Transcript Early afterglows, X-ray flares, and GRB cosmology

Gamma-Ray Bursts:
Early afterglows, X-ray flares,
and GRB cosmology
Zigao Dai
Nanjing University
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Outline
• Shallow decay of X-ray afterglows
 Observations
 Popular models
 Prediction on high-energy emission
• X-ray flares in early afterglows
 Observations
 Late internal shock model
 Prediction on high-energy emission
 Model for X-ray flares of short GRBs
• Gamma-ray burst cosmology
• Summary
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What are GRBs?
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Light Curves and Spectra
Temporal features: diverse and
spiky light curves.
Spectral features: broken power laws
with Ep of a few tens to hundreds of keV
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How to understand?
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Six eras
1) “Dark” era (1973-1991): discovery
Klebesadel, Strong & Olson’s discovery (1973)
2) BATSE era (1992-1996): spatial distribution
Meegan & Fishman’s discovery (1992),
detection rate: ~1 to 3 /day, ~3000 bursts
3) BeppoSAX era (1997-2000): afterglows, redshifts
van Paradijs, Costa, Frail’s discoveries (1997)
4) HETE-2 era (2001-2004): origin of long bursts
Observations on GRB030329/SN2003dh
5) Swift era (2005-): early afterglows, short-GRB
afterglows, high-redshift GRBs, GRB cosmology
6) Fermi era (2008-): high-energy gamma-rays
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Which satellites detect now?
Swift: Gehrels et al. (2004)
Launch on 20 Nov 2004
Burst Alert Telescope: 15-150 keV
X-Ray Telescope: 0.2-10 keV
Ultraviolet/Optical Telescope: (5-18)1014 Hz
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Fermi: Launch on 11 June 2008
Two instruments:
Fermi Burst Monitor (GBM)
10 keV-25 MeV, dedicated to
detecting GRBs;
Large Area Telescope (LAT)
20 MeV-300 GeV.
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Discoveries and studies in the Swift-Fermi era (2005－)
1. Prompt emission and very early afterglows in
low-energy bands
2. Early steep decay and shallow decay of X-ray
afterglows
3. X-ray flares from long/short bursts
4. Highest-redshift (z=8.2) GRB090423
5. Afterglows and host galaxies of short bursts
6. Some particular bursts: GRB060218 /
SN2006aj, GRB060614 / no supernova,
GRB080109 / SN2008D, GRB080319B, …
7. High-energy gamma-ray radiation by Fermi
8. Classification and central engine models
9. GRB cosmology
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I. Shallow decay of X-ray afterglows
GRB050319
t -5.5ν-1.60.22
t -1.14ν-0.800.08
t -0.54ν-0.690.06
Cusumano et al. 2005, astro-ph/0509689
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See Liang et al. (2007) for
a detailed analysis of Swift
GRBs: ~ one half of the
detected GRB afterglows.
Why shallow decay?
─ big problem!
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Popular models
Initial steep decay:
High-latitude emission from relativistic shocked ejecta, e.g. curvature
effect (Kumar & Panaitescu 2000; Zhang et al. 2006; Liang et al.
2006): flux density  (t-t0)-(2+β) with the t0 effect.
Shallow decay:
Continuous energy injection (Dai & Lu 1998a, 1998b; Dai 2004;
Zhang & Meszaros 2001; Zhang et al. 2006; Fan & Xu 2006)
or initially structured ejecta (Rees & Meszaros 1998; Sari &
Meszaros 1998; Nousek et al. 2006) ……
Normal decay:
Forward shock emission (e.g., Liang et al. 2007)
Final jet decay in some cases
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Injected energy = E/2
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Following the pulsar energy-injection model, numerical simulations by
some groups (e.g., Fan & Xu 2006; Dall’Osso et al. 2010) provided fits
to shallow decay of some GRB afterglows with different slopes.
