AGN Accretion and Gravitational Wave Background for LISA

1. AGN Accretion and Gravitational Wave Background for LISA Sigl, Schnittman, Buonanno, to be finished Günter Sigl GReCO, Institut d’Astrophysique de Paris, CNRS et Fédération de Recherche Astroparticule et Cosmologie, Université Paris 7 http://www2.iap.fr/users/sigl/homepage.html

2. Active Galactic Nuclei as Photon and Gravitational Wave Sources The bolometric luminosity Lbol of an AGN is related to the accretion rate Lacc and the Eddington rate LEdd by of which a fraction fX is in X-rays between 2 and 10 keV, LX= fX Lbol. Assume that a fraction ηc of accretion is in the form of compact objects of typical mass m ~ 100 Msun. These objects release a fraction α ~ 1/12 of their mass m in gravitational waves during inspiral to the last stable orbit: Thus, from the observed X-ray luminosity function dn/dLX for AGNs, we can compute the cosmological gravitational wave background.

3. For ΩSMBH = fraction of critical density in SMBHs, Ωacc = fraction of critical density in accreted gas, ΩX = fraction of critical density of X-rays in the 2-10 keV band, facc = fraction of SMBH mass due to accreted gas, fobsc = fraction of obscured emission ~ 0.3, one has faccΩSMBH ~ (1 – ηem) Ωacc ΩX ~ <(1+z)-1> fobsc fX ηem Ωacc Since ΩX/ΩSMBH ~ 1.3x10-3, <(1+z)-1> ~ 0.4from AGN evolution data, one obtains the condition fobscfaccfXηem ~ 3x10-3 Observations then imply that facc ~ 1 and , thus, that SMBH growth is accretion dominated, which will be our standard case. Observations suggest that ηem is not much smaller than 0.1, and that SMBH build-up is dominated by accretion facc ~ 1 and NOT by mergers => fX ~ 0.1: bolometric emission dominated by infrared.

4. The universal photon spectrum

5. Diffuse X-ray background AGN+galaxy clusters Compton thin Compton thick unobscured Comastri, Gilli, Hasinger. astro-ph/0604523

6. X-ray luminosity function The X-ray background between ~1 and ~100 keV is explained by AGNs.

7. compare fX = 0.1, ηem = 0.1, (infrared emission dominated, solid line) with fX = 0.3, ηem = 0.03 (X-ray emission dominated, dashed line), facc = 0.5, fco = 0.01, α = 1/12 in both cases

8. The duty factor is the event rate times the time tcoh ~ f/(df/dt) ~ f -8/3 spent emitting at frequency f. Below a few milli-Hertz > 1 event contributes at any given time and the signal is gaussian. At higher frequencies one would see individual events at final stages of inspiral.

9. The observable inspiral rate as function of frequency f.

12. compare fX = 0.1, ηem = 0.1, (infrared emission dominated), facc = 0.5, fco = 0.01, α = 1/12, m=105Msun This could, e.g., be recently much discussed intermediate mass black holes Individual events stochastic

13. SN and PopIII Compare this with upper limits, sensitivities, and cosmological predictions BBO BBO correlated Giovannini

14. Conclusions 1.) The accretion powering Active Galactic Nuclei give rise to electromagnetic emission from the infrared to γ-rays and at the same time to gravitational waves from inspiral of compact objects. This gravitational wave background may be observable by LISA.