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A natural mechanism to induce an electric charge into a black hole

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A natural mechanism to induce an electric charge into a black hole
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    a  r   X   i  v  :  a  s   t  r  o  -  p   h   /   0   4   0   5   2   3   7  v   1   1   2   M  a  y   2   0   0   4 A natural mechanism to induce an electric charge into a blackhole Jos´e A. de Diego a , Deborah Dultzin-Hacyan a , Jes´us Galindo Trejo a , Dar´ıo N´u˜nez ba Instituto de Astronom´ıa - UNAM, Apdo. 70-264,Ciudad Universitaria, 04510 Mexico, D.F. b Instituto de Ciencias Nucleares - UNAM, Apdo. 70-543,Ciudad Universitaria, 04510 Mexico, D.F. ∗ We present a natural mechanism which may induce an electric charge into an ac-cretting black hole in the presence of a strong, high energy radiation field. We studythis mechanism using Newtonian physics, and we also discuss the process withinthe context of a Kerr Newman black hole. Finally, we consider possible astrophysi-cal applications in X-ray variability and jet formation in the Active Galactic Nuclei(AGN). PACS numbers: 95.35.+d, 95.35.G I. INTRODUCTION The standard picture for active galactic nuclei (AGN) is based on a black hole surroundedby an accretion disk [1] and, at least for radio loud objects, a jet of plasma ejected proba-bly through magnetohydrodynamical mechanisms (e.g. Contopoulos [2], [3], [4]; Kudoh &Shibata [5]; Kudoh et al. [6], [7]).Little importance has been given in astrophysics to mechanisms that might produce anelectric charge in a black hole, as it has been assumed that a significant charge cannot beachieved. For most physical situations, the electron-proton coupling in a plasma is so strongthat radiation pressure cannot break this coupling, although it may introduce some kindof electric charge polarization and plasma oscillations. Levich, Sunyaev & Zeldovich [8]considered first this problem in AGN, taking into account quantum effects which introduce ∗ Electronic address: jdo, dultzin, galindo@astroscu.unam.mx, nunez@nuclecu.unam.mx  2an extra term for the radiation pressure. This new term is proportional to the second powerof the radiation flux and is inversely proportional to the sixth power of the distance. Intheir paper, Levich et al [8] showed that gravitational force may be in fact smaller thanradiation pressure with this additional term, and that matter may escape from the nuclei of Seyfert galaxies and quasars. However, probably due to the poor knowledge of AGN spectralenergy distribution (SED) in 1972, they considered sources where infrared luminosity wasdominant, and they argued that:  ‘It is evident that both the nuclei and the electrons should move simultaneously. Thus, the matter as a whole should move away from the sources.’  Nowadays, we know that the inner funnel and the free falling matter into the black holemust glow with hard radiation, although most AGN are optically deep to  γ  -rays. Electron-positron pairs are created, which implies the existence of   γ   photons of at least 10 − 11 J (i.e.,100 MeV or 10 22 Hz) interacting with soft X-ray photons. Some electrons may be Comp-tonized by hard X-rays and  γ  -rays. An electron Comptonized by a 10 22 Hz photon wouldacquire an energy  ∼  10 − 12 J, i.e. two orders of magnitude larger than the electron energyat rest. Under these circumstances, the velocity of the electron is such that basic ideas of plasma, such as Debye shielding, do not apply. The behaviour of a very fast particle movingthrough a plasma is an important aspect of plasma physics which has been investigatedrecently [e.g., Meyer-Vernet [9], Shivamoggi & Mulser [10]], even in the case of nuclear re-actions in stellar plasmas [Shaviv & Shaviv [11]]. In fact, electrons moving with velocitiesone order of magnitude larger than the thermal electrons of the plasma, are practically notaffected by collisions or even magnetic fields. An astronomical example of such fast particlesare cosmic rays, both ions and electrons, which can travel through and among the galaxiesand reach detectors on earth.On the other hand, even though the exact solutions for charged black holes,  i. e.  theReisnner-N¨ordstrom and the Kerr Newman metrics, have been known for more than fortyyears, (see [12]), and several analysis about their properties have been performed (see [13]and references there in), those charged black holes have always been consider of purelytheoretical interest, as long as Nature in general is neutral and the charge to mass radio,even for an electron is so huge: em e = 1 . 3810 − 34 cm6 . 7610 − 56 cm = 2 . 0410 21 ,  (1)that even if, by some unknown process, there could be a cloud of electrons or even protons  3which accreted and that, surmounting the enormous electric repulsion, form a charged blackhole, this will not be a black hole but a naked singularity, as long as the horizon radius,even for a non rotating black hole,  r H   =  M   1 +   1 − ( eQM   ) 2  , turns out to be imaginary.In this paper, we explain how the high energetic radiation field, generated in the inneredge of the accretion disk, can accelerate the electrons in the outer edge of the accretion flow,allowing them to escape from the accreting plasma. Hard X-rays and  γ  -rays, can acceleratethe electrons up to relativistic velocities through Compton scattering. These charges, movingmuch faster than the thermal electrons, cannot be shielded, and their impact parameter isgreatly reduced, moving almost freely through the plasma [14]. The released electrons caneventually be incorporated into the outflow material in the inner funnel and reach the diskcorona. The efficiency of this process, estimated from the periodicity of the X-ray variability,is very small (approximately one out of 10 25 electrons can escape). These electrons will besupported in the corona by radiation pressure from the disk, until the electric force fromthe charged black hole overwhelms this pressure. This black hole charge is produced by thedecoupled accreted protons, and it will eventually increase enough to reduce the accretionrate. This, in turn, will reduce the radiation