A growing population ofstellar explosions discovered in the radio
and what we can learn from them

Dillon Dong
Jansky Fellow
National Radio Astronomy Observatory1Outline
PART I

PART II

What we can learn froma SN’s radio spectrum?

Shock properties:• Radius• Velocity• Energy• CSM density

How to nd radio
supernovae

Three detection methods:• Followup observations• “Serendipitous” detection• Direct detection

PART III

Deep dive into VT
1210+4956

PART IV

A systematic search for~decade timescale radiotransients within 200 Mpc

A candidate merger-drivenexplosion• 10-2 M☉ / yr ~centuries
before explosion• VT 1210: merger-driven
explosion candidate

29 stellar explosion candidates• 7 associated with optical•

PART VConclusions

supernova emission
Multiple lines of evidence for
CSM asphericity

fi2

Part I:
What we can learn from radio emission

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSRadio emission from supernovae is caused by synchrotron emission
from the shock of ejecta (or jets) interacting with the CSM

VLBI observations of SN 1993J
Bietenholz+094SHOCKPROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Most supernovae are hard to detect in the radio• 290 literature supernovae
at D < 100 Mpc• 90 detections• Median ~1026.5 erg/s/Hz(IIn)• 30 min with the VLA at
100 Mpc• Median ~1025.5 erg/s/Hz
Ib/c

Bietenholz+21

IInother type II(all CCSNe)• too faint for VLA at 100Mpc

5SHOCKPROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

High-quality radio observations can teach us a lot about the shock
Measure:peak frequency νp(t), luminosity Lp(t)6SHOCKPROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

High-quality radio observations can teach us a lot about the shock
Measure:peak frequency νp(t), luminosity Lp(t)Derive:Radius R(t), magnetic eld B(t)Velocity: v(t) = dR/dt
Energy in shock: U(t) = (1/6εB) R3 B2Pre-shock density
Assume:fraction of energy in electrons & magnetic eld: εe, εB
fraction of spherical volume emitting: ffifi

Chevalier+98, Ho +19, Dong+21, deMarchi+22

7

DETECTIONMETHODSSHOCKPROPERTIESDEEP DIVESYSTEMATIC
SEARCHCONCLUSIONS

Example: SN 1993J

r-2 pro le consistent with windfifififi

Fransson & Bjornsson (1998)

Magnetic eld pro le consistent
with turbulent ampli cation behind shock

8fi

Part II:
How to nd radio supernovae

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

3 ways to nd radio supernovae
Targeted follow up(1) Find transients atother wavelengths(2) Pick a subset for radio
followup observations(3) Interpret the detections(or nondetections)fi10

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

3 ways to nd radio supernovae
Targeted follow up

Serendipitous follow up(1) Find transients atother wavelengths(2) Pick a subset for radio
followup observations(2) Check to see if
those transients
happen to be
detected in aradio sky survey(3) Interpret the detections(or nondetections)fi11

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

3 ways to nd radio supernovae
Targeted follow up

Serendipitous follow up(1) Find transients atother wavelengths(2) Pick a subset for radio
followup observations

Direct detection(1) Find transients bydirect comparison of2+ radio epochs(2) Check to see if
those transients
happen to be
detected in aradio sky survey(2) Gather contextual
information, takemultiwavelength
followup observations,(or both)(3) Interpret the detections(or nondetections)fi12

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSHistorical attempts at direct detection▸ Decades ofpioneering surveys▸ Scales probed:▸ ~10mJy▸ ~100 deg2
▸ large range oftimescales▸ Many upper limits
Lacy+202013

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

First successful direct search: theCaltech NRAO Stripe 82 Survey (CNSS)SYSTEMATIC
SEARCHCONCLUSIONS~300 deg2 — 2’’ resolution
3 epochs — 80μJy RMS per epoch

SDSS Stripe 82

O(10) transients found(1 unpublished SN)

Mooley+16, Mooley+18, Anderson+19,
Kunert-Bajraszewska+20, Wołowska+21

14

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATICSEARCHCONCLUSIONSFIRST: a 2 decade-timescale reference epoch~10,000 deg2 — 6’’ resolution
1 epoch — 140μJy RMS per epoch~34,000 deg2 — 2’’ resolution
3 epochs — 140μJy RMS per epoch

FIRST: 1993-2011

VLASS E1: 2017-2020

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Part III:A candidate merger-driven explosionSHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSVT 1210+4956:
1000x more radio luminous than the median IInlocated in star forming arm of higher metallicity LMC-like galaxyd = 154 Mpc~ 1029.5 erg/s/Hz< 0.41 mJy (1997)

