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The immediate environment of an astrophysical black hole

The immediate environment of an astrophysical black hole MNRAS 473, L146–L148 (2018) doi:10.1093/mnrasl/slx179 Advance Access publication 2017 November 7 1,2‹ I. Contopoulos Research Centre for Astronomy and Applied Mathematics, Academy of Athens, Athens 11527, Greece National Research Nuclear University, 31 Kashirskoe Highway, 115409 Moscow, Russia Accepted 2017 November 2. Received 2017 November 1; in original form 2017 October 12 ABSTRACT In view of the upcoming observations with the Event Horizon Telescope (EHT), we present our thoughts on the immediate environment of an astrophysical black hole. We are concerned that two approximations used in general relativistic magnetohydrodynamic numerical simulations, namely numerical density floors implemented near the base of the black hole jet, and a magnetic field that comes from large distances, may mislead our interpretation of the observations. We predict that three physical processes will manifest themselves in EHT observations, namely dynamic pair formation just above the horizon, electromagnetic energy dissipation along the boundary of the black hole jet, and a region of weak magnetic field separating the black hole jet from the disc wind. Key words: magnetic fields – Galaxy: nucleus – quasars: supermassive black holes – submillimetre: stars. (i) Dynamic electron-positron pair formation at the base of the 1 EXPECTATIONS FROM THE EHT black hole jet right above the black hole horizon, The Event Horizon Telescope (EHT) is an international collabora- (ii) Electromagnetic energy dissipation along the last open flux tion whose primary goal is to image the immediate environment of surface through the black hole horizon, and the two supermassive black holes with the largest apparent event (iii) Reversal of the large-scale magnetic field polarity outside horizons, the 4 × 10 M one at the centre of our Galaxy in Sagit- the black hole jet. tarius A, and the 7 × 10 M one in M87 (see eventhorizontele- In the following sections, we will discuss how the above three scope.org for details and information). EHT opens the possibility physical processes might affect the observations of the two super- of imaging extreme light lensing near the horizon, as well as the massive black holes at the centres of our Galaxy and M87. dynamic evolution of black hole accretion, by pushing the limits of global millimeter and submillimeter very long baseline interferom- etry (VLBI). The first observations of Sagittarius A were obtained 2 DYNAMIC PAIR FORMATION in 2017 April, but the publication of the first images will require GR MHD simulations of black hole accretion consistently yield several more months of reduction and analysis. a region above the black hole horizon, where magnetic forces are In this work, we would like to express our concern about the balanced by the gravity of the black hole (e.g. Takahashi et al. 1990; interpretation of the EHT observations with respect to the morphol- Pu et al. 2016). As a result, the flow is separated into inflow and ogy of accretion and jet formation near the black hole horizon. The outflow, and this region is quickly (at light crossing times) emptied EHT Team employs the most comprehensive current theoretical of material. When that takes place, i.e. when the local density drops and numerical models of general relativistic (GR) magnetohydro- below a certain threshold that the code cannot handle, MHD codes dynamic (MHD) accretion and ejection in order to account for the implement so-called density floors. The employment of density expected morphology, spectra and variability of the source (e.g. Lu floors in this region (so-called stagnation or separation surface) is et al. 2016;Chanetal. 2015, and references therein). These fall into tantamount to a continuous artificial supply of material for the jet two general categories: models in which a large-scale magnetic field above the black hole horizon. The simulations do not yield anything of a definite polarity is advected with the accretion flow and gener- special in the region that is emptied of material, and a low-density ates large-scale winds and jets, and models with a random magnetic funnel of the same material as the rest of the material of the accretion field component introduced just to trigger the magneto-rotational flow, namely electron–proton/ion plasma, forms along the magnetic instability that generates accretion but no large-scale ejection flows. field lines that cross the black hole horizon. The following three physical