Fermi Several Constraints by Fermi Zhuo Li ( 黎卓 ) Department of Astronomy, Peking University Kavli Institute of Astronomy and Astrophysics 23 August, Xiamen Thanks to: Xiao-Hong Zhao

Q’s on GRB Synchrotron or inverse-Compton? One-zone or multi-zone emission region? Shock microphysics: magnetic field? Bulk Lorentz factor?

Q1: synchrotron vs IC

Still Peak at MeV Still, peak at MeV by Fermi If MeV dominated, then MeV by synch origin [Derishev 01, Wang, LZ etal 09] GRB C

Where the cutoff? MC simulation Assume Criterion Cutoff, if exist, >100 GeV E cutoff MeV C A Probability

Assume cutoff E = gX13.2GeV 1/145 prob of 13.2-GeV phot => cutoff E~10X13.2GeV Assume acceleration time = fX Lamor time, accel time = synch cooling time => max synch E max synch E = cutoff E => Gamma = 1.5e+3 f*g =1.5e+4(f*g/10) GRB C: one photon, sec; 145 photons >100MeV [Li 08, arXiv: ] Max E photon might not be synch origin Upper bound –R IS <R dec –Fast cool –E peak <1MeV –FB accel

Q2: one-zone vs multi-zone

145 phot with >100MeV 14 >1GeV 1 >10GeV 1 sec time delay for >100MeV emission GRB C

INTEGRAL MeV LC: variability <100ms Photometric redshift: z=4.35 L~10 54 erg/s E~10 55 erg High E time delay >> MeV variability time

250ms delay at >30 MeV 12ms variability at MeV range GRB : HE delay >> MeV variability [Fermi collaboration 09, Nature] delay energy

High E time delay t d >> MeV variability time dt Different region/radius MeV H.E. 1/G [Li 08]

Emit first MeV then GeV within one dynamical time (No) –MeV pulses smeared out within angular spreading time (dt) Inhomogeneous model: relativistic hot spots (maybe) –Sec scale, too long, delay of GeV? High E time delay >> MeV variability time: one-zone options? [Lyutikov & Blandford 02 Narayan & Kumar 08]

Evidence in optical: Naked-eye GRB B Correlated at large time scale –Related processes Optical decay timescale much longer than in gamma-rays –different emission regions [Racusin et al 08]

Optical delay >> MeV variability Optical delay: 2 sec MeV variability: 0.1 sec  different regions [Beskin+ 09]

Internal shock ~10 13 cm X O Meszaros 01 Compact source; relativistic, fluctuating outflow ?

Residual collisions at large radii Velocity fluctuation Primary collisions Gamma-rays Residual collisions Long wavelength Radius Collisions smooth out the fluctuation [Li & Waxman 08]

IC emission from residual collisions Scattering primary MeV photons Cutoff increases with R –HE escapes only at large R Time-average spectrum Time delay [Li 08]

GRB C HE time delay constrains Gamma factor Double check by the transition energy flat spectrum?  Larger scale activity of central engine  energy release at larger distance (energy) [Li 10]

Q3: preshock magnetic field

Constraint by X-ray observations No X cutoff  pre-shock amplification to ~mG [Li & Waxman 06] e ~1/    shock front upstream: B, ndownstream t residence <t IC

Fermi observe >100MeV afterglow up to 10 3 sec B

Assume synch origin of >100MeV phot at 10 3 s –Flux, spectrum & temporal index consistent with synch model Acceleration time vs radiative cooling time –Upstream residence time: the time the electron deflected by an angle of 1/Gamma –Less than synch/jitter cooling time –Less than IC cooling time Klein-Nishina effect

Implication –B>1mG, B/B ISM > 10 3 or B 2 /B 2 ISM >10 6 Origin has nothing to do with far upstream field Formed by streaming CRs(?) Postshock field might form by shock compression of preshock field –B<100mG, could be close to equipartition

Q4: bulk Lorentz factor

Lower Gamma limit Observed highest energy (keV) Observations: >1GeV or >10GeV phot usually observed and no cutoff, avoiding gamma-gamma absorption One zone case: assume Tau(max E)<1, lead to Gamma>1000 [Zhao, LZ & Bai 10]

Two zone case Suppression factor vs obs energy Power-law, other than exponential, suppression 1.larger radius 2.beamed target phot MeVHE Observer frame HE comoving frame [Zhao, LZ & Bai 10]

GRB , td=0.2sGRB B, td=1-5s Two zone case: Gamma ~500

GRB C, Gamma~500 threshold

Remarks Synchrotron or inverse-Compton –But not for highest E phot One-zone or multi-zone emission region Preshock >mG field –formed intrinsically by shock –Postshock field by shock compression Bulk Lorentz factor <~500 LZ 10 ApJ LZ, Zhao 11 JCAP Zhao, LZ & Bai, 11 ApJ Li, LZ, MN subm

Open positions for postdocs and graduated students in our GRB/CR group Contact: Zhuo Li, Welcome to KIAA!