pressure until the electrons can no longerbe supported in the corona and fall to the black hole attracted by the electric force, thusneutralizing its charge. This process can be repeated producing low amplitude quasiperiodicor, under some circumstances, strictly periodic variations in the X-ray luminosity srcinatedin the accretion disk, which may be superposed to another component arising, for example,from the disk corona. Moreover, we are able to compute an expression for the ratio of the maximum charge acquired to the mass of the black hole, and show that the black holeconserves a well defined horizon.In section II we present the model. In section III, we write down the Kerr Newmansolution and discuss some possible consequences of our model within the contex of such anexact solution. In section IV we apply the model to an AGN that show X-ray periodic orquasiperiodic variability, an compare our model with other proposed models for explainingthe observed variability. And finally our conclusions are summarized sin section V.  4 II. THE MODEL The region between the inner edge of an accretion disk and the event horizon of a supermassive black hole contains a very high energy radiation field of X-ray and  γ  -ray photonsemitted at the inner edge of a thick disk. In the funnel, the radiation is high enough togenerate pairs of particles. Thus, in the inner region, a high number of   γ  -ray photons is alsoproduced, with energies of at least 10 − 11 J  . Although high energy radiation cannot escapefreely from the optically thick accretion flow, an electron near the edge of the infalling plasmahas a probability of being scattered by an energetic photon. In this paper we consider thatthe funnel is virtually empty of matter, as in Begelman, Blandford & Rees [15]. Thus, thereleased electron can move freely along the funnel. However, the density inside the funnelis an open unsolved question. The formation of an electron-positron jet inside the funnel[15] is compatible with the electron-proton decoupling as far as the  γ   ray collisions whichproduce electron-positron pairs imply that photons can move freely inside the funnel, thusthe funnel has an extremely low particle density. For electron-proton plasma jets formedinside the funnel, however, optical depth effects can become very important.In this section we will show that, in the radiation field of very high energy photons wherethe accretion flow is embedded, the pushing force of Compton scattering on electrons andthe pulling force of gravity on protons are so intense, that they can occasionally overcomethe electron-proton coupling force in the accreting plasma. After being released from thefreely falling plasma, the electron may remain as an isolated charge and eventually reach thedisk corona, while the decoupled proton is accreted unto the black hole inducing a positiveelectric charge in the singularity. It must be noted that the main cause underlying thisprocess, apart from the strong gravitational attraction of the black hole, is the high energyof the photons, high enough to induce electron-proton decoupling, which means that thisprocess occurs at a microscopic rather than macroscopic scale, as would be the case fora process induced by radiation pressure. The efficiency of this decoupling process mustnecessarily be extremely low, since a break of the global electric coupling in a plasma is notpossible. From observational data, we will show that approximately one in 10 25 electronsmust be scattered in such a way, in order to reach the disk corona and to trigger the process.Once the scattered electron has reached the corona, it will be decelerated by the ambientpressure and subject to a combined pulling electric force from the charged black hole and  5a pushing radiation pressure from the disk. At the beginning of the process, the radiationpressure is very intense, while the electric force is weak. As the positive charge in theblack hole increase by the infall of decoupled protons, the electric repulsive force on otherprotons also increase and thus the net attraction on these particles decreases, diminishingthe accretion rate and the X-ray luminosity component from the disk. Long before the netforce on the protons vanishes preventing the accretion, the luminosity emitted in the thickpart of the disk will drop so much that the radiation pressure will no longer support thedecoupled electrons in the corona. These electrons will fall onto the black hole at once,neutralizing it and allowing again a higher rate of accretion, thus raising the luminosity toits former high value. The onset of this process is periodic, or quasiperiodic in the presenceof small perturbations.In the next subsections, we shall develop the ideas outlined above adopting a classical orspecial relativity approach. These approaches are justified because, for electrons escapingthrough the inner funnel, after moving a few Schwarzschild radii, relativistic corrections arenegligible for our purpose. During the scattering process, the electrons receive the photonswith the same energy as they had when they were emitted, with negligible gravitationaleffects on the photon energy. All this allows us to study the Compton scattering of theelectrons in the infalling plasma from energy considerations. We consider that, at leastin the edge of this plasma, the electrons can escape without any interaction with otherparticles. Abramowicz & Piran [16] and Sikora & Wilson [17] have calculated the radiativeacceleration of a single particle in an empty funnel. However, detailed descriptions of thegeodesic trajectories of the matter is beyond the scope of this paper. A. Compton scattering on the free fall matter The particles in the inner accretion disk that leave the last stable orbit fall to the blackhole embedded in a piece of free fall plasma. An electric charge in this plasma is shielded ina distance called Debye’s wavelength, which is given by: λ D  =   ǫ 0 kT n e e 2  . For a black hole of 10 8 M  ◦  ( M  ◦  being the solar mass), accreting at the Eddington rate,the characteristic particle density near the horizon is 10 11 cm − 3 [15], and the electron tem-
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