Radio and X-ray observations of LFBOTs:
Engine-powered explosions with dense asymmetric CSM

fiImage credit: Ron Miller, Scienti c American

Anna Y. Q. HoAssistant Professor, Cornell

LFBOT Recap (talk by Dan Perley)
Primer: Radio & X-ray emission
LFBOT characteristics & puzzles
Possible explanations
Open questions & observational future

Anna Y. Q. Ho“FBOTs:” durations < established SN classes

Figure from Inserra (2019) extreme SN review

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Anna Y. Q. Ho“FBOTs:” primarily core-collapse supernovae
Individual objects: IIn (Ofek+2010), Ib (Poznanski+2010), .Iax (Kasliwal+2010), Ibn (Matheson+2000), Ic-BL (Whitesides+ 2017)
Connection between Ibn SNe and rapidly evolving transients (Karamehmetoglu+ 2021, Fox & Smith 2019, Xiang+ 2021)

Figure from AYQH et al. 2021

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Anna Y. Q. Ho

Luminous FBOTs (“LFBOTs")
Term from Metzger (2022)

νLν (erg/s)

1043

10411039- Luminous optical, X-ray,

Day 5
Day 14
Day 22
Day 24radio∝ ν 0.54
νLν ∝ ν- Seven so far (Prentice+18,0.25

AYQH+20, Coppejans+20,Perley+21, Yao+22)∝ ν010371035- 0.01-0.1% of CC SN rate
(AYQH+21)
79Dwarf(10-10Msun) S.F.
(10-3-7 Msun/yr) galaxies;
1-2 kpc offset (Perley+2019,

10101012Figure from AYQH et al. 2019

1410Frequency [Hz]

1016

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1018

Ho+2020, Coppejans+2020,Yao+2022, Perley+2021)

Anna Y. Q. Ho

Optical Recap(Talks by Keiichi Maeda
Primer: Radio & X-ray emission & Dillon Dong)LFBOT characteristics & puzzles
Possible explanations
Open questions & observational future

Anna Y. Q. HoPrimer: SN radio emission (with SSA)
Explosion in pre-SN wind —> relativistic electrons, magnetic elds —> synchrotron radiation
Net (observed) spectrum

log( ux)

Single-electron spectrum

ν-αν-αν5/2Opt. Opt.
thick thinlog( ux)log(frequency)

IF electrons have apower-law energy
distribution
Mod. from Fig 28.5, Carroll & Ostlielog(frequency)

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Anna Y. Q. Hofiflfl

Rybicki & Lightman Ch. 6, Chevalier 1998
Review: Chevalier & Fransson 2016

Flux Density (mJy)

300
10060
3 weeks afterexplosion

1 year after explosionlog( ux)

SN radio emission (with SSA)

ν-αν-αν5/2Opt. Opt.
thick thinlog(frequency)

10

10

10.1

110Frequency (GHz)fl

Figure from Soderberg et al. (2006); SN2003bg
SSA in SNe: Slysh (1990), Chevalier (1998)

100

1

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0.1

110Frequency (GHz)100

Anna Y. Q. Ho

SN radio emission (with SSA)

Figure from Soderberg et al. (2005); SN2005L

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Anna Y. Q. HoInferring physical parameters from the SED

log( ux)

Net (observed) spectrum

ν5/2

At any given time tp, peak ux Fp & frequency νp
set by shock radius R, magnetic eld strength B
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1R / Fp ⌫ pν-αν-αOpt. Opt.
thick thin

B / FpR
9/19
1v=/ Fp (⌫p tp )tplog(frequency)

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⌫p

UB
U=
✏B

Conserve momentum:
22/19 4 2
n e / Fp
⌫ p tpAssume wind velocity vw

Ṁ
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2/ Fp
(⌫p tp )
vw

fi

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flfl

Chevalier (1998)

Anna Y. Q. Hoincreasing CSM density (mass-loss rate)

Inferring physical parameters
Decades of radio obs.

