1. GRB Science with XTP Xuefeng Wu Purple Mountain Observatory collaborators: Zigao Dai (NJU), Feng Yuan (SHAO), Enwei Liang (GXU), Yunpeng Men (PKU), & XTP+eXTP science team eXTP Meeting, Beijing, IHEP, Oct , 2015
I am Dai Zigao in Astron. Dept. of Nanjing University. Today I talk about GRBs.

2. Physical Picture of Gamma-Ray Bursts
v ≥ c
Gehrels et al., 2002, Scientific American

3. Magnetic Field Configuration
I. Ordered B-field in large-scale from central engine
Large-scale B-field may exist in
Prompt GRB emission;
Early afterglows (reverse shock)
X-ray flares
X-ray plateaus
Moll 2009

4. Magnetic Field Configuration
II. Disordered B-field by shock amplification
Relativistic Weibel instability
electron saturation
ion saturation
Electrons and ions separates into different currents in a disturbed seed B-field, which amplify the B-field in a positive feedback way.
Medvedev & Loeb (1999)

5. Numerical simulation of the Weibel instability
Frederiksen et al. (2004)
random B-field may exist in
Baryon dominated GRB fireball (prompt+flares);
late afterglows (forward shock)
X-ray plateaus with matter-dominated energy injection

6. Models on GRB polarization
Nonthermal emission:
- synchrotron
- (inverse-) Compton
large-scale B and syn:
PL=(p+1)/(p+7/3)~70%
Shock accelerated electrons
(p=2)
Waxman, 2003, Nature, 423, 388

7. Milestones in GRB polarization detections
1、Prompt gamma-rays：
GRB ：P =80+/-20% (Coburn & Boggs, 2003, Nature)
optical flash: reverse shock
Log(Flux)
t^-2
forward shock
2、Optical afterglows：
Log(t)
GRB060418: 1st poln upper limit on the forward shock emission
P<8% (Mundell et al., 2007, Science)
GRB090102: 1st polarization detection of reverse shock emission
P=10.1%+/-1.3% (Steele et al., 2009, Nature)
GRB120308A: 1st poln detection of reverse+forward shock emission
(Mundell et al., 2013, Nature)
GRB121024A: 1st circular poln detection of forward shock emission
Pcirc=0.64%+/-0.13%，Pcirc/Plinear=0.15
(Wiersema et al., 2014, Nature)

8. Swift Canonical X-ray lightcurve
X-ray polarimeter – a new era for GRBs！
I. steep decay
V. X-ray flare
III. Normal decay
II. plateau
IV. jet phase
Zhang, Fan, Dyks et al. 2006, ApJ, 642, 354

9. GRB Science with XTP Polarization of GRB X-ray emission
X-ray observation of ultra-long GRBs
X-ray micro-flares in GRBs
Late X-ray observation and jet breaks

10. GRB Science with XTP Polarization of GRB X-ray emission
X-ray observation of ultra-long GRBs
X-ray micro-flares in GRBs
Late X-ray observation and jet breaks

11. X-ray flares Burrows et al. 2005, Science, 309, 1833
Explanation: late internal shocks (Burrows et al. 2005; Fan & Wei 2005; Zhang et al. 2006; Wu, Dai, Wang et al. 2005),
implying a long-lasting central engine.

12. fast rise + steep decay Chincarini et al. (2007, ApJ, 671, 1903):
~ 1/3 of the detected GRB afterglows.
fast rise + steep decay

13. X-ray flare sample duration ~ peak time
Swift XRT observations (May – April )
476 flares in 201 GRBs (1/5 Swift triggered GRBs in 10 years)
duration ~ peak time
duration time centered at ~300 s；
for a small fraction, duration > 1 ks
Peak time centered at ~300 s；
for a small fraction, peak time > 1ks

14. X-ray flare sample Swift XRT observations (May 1 2005 – April 30 2015)
476 flares in 201 GRBs (1/5 Swift triggered GRBs in 10 years)
Peak time centered at ~300 s；
for a small fraction, peak time > 10 ks
Peak flux centered at ~3e-9 erg/cm^2/s

15. X-ray flare sample: expected XTP detection possibility
Swift XRT observations (May – April )
476 flares in 201 GRBs (1/5 Swift triggered GRBs in 10 years)
Assume
XTP flux sensitivity = 4.4e-15 (t/10^4 s)^(-1/2) erg/cm^2/s
XTP slewing speed = (3 – 10) degrees per minute
whole flare
peak flare
(angle between the GRB and XTP boresight)

