AT2018cow: the poster-child explosion for high-frequency* time-domain astronomy Anna Ho with Sterl Phinney, Vikram Ravi, Shri Kulkarni, Dan Perley, and the SMA, ALMA, and ATCA teams Journal Club | 16 November 2018 1 Based on Ho et al. (2018), arXiv:1810.10880 see also Margutti et al. (2018), arXiv:1810.10720 Perley et al. (2018), arXiv:1808.00969 * > 90 GHz The Submillimeter Array image credit Nimesh Patel 

A suite of optical surveys are monitoring the night sky for transient and variable phenomena. Full sky / 2 nights 20th mag ATLAS Full sky / night 17th mag 2 Full sky / 3 nights 1600 deg2, 6x/night 20.5 mag 24º x 96º every 30 minutes Gaia TESS 30 deg2 once per week Full sky, mixed cadence, 20th mag Goal of these surveys: * census of extragalactic explosions (SNe, tidal disruption events, …) * census of galactic variable sources * asteroid impact early warning * dark energy constraints * discovering new phenomena 

Stephen Smartt/ATLAS On June 16, ATLAS issued an alert reporting a bright new transient: AT2018cow 3 At 60 Mpc, M = -19.2 

From recent non-detections, we learned that this rose < 4.2 mag in < 1.3 days ATLAS discovery r-band i-band ZTF 4 ATLAS ASAS-SN g-band Days after discovery 0 10 20 30 40 Apparent (AB) magnitude 20 18 16 14 12 Absolute magnitude -14 -16 -18 -20 -22 

If extragalactic, this would be one of the most rapidly rising, luminous explosions ever discovered. Modified from Margutti et al. (2018) Type I superluminous supernovae Stripped-envelope (Type Ibc) core- collapse supernovae Type IIP supernovae IIL supernovae IIn supernovae “fast-blue-opt.” transients 5 

Days after discovery 0 10 20 30 40 Apparent (AB) magnitude 20 18 16 14 12 Absolute magnitude -14 -16 -18 -20 -22 r-band i-band 6 g-band Daily spectroscopic monitoring revealed (1) that the transient was in the galaxy, and (2) the emergence of a broad feature. Modified from Perley et al. (2018) 

I thought that this was going to become a supernova, so I triggered radio follow-up. Why radio? Mod. from Fig 28.5, Carroll & Ostlie * When you have relativistic electrons and magnetic fields, you get synchrotron radiation log(frequency) log(flux) Single-electron spectrum 7 

I thought that this was going to become a supernova, so I triggered radio follow-up. Why radio? Mod. from Fig 28.5, Carroll & Ostlie * When you have relativistic electrons and magnetic fields, you get synchrotron radiation log(frequency) log(flux) Single-electron spectrum Mod. from Fig 6.12, Rybicki & Lightman ν-α Net (observed) spectrum log(frequency) log(flux) ν5/2 Opt. thin Opt. thick ν-α 8 

Why radio? * When you have relativistic electrons and magnetic fields, you get synchrotron radiation ν-α Net (observed) spectrum log(frequency) log(flux) ν5/2 Opt. thin Opt. thick ν-α * From the position and flux of the self-absorption peak, you can infer key physics of the shock 9 I thought that this was going to become a supernova, so I triggered radio follow-up. 

A variety of energetic explosions are observed at radio frequencies ∆t=23 days ∆t=351 days SN2003bg (Soderberg 2006) ν-α Net (observed) spectrum log(frequency) log(flux) ν5/2 Opt. thin Opt. thick ν-α 10 

A variety of energetic explosions are observed at radio frequencies ∆t=23 days ∆t=351 days SN2003bg (Soderberg 2006) Ho (2018) 11 

It is much less common to catch explosions at sub-millimeter wavelengths (high frequencies) Ho (2018) 12 

Days after discovery 0 10 20 30 40 Apparent (AB) magnitude 20 18 16 14 12 Absolute magnitude -14 -16 -18 -20 -22 Without knowing where the peak would be, we triggered a variety of facilities. 13 r-band i-band g-band Day 4: began observing with the submillimeter array (SMA) at 230 GHz and 340 GHz + VLA proposal (<10 GHz) rejected 

Days after discovery 0 10 20 30 40 Apparent (AB) magnitude 20 18 16 14 12 Absolute magnitude -14 -16 -18 -20 -22 Without knowing where the peak would be, we triggered a variety of facilities. 14 r-band i-band g-band Day 9: Atacama Large Millimeter Array (ALMA) proposal accepted, > 90 GHz SMA monitoring + the broad features disappear! 

