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Mysterious transient objects
Poonam Chandra
Royal Military Collage of Canada
Universe has > 125 billion galaxies
Each galaxy has ~100 billion stars
Astronomical time scales
•Age of Universe ~14 billion years
•Life time of stars ~ millions to
billions of years
Some sources appear in the sky
for few seconds to few months to
few years…. Transient objects
Observing, modeling and
understanding these
transient objects
SUPERNOVAE (SNe)
Few months to few years timescale
Massive explosions in the universe
Energy emitted 1051 ergs (1029 times more than an
atmospheric nuclear explosion)
Shines brighter than the host Galaxy
As much energy in 1 month as sun in ~1 billion years
In universe 8 supernova explosions every second
 Thermonuclear and gravitational collapse
GAMMA-RAY BURSTS (GRBs)
Most luminous events in the universe since big bang
Flashes of gamma-rays from random directions in sky
Few milliseconds to few seconds timescale
Even 100 times more energetic than supernovae
Brightest sources of cosmic gamma-ray photons in the
universe
In universe roughly 1 GRB is detected per day
Short duration (< 2 sec) and long duration (> 2sec)
Soft Gamma-Ray Repeaters (SGR)
Time scale of few days
Repeated flares in Soft Gamma Ray or hard X-ray band
Less energetic then supernovae and GRBs but Galactic
In 1/10 of a second as much energy as sun emits in
100,000 years continuously.
1000 times more bright than combining all the stars of
Milky Way together.
Only handful of SGRs are known
Common origin: Massive stars
Nuclear reactions inside a star
4-8 Msun : thermonuclear supernovae
•4-8 Massive star: Burning until Carbon
•Makes Carbon-Oxygen white dwarf
•White Dwarf in binary companion accretes mass
•Mass reaches Chandrashekhar mass
•Core reaches ignition temperature for Carbon
•Merges with the binary, exceed Chandrasekhar mass
•Begins to collapse. Nuclear fusion sets
•Explosion by runaway reaction – Carbon detonation
• Nothing remains at the center
• Energy of 1051 ergs comes out
• Standard candles, geometry of the Universe
Thermonuclear Supernovae
M >8 Msun : core collapse supernovae
• Burns until Iron core is form at the center
• No more burning
• Gravitational collapse
• First implosion (increasing density and temperature at
the center)
• Core very hard (nuclear matter density)
• Implosion turns into explosion
• Neutron star remnant at the centre.
• Explosion with 1053 ergs energy
• 99% in neutrinos and 1 % in ElectroMagnetic
• Scatter all heavy material required for life
Core Collapse
Supernovae
M > 30 Msun : Gamma Ray Bursts
• Forms black hole at the center
•Rapidly rotating massive star collapses into the black
hole.
•Accretion disk around the black hole creates jets
•GRBs are collimated.
• All GRBs extragalactic
• Some GRBs associated with supernovae
(GRB980425/SN1998bw, GRB030329/SN2003dh etc.)
• Dedicated instruments (BATSE, BeppoSax, Swift)
• These GRBs last for few seconds
• For longer duration in lower energy bands
Short Hard Bursts
•Neutron stars or black holes
formed during end stages of
massive stars
•Merger of two neutron stars or a
black hole and a neutron star
colliding
•Less energetic than collapsar GRBs
•Duration less than < 2 seconds.
Soft Gamma Ray Repeater
•When the neutron star in initial formation stages gains very
high magnetic field
•It becomes a magnetar with 1015 Gauss magnetic field
•Global rearrangement in its magnetic structures give SGRs
•Only Galactic sources with energies ~1041-46 ergs
 B.dl 
4
I
c
One common origin
DEATH OF
MASSIVE STARS
•How do massive stars die?
•How are these extreme conditions reached in these events?
•Does the known physical laws work in these extreme conditions?
•Why does small difference in initial conditions lead to such
drastic differences?
•Does nature really need so much fine tuning?
Specific problems:
Interaction of the ejected material from the
supernovae and GRBs with their surrounding
medium and study them in multiwavebands.
Shock velocity of typical SNe are ~1000 times the
velocity of the (red supergiant) wind. Hence, SNe
observed few years after explosion can probe the
history of the progenitor star thousands of years back.
105K
107K
Circumstellar
environment
109K
SN/GRB explosion centre
Photosphere
Outgoing ejecta
Reverse shock shell
Contact discontinuity
Forward shock shell
Radio Emission
Radio emission is synchrotron emission due to energetic electrons
in the presence of the high energy magnetic fields.
Radio emission is absorbed either by free-free absorption
from the circumstellar medium or synchrotron self
absorption depending upon the mass loss rate, ejecta velocity
and electron temperature, magnetic field. Both absorption
mechanisms carry relevant information.
Free-free absorption:
absorption by external
medium
Information about mass loss rate.
2
 .

