The discovery of repeating FRBs, such as FRB 121102, has added to the complexity, with evidence of persistent radio emission and high burst rates. These findings have questioned the assumption of a single FRB progenitor and implied the existence of various subclasses, possibly at different evolutionary stages. Some repeating FRBs have been associated with compact PRSs, highlighting the potential link between burst activity and the presence of a local, magnetized environment. Other studies have explored the correlations between burst rate, morphology, and the presence of PRSs, seeking to better understand the underlying physics of FRB emission. While many FRBs have been localized, only a few have been associated with compact PRSs. This paper seeks to add to this knowledge by precisely localizing and characterizing FRB 20190520B, which exhibits characteristics that link it to previous observations of this type.

The research utilized multi-frequency observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Karl G. Jansky Very Large Array (VLA). FAST, operating in drift-scan and tracking modes, detected a significant number of bursts from FRB 20190520B. VLA observations, using the 'realfast' system, localized the FRB with high precision and revealed a co-located compact PRS. The localization was refined through careful calibration and imaging techniques in CASA, using phase-reference switching to minimize short-timescale phase variations. Deep images from VLA data provided constraints on the size and spectrum of the PRS. Optical and near-infrared (NIR) images from the Canada-France-Hawaii Telescope (CFHT) and Subaru Telescope, respectively, were used to identify and characterize the host galaxy. Optical spectroscopy, employing the Double Spectrograph on the Palomar 200-inch Hale Telescope and the Low Resolution Imaging Spectrometer (LRIS) on the Keck I Telescope, confirmed the host galaxy's redshift. The DM was analyzed by separating it into components from the Milky Way, the IGM, and the host galaxy, accounting for uncertainties in each component's contribution. The host galaxy's DM was independently estimated using the observed Hα emission. The FRB's repetition rate was modeled using a Weibull distribution. Analysis also investigated short- and long-timescale periodicity searches and burst energy distributions. The scattering properties were analyzed using models of Gaussian pulses convolved with one-sided exponentials. Finally, the chance coincidence probabilities for the association between the FRB, the PRS, and the host galaxy were estimated using statistical methods.

FRB 20190520B was precisely localized with the VLA to (RA, dec.) (J2000) = (16 h 02 min 04.272 s, −11° 17′ 17.32″) with a positional uncertainty of (0.10″, 0.08″). A compact persistent radio continuum counterpart was discovered at (RA, dec.) (J2000) = (16 h 02 min 04.261 s, −11° 17′ 17.35″), less than 0.36″ in size and with a flux density of 202 ± 8 µJy at 3.0 GHz. The FRB's host galaxy, J160204.31–111718.5, was identified with a redshift of z = 0.241 ± 0.001, demonstrating a high specific star-formation rate. The estimated host-galaxy DM of about 903 pc cm⁻³ is unusually high, nearly an order of magnitude greater than the average for FRB host galaxies, implying the IGM is not the sole contributor to DM. The FRB showed a high burst rate (R = 4.5^+1.3_−1.2 h⁻¹), and a complex frequency-time intensity structure. The observed scattering time (10 ± 2 ms at 1.25 GHz) is much smaller than expected if the identified galaxy were a foreground object to a more distant host. The chance coincidence probabilities for the association between the FRB, the PRS, and the host galaxy were estimated to be very low (0.8% for the galaxy), strongly suggesting a genuine physical association. The PRS's spectral index is −0.41 ± 0.04. The high host DM, along with the high burst rate and the compact PRS, make FRB 20190520B similar to FRB 20121102A. The burst energy distribution exhibits a log-normal behavior.

The extremely high host-galaxy DM of FRB 20190520B challenges the conventional method of estimating FRB redshifts based solely on the IGM DM contribution. The results emphasize the necessity of precisely identifying and characterizing the host galaxy to obtain accurate redshift estimates. The discovery of a compact PRS co-located with FRB 20190520B strengthens the emerging association between repeating FRBs, high host-galaxy DMs, and the presence of compact, persistent radio emission. The similarity to FRB 20121102A suggests that a fraction of repeating FRBs may originate from young sources still embedded within their complex natal environments, characterized by both thermal and relativistic plasma. The high burst rate, along with the other characteristics, suggests a potential connection between the properties of the host environment and the FRB emission mechanism. While the exact nature of this connection remains unclear, it highlights the need for further investigation into the interplay between the FRB progenitor, its environment, and the observed properties.

This study presented a detailed analysis of the repeating FRB 20190520B, revealing a remarkably high host-galaxy DM and a compact, persistent radio source. The findings challenge assumptions about DM contributions in FRB redshift estimations and highlight the importance of accurate host-galaxy identification. The observed properties suggest a potential connection between repeating FRB activity, the presence of PRSs, and the complex, dense environment surrounding young FRB sources. Further investigation into the properties of a larger sample of localized FRBs with measured host-galaxy redshifts is crucial for characterizing the distribution of host-galaxy DMs and their influence on redshift determinations. Future work could focus on higher-resolution VLBI observations to better constrain the size and structure of the PRS and explore the physical mechanisms underlying the observed correlations between FRB activity and its environment.

The study's limitations include potential uncertainties in the DM components' separation and the assumed models for IGM DM and host-galaxy DM contributions. The estimated scattering time, while indicating an unlikely foreground scenario, relies on assumed parameters for the intervening galaxy's gas properties and turbulence. The optical observations are seeing limited, which could affect the size measurement of the Hα-emitting region. The analysis of burst energy is based on assuming a spectral index of approximately 0.