processes have not yet been included In reality, however, as the region above the black hole hori- in these numerical models and need to be taken into account in the zon is emptied of the accretion flow material, it is left only with interpretation of the observations: the accumulated magnetic field and the electric fields associated with black hole rotation. This configuration is akin to that in the E-mail: icontop@academyofathens.gr pulsar magnetosphere. It is common belief in the study of pulsar 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L146/4600553 by Ed 'DeepDyve' Gillespie user on 16 March 2018 EHT L147 magnetospheres (e.g. Goldreich & Julian 1969) that whenever the magnetosphere is emptied of plasma, strong electric fields develop along the magnetic field, accelerating electrons and positrons to relativistic energies sufficient to generate electron–positron pairs through various astrophysical processes. These pairs cascade down to more and more pairs that quickly (at light crossing times) fill the magnetosphere with a force-free electron–positron (not electron– proton/ion) plasma. The very same process has been proposed in the original paper on the extraction of matter and energy from a rotating black hole Blandford & Znajek (1977) to take place also in black hole magnetospheres. Several researchers have identified the region of pair formation (which is indeed the source of the black hole jet) to be associated with the zero space-charge surface at roughly 2r ,where r ≡ GM/c ,and M is the black hole mass g g (e.g. Ptitsyna & Neronov 2016). Notice that the expected region of pair formation has nothing to do with the stagnation surface of GR MHD simulations. Figure 1. Sketches of our prediction (right-hand side) and the prediction Particle formation and acceleration along magnetic field lines of GR MHD simulations (left-hand side). Black circle: black hole. Dashed that thread the black hole horizon have been considered by mem- line: stagnation surface. Dotted region: electron–positron black hole jet. bers of the EHT Team (e.g. Moscibrodzka ´ et al. 2011). Other teams Everywhere else: electron–proton plasma. Stars: particle acceleration, high have also started to discuss this issue (e.g. Pu et al. 2017). More- energy emission. Grey region between the black hole jet and the extended over, the study of the dynamics of electrostatic gaps in force-free disc wind: weak magnetic field, low radio emission. magnetospheres is very limited, even in the case of pulsars (e.g. Timokhin 2010). Overall, we expect that the source of the jet in along open magnetic field lines and that of the angular velocity the magnetospheric gaps will be sporadic and intermittent, with a of rotation of the magnetic field. In other words, the solution of variability time-scale of the order of the light crossing time 2r /c. the steady-state axisymmetric black hole magnetosphere problem This is equal to about 1 min in Sagittarius A and about 1 d in M87, is uniquely determined as an eigenfunction problem. As a result, the thus only the latter is discernible with a typical VLBI imaging electric current distribution along open magnetic field lines leads to experiment that lasts a couple of hours. a total electric current through the black hole jet that does not, in We conclude this section with the prediction that the funnel region general, go to zero along the last open magnetic flux surface. As in above the horizon will be analogous to a pulsar magnetosphere, with the case of pulsars, it must therefore close just outside it in the form pair formation and injection at its base right above the horizon. This of a current sheet (see also Contopoulos, Kazanas & Fendt 1999). is very different from what GR MHD simulations surmise, namely Notice that it is hard to see the current sheet in global GR MHD sim- non-thermal electron–proton/ion injection higher up along the stag- ulations since these fail to adequately resolve the region immediatly nation surface resulting in both an outflow to large distances and an adjacent to the black hole horizon where the inner light cylinder re- inflow towards the black hole (see e.g. fig. 2 in Pu et al. 2017). In sides. On the other hand, this current sheet is a standard feature analogy to the origin of the pulsar wind in the pulsar magnetosphere of force-free electrodynamic jets (e.g. Tchekhovskoy et al. 2008; (as manifested in the rich sub-pulse structure and variability), the Nathanail & Contopoulos 2014). electron–positron (and not electron–proton/ion) black hole jet will It is thus natural to expect that instabilities of various types not be