M ~ 109.3 M☉
SFR ~ 0.3 M☉/yr
Z ~ 0.8 Z☉

2.7 mJy (2017)
Dong et al. (2021), Science17

DETECTIONMETHODSSHOCKPROPERTIESVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Archival X-ray ash (August 14, 2014)• GRB compactness

problem calculation
implies relativistic jet(Γ > 2.5)
on explosion dateshort, soft, and luminous~5 keV
fl~15s~1046 erg/s

18

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Nebular H alpha in follow-up Keck spectra (4 years later)~1400 km/s

‣ Indicates interaction
with dense CSM‣ Never before seenalongside a jet!
(e.g. Peters+2019)19

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSRadio SED evolution tells the same story (2017-2019)

Single epoch implies
R ~ 1017 cm
B ~ 0.3 G
U ~ 1049 erg
Two epochs implyv ~ 1800 km/s
n ~ 106 cm-3
20

SHOCK
PROPERTIESDETECTION
METHODS

Mystery #1

VT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSWith a density of ~106 cm-3, we would
expect strong free-free absorption

But free-free absorption would drop the spectrum
super-exponentially at low frequencies, which we do not observe
21

SHOCK
PROPERTIESDETECTION
METHODS

Mystery #2

VT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Where did the CSM come from?
How do you get such a high density at ~1017 cm?
Time before explosion~centuries
Average mass loss rate
> 10-2 M☉ yr-1

Eruptive mass loss

Ordinary stellar winds
Radio non-detections

Supergiant winds

Wolf-Rayet winds

22

SHOCK
PROPERTIESDETECTION
METHODS

A solution for 1 and 2:
aspherical CSM frombinary interaction

VT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Low density,
already hot frombeing shockedHigh density,
slow shock,strong radio emitter

23

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Problem: coincidental binary mass ejection within a few
centuries of explosion is exceedingly rare at high Z

Klencki+20 24

SHOCK
PROPERTIESMystery #3

DETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Dense CSM interaction at ~1017 cm is rare ~0.1% of CC rate
Jets are even more rare
Why did they occur together?

25

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSOne model that predicts all of the observables: a merger-driven explosion•Explosion caused by the merger ofa black hole or neutron star with amassive star•

The only(?) model of eruptive
mass loss which naturally explainsa relativistic jet

Chevalier+2012, see also Soker+2017, Schrøder+2020

26

SHOCK
PROPERTIESDETECTION
METHODS

1. High mass X-ray binary(centuries before explosion)

VT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

2. Compact object plunges into companion

3. Jet launched at explosion (2014)
4. Dense CSM interaction years
after explosion (2017 — present)27

Part IV:
Systematic search #1 for more radio supernovae

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

64 transientsassociated withgalaxies within
200 Mpc

Dong+2023b in prep

29

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS

Nuclear transientsare predominantly on
red & dead galaxies

30

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATICSEARCHCONCLUSIONS

Off-nuclear transientsare predominantly onstar forming galaxies

31

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONS▸ 29 stellar explosion
candidates:~0.3% of core-collapse
supernova rate

▸ Where data are available:
Evidence of eruptive
mass loss ~10-2 M☉/yr at~1017 cm
Evidence that the CSM isaspherical▸ 7 with optical counterparts▸ 3 previously known▸ 4 identi ed throughfollowup Keck spectroscopy

fi32

SHOCK
PROPERTIESDETECTION
METHODSVT 1210+4956

SYSTEMATIC
SEARCHCONCLUSIONSSupernova remnant -like lines from the most luminous radio supernovaeH⍺ (forward shock)

Asymmetric structure in forward shock

OIII (reverse shock)Even more complicated velocitystructure in reverse shock33

Part IV:Conclusions

SHOCK
PROPERTIESDETECTION
METHODSDEEP DIVESYSTEMATIC
SEARCHCONCLUSIONS• Radio observations of supernovae are powerful
diagnostics of CSM interaction at large radii.
• The lower the frequency (and the higher theluminosity) the larger the radius.

• With modern radio surveys, we are
now starting to nd them systematicallyLower-luminosity radio supernovae seem to trace winds
but the highest luminosity ones seem to be associatedwith:• Rapid transitions to supernova remnants• very high mass loss rates, centuries before explosion• asymmetric CSM• likely binary interaction• Chevalier+12 style compact object inspirals for a subsetfi35

SHOCK
PROPERTIESDETECTION
METHODSDEEP DIVESYSTEMATIC
SEARCHCONCLUSIONS

Stay tuned — many more transients to come from the full VLA Sky Survey!

O(103) transients per epoch

Dong et al. 2023c in prep36