Signature of the
Photosphere in theGRB Prompt Emission
Rupal BasakSupervisor: A. R. Rao
TIFR, Mumbai, India
SN-GRB Workshop 4-8 November, 2013

Outline
Background
Thermal and non-thermal components- 2BBPL model: 090902B and 081221- Parametrized joint fit: 081221, 090618- A Spine-sheath Jet- Independent Detection- Predicting GeV emission

Conclusions and future works
Rupal Basak

Photosphere in GRB prompt emission

The Fireball Model
Goodman (1986), Paczynski(1986) gave standard modeleven before the BATSE waslaunched
If a high energy density iscreated at a point of space itwill drive e-p plasma andradiation.
The radiation will be thermalized before reaching the photosphere. Hence,the spectrum should be blackbody like. There will be boosting by Lorentz
factor towards the observer, and the temperature should decrease due toadiabatic expansion
The light curve is predicted as a simple pulse
Rupal Basak

Photosphere in GRB prompt emission

Light Curve and Spectrum
The zoo of GRB Light curves

Single variant of spectrum
Lazzati, GRB conference 2012, Munich

Blue: Blackbody (BB)Green: Inverse Compton (IC)Red: Data
Credit: J.T. Bonnell (NASA/GSFC)

Rupal Basak

Photosphere in GRB prompt emission

Internal-External Shock
Rees & Meszaros 1994; Woods & Loeb 1995;
Sari & Piran 1997; Kobayashi et al. 1997

Rupal Basak

Photosphere in GRB prompt emission

Band Model
The internal shock produces
relativistic electrons, which intheir rest frame produce
synchrotron emission. Thismodel is phenomenologically
represented by Band model --two smoothly joined power-laws.

Rupal Basak

Photosphere in GRB prompt emission

Issues in the Synchrotron
ModelBand model is a phenomenological model.
Low energy index crosses the ''line of death''
The actual spectrum can be more complicated.
IS model have its own problems: (i) efficiency is at most 20%, (ii)the situation of inner shell moving faster than the outer one isunstable (Waxman & Piran 1994, ApJL, 433, 85)

Rupal Basak

Photosphere in GRB prompt emission

Back to Blackbody ?
Good news: There are provisions in the original fireball modelOnce the photons go
into the optically thinregion, they are free toescape. With areasonable guess workand assuming very highLorentz factor,
Goodman (1986)
suggested broadeningof the Planck spectrum

Observed spectrum(geometric effect)

Planck model

But, this is still far fromthe actual observation
Rupal Basak

Photosphere in GRB prompt emission

Time-resolved Spectrum

Ryde et al. 2009, ApJ, 702,1211
BBPL=blackbody+powerlw

Rupal Basak

Photosphere in GRB prompt emission

Time-resolved Spectrum
BBPL sometimes iseven better than Band
model

BandBBPL

The kT evolves withtime. This can be
explained by thestandard fireball model

KT (keV)

TimeRupal BasakPhotosphere in GRB prompt emission

Time-resolved Spectrum
BBPL sometimes iseven better than Bandmodel

BandBBPL

The kT evolves withtime. This can be
explained by thestandard fireball model

KT (keV)

TimeRupal Basak

Photosphere in GRB prompt emission

Various Other Possibilities

Time (s)Subphotosphericdissipation ---Multicolour BB+PL(mBBPL)
Time resolved spectra of GRB 090902B (Ryde + 2010). The light curve does
not contain a smooth or separable pulses. For the same GRB Zhang et al.
(2011) claim a BBPL fit for fine enough temporal bins.
Rupal Basak

Photosphere in GRB prompt emission

Various Other Possibilities

Double hump
structure ---Band+BB

GRB 110721A, fitted with Band+BB model (Axelsson et al. 2011). One notices the double
hump in the spectrum (Also see Guiriec et al. 2011; Ryde et al. 2010, 2011; Burgess et al.
2011, McGlynn et al. 2012).
Rupal Basak

Photosphere in GRB prompt emission

The Final Picture?

Lazzati, GRB
conference
2012, Munich

Rupal Basak

Photosphere in GRB prompt emission

The Model
I. GRB 090902B and GRB 081221

2BBPL Model: GRB 090902BBandDouble hump is clearly visible
in the residual of BBPL fit.
BBPL, notethe residual

2BBPL

mBBPL, as done by Ryde +(2010), takes a continuous
temperature distribution over adisk. In contrast 2BBPL modelis truly double hump.

Rao, Basak, Bhattacharya et al. 2013

Rupal Basak

Photosphere in GRB prompt emission

2BBPL Model: GRB 090902B
Band

Double hump is clearly visible
in the residual of BBPL fit.
BBPL, notethe residual

2BBPL

mBBPL, as done by Ryde +(2010), takes a continuous
temperature distribution over adisk. In contrast 2BBPL modelis truly double hump.