continuous and will never be completely filled. It will instead (Rayleigh–Taylor, kink, sausage, reconnection, anomalous electric be sporadic and intermittent (see cartoon in Fig. 1). In the case of current dissipation, etc.) will take place alongside it. Such instabil- M87, this variability may be observable with the EHT. ities are expected to dissipate electromagnetic energy into particle acceleration and radiation, all the way from the equatorial region 3 ELECTROMAGNETIC ENERGY of the black hole horizon to infinity. We thus conclude this section DISSIPATION with the prediction that the EHT will observe energy dissipation along the boundary of the electron–positron black hole jet (see car- The second physical process that we would like to emphasize is toon in Fig. 1). In other words, we expect that the electron–positron the development of a strong current sheet along the last open black hole jet will be strongly edge brightened. magnetic flux surface that crosses the black hole horizon. This is a fundamental feature of black hole and pulsar magnetospheres. Nathanail & Contopoulos (2014) showed that the structure of the 4 MAGNETIC FIELD AND WIND TOPOLOGY steady-state axisymmetric black hole magnetosphere is determined ACCORDING TO THE COSMIC BATTERY by the condition of smooth crossing of the two light cylinders that appear in the problem, namely the outer one analogous to the pulsar Current GR MHD numerical simulations of black hole accretion light cylinder, and the inner general relativistic one inside the black and ejection flows implement an initial magnetic field configura- hole ergosphere. These are the two Alfven ´ surfaces of the flow, one tion in which the plasma is threaded by a large-scale magnetic field for the negatively/positively charged outflow outside the zero space of a definite polarity. In these simulations, the system generates charge surface, and another for the positively/negatively charged large-scale winds and jets, and eventually reaches a so-called mag- inflow inside for black hole rotation that is aligned/counter-aligned netically arrested disc (MAD) state. Simulations with a random with the magnetic field, respectively. The presence of two light initial magnetic field have also been performed such as the stan- cylinders imposes two restrictions, which determine uniquely two dard and normal evolution (SANE; e.g. Chan et al. 2015; Narayan unknown distributions of the problem, that of the electric current et al. 2015), but in these the field is introduced only to establish MNRASL 473, L146–L148 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L146/4600553 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L148 I. Contopoulos the magnetorotational instability, which is needed for accretion to jet originates in the disc and not in the black hole. Furthermore, develop. Obviously, SANE simulations do not lead to large-scale all such detections of Faraday rotation measure gradients (firm or ejection flows. tentative) yield axial electric currents that in all cases point away Most recently, we performed the first ever GR radiation MHD from the central black hole. This particular break of symmetry is simulation with an extra electric field component due to the ab- consistent with the Cosmic Battery, provided the field polarity as- beration of radiation in the induction equation Contopoulos et al. sociated with the large-scale jet is indeed that of the return field (2017). This is the GR manifestation of the so-called Cosmic Bat- that threads the accretion disc around the central black hole (see tery introduced almost 20 yr ago by Contopoulos & Kazanas (1998). Christodoulou et al. 2016 for details). The result of this simulation was that a large-scale magnetic field 5 SUMMARY component develops on top of the SANE evolution, which eventu- ally has a strong dynamical effect on the accretion flow (it evolves We expect surprises from the EHT. In particular, we expect to ob- towards a MAD state). serve intermittent variability that is due to particle acceleration It is important to notice here that, if indeed the magnetic field that electrostatic gaps right above the black hole horizon and not further threads the black hole is generated by the Cosmic Battery (instead up in the stagnation surface of GR MHD numerical simulations. We of being brought in from large scales as in all previous GR MHD also expect electromagnetic energy dissipation along the boundary simulations), it attains a very particular topology: of the black hole jet. Such particle acceleration processes have not yet been considered in the numerical simulations used by