    M uw T


2
ff
3
2
s
R 3
Synchrotron self absorption: absorption by
internal medium
Information about magnetic field and the size.

ssa

2.5
1.5
B
N
rel
X-ray emission from supernovae
Thermal X-rays
versus
Non-thermal X-rays
Date of Explosion :
28 March 1993
SN 1993J
Type : IIb
Parent Galaxy :M81
Distance : 3.63 Mpc
“X-rays from explosion site: 15 years of light curves of SN 1993J”,
P. Chandra, et al. 2008, submitted to ApJ
“Modeling the light curves of SN 1993J”,
T. Nymark, P. Chandra, C. Fransson 2008, accepted for publication in A&A
“Synchrotron aging and the radio spectrum of SN 1993J”,
P. Chandra, A. Ray, S. Bhatnagar 2004 ApJ Letters 604, 97
“The late time radio emission from SN1993J at meter wavelengths”,
P. Chandra, A. Ray, S. Bhatnagar 2004 ApJ Letters 604, 97
Understanding the physical mechanisms in the forward
shocked shell from observations in low and high frequency
radio bands with the GMRT and the VLA.
Radio emission in a supernova arises due to
synchrotron emission, which arises by the
ACCELERATION OF ELECTRONS
in presence of an
ENHANCED MAGNETIC FIELD.
Giant Meterwave Radio Telescope, India
Very Large Array, USA
On Day 3200…… GMRT+VLA spectrum
Chandra, P. et al. 2004
F
l
u
x
Synchrotron
cooling break at 4
GHz
GMRT
VLA
Frequency
1.5 years later…………. ~Day 3750
Synchrotron
cooling break at
F
l
u
x
~5.5 GHz
GMRT
VLA
Frequency
Synchrotron Aging
Due to the efficient synchrotron
radiation, the electrons, in a
magnetic field, with high energies
are depleted.
.
4
dE
2
e


2
2
2

B sin E


4 7
3m c
 dt  Sync
b
Q(E)E-g
N(E)=kE-g
N(E)
steepening of spectral index from =(g-1)/2 to g/2 i.e. by 0.5
Ecut off
3e
2
 
B
sin

E
3 5
4

m
c
.
E
1
 2
bB t
On day 3200
B=330 mG
On day 3770
B=280 mG
Magnetic Field follows 1/t decline trend
Equipartition
magnetic field~
30 mG
Equipartition magnetic field is 10 times
smaller than actual B, hence magnetic energy
density is 4 order of magnitude higher than
relativistic energy density
d break
dt
2
2



