22 nd february 2007 poonam chandra unusual behavior in radio supernovae poonam chandra jansky...
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22nd February 2007 Poonam Chandra
Unusual Behavior in Radio Supernovae
Poonam Chandra
Jansky Fellow, National Radio Astronomy Observatory
Astronomy Department , University of Virginia
22nd February 2007 Poonam Chandra
SN 1993J
Unusually well understood
22nd February 2007 Poonam Chandra
SN 1993J: Why so special???
• Exploded just 3 Mpc away, extremely bright
• First clear case of transition between supernova Type IIP to Ib.
• Was detected in X-rays and Radio bands just 5 days after the explosion.
• Maund et al (2004) detected unambiguously the signature of the massive binary companion in HST observations, 10 years after the explosion.
• Excellent VLBI measurements, Radio emitting zone resolved.
22nd February 2007 Poonam Chandra
Still lot more to learn from SN
1993J
My talk is based on the new surprises, it is offering to us
22nd February 2007 Poonam Chandra
Radio Emission in a Supernova
Radio emission in a supernova arises due to synchrotron emission, which arises by the
ACCELERATION OF ELECTRONSACCELERATION OF ELECTRONS
in presence of an
ENHANCED MAGNETIC FIELDENHANCED MAGNETIC FIELD.
?
?
22nd February 2007 Poonam Chandra
Radio data:
We found synchrotron cooling break in combined GMRT and VLA (0.2 GHz to 44 GHz) near simultaneous observations on day 3200.
22nd February 2007 Poonam Chandra
On Day 3200…… GMRT+VLA spectrum
GMRT
VLA
Synchrotron cooling break at
4 GHz
Chandra, P. et al. 2004
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Is synchrotron cooling break real?
Unsettled Issue!!!
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Observations of SN 1993J on Day 3770
Date of observation
Frequency GHz Flux density mJy
June 17, 03 0.239 58.2 ± 11.0
June 17, 03 0.619 33.4± 4.3
June 13, 03 1.280 20.2± 2.1
June 26, 03 4.885 8.35 ± 0.42
June 26, 03 14.940 0.98 ± 0.32
June 26, 03 22.485 0.82 ± 0.27
But on day 3770…………..
GM
RT
VLA
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1.5 years later…………. ~Day 3750
Synchrotron cooling break at
~5.5 GHz
GMRT
VLA
22nd February 2007 Poonam Chandra
We predicted the evolution of break frequency with roughly 1.2
GHz per year. Our shift in the synchrotron cooling frequency is consistent with this calculation.
(PC, Ray, Bhatnagar, 2004)
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F=
Frequency
Flu
x Den
sity
1= 2= 3
1
23
1< 2< 3
2
3
1
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On Day 3200…… GMRT+VLA spectrum
GMRT
VLA
Chandra, P. et al. 2004
22nd February 2007 Poonam Chandra
1.5 years later…………. ~Day 3750
GMRT
VLA
22nd February 2007 Poonam Chandra
Synchrotron Aging in SN 1993J
Synchrotron losses
Adiabatic expansion
Diffusive Fermi acceleration
Energy losses due to adiabatic expansionEnergy losses due to adiabatic expansion
t
EE
R
V
dt
dE
Adia
V
Ejecta velocity
Size of the SN
R
22nd February 2007 Poonam Chandra
v3
)vv(4 21
21 v
1
v
1
v
4ct
1v2v
Upstream velocity
Downstream velocity
Spatial diffusion coefficient of the test particles across ambient magnetic field
Particle velocity
20
)/(
20
22 tREEV
t
E
dt
dE
cFermi
v
Energy gain due to diffusive Fermi acceleration Energy gain due to diffusive Fermi acceleration
22nd February 2007 Poonam Chandra
EtEbBEtR
E
dtdE
E
Total12222 )20/(/
t tBB /0For and
Break frequency Break frequency
.
.
.
.
2
2/12/12
30 2
20
ttR
Bbreak
(Fransson & Bjornsson, 1998, ApJ, 509, 861)
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On day 3200
B=330 mG
On day 3770
B=280 mG
Magnetic Field follows 1/t decline trend
22nd February 2007 Poonam Chandra
ISM magnetic field is few microGauss. Shock wave will compress magnetic field at most by a factor of 4, still few 10s of
microGauss. Hence magnetic field inside the forward shock is highly enhanced,
most probably due to instabilities
Equipartition magnetic field is 10 times smaller than actual B, hence magnetic energy density is 4 order of magnitude higher than relativistic energy density
22nd February 2007 Poonam Chandra
.
.
.
.
2
2/12/12
30 2
20
ttR
Bbreak
Acceleration diffusion constant
Ball & Kirk 1992
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For SN 1987A 24102 cm2 sec-1
2424 1096.21027.2
4 cm2 sec-1
Scaled value of diffusion coefficient for 1993J
(Ball & Kirk, 1992, ApJL)
22nd February 2007 Poonam Chandra
2/12/32
2/12/12
30 2
202
20tt
Rtt
RB
dt
d break
From VLBI (Bartel, 2002, ApJ),
on day 3200
R=2.65 x 1017 cm
On day 3770
R=3.8 x 1017 cm
break(3200)=4 GHz
break(3770)=5.5 GHz
22nd February 2007 Poonam Chandra
Diffusion acceleration coefficient
=(5.3 +/- 3.0) x 1024 cm2 s-1
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First time direct measurement of
ENHANCED MAGNETIC FIELD
and
ACCELERATION DIFFUSION COEFFICIENT
responsible for radio emission
by observing the synchrotron cooling break and its evolution with time
Result:
22nd February 2007 Poonam Chandra
Radio Emission in a Supernova
Radio emission in a supernova arises due to synchrotron emission, which arises by the
ACCELERATION OF ELECTRONSACCELERATION OF ELECTRONS
in presence of an
ENHANCED MAGNETIC FIELDENHANCED MAGNETIC FIELD.
Take it with caution!!!!Take it with caution!!!!
We need at least 4-5 simultaneous We need at least 4-5 simultaneous datasets showing the synchrotron datasets showing the synchrotron
cooling break to feel more cooling break to feel more confident!!!confident!!!
22nd February 2007 Poonam Chandra
Thanks to:
Kurt Weiler: NRL, Washington
Chris Stockdale: Marquette Univ.
Alak Ray: TIFR, Mumbai, India
Roger Chevalier: Univ Virginia
Juan Uson: NRAO, Charlottesville
22nd February 2007 Poonam Chandra
Thanks