Rao, Basak, Bhattacharya et al. 2013

Rupal Basak

Photosphere in GRB prompt emission

2BBPL Model: GRB 090902B
Band model
Rao, Basak, Bhattacharya et al. 2013

BBPL model

2BBPL model
2 BBs with a power-law
0102030

Time (s)Rupal Basak

Photosphere in GRB prompt emission

2BBPL Model: GRB 090902B
Some interesting features of 2BBPL

0102030

Time (s)Rupal Basak

Photosphere in GRB prompt emission

Case Study: GRB 081221
Basak & Rao 2013a, ApJ, 768, 187

Light Curve

Parameter evolutionRupal BasakkT & peak energy evolution

2BBPL and BBPL fit

Chi squares

2BBPL and Band fit

Photosphere in GRB prompt emission

Conclusion I
Higher BB

EF(E)Lower BB

Energy (keV)Power-law

Conclusion I
Higher BB

EF(E)Lower BB

Energy (keV)Conclusion IEF(E)At a later time:

Energy (keV)

EF(E)

Conclusion I

Energy (keV)The Model
II. Parametrized Joint Fit
See poster P31 in the GRB conferenceChoices of Spectral Models
Spectral models
Fully non-thermalBand model.

Thermal+non-thermal
BBPL
mBBPL
2BBPLBBPL=Blackbody+powerlaw
mBBPL=Multicolor BB+PL
2BBPL=2BBs+PL

Rupal Basak

Photosphere in GRB prompt emission

Choices of Spectral Models
Spectral models
Thermal+non-thermal
BBPL
mBBPL
2BBPL

Fully non-thermalBand model.

A proper choice of time bins
Re-binning of spectral channels
Parametrization of spectral model

Rupal Basak

Photosphere in GRB prompt emission

Parametrized Joint Fit
Fully non-thermalCount Rate

Band model.

See poster P31 in
the GRB conferenceTime (s)Rupal Basak

Photosphere in GRB prompt emission

Parametrized Joint Fit
Fully non-thermalTime binningaccording to thecount per bin. Notice
the unequal bin size

Count Rate

Band model.

810

If a 4 parametermodel (Band) is fitted
at each bin, total 72(18x4) parameters
are required.

Time (s)Rupal Basak

Photosphere in GRB prompt emission

Parametrized Joint Fit
Fully non-thermal

Peak energy of theBand model varies
smoothly with time

Count Rate

200100Peak energy

Band model.

50

Time (s)Rupal Basak

Photosphere in GRB prompt emission

Parametrized Joint Fit
Fully non-thermal

Photon indicesremain more or lessconstant over asection-1

Alpha or Beta

Peak energy of theBand model varies
smoothly with time

Count RateBand model.

-5Time (s)Rupal Basak

Photosphere in GRB prompt emission

Parametrized Joint Fit
Band model.

Perform a simultaneous fit, parametrizing thepeak evolution and tying the photon indices.
From 72 parameters, this will
reduce to 26 parameters
Similar treatment for othermodels
For example, BBPL will havesame no. Of parameters as
Band. MBBPL will have one
extra and 2BBPL will have two
extra free parameters,

Rupal Basak

Photosphere in GRB prompt emissionSpectrum at different Regions

Rupal Basak

Band

mBBPL

BBPL/mBBPL

Comparison b/w Band, BBPL and mBBPLBand

Photosphere in GRB prompt emission

Values of Alpha

Rupal Basak

Photosphere in GRB prompt emission

Values of Alpha is within the ''line of death” where Band
Model is the best.

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Photosphere in GRB prompt emission

Results: GRB 081221
RegionModel2/Model1p

Pulse1,
Rising

BBPL/BandmBBPL/Band
2BBPL/Band
2BBPL/BBPL

0.38
0.41
0.30
0.03

0.87
0.81
1.04
2.19

61.5%
58.5%
70.2%
97.1%

Pulse1,
Falling

Band/BBPLmBBPL/BBPL
2BBPL/BBPL
Band/2BBPL

0.30
0.33
1E-5
0.48

1.03
0.97
4.4
0.70

69.9%
66.6%
100%
51.9%

Pulse2,
Rising

Band/BBPLmBBPL/BBPL
2BBPL/BBPL
2BBPL/Band

0.02
0.01
1E-20
0.39

2.37
2.55
9.29
0.86

98.2%
98.9%
100%
60.9%

Pulse2,
Falling

Band/BBPLmBBPL/BBPL
2BBPL/BBPL
2BBPL/Band
2BBPL/mBBPL

1E-5
8E-5
2E-66
0.30
0.16

4.41
3.95
17.2
1.03
1.39

83.5%

Rupal BasakσC.L.