Days after discovery 0 10 20 30 40 Apparent (AB) magnitude 20 18 16 14 12 Absolute magnitude -14 -16 -18 -20 -22 Without knowing where the peak would be, we triggered a variety of facilities. 15 r-band i-band g-band SMA monitoring ALMA snapshot observations 

Days after discovery 0 10 20 30 40 Apparent (AB) magnitude 20 18 16 14 12 Absolute magnitude -14 -16 -18 -20 -22 Without knowing where the peak would be, we triggered a variety of facilities. 16 r-band i-band g-band SMA monitoring ALMA snapshot observations ATCA monitoring < 34 GHz 

We also downloaded the publicly available X-ray observations Swift began observing at ∆t=3 with the X-ray Telescope (XRT) (0.3-10 keV) NuSTAR took four epochs of observations of AT2018cow (3-80 keV) 17 

This was a luminous millimeter and X-ray explosion, unfolding in two stages Ho et al. (2018) 18 Radio/millimeter light curve X-ray light curve 

The peak persisted at high frequencies for several weeks Ho et al. (2018) ν-α Net (observed) spectrum log(frequency) log(flux) ν5/2 Opt. thin Opt. thick ν-α 19 

The peak persisted at high frequencies for several weeks Ho et al. (2018) Size of shock Velocity of shock Energy behind shock Density of medium 20 

In energy-velocity space, AT2018cow is not particularly special Ho et al. (2018) Sidebar: your choice of εB matters! Utotal=UB/εB 21 

In peak frequency-luminosity space, AT2018cow is special Ho et al. (2018) 22 

Physical interpretation (thanks, Sterl): high energy, high density Ho et al. (2018) 23 

Whither the X rays? * Not an extension of the synchrotron spectrum * Not inverse Compton Engine emerges Ho et al. (2018) 24 

and a poster-child explosion for high-frequency time-domain astronomy! Ho et al. (2018) 25 

Looking to the future * Radio observations probe the outer blast wave surrounding an explosion * AT2018cow gave rise to a sub-relativistic (v=0.13c) shock plowing into a dense (ne ~ 3x105 cm-3) medium * X-rays reveal a newborn central engine, an accreting BH or a spinning NS * Other explosions would have been luminous millimeter transients at early times * We need a high-frequency facility dedicated to the pursuit of transients! * Optical surveys (like ZTF) will deliver a population of AT2018cow analogs… 26 

Looking to the future * Radio observations probe the outer blast wave surrounding an explosion * AT2018cow gave rise to a sub-relativistic (v=0.13c) shock plowing into a dense (ne ~ 3x105 cm-3) medium * X-rays reveal a newborn central engine, an accreting BH or a spinning NS * Other explosions would have been luminous millimeter transients at early times * We need a high-frequency facility dedicated to the pursuit of transients! * Optical surveys (like ZTF) will deliver a population of AT2018cow analogs… ? ZTF18abvkwla (“The Koala”) 27 

Thank you to… * Mansi, for encouraging me to work on this project * Shri, for helping me strategize and manage the scope of the paper * Vikram, for helping with the proposal-writing and initial modeling * Sterl, for late nights at the chalkboard and carefully checking the manuscript * The SMA, ALMA, and ATCA teams for observing & reducing the data 28 The Submillimeter Array image credit Nimesh Patel 

An emerging central engine viewed at an angle Margutti et al. (2018) 29 

I thought that this was going to become a broad-lined Ic supernova. 1%: Ic-BL “broad-lined*” *vph > 20,000 km/s Ek > 1052 erg Core-collapse supernova (1051 erg) 10%: Ic Stripped of H, He 30 

31 Ho et al. (2018)