the EHT (i) The magnetic field that is generated around the innermost sta- Team to interpret the observations. Finally, we expect that a re- ble circular orbit (ISCO) is brought to the centre by the freely falling gion of weaker magnetic field separates the black hole jet from the accretion flow and threads the black hole horizon. The field returns magnetically driven disc wind at larger distances, as dictated by and closes at larger distances through the viscous and turbulent disc. the Cosmic Battery. All these elements yield a configuration that (ii) The return magnetic field diffuses outward through the consists of an intermittent edge-brightened electron–positron spine disc, all along driving a disc wind (Blandford & Payne 1982; jet, a slower and initially weaker electron–proton/ion disc wind at Contopoulos & Lovelace 1994; Contopoulos, Kazanas & Fuku- larger cylindrical radii and a weakly emitting region in between. mura 2017). We are eagerly awaiting the first EHT observations, especially (iii) At a distance of optical depth unity (this is how far radiation, those of the M87 jet, that will either confirm or disprove our pre- the driving force of the Cosmic Battery, penetrates in the accretion dictions. flow), the magnetic field reverses. We thus expect that a region with weak magnetic field develops at that distance in the disc. ACKNOWLEDGEMENTS There are several important corrolaries derived from the above We acknowledge interesting discussions with Drs. George magnetic field configuration. First, magnetic field loops will con- Contopoulos, Christos Efthymiopoulos and Nick Kylafis. tinuously originate around a distance of optical depth unity at the slow rate dictated by the Cosmic Battery (see Contopoulos REFERENCES & Kazanas 1998 for details). The loops will be stretched in the azimuthal direction by the differential rotation, and as a result Asada K., Nakamura M., Pu H.-Y., 2016, ApJ, 833, 56 they will open up to large axial distances (as in Contopoulos, Blandford R. D., Payne D. G., 1982, MNRAS, 199, 883 Nathanail & Katsanikas 2015). The opening up of the magnetic Blandford R. D., Znajek R. L., 1977, MNRAS, 179, 433 Chan C.-K., Psaltis D., Ozel F., Medeiros L. Marrone D., Sadowski A., field may be observable by the EHT at the base of the M87 jet on Narayan R., 2015, ApJ, 812, 103 time-scales of the order of days. Secondly, a region with lower mag- Christodoulou D. M., Gabuzda D., Knuettel S., Contopoulos I., Kazanas D., netic field will develop between the strongly magnetized black hole Coughlan C. P., 2016, A& A, 591, 61 jet and the surrounding more weakly magnetized disc wind (see Contopoulos I., Kazanas D., 1998, ApJ, 508, 859 cartoon in Fig. 1; see also the last panel of fig. 3 in Contopoulos Contopoulos J., Lovelace R. V. E., 1994, ApJ, 429, 139 et al. 2017 for a clear numerical indication of this effect). Contopoulos I., Kazanas D., Fendt C., 1999, ApJ, 511, 351 We find hints of the above configuration in the observations of Contopoulos I., Nathanail A., Katsanikas M., 2015, ApJ, 805, 105 a spine component in the M87 radio jet by Asada et al. 2016.We Contopoulos I., Nathanail A., Sadowski A., Kazanas D., Narayan R., 2018, interpret their observations in the context of the Cosmic Battery MNRAS, 473, 721 Contopoulos I., Kazanas D., Fukumura K., 2017, MNRAS, 472, L20 as follows. The M87 jet consists of three sub-components: an in- Goldreich P., Julian W. H., 1969, ApJ, 157, 869 nermost electron–positron Blandford–Znajek black hole jet (spine), Lu R. -S. et al., 2016, ApJ, 817, 173 an outer (sheath) electron-proton/ion disc wind and a region in be- Moscibrodzka ´ M., Gammie C. F., Dolence J. C., Shiokawa H., 2011, ApJ, tween with weaker radio emission. The electron–positron spine jet 735, 9 becomes unobservable beyond about 1 pc due to radiative cooling Narayan R., Sadowski A., Penna R. F., Kulkarni A. K., 2015, MNRAS, 426, and/or Doppler beaming (Asada, Nakamura & Pu 2016), and what we are left with at large scales is the innermost part of the mag- Nathanail A., Contopoulos I., 2014, ApJ, 788, 186 netically driven disc wind. This is what will eventually become the Ptitsyna K., Neronov A., 2016, A&A, 593, 8 kpc-scale M87 jet. Pu H.-Y., Yun K., Younsi A., Yoon S.-J., 2016, ApJ, 820, 105 The latter conclusion (that the kpc-scale jet is not the electron– Pu H.