R 1/ 2
R 3 / 2
3
1/ 2
1/ 2 
 B0 
t  2t  
t  2t 
 20 
  20 

Diffusion acceleration coefficient
=(5.3 +/- 3.0) x 1024 cm2 s-1
On Day 3200…… GMRT+VLA spectrum
Chandra, P. et al. 2004
F
l
u
x
Synchrotron
cooling break at 4
GHz
GMRT
VLA
Frequency
X-ray studies of SN 1993J
(Chandra et al 2008;
Nymark, Chandra, Fransson 2008)
X-ray telescopes
ROSAT
ASCA
Chandra
XMM-Newton
Swift
ROSAT
ASCA
Swift
XMM
Chandra
X-ray studies of SN 1993J (Chandra et al 2008;
Nymark, Chandra, Fransson 2008)
L ~ t-(0.8-1): adia
L ~ t-1/(n-2): rad.
Density index ~ 12
X-ray spectrum of SN 1993J (Chandra et al 2008;
Nymark, Chandra, Fransson 2008)
CONCLUSIONS
•All the X-ray emission below 8 keV is coming from
reverse shock.
•Reverse shock is adiabatic and clumpy.
•The clumps are producing slow moving radiative
reverse shock.
•The ejecta density profile is Density ~ R-12
•The reverse shock has travelled upto CNO zone in
the ejecta.
SN 1995N in radio and X-ray bands
(Chandra et al 2008, to appear in ApJ;
Chandra, P. et al. 2005, ApJ)
SN 1995N
A type IIn supernova
Discovered on 1995 May 5
Parent Galaxy MCG-02-38-017
(Distance=24 Mpc)
Bremsstrahlung (kT=2.21 keV, NH=2.46 x 1021/cm2. )
Gaussians at 1.03 keV (N=0.34 +/- 0.19 x 10-5) and 0.87 keV
(N=0.36 +/- 0.41 x 10-5)
NeX
NeIX?
99.9%
90%
67%
NeX
99.9%
90%
67%
NeIX
Constraining the progenitor mass
LNeX     j NeX ddV
 ne nNeXI 
eff
h NeX
4
Luminosity of Neon X line
ne 
6.77  105
f
Fraction of NeXI to
total Neon
1
2
M Ne  0.016M sun
Cascade
factor
Emissivity of
neon X line
Number density of
neon is ~ 600 cm-3.
Compatible with
15 solar mass
progenitor star
SN 1995N Chandra observations
Total counts
758 counts
Temperature
2.35 keV
Absorption column
Depth
1.5 x 10-21 cm-2
0.1-2.4 keV
Unabsorbed flux 0.6-1.0 x 10-13 erg cm-2 s-1
0.5-7.0 keV
Unabsorbed flux 0.8-1.3 x 10-13 erg cm-2 s-1
Luminosity (0.1-10 keV) 2 x 1040 erg s-1
•How fast ejecta is decelerating?
R~t-0.8
•What is the mass loss rate of the progenitor star?
M/t = 6 x 10-5 Msun yr-1
•Density structure
Density ~ R-8.5
•Density and temperature of the reverse shock
Forward shock: T=2.4 x 108 K, Density=3.3 x 105 cm-3
Reverse shock: T=0.9 x 107 K, Density= 2 x 106 cm-3
SN 2006X, Patat, Chandra, P. et al. 2007, Science
•Type Ia supernova (Thermonuclear supernova)
•True nature of progenitor star system?
•What serves as a companion star?
•How to detect signatures of the binary system? Single
degenerate or double degenerate system?
Observations of SN 2006X:
•Observations with 8.2m VLT on day -2, +14, +61,
+121
•Observations with Keck on day +105
•Observations with VLA on day ∼ 400 (Chandra
et al. ATel 2007).
•Observations with VLA on day ∼ 2 (Stockdale,
ATel 729, 2006).
•Observations with ChandraXO on day ∼ 10
(Immler, ATel 751, 2006).
Na I D2
line
Na vs Ca
RESULTS
•First ever supernova followed regularly till 4 months.
• Variability not due to line-of-sight geometric effects.
•Associated with the progenitor system.
•Estimate of Na I ionizing flux: SUV ∼ 5 × 10 50 photons s − 1
• This flux can ionize Na I up to ri ∼ 1018 cm.
•This implies ne ∼ 10 5 cm − 3
(ONLY PARTIALLY IONIZED HYDROGEN CAN PRODUCE SUCH
HIGH NUMBER DENSITY OF ELECTRONS )
•Confinement: rH ≈ 10 16 cm
•Ionization timescale τi < Recombination timescale τr .
Increase in ionization fraction till maximum light.
Recombination star ts.
• When all Na II recombined, no evolution. Agree with
results.
Mass estimation
From spectroscopic data:
Na I column density N (Na I) ≈ 1012 cm − 1
log(Na/H)= −6.3.
For complete recombination,
M (H) ≤ 3 × 10−4 ⊙ M.
From radio:
3 − σ upper limit on flux density F (8.46GHz) <
70 µJy.
Mass loss rate ≤ 10 − 8 ⊙ M year − 1
CSM mass < 10 − 3 ⊙ M Below detection limit.
Nature of the progenitor star
•CSM expansion velocity ∼ 50 − 100 km s − 1 .
•For R ∼ 1016 cm, material ejected ∼ 50 year before!
•Double-degenerate system not possible. Not enough
mass.
•Single degenerate. Favorable.
•Not main sequence stars or compact Helium stars.
•High velocity required.
•Compatible with Early red giant phase stars.
•Possibility of successive novae ejection.
COLLABORATORS
Claes Fransson (Stockholm Obs)
Tanya Nymark (Stockholm Obs)
Roger Chevalier (UVA)
Dale Frail (NRAO)
Alak Ray (TIFR)
Shri Kulkarni (Caltech)
Brad Cenko (Caltech)
Kurt Weiler (NRL)
Christopher Stockdale (Marquette)
…and …. more
GRB 070125: Chandra et al. 2008 ApJ
•Detected by inter-Planetary Network of GRB detectors
•Triangulated by Odyssey, Suzaku, Integral
•RHESII, Konus-Wind observed
•Swift was slewing, BAT marginal detection at t=4min
•RHESSI: Epeak =980+/-300 keV and
•Fluence (30keV-10MeV) =1.5 x 10-4 erg cm-1
•Konus-Wind: Epeak=367+/-~60 keV and
•fluence (20keV-10MeV)= 1.74 x 10-4 erg cm-1
•Redshift z=1.5477, Eiso = 1054 erg
GCN 6028,6102,6071,6049,6047,6041,6096,6030,6039,6064,6042
GRB 070125: observations
Observed by Swift-XRT, Swift-UVOT, P60,
SARA 0.9m, Lick 3m, Keck/LRIS, TNT 0.8m,
Prompt, VLT, GMRT, WSRT, VLA , IRAM
Follow up Observatiions:
•P60 observations until day ~25
•(Swift-XRT followed it until day 14)
•Chandra observations on day ~39
•Submm observations until day ~15
•VLA observations until day ~280
POONAM CHANDRA
Jansky Fellow, NRAO
University of Virginia
•Synchrotron emission
•Corrections to Inverse Compton
•Inverse Compton important in X-rays only
•IC important throughout the evolution
•Role of IC in GRB Light curve
only the synchrotron model for
the GRB afterglow and derive
various parameters
spectrum due to IC scattering has
the same shape as that of the
synchrotron model.
F
F
IC
IC
 Fmax
 Fmax
IC
IC