In all cases 2BBPL is superiorthan BBPL. In all cases 2BBPL iscomparable or better than Band.
mBBPL is inferior than Band and2BBPL
In summary, 2BBPL is the bestmodel

Same results are obtained fortwo pulses of GRB 090618. These
are the two brightest GRBs having
100% separablepulses. It is unlikely
100%
100% that we can find better result in
69.7% any other GRBs
Basak & Rao 2013a, ApJ, 768, 187
Photosphere in GRB prompt emission

The Model
III. A Spine-sheath Jet

Origin of 2BBPL: Spine-sheath Jet
Ito, Nagataki, Ono et al. 2013
(Talk by H. Ito in GRB conf.)

Peak of the spectrum,

E p =r1/ 6i

Rupal Basakη

8/ 3−5/12

L

Peak luminosity,

L p =r

2/ 3i

8 /3

η

1/ 3L

Photosphere in GRB prompt emission

Origin of 2BBPL: Spine-sheath Jet
Ito, Nagataki, Ono et al. 2013
(Talk by H. Ito in GRB conf.)

Lazzati &Begelman(2008)

Peak of the spectrum,

E p =r1/ 6i

Rupal Basakη

8/ 3−5/12

L

Peak luminosity,

L p =r2/ 3i8 /3

η

1/ 3L

Photosphere in GRB prompt emission

The ModelIV. Independent Observation
Fermi
GBM: 8keV- 30 MeV
XRT: 0.3-10 keV
Swift

BAT: 15-150 keV

LAT >30MeVXRT Observation of
the Second BB

090618081221
XRTPage et al. (2011)

Basak & Rao 2013a, ApJ, 768, 187

Rupal Basak

Photosphere in GRB prompt emission

XRT Observation of
the Second BB
BB temperatures in the XRT time-resolved spectrum
Note that temperature falls as a power-law with time

Rupal Basak

Photosphere in GRB prompt emission

XRT Observation of
the Second BBPreliminBB temperature in thearBAT/ GBM spectrum iny118-125 s

Rupal Basak

Photosphere in GRB prompt emission

XRT Observation of
the Second BBPrelimin
Higher BB Temperature
aryLower BB Temperature

Rupal Basak

Photosphere in GRB prompt emission

XRT Observation of
the Second BBPreliminary
BB temp. In BAT

Rupal Basak

Photosphere in GRB prompt emission

XRT Observation of
the Second BBPr
eliminary

To emphasize, these are
independently detected
and falls perfectly in ourpredicted line

Rupal Basak

Photosphere in GRB prompt emission

The ModelV. Predicting GeV emission

Predicting GeV Emission
First detection: GRB 940217 (Hurley etal. 1994), observed by CGRO/EGRET,
90 minutes after the CGRO/BATSE
detection.

r=0.537

Features: (a) Delayed onset, (b) longer
lasting, (c) late time power-law
temporal evolution, (d) spectrum:
either an extrapolation of Band or
significantly different component (mostnotable for bright cases)
Problems: (1) No unified spectral
model found for MeV-GeV data (e.g.,Zhang et al. 2011 found fivecombinations) (2) Very weak MeV-GeVcorrelation to draw inferences
Rupal BasakZheng et al. 2012

Solution? Try multi-component
spectral model. We select two pair ofGRBs with comparable GBM fluence,
but widely different LAT fluence.
Photosphere in GRB prompt emissionMorphological Difference
Basak and Rao 2013b, ApJ 775, 31

High LAT count:(A) 090902B,(B) 090926A

Low LAT count:(A) 100724B,(B) 091003

Rupal Basak

Photosphere in GRB prompt emission

MeV-GeV Correlation
Basak and Rao 2013b, ApJ 775, 31

LAT photon fluence

Correlations between the LAT fluence with the GBM fluence and GBM PL fluence

Total GBM fluence

Non-thermal GBM fluence

Correlation

Pearson r

Pearson P

Spearman r

Total GBMLAT fluence

0.87

5.66E-6

0.75

5.61E-4-1.4

Non-thermal
GBM-LAT

0.88

3.21E-6

0.81

9.23E-5

2..3

Rupal BasakSpearman P D-parameter

Photosphere in GRB prompt emission

Conclusions and Future Works
Conclusions:(1) The peak of the prompt emission spectrum of GRB has a photosphricorigin. However, in most general case it has another weaker peak due to thestructure of the jet itself (Rao, Basak, Bhattacharya et al. 2013 arxiv:
1308.2506; Basak & Rao et al. 2013 ApJ 768, 187)
(2) The 2BBPL model is always statistically better than other models. ThePL component might be a combination of Fermi acceleration within the jetand synchrotron radiation in the IS regions(3) GeV emission is the non thermal part of the MeV emission. (Basak & Raoet al. 2013 ApJ 775, 31)

Future work:(1) Production of PL component along with very high GeV emission has tobe investigated with greater detail(2) A full simulation of the prompt emission with varying jet profile as well
as optically thin synchrotron emission to be performed
Rupal Basak

Photosphere in GRB prompt emission

XRT Observation ofthe Second BB
At later times,
looks like two
BBs and hence,
the last threepoints can split

Rupal Basak

Photosphere in GRB prompt emission