-Y., Wu K., Younsi Z., Asada K., Mizuno Y., Nakamura M., 2017, positron black hole jet, but is the electron–proton disc wind) is ApJ, 845, 160 Takahashi M., Nitta S., Tatematsu Y., Tomimatsu A., 1990, ApJ, 363, 206 consistent with the observation of Faraday rotation measure gradi- Tchekhovskoy A., McKinney J. C., Narayan R., 2008, MNRAS, 388, 551 ents transverse to the axis of the jet (Christodoulou et al. 2016 ,and Timokhin A. N., 2010, MNRAS, 408, 2092 references therein). Such gradients, wherever they are observed, are intrinsic to the jet, and therefore they indirectly prove that the jet consists of electrons and protons/ions (electron–positron plasmas do not generate Faraday rotation). We conclude that the kpc-scale This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 473, L146–L148 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L146/4600553 by Ed 'DeepDyve' Gillespie user on 16 March 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Letters Oxford University Press

The immediate environment of an astrophysical black hole

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Oxford University Press
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© 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
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Abstract

MNRAS 473, L146–L148 (2018) doi:10.1093/mnrasl/slx179 Advance Access publication 2017 November 7 1,2‹ I. Contopoulos Research Centre for Astronomy and Applied Mathematics, Academy of Athens, Athens 11527, Greece National Research Nuclear University, 31 Kashirskoe Highway, 115409 Moscow, Russia Accepted 2017 November 2. Received 2017 November 1; in original form 2017 October 12 ABSTRACT In view of the upcoming observations with the Event Horizon Telescope (EHT), we present our thoughts on the immediate environment of an astrophysical black hole. We are concerned that two approximations used in general relativistic magnetohydrodynamic numerical simulations, namely numerical density floors implemented near the base of the black hole jet, and a magnetic field that comes from large distances, may mislead our interpretation of the observations. We predict that three physical processes will manifest themselves in EHT observations, namely dynamic pair formation just above the horizon, electromagnetic energy dissipation along the boundary of the black hole jet, and a region of weak magnetic field separating the black hole jet from the disc wind. Key words: magnetic fields – Galaxy: nucleus – quasars: supermassive black holes – submillimetre: stars. (i) Dynamic electron-positron pair formation at the base of the 1 EXPECTATIONS FROM THE EHT black hole jet right above the black hole horizon, The Event Horizon Telescope (EHT) is an international collabora- (ii) Electromagnetic energy dissipation along the last open flux tion whose primary goal is to image the immediate environment of surface through the black hole horizon, and the two supermassive black holes with the largest apparent event (iii) Reversal of the large-scale magnetic field polarity outside horizons, the 4 × 10 M one at the centre of our Galaxy in Sagit- the black hole jet. tarius A, and the 7 × 10 M one in M87 (see eventhorizontele- In the following sections, we will discuss how the above three scope.org for details and information). EHT opens the possibility physical processes might affect the observations of the two super- of imaging extreme light lensing near the horizon, as well as the massive black holes at the centres of our Galaxy and M87. dynamic evolution of black hole accretion, by pushing the limits of global millimeter and submillimeter very long baseline interferom- etry (VLBI). The first observations of Sagittarius A were obtained 2 DYNAMIC PAIR FORMATION in 2017 April, but the publication of the first images will require GR MHD simulations of black hole accretion consistently yield several more months of reduction and analysis. a region above the black hole horizon, where magnetic forces are In this work, we would like to express our concern about the balanced by the gravity of the black hole (e.g. Takahashi et al. 1990; interpretation of the EHT observations with respect to the morphol- Pu et al. 2016). As a result, the flow is separated into inflow and ogy of accretion and jet formation near the black hole horizon. The outflow, and this region is quickly (at light crossing times) emptied EHT Team employs the most comprehensive current theoretical of material. When that takes place, i.e. when the local density drops and numerical models of general relativistic (GR) magnetohydro- below a certain threshold that the code cannot handle, MHD codes dynamic (MHD) accretion and ejection in order to account for the implement so-called density floors. The employment of density expected morphology, spectra and variability of the source (e.g. Lu floors in this region (so-called stagnation or separation surface) is et al. 2016;Chanetal. 