IC 

 c 
1 / 2



IC 


m


;
p/2
IC
c
 



IC 


c


IC
m
IC
m
1 / 2
;  
IC
m
1/ 8
F
F
F
IC
IC
IC
 t 
 0.0079
 Jy; 2.8  t  3.7
 2.8d 
 t 
 0.0082

 3.7d 
1 / 2
 t 
 0.0066

 3.7d 
 2.4
Jy; 3.7  t  5.7
Jy; t  5.7
CONCLUSIONS: GRB070125
Inverse Compton Scattering flattens the X-ray
light curve, at least in some GRBs.
Jet break in X-ray may get delayed beyond
Swift observations.
It may be a major cause for the absence of jet
break in X-ray bands.
•Radio scintillation detection
•8 hours observation with VLA in 8 GHz
•Mapped every 20 minutes
  
 src  2.25

 10GHz 
6/5
 Dscr 


 kpc 


SM
 3.5 20 / 3

kpc 
 10 m
    vISS 
 6.7 10 
 
-1 
 10GHz   50 km s 
6/5
tdiff
1
4
1
3 / 5
as


SM
 3.5 20 / 3

kpc 
 10 m
(Goodman 1997)
Size of the Fireball
R  5.7 10 cm
17
3 / 5
s
SGR 1806-20, Cameron, Chandra et. al. Nature
SGR 1806-20
Giant flare on Dec 27, 2004
Detected by INTEGRAL, RHESSI, Wind
Spacecraft, SWIFT, GMRT, VLA, ATCA etc.
80,000 counts/sec (RHESSI)
13th July 2005
Poonam Chandra
27th December 2004 at 4:30:26.65 pm EST
13th July 2005
Poonam Chandra
Courtesy: NASA
Precursor Spike
Tail
Duration
1 sec
0.2 sec
382 sec
Temp
15 keV
175 keV 3-100 keV
Fluence 1.8x10-4
(erg/cm2)
Energy
(ergs)
13th July 2005
2.4x1042
1.36
4.6x10-3
1.8x1046 1.2x1044
Poonam Chandra
GMRT observations of SGR 1806-20
•From January 4, 2005 to February
24, 2005
•Initially very frequently, almost
everyday
•Snapshots, 40-60 minutes.
•Mostly in 240 and 610 MHz and in
1060 MHz at some occasions.
13th July 2005
Poonam Chandra
Distance estimation of SGR 1806-20 from the HI absorption
spectra
HI emission
spectrum
13th July 2005
Poonam Chandra
Source
HI absorption spectrum
13th July 2005
Poonam Chandra
SGR 1806-20
21cm HI spectrum
100
0  220 km/s
13th July 2005
Flux density (Jy)
R0  8 kpc
d (kpc) Flux density (Jy)
Brightness temp (K)
80
-
60
40
20
Lower limit d=6.4 kpc
0.08
0.04
0
10
20
Upper limit d=9.8 kpc
0.8
0.6
0.4
0.2
-50
0
50 100 150
Poonam Chandra
Velocity (km/s)
Association with the heavy mass cluster and
Luminous Blue Variable?
What kind of stars produce magnetars which forms
SGRs?
13th July 2005
Poonam Chandra
COLLABORATORS
Claes Fransson (Stockholm Obs)
Tanya Nymark (Stockholm Obs)
Roger Chevalier (UVA)
Dale Frail (NRAO)
Alak Ray (TIFR)
Shri Kulkarni (Caltech)
Brad Cenko (Caltech)
Bryan Cameron (Caltech)
…and …. more