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Variants of the pulsar energy-injection model:
1. Luminosity as a power-law function of time
Generally,
(Zhang & Meszaros 2001; Zhang et al. 2006)
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GRB060729: Grupe et al. (2007, ApJ, 662, 443)
GRB070110: Troja et al. (2007, ApJ, 665, 599)
GRB050801: De Pasquale et al. (2007, MNRAS, 337, 1638)
q=0
millisecond pulsars
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Variants of the pulsar energy-injection model:
2. Relativistic wind bubble (RWB)
Ambient gas
(zone 1)
Shocked
ambient gas
(zone 2)
Shocked wind (zone 3)
A relativistic e-e+ wind
(zone 4)
External
shock (ES)
Black hole
Termination
shock (TS)
Contact
discontinuity
Dai (2004, ApJ, 606, 1000)
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Dai 2004
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Yu & Dai (2007, A&A, 470, 119)
Variants of the pulsar energy-injection model:
3. RWB with a Poynting-flux component
~ const.
Mao, Yu, Dai et al. (2010): TS-dominated
and ES-dominated types for different
σ =ησ* (where σ* ~ 0.05).
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Structured ejecta model: protonic-component-dominated energy injection
Structured ejecta model:
initial ejecta with a
distribution of Lorentz
factors
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Tests of energy injection models:
1. High-energy emission
Structured ejecta model
Yu, Liu & Dai (2007, ApJ, 671, 637)
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GeV flux: Yu, Liu & Dai (2007, ApJ, 671, 637)
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Tests of energy injection models:
2. Gravitational radiation
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Summary: Shallow Decay of Afterglows
• Several explanations for the shallow decay of
early X-ray afterglows: energy injection
models (electronic- and protonic-componentdominated), and so on.
• Detections of high-energy emission (by Fermi)
and gravitational radiation (by advancedLIGO) are expected to test energy injection
models.
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II. X-ray flares from long bursts
Burrows et al. 2005, Science, 309, 1833
Explanation: late internal shocks (Fan & Wei 2005;
Zhang et al. 2006; Wu, Dai, Wang et al. 2005),
implying a long-lasting central engine.
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Chincarini et al. (2007, ApJ,
671, 1903): ~ one half of
the detected GRB
afterglows.
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Short GRB050724: Barthelmy et al. 2005, Nature, 438, 994
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Why internal dissipation models?
Lazzati & Perna (2007): Flare duration vs. occurrence time in different
dynamical settings as a function of the spectral index. The shaded area
represents the observed distribution of Δt/t from Chincarini et al. (2007).
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Why internal dissipation models?
Liang et al. (2006) tested the curvature effect of X-ray flares and showed
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that t0 is nearly equal to tpk.
The Internal-External-Shock Model
How to produce X-ray flares?
XRFs
Afterglow
GRB
Central Relativistic Internal
Engine
Shocks
Wind
Late Internal
Shocks
External
Shock
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Late-internal-shock model for X-ray flares
• Two-shock structure:
Reverse
Contact
shock (S2) discontinuity
unshocked
shell 4
shocked materials
3
2
Forward
shock (S1)
unshocked
shell 1
Gamma_3 = Gamma_2
Dynamics
P_3 = P_2
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Yu & Dai (2008): spectrum and light curve
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Energy source models of X-ray/optical flares
How to restart the central engine?
1. Fragmentation of a stellar core (King et al.
2005)
2. Fragmentation of an accretion disk (Perna
Armitage & Zhang 2005)
3. Magnetic-driven barrier of an accretion disk
(Proga & Zhang 2006)
4. Magnetic activities of a newborn millisecond
pulsar (for short GRB) (Dai, Wang, Wu &
Zhang 2006)
5. Tidal ejecta of a neutron star-black hole
merger (Rosswog 2007)
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Basic features of short GRBs
1. low-redshifts (e.g., GRB050724, z=0.258;
GRB050813, z=0.722)
2. Eiso ~ 1048 – 1050 ergs;
3. The host galaxies are very old and short GRBs
are usually in their outskirts.
 support the NS-NS merger model！
4. X-ray flares challenge this model!
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Rosswog et al., astro-ph/0306418
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Kluzniak & Ruderman (1998)
Lazzati (2007)
1. Many flares after a GRB
2. Spectral softening of flares
3. Average flare-L decline
Dai, Wang, Wu & Zhang 2006, Science, 311, 1127: a differentiallyrotating, strongly magnetized, millisecond pulsar after the merger.
