Chapter II: A systematic survey of luminous, extragalactic radio transients in
2.2 Introduction
Extragalactic eruptions, such as stellar explosions and supermassive black hole (SMBH) accretion flares, can produce dramatic radio transients. These transients occur when large populations of charged particles are suddenly accelerated to rel- ativistic speeds. In some cases, the particles are accelerated by shocks from newly launched jets traveling at large fractions of the speed of light. Such newborn jets have been observed from SMBHs in jetted tidal disruption events (TDEs) and active galactic nucleus (AGN) flares (e.g., Zauderer et al., 2011; Nyland, Dillon Z. Dong, et al., 2020; Yvette Cendes et al., 2022; Somalwar, Ravi, Dillon Z. Dong, et al., 2022), and during the birth of stellar mass compact objects in supernovae (SNe) and compact object mergers (e.g., Kulkarni, Frail, et al., 1998; G. Hallinan et al., 2017). In other cases, the accelerators are lower-velocity outflows that dissipate their energy by shocking dense circum-explosion gas. This has been seen in TDE outflows where the dense gas may be part of a complicated structure surrounding the black hole (e.g., M. M. Anderson et al., 2019; Kate D. Alexander et al., 2020;
Somalwar, Ravi, D. Dong, et al., 2021) and SNe where the dense gas was likely ejected from the massive star in a period of intense pre-supernova mass loss (e.g., Margutti, Kamble, et al., 2017; Palliyaguru et al., 2019; D. Z. Dong et al., 2021).
The full diversity of radio-transient-producing particle accelerators is just beginning to be uncovered.
The luminosity, evolution, and spectral features of a radio transient encode in- formation about both the explosion and its immediate surroundings, much of which is inaccessible at shorter wavelengths. The most intrinsically luminous transients are produced by high-velocity shocks and/or high-density environments. At early times, this luminosity is often obscured by synchrotron self-absorption (SSA) or free-free absorption (FFA). In particular, a shock dominated by SSA can suddenly
become detectable once it expands to a sufficiently large radius, provided that it is still capable of accelerating a sufficient number of particles at that radius. This typically occurs when there is a dense reservoir of gas to shock at that radius.
Likewise, a shock (or any other type of particle-acceleration region) dominated by FFA may suddenly appear if there is a rapid change in the external opacity. A decrease in free-free absorption may explain the appearance of radio emission in some supernovae surrounded by particularly compact and dense gas (e.g., Chandra, Roger A. Chevalier, N. Chugai, Fransson, et al., 2015) and has recently been seen in the emergence of an extremely radio-luminous nebula likely powered by an ener- getic neutron star within a shell of supernova ejecta (D. Dong and Gregg Hallinan, 2022). In both cases, there may be a delay of years or even decades before the radio emission becomes detectable.
To date, most radio transients with durations of ∼days to ∼decades have been found through targeted follow-up observations of transients first discovered in op- tical or high-energy surveys. Such follow-up has enabled a number of discoveries, including deep constraints on the circumstellar gas density of Type Ia SNe (and therefore limits on the presence of stellar-wind-emitting companion stars; Chomiuk et al., 2016) and the detection of a faint (∼50𝜇Jy) radio counterpart to a Type I superluminous supernova by Eftekhari, E. Berger, B. Margalit, Blanchard, et al.
(2019), perhaps indicative of a central engine. Radio follow-up campaigns are, however, limited by available telescope time: it is not possible to follow-up every optically- or high-energy-selected event in the radio for years. Only targets that are perceived to have particularly high scientific value can have such extensive radio follow-up. The scarcity of follow-up resources (both in telescope time and human time) imposes a strong and difficult-to-quantify selection bias on our understanding of radio transients. Sources that are not a-priori expected to be luminous radio emitters (e.g., due to a lack of observational precedent, theoretical predictions, early diagnostic features, or a combination of the above) are unlikely to be targeted for follow-up in the first place. Even if a target is selected for follow-up, it may be observed too early or too late to catch the radio emission (see e.g., discussions in Kate D. Alexander et al., 2020; Bietenholz, Bartel, et al., 2021, about the delayed emission observed in TDEs and SNe).
