A Radio Study of Persistent Radio Sources in Nearby Dwarf Galaxies: Implications for Fast Radio Bursts

Yuxin (Vic) Dong

Most extragalactic radio sources arise from recent star formation and active galactic nuclei (AGN). However, over the past decade, a unique third category has emerged: compact persistent radio sources (PRSs) that are predominately found in dwarf host galaxy environments. They were initially linked to fast radio bursts (FRBs), which are bright, millisecond-duration flashes of coherent radio emission with an enigmatic origin. The PRSs appear to fall into two distinct families: one potentially tied to intermediate-mass black holes (IMBHs) and the other to FRBs and other transient phenomena.

To date, two luminous compact PRSs have been unambiguously associated with FRBs in dwarf galaxies. These PRSs are believed to be emission from nebulae surrounding a central engine such as a magnetar or an accreting compact object. Reines et al. (2020) presented a sample of PRSs in dwarf galaxies and suggested they are off-nuclear accreting IMBHs. However, Eftekhari et al. (2020) argued that some of these sources are instead, analogs of those associated with FRBs, based on similarities in the radio properties like those observed for the PRS associated with FRB20121102A. In this work, we further explore the possibility that these radio sources share the same origin as FRB-PRSs.

We obtained new multi-frequency (L—X band) Karl G. Jansky Very Large Array (VLA) observations for 12 radio sources. This dataset is complemented by the European VLBI Network (EVN) observations at 1.7 GHz for three sources in the sample (Figure 1), thereby constraining the size down to milliarcsecond (mas) scales. The radio properties of these sources, including luminosities, spatial offsets, and spectral evolution, are compared to known PRSs and models of central-engine powered emission. Several radio transients have also been discovered in dwarf host galaxies, including the decades-long transient J1419+3940 (Law et al. 2018), the PRS associated with the superluminous supernova PTF10hgi (Eftekhari et al. 2019), and the pulsar wind nebula candidate VT 1137−0337 (Dong & Hallinan 2023). Noteworthy for their similar host environments, these transients also exhibit radio properties reminiscent of PRSs, hinting at a potential common origin with FRB-associated PRSs. We examined the spectral luminosities of the FRB-PRS candidates relative to known FRB-PRSs and other radio transients (Figure 2). While the sample is less luminous than the two known FRB-PRSs, it is comparable to sources like VT 1137−0337 and PTF10hgi and brighter than J1419+3940. Additionally, the radio luminosities for this sample are comparable to existing PRS limits for well-localized FRBs, suggesting that these FRBs could harbor lower luminosity PRSs.

Among the sample, a single source, J1136+2643, stands out as the most promising FRB-PRS candidate. Its compact size, proximity to the host galaxy center, and spectral energy distribution are broadly consistent with models involving a neutron star wind nebula and hypernebula. Additionally, two other sources, J0019+1507 and J0909+5655, exhibit similar sizes but are likely background AGN due to their large offsets and flat spectral indices. Our work highlights the diversity in the observed radio properties of PRSs and the importance of high-resolution VLBI observations, such as those from the EVN, in unraveling the nature of PRSs and their connections to FRBs and dwarf host environments.

Figure 1. One of the EVN-detected sources, J0019+1507, at 1.7 GHz with contour lines depicting rms levels starting from 3.5σ, where σ is 50 μJy/beam. A small bar in the top-right of the image shows the physical scale assuming that the sources are at the same redshift of the associated dwarf galaxy. The synthesized beam of size 6 mas × 5 mas is represented by the grey ellipse in the lower left corner.

Figure 2: Radio spectral luminosities of the sample of PRS candidates (blue), with J1136+2643, the most promising candidate highlighted. Also shown for comparison are PRS 20121102A, PRS 20190520B, both of which associated with a FRB, VT 1137−0337 (orange), PTF10hgi (purple), and J1419+3940 (dark orange). Grey triangles represent all existing PRS luminosity upper limits. The radio sources in this sample are overall less luminous than the FRB-PRSs, with the exception of J0931+5633, but comparable to other radio transients.

References:

Reines et al. 2020, ApJ, 888, 36

Eftekhari et al. 2020, ApJ, 895, 98

Law et al. 2018, ApJL, 866, L22

Eftekhari et al. 2019, ApJL, 876, L10

Dong & Hallinan 2023, ApJ, 948, 119

More information: Y. Dong, et al. 2024, ApJ, 973, 133

Contact:

Yuxin (Vic) Dong, CIERA, Northwestern University, yuxin.dong@northwestern.edu