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Implications for central engines
• X-ray flares after some GRBs may be
due to a series of magnetic activities of
highly-magnetized millisecond pulsars.
• The GRBs themselves may result from
hyperaccretion disks surrounding the
pulsars via neutrino or magnetic
processes (Zhang & Dai 2008, 2009, 2010).
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III. GRB cosmology
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Disadvantages in SN
cosmology:
1. Dust extinction
2. ZMAX ~ 1.7
zT~0.5
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Two advantages of GRBs relative to SNe
①
GRBs can occur at very high redshifts and thus could be
more helpful in measuring the slope of the Hubble
diagram than SNe Ia.
②
Gamma rays are free from dust extinction, so the
observed gamma-ray flux should be a direct measurement
of the prompt emission energy.
So, GRBs are an attractive and promising probe of the
universe.
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The afterglow jet model (Rhoads 1999; Sari et al. 1999;
Dai & Cheng 2001 for 1<p<2):
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M=0.27, =0.73
Ghirlanda correlation
Ghirlanda et al. (2004a); Dai, Liang & Xu (2004): a tight
correlation with a slope of ~1.5 and a small scatter of 2~0.53,
suggesting a promising and interesting probe of cosmography.
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Dai, Liang & Xu (2004, ApJ, 612, L101)
Red: GRB
Blue: SNIa
Concordance cosmology
The Hubble diagram of GRBs is consistent with that of SNe Ia.
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Dai, Liang & Xu (2004) assumed a cosmology-independent correlation.
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Recent works
 Schaefer (2007): 69 GRBs including Swift bursts + 5 correlations
 Li et al. ( 2007), Wright (2007), Liang et al. (2008): GRBs + some
other probes, DL calculated for the concordance cosmology or SNe
 Wang, Dai & Zhu (2007): 69 GRBs + more other probes, DL by
simultaneous fitting of 5 correlations for any given cosmology
GRBs provide a much longer arm for measuring changes
in the slope of the Hubble diagram than SNe Ia.
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Wang, Dai & Zhu (2007, ApJ)
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Constraints on evolution of w(z) (Wang, Qi & Dai 2011)
115 GRBs
1.
2.
3.
The addition of GRBs leads to a stronger constraint on w(z)
at the 3rd redshift bin.
EOS of dark energy w(z)>0 at z>1.0.
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Parameter w(z) deviates from -1.
Comparison of Two Cosmological Probes
Explosions
SNe Ia
GRBs
Astrophysical
energy sources
Thermonuclear explosion
of accreting white dwarfs
Core collapse of massive
stars
Standardized
candles
Colgate (1979):
Lp constant
Frail et al. (2001):
E jet constant
More standardized
candles
Phillips (1993):
Lp~Δm15 (9 low-z SNe Ia)
Ghirlanda et al. (2004a):
E jet~Ep (14 high-z bursts)
Other correlations
Riess et al. (1995);
Perlmutter et al. (1999) …
Liang & Zhang (2005),
Schaefer (2007) …
Recent observations 37 HST-detected SNe Ia up A large Swift-detected
to z~1.7 (Riess et al. 2007)
sample up to higher z~8.2
Comments on
research status
From infancy to childhood At babyhood (to childhood
(1998) to adulthood (SNAP) by future missions?) 48
Summary on GRB cosmology
 Finding: There have been >150 papers on GRB cosmology,
which show that GRBs might provide a complementary and
promising probe of the early universe and dark energy.
 Advantages: 1) GRBs can occur at very high redshifts;
2) Gamma rays are free from dust extinction.
 Disadvantages: The correlations have not been calibrated
with low-z bursts (but also Liang, N. et al. 2008).
 Status: The current GRB cosmology is at babyhood.
 Prospect: In the future, the GRB cosmology could progress
from its infancy to childhood, if a larger sample of GRBs (or
some subclass) and a more standardized candle are found.
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Summary of this talk
 Shallow decay of early afterglows and X-ray flares
seem to imply a long activity of the central engine
(e.g., highly-magnetized millisecond pulsars).
 Future detections by Fermi and advanced-LIGO
are expected to test this implication.
 We expect possible progress in GRB cosmology in
the Swift, Fermi, SVOM … eras.
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