Radio sky surveys provide a promising method of overcoming some of these difficul- ties. They can illuminate many transients that would not have been otherwise found,
such as those with no multiwavelength counterpart. Examples of transients that may be intrinsically radio-only include Galactic Center Radio Transients (GCRTs;
Hyman, T. Joseph W. Lazio, Kassim, and Bartleson, 2002; Hyman, T. Joseph W.
Lazio, Kassim, Ray, et al., 2005; Hyman, Wijnands, et al., 2009; Chiti et al., 2016;
Wang et al., 2021), and Fast Radio Bursts (FRBs; see James M. Cordes and Shami Chatterjee, 2019, and references therein). Other examples that were not identi- fied in detection surveys include VT 1210+4956, the radio afterglow of a compact object/massive star merger associated with a previously unidentified X-ray burst (D. Z. Dong et al., 2021), and VT 1137-0337, a luminous emerging radio nebula likely powered by a young neutron star where the associated supernova occurred a few decades ago (D. Dong and Gregg Hallinan, 2022). Radio transient surveys are also sensitive to those transients that were not classified by high-energy surveys as warranting follow-up. For example, Stroh et al. (2021) checked the locations of
∼ 70,000 optical supernovae observed before Epoch 1 of VLASS (2017 - 2019) and found 19 with late-time radio counterparts. Of these, 10 were not present in the Bietenholz, Bartel, et al. (2021) compilation of supernovae with targeted follow-up.
The primary challenges faced by direct radio transient surveys are that they are typically not as sensitive as targeted follow-up and are subject to uncertainties in in- terpreting the transients because multiwavelength counterparts may not exist. These obstacles are orthogonal to those faced by targeted follow-up so, in spite of them, radio transient surveys will still probe a transient population complementary to that found through targeted follow-up. Moreover, these challenges are surmountable.
The limited sensitivity will be overcome with future, more sensitive instruments. As seen with VT 1210+4956 and VT 1137-0337 among other examples, rich diagnostic information about radio transients can often be obtained through multi-wavelength follow-up observations even (and sometimes only) years after explosion.
In this paper, we present results from a direct search for radio transients in two surveys covering an overlapping area of∼10000 deg2and separated by∼2 decades:
the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST; Becker, R. L.
White, and Helfand, 1995) and Epoch 1 of the VLA Sky Survey (Lacy et al., 2020).
In an effort to assemble a statistically significant sample while limiting selection biases, we restrict our search to sources associated with galaxies with a luminosity distance 𝑑𝐿 < 200 Mpc. In Section 2.3, we discuss our transient selection proce- dure along with the sources of incompleteness, false positives, and selection biases,
which influence our final sample of 64 sources. We additionally provide validation for a subset of transients through association with archival transients and follow-up optical spectra. In Section 2.4, we present our scheme for assigning initial classifica- tions to these transients using their host galaxy context. In Section 2.5, we compare those host galaxies with the overall population of 𝑑𝐿 < 200 Mpc spectroscopic galaxies and identify some preliminary trends to be confirmed with larger future samples. In Section 2.6, we provide a joint fit for the volumetric rate and observed luminosity function of each class. In Section 4.7, we discuss the physical nature of transients in each sub class (stellar explosions, AGN flares, and TDEs). Finally, in Section 2.8, we summarize our results and discuss the implications for future radio transient searches.
Throughout this paper, we assume a flatΛCDM cosmology where𝐻0= 69.6 km s−1Mpc−1 such that our distance cutoff 𝐷𝐿 < 200 Mpc corresponds to a redshift limit 𝑧 < 0.04487 (Wright, 2006).