Influence of shock asymmetry on spectrum and light curve of shock breakout in a circumstellar mediumYukari Ohtani (NAOJ), Akihiro Suzuki (Kyoto Univ.), and Toshikazu Shigeyama (Univ. of Tokyo)

Supernovae through the ages@Easter Island, Chile, from 9th to 13th August 2016

Abstract We study the influences of aspherical shape of the shock on the emission properties of a core-collapse supernova at its explosion, aiming at obtaining information on the explosion mechanisms in future. By using a toy-model for the motion of an ellipsoidal shock front and a Monte-Carlo method, we calculate the influence of bulk-Compton scattering on the light curve and spectrum of shock breakout emission. We compare the results with the observation of XRO 080109/SN 2008D. In order to explain the observed outburst by considering a 2D ellipsoidal shock, it is necessary that the ratio between the maximum and minimum velocities of the shock is about 1.6, the viewing angle is at least 30 degree off the axis of symmetry, and the shock radial velocity is higher than 50 % of the light speed along the line of sight.

1. Introduction

Motivation : Core-collapse supernovae are believed to have aspherical shapes based on the recent observations and the theories of the explosion mechanisms. In order to obtain information on the shock asymmetry, it would be worth while to investigate the emission properties of shock breakout.

4. Conclusions We found that the information on the motion/shape of the shock could be obtained by the overall shape of the light curve and the gradient of the non-thermal spectral component of emission at the moment of supernova explosion (such as observation of XRO 080109/SN 2008D). Though there remains some questions such as the relation between the explosion energy and the shock velocity in the various directions, the studies the observable properties of shock breakout might provide some pieces of information on the explosion mechanisms of supernovae in future.

Reference Soderberg et al., 2008, Nature, 453, 469 Suzuki and Shigeyama 2010, ApJ, 717, L154 Suzuki and Shigeyama 2010, ApJ, 719, 881 Couch et al., 2011, ApJ, 727, 104

Problem: There are no studies which explain both of the observed light curve and spectrum by considering bulk-Compton scattering.

3. Results We investigate a/b and Θ dependence of the light curve and the non-thermal spectral component.

e.g., XRO 080109/SN 2008D

a/b ↗ ： • ∫ Ldt ↘︎ (energies of scattered photons ↘︎) • ∆tp ↗ (time of breakout ↗)

SpectrumLight curve (0.3-10 keV)

Θ ↗： • the onset delays, • ∆tp ↗ (light crossing time ↗)

a/b dependence (60 deg < Θ < 90 deg)

0x100

1x1044

2x1044

3x1044

0 100 200 300 400 500 600

L [erg s-1]

tobs [sec]

α=0α=0.2α=0.5α=0.8

Δtp

Δtdecay

a/b=1 0.87 0.64 0.47

tobs [sec]

L [e

rg s

-1]

0x1001x10432x10433x10434x10435x10436x10437x10438x10439x1043

0 100 200 300 400 500 600

L [erg s-1]

tobs [sec]

0 deg<θ<30 deg30 deg<θ<60 deg60 deg<θ<90 deg

0 deg < Θ < 30 deg 30 deg < Θ < 60 deg 60 deg < Θ < 90 deg

Δtp

tobs [sec]

L [e

rg s

-1]

a/b ↘︎ (Vej ↘︎ in the model) ： • dlog N/dlog E ↘︎

Comparison with XRO 080109/SN 2008D

0x100

1x1043

2x1043

3x1043

4x1043

5x1043

6x1043

7x1043

8x1043

0 100 200 300 400 500 600

L [erg s

-1]

tobs[sec]

30deg<Θ<60deg α=0.5XRO 080109

tobs [sec]

a/b = 1.6 0 deg < Θ < 30 deg

L [e

rg s

-1]

Light curve • a/b ~ 1.6 • Θ > 30 deg

Spectrum • Vej > ~ 0.5 c

2. Method Settings • Ejecta density : uniform (total mass: 10M⊙) • Ejecta velocity : Vej =0.7 c at the top (not changes with time)

• The shape of the shock front is like an ellipsoid of revolution • Angle dependence : 1+α cos2Θ

• CSM density : nCSM = A x r -2 (A <- τ=c/Vej ; Vej : ejecta velocity)

Monte-Carlo calculation (radiative transfer)

ba

�

Symmetry axis

Photon

Line of sight

Shock front

Position of breakout • Rb = 3×1012 cm • τb=c / Vej ~ 1.4

CSM

Interaction • inverse-Compton

scattering - Klein-Nishina cross

section

Photon generation • blackbody emission at

temperature of 0.1 keV

E�2.6E�2.0

E�2.6E�2.0

E [eV]

E [eV]

dN/d

t [eV

-1 s

-1]

dN/d

t [eV

-1 s

-1]

0.57 c < Vej < 0.70 c

0.45 c < Vej < 0.49 c

�

Rb

�

Rb

(a/b = 2) :

�

photonΘ [deg]= 0 - 30 30 - 60 60 - 90

Θ dependence (a/b = 2)

a/b= 1 1.1 1.6 2.1

top : Early beakout. But photons are distant from the observer.

side : Late breakout. But photons are close to the observer.

e.g. off-axis observer

Power-law spectrum :

Shape of the shock ? (e.g., Suzuki & Shigeyama 2010a, Couch et al., 2011)

coun

t [s

-1 k

eV-1

]

0.5 1 2 5E [keV]

dN � E�2.3±0.3600 sec

coun

t rat

e [s

-1]

0 200 400 600time [sec]

Shape of the light curve (0.3-10 keV)

▷▷ radius of visible region : ~ 1012 cm

Shock velocity

bulk-Comptonization(e.g., Suzuki & Shigeyama 2010b)

Light curve Spectrum bulk-Compton scattering

Suzuki+ 2010a ◯ × ×Suzuki+ 2010b × ◯ ◯

Couch+ 2011 ◯ ◯ ×This study ◯ ◯ ◯

This study: To obtain information on shock asymmetry in future, we investigate the relation between the properties of shock breakout emission and the shock behavior (asymmetry, velocity)