Mysterious transient objects

Poonam ChandraRoyal Military Collage of Canada

Universe has > 125 billion galaxiesEach 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 timescaleMassive explosions in the universeEnergy emitted 1051 ergs (1029 times more than an atmospheric nuclear explosion)Shines brighter than the host GalaxyAs much energy in 1 month as sun in ~1 billion yearsIn universe 8 supernova explosions every second Thermonuclear and gravitational collapse

GAMMA-RAY BURSTS (GRBs)

Most luminous events in the universe since big bangFlashes of gamma-rays from random directions in skyFew milliseconds to few seconds timescaleEven 100 times more energetic than supernovaeBrightest sources of cosmic gamma-ray photons in the universeIn universe roughly 1 GRB is detected per dayShort duration (< 2 sec) and long duration (> 2sec)

Soft Gamma-Ray Repeaters (SGR)

Time scale of few daysRepeated flares in Soft Gamma Ray or hard X-ray bandLess energetic then supernovae and GRBs but GalacticIn 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

Ic

dlB 4

.

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:

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.

Interaction of the ejected material from the supernovae and GRBs with their surrounding medium and study them in multiwavebands.

SN/GRB explosion centre

Photosphere

Outgoing ejecta

Reverse shock shell

Contact discontinuity

Forward shock shell

Circumstellar environment

105K

109K107K

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.

Radio Emission

Free-free absorption: absorption by external medium

Information about mass loss rate.

Synchrotron self absorption: absorption by internal medium

Information about magnetic field and the size.

32

3

2

2.

Rw TuM sff

NB relssa

5.15.2

X-ray emission from supernovae

Thermal X-rays

versus

Non-thermal X-rays

Date of Explosion : 28 March 1993

Type : IIb

Parent Galaxy :M81

Distance : 3.63 Mpc

SN 1993J

“Modeling the light curves of SN 1993J”, T. Nymark, P. Chandra, C. Fransson 2008, accepted for publication in A&A

“X-rays from explosion site: 15 years of light curves of SN 1993J”, P. Chandra, et al. 2008, submitted to ApJ

“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

GMRT

VLA

Synchrotron

cooling break at 4 GHz

Chandra, P. et al. 2004

Frequency

Flux

1.5 years later…………. ~Day 3750

Synchrotron cooling break at

~5.5 GHz

GMRT

VLA

Frequency

Flux

Synchrotron Aging

Due to the efficient synchrotron radiation, the electrons, in a

magnetic field, with high energies are depleted.

tbBE offcut 2

1

bN

(E)

E

N(E)=kE-g

.

Q(E)E-g

steepening of spectral index from a=(g-1)/2 to g/2 i.e. by 0.5

.

253

sin4

3EB

cm

e

22274

4

sin3

2EB

cm

e

dt

dE

Sync

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

2/12/32

2/12/12

30 2

202

20tt

Rtt

RB

dt

d break

Diffusion acceleration coefficient

k=(5.3 +/- 3.0) x 1024 cm2 s-1

On Day 3200…… GMRT+VLA spectrum

GMRT

VLA

Synchrotron

cooling break at 4 GHz

Chandra, P. et al. 2004

Frequency

Flux

X-ray studies of SN 1993J (Chandra et al 2008; Nymark, Chandra, Fransson 2008)

ROSATASCAChandraXMM-NewtonSwift

X-ray telescopes

ROSAT SwiftASCA

Chandra

XMM

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)

NeXNeIX?

NeX

NeIX

99.9%90%67%

99.9%90%67%

Constraining the progenitor mass

4NeXeff

NeXIe

NeXNeX

hnn

dVdjL

2

1

51077.6

f

ne

sunNe MM 016.0Compatible with 15 solar mass progenitor star

Luminosity of Neon X line

Cascade factor

Emissivity of neon X line

Number density of neon is ~ 600 cm-3.

Fraction of NeXI to total Neon

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 10∼ 18 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.

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. ⊙

Mass estimation

•CSM expansion velocity 50 − 100 km s ∼ − 1 . •For R 10∼ 16 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.

Nature of the progenitor star

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

•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 ergGCN 6028,6102,6071,6049,6047,6041,6096,6030,6039,6064,6042

GRB 070125: Chandra et al. 2008 ApJ

GRB 070125: observationsObserved 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

MULTIW

AVEBAND

MODELIN

G OF

BRIGHTEST R

ADIO G

RB

070125 IN SW

IFT E

RA

POONAM CHANDRAJansky Fellow, NRAOUniversity 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.

ICmIC

c

ICm

p

ICm

ICIC

ICm

ICcIC

c

ICIC

FF

FF

;

;

2/12/

max

2/1

max

7.5 Jy;d7.3

0066.0

7.57.3 Jy;d7.3

0082.0

7.38.2 Jy;d8.2

0079.0

4.2

2/1

8/1

tt

F

tt

F

tt

F

IC

IC

IC

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.

CONCLUSIONS: GRB070125

•Radio scintillation detection•8 hours observation with VLA in 8 GHz•Mapped every 20 minutes

askpcm10kpcGHz10

25.25/3

3/205.3

15/6

SMDscrsrc

skpcm10s km 50GHz10

107.65/3

3/205.3

1

1-

5/64

SMvt ISSdiff

Size of the Fireball

cm107.5 17R

(Goodman 1997)

Poonam Chandra13th July 2005

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)

SGR 1806-20, Cameron, Chandra et. al. Nature

Poonam Chandra13th July 2005

27th December 2004 at 4:30:26.65 pm EST

Courtesy: NASA

Poonam Chandra13th July 2005

Precursor Spike Tail

Duration 1 sec 0.2 sec 382 sec

Temp 15 keV 175 keV 3-100 keV

Fluence (erg/cm2)

1.8x10-4 1.36 4.6x10-3

Energy (ergs)

2.4x1042 1.8x1046 1.2x1044

Poonam Chandra13th July 2005

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.

Poonam Chandra13th July 2005

Distance estimation of SGR 1806-20 from the HI absorption spectra

HI emission spectrum

Poonam Chandra13th July 2005

Source

HI absorption spectrum

Poonam Chandra13th July 2005

SGR 1806-20

Flux

den

sity

(Jy)

d

(kpc

) F

lux

dens

ity (J

y)

Brig

htne

ss te

mp

(K)

100

20

40

60

80

Velocity (km/s)- -50 0 50 100 150

0.2

0

0.4

0.6

0.8

0.08

0.04

2010

Lower limit d=6.4 kpc

Upper limit d=9.8 kpc

km/s 220

kpc 8

0

0

R

21cm HI spectrum

Poonam Chandra13th July 2005

Association with the heavy mass cluster and Luminous Blue Variable?

What kind of stars produce magnetars which forms SGRs?

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