2015, and references therein). These fall into tantamount to a continuous artificial supply of material for the jet two general categories: models in which a large-scale magnetic field above the black hole horizon. The simulations do not yield anything of a definite polarity is advected with the accretion flow and gener- special in the region that is emptied of material, and a low-density ates large-scale winds and jets, and models with a random magnetic funnel of the same material as the rest of the material of the accretion field component introduced just to trigger the magneto-rotational flow, namely electron–proton/ion plasma, forms along the magnetic instability that generates accretion but no large-scale ejection flows. field lines that cross the black hole horizon. The following three physical processes have not yet been included In reality, however, as the region above the black hole hori- in these numerical models and need to be taken into account in the zon is emptied of the accretion flow material, it is left only with interpretation of the observations: the accumulated magnetic field and the electric fields associated with black hole rotation. This configuration is akin to that in the E-mail: icontop@academyofathens.gr pulsar magnetosphere. It is common belief in the study of pulsar 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L146/4600553 by Ed 'DeepDyve' Gillespie user on 16 March 2018 EHT L147 magnetospheres (e.g. Goldreich & Julian 1969) that whenever the magnetosphere is emptied of plasma, strong electric fields develop along the magnetic field, accelerating electrons and positrons to relativistic energies sufficient to generate electron–positron pairs through various astrophysical processes. These pairs cascade down to more and more pairs that quickly (at light crossing times) fill the magnetosphere with a force-free electron–positron (not electron– proton/ion) plasma. The very same process has been proposed in the original paper on the extraction of matter and energy from a rotating black hole Blandford & Znajek (1977) to take place also in black hole magnetospheres. Several researchers have identified the region of pair formation (which is indeed the source of the black hole jet) to be associated with the zero space-charge surface at roughly 2r ,where r ≡ GM/c ,and M is the black hole mass g g (e.g. Ptitsyna & Neronov 2016). Notice that the expected region of pair formation has nothing to do with the stagnation surface of GR MHD simulations. Figure 1. Sketches of our prediction (right-hand side) and the prediction Particle formation and acceleration along magnetic field lines of GR MHD simulations (left-hand side). Black circle: black hole. Dashed that thread the black hole horizon have been considered by mem- line: stagnation surface. Dotted region: electron–positron black hole jet. bers of the EHT Team (e.g. Moscibrodzka ´ et al. 2011). Other teams Everywhere else: electron–proton plasma. Stars: particle acceleration, high have also started to discuss this issue (e.g. Pu et al. 2017). More- energy emission. Grey region between the black hole jet and the extended over, the study of the dynamics of electrostatic gaps in force-free disc wind: weak magnetic field, low radio emission. magnetospheres is very limited, even in the case of pulsars (e.g. Timokhin 2010). Overall, we expect that the source of the jet in along open magnetic field lines and that of the angular velocity the magnetospheric gaps will be sporadic and intermittent, with a of rotation of the magnetic field. In other words, the solution of variability time-scale of the order of the light crossing time 2r /c. the steady-state axisymmetric black hole magnetosphere problem This is equal to about 1 min in Sagittarius A and about 1 d in M87, is uniquely determined as an eigenfunction problem. As a result, the thus only the latter is discernible with a typical VLBI imaging electric current distribution along open magnetic field lines leads to experiment that lasts a couple of hours. a total electric current through the black hole jet that does not, in We conclude this section with the prediction that the funnel region general, go to zero along the last open magnetic flux surface. As in above the horizon will be analogous to a pulsar magnetosphere, with the case of pulsars, it must therefore close just outside it in the form pair formation and injection at its base right above the horizon. This of a current sheet (see also Contopoulos, Kazanas & Fendt 1999). is very different from what GR MHD simulations surmise, namely Notice that