1

Gamma-Ray Bursts:Early afterglows, X-ray flares,

and GRB cosmology

Zigao Dai

Nanjing University

2

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

3

What are GRBs?

4

Spectral features: broken power laws

with Ep of a few tens to hundreds of keV Temporal features: diverse and

spiky light curves.

Light Curves and Spectra

5

How to understand?

6

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

7

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

Which satellites detect now?

8

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.

9

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 bursts4. Highest-redshift (z=8.2) GRB0904235. Afterglows and host galaxies of short bursts6. 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

10

I. Shallow decay of X-ray afterglows

Cusumano et al. 2005, astro-ph/0509689

t -5.5ν-1.60.22

GRB050319

t -0.54ν-0.690.06

t -1.14ν-0.800.08

11

See Liang et al. (2007) for a detailed analysis of Swift GRBs: ~ one half of the detected GRB afterglows.

Why shallow decay?

─ big problem!

12

Popular modelsInitial 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

13

Injected energy = E/2

14

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.

15

Generally,

(Zhang & Meszaros 2001; Zhang et al. 2006)

Variants of the pulsar energy-injection model: 1. Luminosity as a power-law function of time

16

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

17

Termination shock (TS)

External shock (ES)

Contact discontinuity

Ambient gas (zone 1)

A relativistic eA relativistic e--ee++ wind wind (zone 4)

Shocked wind (zone 3)

Shocked ambient gas (zone 2)

Variants of the pulsar energy-injection model: 2. Relativistic wind bubble (RWB)

Black hole

Dai (2004, ApJ, 606, 1000)

18Yu & Dai (2007, A&A, 470, 119)

Dai 2004

19

Variants of the pulsar energy-injection model: 3. RWB with a Poynting-flux component

Mao, Yu, Dai et al. (2010): TS-dominated and ES-dominated types for different σ =ησ* (where σ* ~ 0.05).

~ const.

20

Structured ejecta model: initial ejecta with a distribution of Lorentz factors

Structured ejecta model: protonic-component-dominated energy injection

21

Yu, Liu & Dai (2007, ApJ, 671, 637)

Tests of energy injection models: 1. High-energy emission

Structured ejecta model

22

GeV flux: Yu, Liu & Dai (2007, ApJ, 671, 637)

23

Tests of energy injection models: 2. Gravitational radiation

24

Summary: Shallow Decay of Afterglows

• Several explanations for the shallow decay of early X-ray afterglows: energy injection models (electronic- and protonic-component-dominated), and so on.

• Detections of high-energy emission (by Fermi) and gravitational radiation (by advanced-LIGO) are expected to test energy injection models.

25

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.

26

Chincarini et al. (2007, ApJ, 671, 1903): ~ one half of the detected GRB afterglows.

27Short GRB050724: Barthelmy et al. 2005, Nature, 438, 994

28

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).

Why internal dissipation models?

29

Why internal dissipation models?

Liang et al. (2006) tested the curvature effect of X-ray flares and showed that t0 is nearly equal to tpk.

30

CentralEngine

Relativistic Wind

The Internal-External-Shock ModelHow to produce X-ray flares?

ExternalShock

Afterglow

InternalShocks

GRB

Late InternalShocks

XRFs

31

Late-internal-shock model for X-ray flares

• Two-shock structure:

Reverse Contact Forward shock (S2) discontinuity shock (S1)unshocked shocked materials unshocked

shell 4 3 2 shell 1

Gamma_3 = Gamma_2

P_3 = P_2Dynamics

32

Yu & Dai (2008): spectrum and light curve

33

Energy source models of X-ray/optical flaresHow 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)

34

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!

35Rosswog et al., astro-ph/0306418

36

Dai, Wang, Wu & Zhang 2006, Science, 311, 1127: a differentially-rotating, strongly magnetized, millisecond pulsar after the merger.

Kluzniak & Ruderman (1998) Lazzati (2007)

1. Many flares after a GRB

2. Spectral softening of flares

3. Average flare-L decline

37

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).

38

III. GRB cosmology

39

Disadvantages in SN cosmology:

1. Dust extinction

2. ZMAX ~ 1.7

zT~0.5

40

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.

41

The afterglow jet model (Rhoads 1999; Sari et al. 1999; Dai & Cheng 2001 for 1<p<2):

42

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.

M=0.27, =0.73

Ghirlanda correlation

43

The Hubble diagram of GRBs is consistent with that of SNe Ia.

Dai, Liang & Xu (2004, ApJ, 612, L101)

Concordance cosmology

Red: GRB Blue: SNIa

44

Dai, Liang & Xu (2004) assumed a cosmology-independent correlation.

45

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.

46Wang, Dai & Zhu (2007, ApJ)

47

1. The addition of GRBs leads to a stronger constraint on w(z) at the 3rd redshift bin.

2. EOS of dark energy w(z)>0 at z>1.0.

3. Parameter w(z) deviates from -1.

115 GRBs

Constraints on evolution of w(z) (Wang, Qi & Dai 2011)

48

Explosions SNe Ia GRBsAstrophysical 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 to z~1.7 (Riess et al. 2007)

A large Swift-detected sample up to higher z~8.2

Comments on research status

From infancy to childhood (1998) to adulthood (SNAP)

At babyhood (to childhood by future missions?)

Comparison of Two Cosmological Probes

49

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.

50

Summary of this talkShallow 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.