Gamma-Ray Burstastro.tsinghua.edu.cn/~xbai/teaching/StudentSeminar2017F/20171229... · power law...
-
Upload
vuongkhanh -
Category
Documents
-
view
213 -
download
0
Transcript of Gamma-Ray Burstastro.tsinghua.edu.cn/~xbai/teaching/StudentSeminar2017F/20171229... · power law...
Summary• What is a GRB(Gamma-Ray Burst)?
• Short and intense pulses of soft gamma rays with non-thermal spectra
• The bursts last from a fraction of a second to several hundred seconds.
• Narrowly beamed
• Long lasting afterglow(in X-ray, optical, radio wavelengths)
• The physical pictures• High energy physical processes
• Relativistic effects
• Synchrotron emission, inverse Compton scattering
• Internal and external shocks
• Stellar collapse & neutron star merger
Spatial Distribution
• BATSE (Burst and Transient Source Explorer)
Cosmic origin!Isotropic
For a constant peak luminosity: 𝑉/𝑉𝑚𝑖𝑛 = 0.5
But the observed value: 𝑉/𝑉𝑚𝑖𝑛 = 0.348
NOT homogeneous!
GRB: observation
• Spatial distribution: isotropic but not homogeneous, cosmic origin
• Prompt Emission• Fast γ-ray emission, together with X-ray flash(XRF) and
possible optical and radio emission(very rare)
• Afterglow
• Association with supernovae
Prompt emission
Spectrum• Non-thermal spectrum peaks at a few hundred
keV, and many events have a long high-energy tail extending up to GeV.
• An empirical fit for the spectrum: broken power-law
• Peak Energy:
• Break Energy:
• Paucity of soft or harder events• Intrinsic or observational artifact? –Not clear
Hardness vs. duration
• NHE(no high energy) bursts:• no emission above 300keV(very negative β)
• Fainter than regular ones
• Many bursts have NHE pulses along with regular pulses
• High energy tail• GRB941017(González et al. 2003)
• Remains roughly constant
• The “tail”(10Mev-200MeV) contains more than 50 times energy than the main γ-ray(30keV-2MeV) energy
• Low Energy tail• Some bursts have steeper low-energy
power spectrum(α>1/3)
Prompt emission: spectrum
-18 to 14 s
14 to 17 s
47 to 80 s
80 to 113 s
113 to 211 s
Spectra of GRB 941017
• GRB duration: T90(T50)• the time in which 90% (50%) of
the counts of the GRB arrive
• Hardness: N(100–300 keV) / N(50–100 keV)
• Hardness – duration correlation• Long & short • Ultra-long
• Variable• Variability time scale(δt) much
shorter than the burst duration• ~80% GRBs show variability
structures• The rest have rather smooth light
curves with a fast-rise exponential decay(FRED)
• Variability luminosity correlation
Prompt emission: Temporal structure
GRB920627
• Pulses
• The bursts are composed of series of individual pulses• Light curve of an individual pulse is a FRED with an average rise-to-
decay ratio of 1:3
• The low-energy emission is delayed compared to the high-energy emission
• The pulses’ low-energy light curves are wider compared to the high-energy light curves(width ~ E-0.4)
• Width-symmetry-intensity correlation: High intensity pulses are (statistically) more symmetric (lower decay-to-rise ratio) and with shorter spectral lags
• Hardness-intensity correlation: The instantaneous spectral hardness of a pulse is correlated to the instantaneous intensity
Prompt emission: Temporal structure
GRB: observation
• Spatial distribution: isotropic but not homogeneous, cosmic origin
• Prompt Emission• Fast γ-ray emission, together with X-ray flash(XRF) and
possible optical and radio emission(very rare)
• Afterglow• slowly fading emission at longer wavelength
• Association with supernovae
• X-ray• 𝑓𝜈(𝑡) ∝ 𝜈−𝛽𝑡−𝛼(α~1.4, β~0.9)
• Normal distribution of the flux in 1-10keV, 11h after burst
• Constant luminosity
• Beam effect
• Optical and IR afterglow• Power-law decay (~t-α), or
broken power law• Fν(t)=f*(t / t*)−α1{1−exp[−(t
/ t*)(α1−α2)](t / t*)(α1−α2)}.
• Power-law spectrum(~ν-β)• Absorption lines• Providing information of
the host galaxy: distance and redshift
• Dark GRBs• ~50% GRBs do not have
optical afterglow• Observational artifact?