16. X-ray flare sample: expected XTP detection possibility
Swift XRT observations (May – April )
476 flares in 201 GRBs (1/5 Swift triggered GRBs in 10 years)
Earth occulation
P=50%, R=8 degree/min

17. fluence distribution of X-ray flares
Minimum Detectable Polarization:
MDP~19.5%
~ (fluence*S) ^-0.5
S=200 cm^2
MDP~6.2%
MDP~62%
XTP can do polarization detection for
bright X-ray flares if it slews to the GRB
within ~1 ks after the trigger!
fluence ~ Fp*Tp/3
peaked at 1e-7 erg/cm^2

18. Two Models for GRB X-ray Polarizations
(1) SO：synchrotron with ordered B-field (2) SR：synchrotron with random B-field
following Toma et al. (2009, ApJ) SO SR

19. Input of X-ray flare simulation
GRB redshift distribution
1997 Feb. – 2015 Oct.

20. Input of X-ray flare simulation
GRB redshift distribution
Gaussian Fit
In our simulation:
xc=0.3,
w=0.1

21. Input of X-ray flare simulation
GRB redshift distribution
GRB jet half-opening angle distribution
Lu et al., 2012, ApJ

22. Input of X-ray flare simulation
GRB redshift distribution
GRB jet half-opening angle distribution
Viewing angle distribution
p(θv) dθv =sin θv dθv, 0<θv <θj+1/Γ

23. Input of X-ray flare simulation
GRB redshift distribution
GRB jet half-opening angle distribution
Viewing angle distribution
X-ray flare energies (analogue to Frail relation)

24. X-ray flares statistics: <Ex,iso> ~ 0.01<Eg,iso>
Prompt:
Margutti, et al. 2011, MNRAS

25. Input of X-ray flare simulation
GRB redshift distribution
GRB jet half-opening angle distribution
Viewing angle distribution
X-ray flare energies (analogue to Frail relation)
Band spectrum and Ep - Eiso relation

26. X-ray flares statistics: Ep - Ex,iso relation
Band spectrum
X-ray flares 10
Amati relation
Prompt GRBs Ep
Falcone et al., 2007, ApJ
Margutti, et al. 2010, MNRAS

27. Input of X-ray flare simulation
GRB redshift distribution
GRB jet half-opening angle distribution
Viewing angle distribution
X-ray flare energies (analogue to Frail relation)
Band spectrum and Ep - Eiso relation
Lorentz factor – peak time relation

28. X-ray flares statistics: Lorentz factor – Tp relation
Prompt GRBs
X-ray flares
Mu, H. J. et al., 2015, ApJ submitted

29. Preliminary Preliminary simulated X-ray flares
X-ray flares simulations: Ex,iso & fluence distribution
simulated X-ray flares
Preliminary
Preliminary

30. Preliminary XTP simulations for X-ray flares: polarization No SR!
XTP can help to distinguish the magnetic field configuration in GRBs
by detecting a large sample of X-ray flare polarizations!

31. Preliminary XTP simulations for X-ray flares: polarization
only 14% of SO flares
XTP with higher polarization sensitivity can increase the detected
number of polarized X-ray flares!

32. X-ray Plateaus - Swift discovery and hydrodynamic origin
external plateau by energy injection:
matter-dominated？-> low poln
Poynting flux – domianted ？-> high poln
Internal plateau
magnetic dissipative process？-> high poln
GRB070110
GRB050319
post-plateau
t -1.14ν-0.800.08
sharp
drop
steep decay
plateau
steep decay
t -5.5ν-1.60.22
plateau
t -0.54ν-0.690.06
Cusumano et al., 2006, ApJ, 639, 316
Troja et al., 2007, ApJ, 665, 599

33. A large sample
~ half of the detected GRB afterglows have shallow decay/plateau
for a detailed analysis of Swift GRBs, see Liang et al., 2007, ApJ, 670, 565,

34. X-ray plateau sample
Swift XRT observations (March October 2010)
46 X-ray plateaus (1/10 Swift triggered GRBs in 5 years)
flux centered at 1e-11 erg/cm^2/s
duration ~ 1 – 100 ks, typically ~10 ks

35. X-ray plateau sample: expected XTP detection possibility
Swift XRT observations (March October 2010)
46 X-ray plateaus (1/10 Swift triggered GRBs in 5 years)
Earth occulation
P=50%, R=2 degree/min