- Relativistic ejecta (e.g., Kulkarni+98,Soderberg+10, Corsi+14)- Mass-loss (e.g., Weiler+86, Fransson+96,Soderberg+06, Margutti+17, Maeda+21,Dong+21)

mass-loss rate in units of10

4M yr

Chevalier (1998), Jencson et al. (2018);
fig. from AYQH et al. 2019

1/1000 km s
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1Anna Y. Q. HoPrimer: X-ray emission(a) Thermal (free-free) bremsstrahlung(b) Non-thermal synchrotron(c) Inverse Compton

LBB
LUVOIR

Review: Chevalier & Fransson 2016

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Anna Y. Q. Ho

Optical Recap
Primer: Radio & X-ray emissionLFBOT characteristics & puzzles
Possible explanations
Open questions & observational future

Anna Y. Q. Ho

Sp. Flux Dens. [mJy]Sp.

∆t [days]=13.36–14.38−1Puzzle
10 #1: Radio emission peaks > 100 GHz for weeks
The Cow
10295.0

10Fν ∝ ν

0−12.5∆t [days]≈ 2290

10Day 24

192.5

10Fν ∝ ννp ≈ 100, Fp ≈ 94

−1.06±0.01∆t [days]≈ 22100

1012

10Frequency [GHz]10ALMA

Figure from AYQH et al. 2019

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3SMA

Anna Y. Q. Ho

(>100 GHz)!"#$%&'#($)*++$,-./
SMA

NOEMA

!"#$%01$)23+$,-./!"#$!&4'&56&5$7#86($)*++$,-./$$

AYQH et al. 2019, 2022, 2023 in prep

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Anna Y. Q. Ho

Puzzle #2: Distinct early high-frequency component

Figure from Margutti et al. (2019)12/25

Anna Y. Q. Ho

v-1.86±0.03

Puzzle #3: Steep high-ν SED

v-1.31±0.03(And shallow low-ν SED)
Net (observed) spectrum(Coppejans et al. 2020)log( ux)

Power-law
Ne~E-pp=3.6 [-0.1, +0.4]

Spectral index:

ν5/2

ν-αν-αOpt. Opt.
thick thinv-2.0±0.2log(frequency)

α=(p-1)/2Expect p=2-3
α=0.5-1

flFigure modified from AYQH et al. (2022)

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Anna Y. Q. Hofν [mJfν ∝ t2

10.0

Puzzle #4: Luminous & variable X-rays
SMA: 230.6–234.6 GHz
SMA: 341.5–349 GHz
ATCA: 34 GHz

Swift (0.3-10 keV)

NuSTAR (3-80 keV)

FX [10−1221043 erg/s

erg/cm /s]

0.5
Decline phase

10.0Plateau phase

1.00.1

XRT 0.3-10 keV
3-10 keV

10-20 keV
20-40 keV

40-80 keV

101102

Days (observer frame) since MJD 58285.0 (2018 June 16 UT)
Figure from AYQH et al. (2019a). See also
Kuin et al. (2018), Rivera Sandoval et al.
(2019), Margutti et al. (2019)14/25

Anna Y. Q. Ho

Puzzle #4: Luminous & variable X-rays
Observed 0.3-10 keV luminosity (erg/s)…lasting hundreds of days

10100
1000
Rest-frame days since explosion
Figure from Yao, AYQH et al. (2022)

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Anna Y. Q. Ho

Optical Recap
Primer: Radio & X-ray emissionLFBOT characteristics & puzzles
Possible explanations
Open questions & observational future

Anna Y. Q. HoFast (sub-rel.) shock, dense medium
v=0.1c-0.6c
18cow (20d): 105 cm-3 (10-3 Msun/yr) AYQH+19
18cow (300d): (10-6 Msun/yr) Nayana+21
10-3 Msun in 1016 cm AYQH+19

Shock properties: AYQH+19, Margutti+19,Nayana+21, Coppejans et al. 2020,Margutti et al. 2019, AYQH et al. 2020,
Bright et al. 2022, AYQH et al. 2022

mass-loss rate in units of104M yr

1/1000 km sFig. modified from Yao, AYQH et al. 2022

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Steep high-frequency SED (+ shallow low-frequency SED)
1. Ultra-long GRB 130925A

2. AGN (“steep-spectrum sources”)

Horesh+15

Hypothesis: steep
electron energy distribution

Interpretation: the central
engine shuts off, the electronscool, the spectrum steepens

3. Oblique shocks (e.g., Xu 2022 for TDEs)
AYQH et al. (2022a)

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Anna Y. Q. Ho

A steep electron energy distribution?
In modeling supernovae, it’s assumed that all electrons are accelerated into a power law…
Fermi acceleration: p=2.3log( ux)Number of electrons

Net (observed) spectrum

Power-law tailNe~E-pν5/2ν-αν-αOpt. Opt.
thick thinlog(frequency)

Spectral index:

γmin

α=(p-1)/2Expect p=2-3Modi ed fromRessler & Laskar (2017)

Electron Lorentz factor (energy)

fifl

Chevalier+98, Kulkarni+98, Björnsson & Franson+04,Soderberg+05, Chevalier & Fransson+06, …

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α=0.5-1

Anna Y. Q. Ho

A steep electron energy distribution?