Absorption, higher z, or intrinsically fainter?
GRB 990510
Days after the burstO
bse
rved
mag
• Radio afterglow• ~50% GRBs have radio
afterglow, among which ~80% have optical counterparts
GRB 970508
GRB: observation
• Spatial distribution: isotropic but not homogeneous, cosmic origin
• Prompt Emission• Fast γ-ray emission, together with X-ray flash(XRF) and
possible optical and radio emission(very rare)
• Afterglow• slowly fading emission at longer wavelengths
• Only Long bursts
• Association with supernovae• Related to stellar death
• Association with
supernovae(SNe)
• A SN bump in the
afterglow
• Only the long bursts
• The GRB SNe are very
different from normal type
Ib/c supernovae
GRB 090618
Physical Process: Relativistic Motion
• For gamma photons to produce e+e- pairs:
• We get a optically thick source!
• Considering relativistic motion, the source is optically thin!
• Relativistic time effect
• 𝛿𝑡 =𝑅2−𝑅1
𝑣−
𝑅2−𝑅1
𝑐≈ (𝑅2 − 𝑅1)/2𝑐Γ
2
• 𝑅 = 2𝛿𝑡𝑐Γ2
• Hugoniot shock jump conditions(when the upstream matter is cold):
Physical Process: Relativistic shocks
• How the electrons been accelerated?
• diffuse shock acceleration model
• The role of magnetic filed
• The acceleration resulted in a power law spectrum in form of : 𝑁 𝐸 d𝐸 ∝ 𝐸−𝑝d𝐸
• With:
Physical Process: Particle acceleration
• Energy source for the prompt emission and afterglow
• To study the synchrotron emission, you need to consider the motion of the electron and the source
• In observer’s frame:
• Power( in local frame):
• Cooling time(in observer’s frame):
Physical Process: Synchrotron
• Synchrotron spectrum(optical thin)
• Sum of power law: 𝐹𝜈 ∝ 𝜈1/3
• Peak power at 𝜈𝑠𝑦𝑛(𝛾𝑒):
• The overall spectrum: sum of emission of all electrons
• Self absorption: 𝜈𝑎
• For intermediate frequency: Cooling of electrons• Fast cooling(𝛾𝑒,𝑚𝑖𝑛 > 𝛾𝑒,𝑐)
• Slow cooling(𝛾𝑒,𝑚𝑖𝑛 < 𝛾𝑒,𝑐)
Physical Process: Synchrotron
• Jets
Considering a spherical shell with constant velocity
• Time delay for light from angle θ: 𝑅(1 − cos 𝜃)/𝑐 ≈ 𝑅𝜃2/2𝑐
• Relativistic beaming: 𝜃~1/Γ
• Jet angle: 𝜃𝑗~1/Γ
• For an instantaneous flash of power law spectrum 𝑓𝜈 ∝ 𝜈−𝛼, the observed flux
will decay as power law with 𝑡−(2−𝛼) at late times
• The photons earn energy through interaction with electrons
• Comptonization parameter
• 𝑌 =𝜖𝑒
𝑈𝐵𝑖𝑓 𝑈𝑒 ≪ 𝑈𝐵
• 𝑌 = 𝑈𝑒/𝑈𝐵 𝑖𝑓 𝑈𝑒 ≫ 𝑈𝐵
• Add ultrahigh energy component to the spectrum(𝛾𝑒2)
• Speed up cooling, shorten cooling time tsyn(by a factor of Y)
Physical Process: Inverse Compton scattering
Core-Collapse of massive stars
• Association with supernovae
SN1998bw SN2002dh etc.
Core-collapse > central engine >
Unanswered questions
• what is the composition of jet/ejecta (baryonic, e± or magnetic outflow)?
• how are γ-rays, particularly of energy less than ∼10 MeV, produced?
• is a black hole or a rapidly rotating, highly magnetized, neutron star (magnetar) produced in GRBs?
• what is the mechanism by which relativistic jets are launched?
• what are the properties of long and short duration GRB progenitor stars?
References
• Kumara, P., Zhang B. The physics of gamma-ray bursts & relativistic jets. PhR, 561, 1-109(2015)
• Piran T. The physics of gamma-ray bursts. RvMP, 76, 1143-1210(2004)
GRB energetics• Isotropic luminosity
function(dlnL)• Related to star formation rate
• NOT isotropic but beamed
• 𝐸𝛾 =𝜃2
2𝐸𝛾,𝑖𝑠𝑜