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Register a new userThe role of magnetic reconnection on jet/accretion disk systems
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Added by
Lu\\'is Henrique Sinki Kadowaki
Publication date
2010
fields
Physics
and research's language is
English
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It was proposed earlier that the relativistic ejections observed in microquasars could be produced by violent magnetic reconnection episodes at the inner disk coronal region. Here we revisit this model, which employs a standard accretion disk description and fast magnetic reconnection theory, and discuss the role of magnetic reconnection and associated heating and particle acceleration in different jet/disk accretion systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).
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Models of jet production in black hole systems suggest that the properties of the accretion disk - such as its mass accretion rate, inner radius, and emergent magnetic field - should drive and modulate the production of relativistic jets. Stellar-mass black holes in the low/hard state are an excellent laboratory in which to study disk-jet connections, but few coordinated observations are made using spectrometers that can incisively probe the inner disk. We report on a series of 20 Suzaku observations of Cygnus X-1 made in the jet-producing low/hard state. Contemporaneous radio monitoring was done using the Arcminute MicroKelvin Array radio telescope. Two important and simple results are obtained: (1) the jet (as traced by radio flux) does not appear to be modulated by changes in the inner radius of the accretion disk; and (2) the jet is sensitive to disk properties, including its flux, temperature, and ionization. Some more complex results may reveal aspects of a coupled disk-corona-jet system. A positive correlation between the reflected X-ray flux and radio flux may represent specific support for a plasma ejection model of the corona, wherein the base of a jet produces hard X-ray emission. Within the framework of the plasma ejection model, the spectra suggest a jet base with v/c ~ 0.3, or the escape velocity for a vertical height of z ~ 20 GM/c^2 above the black hole. The detailed results of X-ray disk continuum and reflection modeling also suggest a height of z ~ 20 GM/c^2 for hard X-ray production above a black hole, with a spin in the range 0.6 < a < 0.99. This height agrees with X-ray time lags recently found in Cygnus X-1. The overall picture that emerges from this study is broadly consistent with some jet-focused models for black hole spectral energy distributions in which a relativistic plasma is accelerated at z = 10-100 GM/c^2.
We attempt to explain the observed radio and gamma-ray emission produced in the surrounds of black holes by employing a magnetically-dominated accretion flow (MDAF) model and fast magnetic reconnection triggered by turbulence. In earlier work, standard disk model was used and we refine the model by focussing on the sub-Eddington regime to address the fundamental plane of black hole activity. The results do not change substantially with regard to previous work ensuring that the details of the accretion physics are not relevant in the magnetic reconnection process occurring in the corona. Rather our work puts fast magnetic reconnection events as a powerful mechanism operating in the core region, near the jet base of black hole sources on more solid ground. For microquasars and low-luminosity active galactic nuclei (LLAGNs) the observed correlation between radio emission and mass of the sources can be explained by this process. The corresponding gamma-ray emission also seems to be produced in the same core region. On the other hand, the emission from blazars and gamma-ray bursts (GRBs) cannot be correlated to core emission based on fast reconnection.
During magnetically dominated relativistic reconnection, inflowing plasma depletes the initial relativistic pressure at the x-line and collisionless plasma heating inside the diffusion region is insufficient to overcome this loss. The resulting pressure drop causes a collapse at the x-line, essentially a localization mechanism of the diffusion region necessary for fast reconnection. The extension of this low-pressure region further explains the bursty nature of anti-parallel reconnection because a once opened outflow exhaust can also collapse, which repeatedly triggers secondary tearing islands. However, a stable single x-line reconnection can be achieved when an external guide field exists, since the reconnecting magnetic field component rotates out of the reconnection plane at outflows, providing additional magnetic pressure to sustain the opened exhausts.
We analyze X-ray light curves of the blazar Mrk 421 obtained from the Soft X-ray Imaging Telescope and the Large Area X-Ray Proportional Counter instrument onboard the Indian space telescope $AstroSat$ and archival observations from $Swift$. We show that the X-ray power spectral density (PSD) is a piece-wise power-law with a break, i.e., the index becomes more negative below a characteristic break-timescale. Galactic black hole X-ray binaries and Seyfert galaxies exhibit a similar characteristic timescale in their X-ray variability that is proportional to their respective black hole mass. X-rays in these objects are produced in the accretion disk or corona. Hence, such a timescale is believed to be linked to the properties of the accretion flow. Any relation observed between events in the accretion disk and those in the jet can be used to characterize the disk-jet connection. However, evidence of such link have been scarce and indirect. Mrk 421 is a BL Lac object which has a prominent jet pointed towards us and a weak disk emission, and it is assumed that most of its X-rays are generated in the jet. Hence, existence of the break in its X-ray PSD may indicate that changes in the accretion disk, which may be the source of the break timescale are translating into the jet, where the X-rays are produced.
We study the disk-jet connection in supermassive black holes by investigating the properties of their optical and radio emissions utilizing the SDSS-DR7 and the NVSS catalogs. Our sample contains 7017 radio-loud quasars with detection both at 1.4~GHz and SDSS optical spectrum. Using this radio-loud quasar sample, we investigate the correlation among the jet power ($P_{rm jet}$), the bolometric disk luminosity ($L_{rm disk}$), and the black hole mass ($M_{rm BH}$) in the standard accretion disk regime. We find that the jet powers correlate with the bolometric disk luminosities as $log P_{rm jet} = (0.96pm0.012)log L_{rm disk} + (0.79 pm 0.55)$. This suggests that the jet production efficiency of $eta_{rm jet}simeq1.1_{-0.76}^{+2.6}times10^{-2}$ assuming the disk radiative efficiency of $0.1$ implying low black hole spin parameters and/or low magnetic flux for radio-loud quasars. But it can be also due to dependence of the efficiency on geometrical thickness of the accretion flow which is expected to be small for quasars accreting at the disk Eddington ratios $0.01 lesssim lambda lesssim 0.3$. This low jet production efficiency does not significantly increase even if we set the disk radiative efficiency of 0.3. We also investigate the fundamental plane in our samples among $P_{rm jet}$, $L_{rm disk}$, and $M_{rm BH}$. We could not find a statistically significant fundamental plane for radio-loud quasars in the standard accretion regime.
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