Luminous Millimeter, Radio, and X-Ray Emission from ZTF 20acigmel (AT 2020xnd)

Abstract

Observations of the extragalactic (z = 0.0141) transient AT 2018cow established a new class of energetic explosions shocking a dense medium, producing luminous emission at millimeter and submillimeter wavelengths. Here we present detailed millimeter- through centimeter-wave observations of a similar transient, ZTF 20acigmel (AT 2020xnd), at z = 0.2433. Using observations from the NOrthern Extended Millimeter Array and the Very Large Array, we model the unusual millimeter and radio emission from AT 2020xnd under several different assumptions and ultimately favor synchrotron radiation from a thermal electron population (relativistic Maxwellian). The thermal electron model implies a fast but subrelativistic (v ≈ 0.3c) shock and a high ambient density (n e ≈ 4 × 103 cm−3) at Δt ≈ 40 days. The X-ray luminosity of L X ≈ 1043 erg s−1 exceeds simple predictions from the radio and UVOIR luminosity and likely has a separate physical origin, such as a central engine. Using the fact that month-long luminous (L ν ≈ 2 × 1030 erg s−1 Hz−1 at 100 GHz) millimeter emission appears to be a generic feature of transients with fast (t 1/2 ≈ 3 days) and luminous (M peak ≈ −21 mag) optical light curves, we estimate the rate at which transients like AT 2018cow and AT 2020xnd will be detected by future wide-field millimeter transient surveys such as CMB-S4 and conclude that energetic explosions in dense environments may represent a significant population of extragalactic transients in the 100 GHz sky.

Extragalactic Millimeter Transients in the Era of Next-generation CMB Surveys

“…The most luminous examples are the relativistic TDEs Swift J164449.3+573451 (hereafter, Sw J1644+57; Bloom et al 2011;Burrows et al 2011;Levan et al 2011;Zauderer et al 2011) and AT2022cmc (Andreoni et al 2022) with a luminosity comparable to typical LGRBs detected over a timescale ∼200 days (Berger et al 2012;Alexander et al 2022;Perley et al 2022;Smith et al 2022). More recently, Cendes et al (2021b) used (Yuan et al 2016), and AT2022cmc (Alexander et al 2022Perley et al 2022;Smith et al 2022), the FBOTs AT2018cow (Ho et al 2019) and AT20202xnd (Ho et al 2022), core-collapse supernovae (stars), and long GRBs (gray circles; see Table 1). Limits correspond to 3σ.…”

“…Finally, in the nearby universe, a small number of supernovae have also been detected in the millimeter (e.g., Weiler et al 2007;Soderberg et al 2008;Horesh et al 2013b), with luminosities of ∼10 37 -10 38 erg s −1 . At higher luminosities (∼10 41 erg s −1 ), a growing number of luminous, rapidly evolving blue transients (see Section 2.2.6) have also been discovered (e.g., Ho et al 2019;Margutti et al 2019;Coppejans et al 2020;Ho et al 2022). Despite the wide diversity of physical phenomena, the production of synchrotron emission in these various transients is a generic feature of shock mediated outflows.…”

CCAT-prime Collaboration: Science Goals and Forecasts with Prime-Cam on the Fred Young Submillimeter Telescope

“…Fainter 3 mJy detection limits (and the sensitivity to ∼50% farther sources) are more suitable for 220 and 280 GHz (see Table 5). Although the displayed CCSNe (SN 1993J, SN 2008D, SN 2011df, and SN 2020oi) are relatively faint millimeter-submillimeter sources, radio lightcurves of other SNe (SNe 2009bb, 2003L, 2003bg, and 2007bg) suggest that there exists a larger sample of SNe that may be similarly luminous (sub)millimeter transients as compared to the FBOTs AT2018cow and AT2020xnd (Ho et al 2019(Ho et al , 2022. In addition, the radio luminosities of other FBOTs have peak radio spectral luminosities that are as bright or brighter than those of AT2018cow and AT2020xnd.…”

“…At 220 GHz and 280 GHz, FYST will be sensitive to objects ∼50% more distant. This image was adapted from lightcurves collated inHo et al (2022), with additional information from de Ugarte Postigo et al (2012),Urata et al (2015).…”

O’TRAIN: A robust and flexible ‘real or bogus’ classifier for the study of the optical transient sky

“…Those transients in part belong to known astrophysical classes such as supernovae or galactic flaring stars, while new classes of transients have been recently discovered. For example, since 2018, the ATLAS and ZTF surveys have already confirmed the existence of a new type of transient called fast blue optical transients, (FBOTs), for which only four events have been robustly identified thus far: ATcow18 (Smartt et al 2018;Prentice et al 2018), ZTF18abvkwla (Ho et al 2020), CSS161010 (Coppejans et al 2020), and AT2020xnd/ZTF20acigmel (Bright et al 2022;Perley et al 2021;Ho et al 2021). Down to minute timescales, the SVOM/GWAC fast cadence survey (Han et al 2021) has also detected new powerful outburts from nearby M dwarf stars (Xin et al 2021;Wang et al 2021).…”