it is hard to see the current sheet in global GR MHD sim- non-thermal electron–proton/ion injection higher up along the stag- ulations since these fail to adequately resolve the region immediatly nation surface resulting in both an outflow to large distances and an adjacent to the black hole horizon where the inner light cylinder re- inflow towards the black hole (see e.g. fig. 2 in Pu et al. 2017). In sides. On the other hand, this current sheet is a standard feature analogy to the origin of the pulsar wind in the pulsar magnetosphere of force-free electrodynamic jets (e.g. Tchekhovskoy et al. 2008; (as manifested in the rich sub-pulse structure and variability), the Nathanail & Contopoulos 2014). electron–positron (and not electron–proton/ion) black hole jet will It is thus natural to expect that instabilities of various types not be continuous and will never be completely filled. It will instead (Rayleigh–Taylor, kink, sausage, reconnection, anomalous electric be sporadic and intermittent (see cartoon in Fig. 1). In the case of current dissipation, etc.) will take place alongside it. Such instabil- M87, this variability may be observable with the EHT. ities are expected to dissipate electromagnetic energy into particle acceleration and radiation, all the way from the equatorial region 3 ELECTROMAGNETIC ENERGY of the black hole horizon to infinity. We thus conclude this section DISSIPATION with the prediction that the EHT will observe energy dissipation along the boundary of the electron–positron black hole jet (see car- The second physical process that we would like to emphasize is toon in Fig. 1). In other words, we expect that the electron–positron the development of a strong current sheet along the last open black hole jet will be strongly edge brightened. magnetic flux surface that crosses the black hole horizon. This is a fundamental feature of black hole and pulsar magnetospheres. Nathanail & Contopoulos (2014) showed that the structure of the 4 MAGNETIC FIELD AND WIND TOPOLOGY steady-state axisymmetric black hole magnetosphere is determined ACCORDING TO THE COSMIC BATTERY by the condition of smooth crossing of the two light cylinders that appear in the problem, namely the outer one analogous to the pulsar Current GR MHD numerical simulations of black hole accretion light cylinder, and the inner general relativistic one inside the black and ejection flows implement an initial magnetic field configura- hole ergosphere. These are the two Alfven ´ surfaces of the flow, one tion in which the plasma is threaded by a large-scale magnetic field for the negatively/positively charged outflow outside the zero space of a definite polarity. In these simulations, the system generates charge surface, and another for the positively/negatively charged large-scale winds and jets, and eventually reaches a so-called mag- inflow inside for black hole rotation that is aligned/counter-aligned netically arrested disc (MAD) state. Simulations with a random with the magnetic field, respectively. The presence of two light initial magnetic field have also been performed such as the stan- cylinders imposes two restrictions, which determine uniquely two dard and normal evolution (SANE; e.g. Chan et al. 2015; Narayan unknown distributions of the problem, that of the electric current et al. 2015), but in these the field is introduced only to establish MNRASL 473, L146–L148 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L146/4600553 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L148 I. Contopoulos the magnetorotational instability, which is needed for accretion to jet originates in the disc and not in the black hole. Furthermore, develop. Obviously, SANE simulations do not lead to large-scale all such detections of Faraday rotation measure gradients (firm or ejection flows. tentative) yield axial electric currents that in all cases point away Most recently, we performed the first ever GR radiation MHD from the central black hole. This particular break of symmetry is simulation with an extra electric field component due to the ab- consistent with the Cosmic Battery, provided the field polarity as- beration of radiation in the induction equation Contopoulos et al. sociated with the large-scale jet is indeed that of the return field (2017). This is the GR manifestation of the so-called Cosmic Bat- that threads the accretion disc around the central black hole (see tery introduced almost 20 yr ago by Contopoulos & Kazanas (1998). Christodoulou et al. 2016 for details). The result of this simulation was that a large-scale magnetic field 5 SUMMARY component develops on