Number of electrons

Expectation from simulations: some % of e-s are accelerated into a power law…
but most end up in a relativistic Maxwellian

Modi ed fromRessler & Laskar (2017)

1%power law
Power-law tailNe~E-p50% inpower law

Electron Lorentz factor (energy)

fiEichler & Waxman (2005), Giannios & Spitkovsky(2009), Ressler & Laskar (2017), Jóhannesson &Björnsson 2018, Warren et al. (2018), Yuan et al. (2003)

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Anna Y. Q. Ho

A steep electron energy distribution?
Expectation from simulations: some % of e-s are accelerated into a power law…
but most end up in a relativistic Maxwellian

Number of electrons

Steeper spectrum: anexponential in frequency

Modi ed fromRessler & Laskar (2017)

1%power law

Ne~e-γ/theta

Power-law tailNe~E-p50% in
power law

Electron Lorentz factor (energy)

Eichler & Waxman (2005), Giannios & Spitkovsky(2009), Ressler & Laskar (2017), Jóhannesson &Björnsson 2018, Warren et al. (2018), Yuan et al. (2003)
fiDiscussed in the context of- GRB afterglows- Sgr A*

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Anna Y. Q. HoAre we seeing the Maxwellian?
Inferred shock properties:v ~ 0.2-0.4c, ne ~ 102 to 105 cm-3% of electrons in powerlaw <20%
Important for v > 0.2c
(Margalit & Quataert 2021)

Camel(AYQH+21b)

AYQH et al. (2022)

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Cow (data from
AYQH+19)

CSS161010(data fromCoppejans+20)

Anna Y. Q. Ho

Inhomogeneous, asymmetric circumburst medium

Tasmanian Devil (AT2022tsd)- Multiple components (radio, optical, X-ray)- Shallow radio SEDs (Björnsson & Keshavarzi 2017)- Steep above-peak spectral index (Xu 2022)

AYQH et al. in prep.

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Anna Y. Q. Ho

X-rays + radio: central engine active for ~102 days
- Not consistent with free-free, synchrotron, inverse Compton- Require additional power source—compact object(AYQH et al. 2019, Margutti et al. 2019, Coppejans et al. 2020)

Engine + asymmetric CSM: merger of NS/BH with WR star? (Metzger 2022)
Figure from AYQH et al. in prep

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Anna Y. Q. Ho

GRB 060218 + Ic-BL SN 2006aj
Breakout radius: 1013-1014 cm!
Waxman et al. (2007), Chevalier &Fransson (2008), Nakar (2015),
Irwin & Chevalier (2016)

Flux (erg cm-2 s-1)

Engine + CSM: also inGRB-SNe? (H & He-poor)
0.3-10 keV X-rays

Swift / UVOT

Choking a jet in CSM(Duffell & AYQH 2020)
VB
U
UVW2
UVW1
UVM1

Figure modified from Campana et al. (2006)

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Anna Y. Q. Ho

Optical Recap
Primer: Radio & X-ray emission
LFBOT characteristics & puzzles
Possible explanations
Open questions & observational future

Anna Y. Q. Ho

Outstanding questions

What we need

- What is the composition?

UV spectroscopy- What is the geometry/rate?
(And is the CSM truly coupled
to the engine?)- What is the very late time(years) behavior?

- What is the diversity? AndX-ray & submmsurveys

Einstein Probe

Sensitive UV/optical imaging

Rubin

UV surveys

ULTRASAT (2025)5-10 yr-1

what is the engine (NS/BH)?

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Takeaway points

1. LFBOT radio + X-ray (+ ares): reveals central engine, not just interaction
2. Millimeter (>100 GHz) observations critical for high CSM regime (ALMA, NOEMA)
3. Be careful when modeling radio emission from fast (v > 0.1c) shocks
4. Combination of CSM + engine (LFBOTs, LLGRBs, Dillon’s MAXI event…)—mergers?

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Anna Y. Q. Ho