A Hyperactive FRB Pinpointed in an SMC-Like Satellite Host Galaxy

TL;DR

The study achieves a milliarcsecond-precision localization of FRB 20240114A with the EVN, firmly placing the burst 0.5 kpc from the nucleus of its low-metallicity dwarf host and revealing a satellite-central configuration akin to the Small Magellanic Cloud–Milky Way system. A comprehensive DM budget, combining Milky Way, IGM, intervening halos, and the host, shows a dominant contribution from foreground halos along the sightline, emphasizing the significant role of intervening structures in shaping FRB dispersion measures for cosmological applications. Deep GTC spectroscopy and Prospector SED modeling characterize the FRB host as a compact, metal-poor, star-forming dwarf ($\sim 10^{8.6} M_\odot$) and identify a more massive central galaxy at the same redshift, with the FRB host residing at $\sim85$ kpc projected distance and in a bound orbit. The results broaden the diversity of FRB environments, inform interpretations of hostless or highly-offset FRBs, and underscore the necessity of incorporating foreground halo information in FRB-based cosmology and local-environment studies.

Abstract

Precise localizations of fast radio bursts (FRBs) are essential for uncovering their host galaxies and immediate environments. We present the milliarcsecond-precision European VLBI Network localization of FRB 20240114A, a hyperactive repeating FRB, achieving <90x30 mas (1-sigma) accuracy. This precision places the burst 0.5 kpc from the nucleus of its low-metallicity star-forming dwarf host at a spectroscopic redshift of z = 0.130287. Our Gran Telescopio CANARIAS spectroscopic follow-up reveals that the dwarf FRB host is gravitationally bound to a more massive, star-forming spiral galaxy. This establishes the first known instance of an FRB residing in a satellite galaxy within a larger galactic system. This configuration, analogous to the Small Magellanic Cloud orbiting the Milky Way (but at a lower overall mass scale), expands the known diversity of FRB host environments and offers important insights for interpreting seemingly "hostless" or highly offset FRBs. Furthermore, our detailed dispersion measure (DM) budget analysis indicates that the dominant contribution to FRB 20240114A's DM likely originates from the foreground galaxy halos. This finding addresses the anomalously high DM observed for this FRB and underscores the significant role of intervening foreground structures in shaping observed FRB DMs, which is important for accurate FRB-based cosmological measurements. Our results highlight the importance of deep, high-resolution optical/infrared observations (e.g., with the Hubble or James Webb Space Telescopes) to fully leverage our precise radio localization and probe the immediate astrophysical birthplaces of FRB progenitors within these complex galactic systems.

Figures (9)

• Figure 1: Each panel shows the absolute value of the dirty maps (i.e., the absolute value of the Fourier transform of the visibilities) for individual bursts. Bursts from Epoch 1 (EK056A/PR318A) are labeled A${n}$ and bursts from Epoch 2 (EK056B/PR319A) are labeled B${n}$. The bursts are sorted on their arrival times. Bursts that pass the visual inspection (as indicated by gold stars) are fitted with a rotated 2-D Gaussian function, for which the $2\sigma$ contour are shown as gold ellipses. The $2\sigma$ region of the best combined position is shown as a white ellipse in all panels. Burst A$02$ was not found to be robustly astrophysical in origin (§ \ref{['subsubsec:burstsearch']}) and no calibration solution was found for B$08$ (§ \ref{['subsubsec:correlation']}). Consequently, these panels are left blank.
• Figure 2: Dynamic spectra (bottom panels) and frequency-averaged temporal profiles (top panels) for all bursts. Each burst is coherently dedispersed to a value of $527.723$ pc cm$^{-3}$. The frequency resolution in each dynamic spectrum is $500$ kHz and the respective time resolutions are indicated in the top right of each panel. Horizontal white bands in the dynamic spectra indicate data that have been masked due to RFI or subband edges. Burst labels are the same as in Figure \ref{['fig:burst_dirty_map']}. Burst A$02$ was not found to be robustly astrophysical in origin (§ \ref{['subsubsec:burstsearch']}). Consequently, this panel is left blank.
• Figure 3: Wide-field RGB view of the FRB 20240114A field using DESI $g$ (blue), $r$ (green), and $z$ (red) band images. The FRB host galaxy at $z = 0.130287$, likely a satellite of a nearby central galaxy at the same systemic redshift, is marked with a cyan box along with its companion; their projected separation of $35.5\hbox{$^{\prime\prime}$}$ ($85$ kpc transverse distance) is marked by the double-headed arrow. Gray dashed boxes indicate the zoom regions shown in the upper (host) and lower (central galaxy) insets. In the host inset, the EVN-derived FRB position is highlighted by a white ellipse whose semi-axes represent the $10\sigma$ localization uncertainty (inflated to be visible on this scale).
• Figure 4: Central panel: OSIRIS spectra of the FRB host (black solid line) and the central galaxy (solid grey). Peripheral panels: These insets show the brightest emission lines detected in each spectrum and their corresponding single-Gaussian fits. On the left, we show the region around the unresolved [O ii]$\lambda\lambda$3726,3629Å doublet, while the H$\beta$ and [O iii]$\lambda\lambda$4959,5007Å lines and their best fits are shown on top. The right panels show the H$\alpha$ line region for each of the two spectra. All spectra correspond to our 2024 observations, as these data provide a wider wavelength coverage.
• Figure 5: Nonparametric star-formation histories (SFHs). The left panel shows the SFH of the FRB host galaxy, and the right panel that of the central companion, both reconstructed with Prospector (see § \ref{['section:host_properties']} and Appendix \ref{['sec:appendix_prospector']}). Solid lines indicate the median SFR in each time bin; shaded envelopes mark the 16th--84th percentile credible intervals.