top of the SANE evolution, which eventu- ally has a strong dynamical effect on the accretion flow (it evolves We expect surprises from the EHT. In particular, we expect to ob- towards a MAD state). serve intermittent variability that is due to particle acceleration It is important to notice here that, if indeed the magnetic field that electrostatic gaps right above the black hole horizon and not further threads the black hole is generated by the Cosmic Battery (instead up in the stagnation surface of GR MHD numerical simulations. We of being brought in from large scales as in all previous GR MHD also expect electromagnetic energy dissipation along the boundary simulations), it attains a very particular topology: of the black hole jet. Such particle acceleration processes have not yet been considered in the numerical simulations used by the EHT (i) The magnetic field that is generated around the innermost sta- Team to interpret the observations. Finally, we expect that a re- ble circular orbit (ISCO) is brought to the centre by the freely falling gion of weaker magnetic field separates the black hole jet from the accretion flow and threads the black hole horizon. The field returns magnetically driven disc wind at larger distances, as dictated by and closes at larger distances through the viscous and turbulent disc. the Cosmic Battery. All these elements yield a configuration that (ii) The return magnetic field diffuses outward through the consists of an intermittent edge-brightened electron–positron spine disc, all along driving a disc wind (Blandford & Payne 1982; jet, a slower and initially weaker electron–proton/ion disc wind at Contopoulos & Lovelace 1994; Contopoulos, Kazanas & Fuku- larger cylindrical radii and a weakly emitting region in between. mura 2017). We are eagerly awaiting the first EHT observations, especially (iii) At a distance of optical depth unity (this is how far radiation, those of the M87 jet, that will either confirm or disprove our pre- the driving force of the Cosmic Battery, penetrates in the accretion dictions. flow), the magnetic field reverses. We thus expect that a region with weak magnetic field develops at that distance in the disc. ACKNOWLEDGEMENTS There are several important corrolaries derived from the above We acknowledge interesting discussions with Drs. George magnetic field configuration. First, magnetic field loops will con- Contopoulos, Christos Efthymiopoulos and Nick Kylafis. tinuously originate around a distance of optical depth unity at the slow rate dictated by the Cosmic Battery (see Contopoulos REFERENCES & Kazanas 1998 for details). The loops will be stretched in the azimuthal direction by the differential rotation, and as a result Asada K., Nakamura M., Pu H.-Y., 2016, ApJ, 833, 56 they will open up to large axial distances (as in Contopoulos, Blandford R. D., Payne D. G., 1982, MNRAS, 199, 883 Nathanail & Katsanikas 2015). The opening up of the magnetic Blandford R. D., Znajek R. L., 1977, MNRAS, 179, 433 Chan C.-K., Psaltis D., Ozel F., Medeiros L. Marrone D., Sadowski A., field may be observable by the EHT at the base of the M87 jet on Narayan R., 2015, ApJ, 812, 103 time-scales of the order of days. Secondly, a region with lower mag- Christodoulou D. 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Pu H.-Y., Yun K., Younsi A., Yoon S.-J., 2016, ApJ, 820, 105 The latter conclusion (that the kpc-scale jet is not the electron– Pu H.-Y., Wu K., Younsi Z., Asada K., Mizuno Y., Nakamura M., 2017, positron black hole jet, but is the electron–proton disc wind) is ApJ, 845, 160 Takahashi M., Nitta S., Tatematsu Y., Tomimatsu A., 1990, ApJ, 363, 206 consistent with the observation of Faraday rotation measure gradi- Tchekhovskoy A., McKinney J. C., Narayan R., 2008, MNRAS, 388, 551 ents transverse to the axis of the jet (Christodoulou et al. 2016 ,and Timokhin A. N., 2010, MNRAS, 408, 2092 references therein). Such gradients, wherever they are observed, are intrinsic to the jet, and therefore they indirectly prove that the jet consists of electrons and protons/ions (electron–positron plasmas do not generate Faraday rotation). We conclude that the kpc-scale This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 473, L146–L148 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L146/4600553 by Ed 'DeepDyve' Gillespie user on 16 March 2018

Journal

Monthly Notices of the Royal Astronomical Society LettersOxford University Press

Published: Nov 7, 2017

Keywords: magnetic fields; Galaxy: nucleus; quasars: supermassive black holes; submillimetre: stars

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