high-energy emission from the tidal disruption of stars by massive black holes xiang-yu wang nanjing...
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High-energy emission from the tidal disruption of stars by massive black holes
Xiang-Yu Wang
Nanjing University, China
Collaborators: K. S. Cheng(HKU), Ruo-Yu Liu(NJU)
---preliminary results
The basic picture• Rees 1988: When r<r_t,
the star is captured by the BH
r_t---tidal radius
• Applicable to 106-7 M
SMBH
• A transient accretion disk is formed
Artists conception of tidal disruption of star
Motivations• A jet may form along the axis of the accretion
disk (Cheng et al. 06)
• This jet may produce high-energy gamma-ray emission
• Use Fermi/LAT to constrain this process
Previous works on the jet emission• Disk may produce x-ray
flares (Halpern et al. 2004)
• Modelling with the jet-shock emission (Wong , Huang & Cheng 07)
The spectrum should be very different
Our case— initial condition•Jet energy (Cheng et al. 06)
•A long injection phase (Halpern et al. 04)
•Initial bulk Lorentz factor
•Density of surrounding medium
Maximum synchrotron photon energy
1d ---a parameter describing the ---a parameter describing the efficiency of the shock accelerationefficiency of the shock acceleration
Synchrotron radiation can not produce photons with energy >50 MeV!
The calculated flux at 100MeV
Fermi/LAT sensitivityparameters• E=10^52 erg• t_b=3*10^6 s• d=50Mpc• ep_e=0.1• ep_B=0.001• n=1000 cm^-3• p=2.5
The expected detection rate by LAT•The rate of capture events
•Within , the number of capture events
Ultra-high energy cosmic rays (UHECRs)
Ultra-high energy cosmic rays
E>10^18-10^19 eV
Extragalactic origin
HiRes result• Summary of spectral indices and break points from the fits to the HiRes monocular data
ankle GZK
slope below 3.22 ±0.03 2.81 ± 0.03
break point (logEeV)
18.65 ± 0.04 19.75 ± 0.04
slope above 2.81 ± 0.03 5.1 ± 0.7
HiRes Collaboration, PRL
D. Bergman and J. Belz, arXiv:0704.3721
Sources: Acceleration
RB eBRBR
R
BR
ccV p
v/
1~
1 2
ce
BRL p
222
2
1v
84
v
v
erg/s102 45220,
2
pL
2R
tRF=R/c)
l =R/
2 2
[Waxman 04]
•AGN: ~ few L>1045 erg/s•GRB: ~ 300 L>1051 erg/s
Super-galactic plane
galacticcoordinates
Border of the f.o.v.
27 eventsE > 57 EeV3.20 radius
Véron &Véron-Cetty catalogue442 AGN (292 in f.o.v.)z<0.017 (71 Mpc)
Relative exposureRelative exposure
Auger result
Auger UHECR correlation withVeron-Cetty Veron galaxies
• VCV catalog -- mostly AGNs, but not pure or complete
• L_bol : Most correlations are with too-weak AGNs (Zaw, Farrar, Greene 08)
• Morphology of correlated galaxies: few have jets (Moskalenko, Stawarz, Porter, Cheung 08)
• Standard Scenarios don’t work!Actually, no observed objects with luminosity >10^45 erg s^-1 within d=100Mpc !
Diffusion of the UHECRs induced by the intergalactic B
•CR dispersion time
•But, there could be past transient sources with a high luminosity above 10^45 erg/s
p
D
B
UHECR production in transient Giant AGN flares (Farrar & Gruzinov, 2008)
•Black Hole tidal disruption of a passing star
– Occurs every 10^4-10^5 yr– In AGN, produces a Super-Eddington jet– Duration ~ debris return time, ~1 month– event energy: ~0.01 Msun > 10^52 ergs•Easily achieves L > 10^45 erg/s required
forUHECR acceleration
UHECR chemical composition--Auger result
Elongation Rate measured over two decades of energy
Pierre Auger Collaboration 2010, PRL
Possible presence of intermediate-mass or heavy nuclei in Possible presence of intermediate-mass or heavy nuclei in UHECRs ? UHECRs ?
But inconsistent with HiRes result
Summary• Stellar capture by massive BH may power a jet
• The jet-driven expanding blast wave can produce high-energy gamma-ray emission through SSC process, which may be detected by Fermi/LAT up to distance ~
• Depending on the energy released, the expected detection rate is ~
• The same jet may also accelerate UHECRs
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