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

Jet-driven blast wave emission in GRB---the afterglows

~300

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

The dynamic of the blast wave

Synchrotron radiation•The magnetic field

•The spectrum

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!

Synchrotron self-Compton emission

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

2. Ultra-high energy cosmic rays from the jet resulted from the tidal disruption

Ultra-high energy cosmic rays (UHECRs)

Ultra-high energy cosmic rays

E>10^18-10^19 eV

Extragalactic origin

Greisen, Zatsepin and Kuzmin(GZK) effect

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

AGNs as a candidate of UHECRs

Hillas Plot

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

The maximum energy of accelerated protons•particle acceleration in the blast wave

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