36. fluence distribution of X-ray plateaus
Minimum Detectable Polarization:
MDP~19.5%
MDP~6.2%
~ (fluence*S) ^-0.5
S=200 cm^2
MDP~62%
XTP can do polarization detection for
bright X-ray plateaus if it slews to the GRB
within ~1 ks – 100 ks after the trigger!
fluence ~ Fb*Tb
peaked at 1e-7 erg/cm^2

37. X-ray plateau polarization: theoretical expectation
Poynting-flux-dominated
energy injection
high polarization (20%-50%)
Matter-dominated energy injection
low polarization (<5%)
random B-field
aligned B-field
Lan, M. X., Wu, X. F., & Dai, Z. G., 2015, ApJ to be submitted

38. Preliminary XTP simulations for X-ray plateaus: polarization MDP
XTP can help to distinguish the magnetic field configuration and the
physical origin of X-ray plateaus if a large sample of polarizations of
X-ray plateaus has been detected!

39. GRB Science with XTP Polarization of GRB X-ray emission
X-ray observation of ultra-long GRBs
X-ray micro-flares in GRBs
Late X-ray observation and jet breaks

40. Ultra-long GRBs
Levan et al., 2013, arXiv:

41. Polarization of ultra-long GRBs
GRB A
（1）Bright X-ray emission lasts for 10 ks (e.g., GRB A)
（2）Fall-back (a massive progenitor) accretion + Blandford-Znajek mechanism was proposed, which successfully interprets the long-lasting X-ray emission in super-long GRB A (Wu, Hou & Lei, 2013, ApJ, 767, L36) -> high polarization of X-ray emission?

42. GRB Science with XTP Polarization of GRB X-ray emission
X-ray observation of ultra-long GRBs
X-ray micro-flares in GRBs
Late X-ray observation and jet breaks

43. GRB optical micro-flares
Swenson, et al., 2013, arXiv:

44. GRB optical micro-flares
flux of flare
Flux ratio =
flux of underlying component
micro-flare: Flux ratio < 0.5
Swenson, et al., 2013, arXiv:

45. GRB X-ray flare amplitudes
Where are microflares?
Chincarini et al., 2010, MNRAS

46. XTP vs Swift/XRT: larger detection area, higher temporal resolution
 identify X-ray micro-flares from underlying external-shock afterglows
 unveiling the central engine activities in smaller time scales
GRB

47. GRB X-ray flares statistics
Differential duration time distributions of solar flares and X-ray flares. The slopes: (-2.00±0.05, -1.10±0.15).
microflares
dominate,
but lack …
Wang & Dai, 2013, Nature Physics

48. GRB X-ray flares statistics
Left: differential energy distribution of solar flares
Right: cumulative energy distribution of X-ray flares
The slopes: (-1.65±0.02, -1.06±0.15)
microflares
dominate，
but lack …
Wang & Dai, 2013, Nature Physics

49. GRB X-ray flares: underlying physics
Self-organized criticality (SOC): subsystems will self-organize to a critical state at which a small perturbation can trigger an avalanche of any size within the system (Bak et al. 1997).
The slopes of frequency distributions for energies and durations depends on the Euclidean dimensions S (Aschwanden 2012):
S ≈ 1 for X-ray flares, and S ≈ 3 for solar flares.
Wang & Dai (2013) suggest that magnetic reconnection from ultra-strongly magnetized millisecond pulsars (Dai et al. 2006) may trigger an S ≈ 1 SOC process.
microflares affect the evaluation of the S value

50. GRB Science with XTP Polarization of GRB X-ray emission
X-ray observation of ultra-long GRBs
X-ray micro-flares in GRBs
Late X-ray observation and jet breaks

51. ~ 90% GRBs with XRT promptly slewing to the GRB position have been detected
with X-ray afterglows since Swift was launched in 2004
(2) Limited by the sensitivity of Swift/XRT, most X-ray afterglows are not well detected
during late times (say, a few days since the trigger), and little X-ray jet breaks have been
discovered, which is important for estimating the beaming factor and calculating the true
energies of GRBs.
GRB ：
longest X-ray afterglow up to date
Chandra deep observations
possible jet break
potential spectral softening
Grupe et al., 2010, ApJ

52. GRB Science with XTP: Summary
Polarization of GRB X-ray emission
most GRBs have X-ray flares or plateaus
large-scale B-field & physical origins
polarization of 10%-20% SO flares can be detected
slewing speed: (3 – 10) degree/min acceptable
X-ray observation of ultra-long GRBs
physical origin and mechanism of ULGRBs
X-ray micro-flares in GRBs
all-scale activities of GRB central engine
Late X-ray observation and jet breaks
jet angle and true energy of GRBs

53. Thanks