Thermal Synchrotron Radiation Research Articles

Dynamics and emission properties of flux ropes from two-temperature GRMHD simulations with multiple magnetic loops

Context. Flux ropes erupting in the vicinity of a black hole are thought to be a potential model for the flares observed in Sagittarius A*. Aims. In this study, we examine the radiative properties of flux ropes that emerged in the vicinity of a black hole. Methods. We performed three-dimensional two-temperature general relativistic magnetohydrodynamic (GRMHD) simulations of magnetized accretion flows with alternating multiple magnetic loops and general relativistic radiation transfer (GRRT) calculations. In the GRMHD simulations, we implemented two different sizes of initial magnetic loops. Results. In the small loop case, magnetic dissipation leads to a weaker excitement of magneto-rotational instability inside the torus, which generates a lower accretion rate compared to the large loop case. However, the small loop case generates more flux ropes due to frequent reconnection by magnetic loops with different polarities. By calculating the thermal synchrotron emission, we found that the variability of light curves and the emitting region are tightly related. At 230 GHz and higher, the emission from the flux ropes is relatively stronger compared to the background, which is responsible for the filamentary structure in the images. At lower frequencies (e.g., 43 GHz), emission comes from more extended regions, which have a less filamentary structure in the image. Conclusions. Our study shows that self-consistent electron temperature models are essential for the calculation of thermal synchrotron radiation and the morphology of the GRRT images. Flux ropes contribute considerable emission at frequencies ≳230 GHz.

Imaging thick accretion disks and jets surrounding black holes

Based on the horizon-scale magnetofluid model developed in [1], we investigate the millimeter-wave images of a geometrically thick accretion disk or a funnel wall, i.e., the magnetofluid that encloses the base of the jet region, around a Kerr black hole. By employing the numerical method to solve the null geodesic and radiative transfer equations, we obtain the optical appearances at various observational angles and frequencies, generated by the thermal synchrotron radiation within the magnetofluid. For the thick disk, we specifically examine the impact of emission anisotropy on images, concluding that anisotropic synchrotron radiation could play an important role in the observability of the photon ring. For the funnel wall, we find that both the outflow and inflow funnel walls exhibit annular structures on the imaging plane. The outflow funnel wall yields a brighter primary image than the photon ring, whereas the inflow one does not. Based on our investigation, the inflow funnel wall model can not be ruled out by current observations of M87*.

Radiative energy from a reconnection region around massive black hole

ABSTRACT In the previous numerical study, we find the blob formation and ejection in the presence of magnetic reconnection in the environment of the hot flow of the accretion disc. Based on those encouraging results, in the present work, we calculate the energy and the spectrum of the emission in the different bands around sagittarius A* (Sgr A*). We assume the electrons in the magnetic reconnection region are non-thermal and emit synchrotron radiation. The electrons in the other region are thermal, which follows the thermal distribution, and the thermal electron emission mechanism is thermal synchrotron radiation. During the whole process of the magnetic evolution and reconnection, we find two peaks in the temporal light curve in the recently observed radio frequencies (230 and 43 GHz) and near-infrared (NIR) wavelengths (3.8 and 2.2 μm). Although the light curve of the NIR band is most prominent in a single peak. The first peak appears because of the blob in the plasma flow, which is formed due to the magnetic reconnection. The second peak appears due to the production of the non-thermal electrons with the evolution of the magnetic flux. Both peaks reach luminosity of more than 1026 erg s−1 for a single plasmoid/blob. For the NIR band, the highest luminosity can reach more than 1028 erg s−1. These luminosities can be high for the large simulation area and the stronger magnetic field with the multiple blobs. We infer that the observed flares are a group of magnetic reconnection phenomena, not a single one.

An Observational Signature of Sub-equipartition Magnetic Fields in the Spectra of Black Hole Binaries

Abstract A common assumption used in the study of accretion disks is that the magnetic energy density and the kinetic energy density should be in equipartition. This assumption relies on the faster growth rate of the magnetic field strength against the kinetic energy of the particles in the flow, for decreasing radius, combined with a dissipation mechanism that tends toward equipartition. In this paper, we examine this assumption by modeling the radio, millimeter, and optical spectra of several black hole binaries in their quiescent state. We use a standard two-component disk model, consisting of an inner geometrically thick and optically thin disk, emitting thermal synchrotron radiation, along with an outer, thin disk, which radiates as a multicolor blackbody. We find that at the low accretion rates typical of the quiescent state, the spectral shape is qualitatively reproduced using magnetic fields that are between 0.1% and 1% of the equipartition value, considerably smaller than previously thought. We discuss our findings in view of (1) the launching of jets in these objects, which is commonly believed to rely on the presence of a strong magnetic field in the central region of the disk, and (2) the role of magnetic dissipation in the structure of the inflow.

Comparison of the ion-to-electron temperature ratio prescription: GRMHD simulations with electron thermodynamics

ABSTRACT The Event Horizon Telescope (EHT) collaboration, an Earth-size sub-millimetre radio interferometer, recently captured the first images of the central supermassive black hole in M87. These images were interpreted as gravitationally lensed synchrotron emission from hot plasma orbiting around the black hole. In the accretion flows around low-luminosity active galactic nuclei such as M87, electrons and ions are not in thermal equilibrium. Therefore, the electron temperature, which is important for the thermal synchrotron radiation at EHT frequencies of 230 GHz, is not independently determined. In this work, we investigate the commonly used parametrized ion-to-electron temperature ratio prescription, the so-called R−β model, considering images at 230 GHz by comparing with electron-heating prescriptions obtained from general-relativistic magnetohydrodynamical (GRMHD) simulations of magnetized accretion flows in a Magnetically Arrested Disc (MAD) regime with different recipes for the electron thermodynamics. When comparing images at 230 GHz, we find a very good match between images produced with the R−β prescription and those produced with the turbulent- and magnetic reconnection-heating prescriptions. Indeed, this match is on average even better than that obtained when comparing the set of images built with the R−β prescription with either a randomly chosen image or with a time-averaged one. From this comparative study of different physical aspects, which include the image, visibilities, broad-band spectra, and light curves, we conclude that, within the context of images at 230 GHz relative to MAD accretion flows around supermassive black holes, the commonly used and simple R−β model is able to reproduce well the various and more complex electron-heating prescriptions considered here.

Constraining Low-luminosity Gamma-Ray Bursts as Ultra-high-energy Cosmic Ray Sources Using GRB 060218 as a Proxy

Abstract We study the connection between low-luminosity gamma-ray bursts (llGRBs) and ultra-high-energy cosmic rays (UHECRs) using the canonical low-luminosity GRB 060218 as a proxy. We focus on the consequential synchrotron emission from electrons that are coaccelerated in the UHECR acceleration region, comparing this emission to observations. Both the prompt and afterglow phases are considered. For the prompt phase, we assume the coaccelerated electrons are injected with a power-law distribution instantaneously (without additional heating or reacceleration), which results in bright optical-UV emission in tension with observations. For the afterglow phase, we constrain the total kinetic energy of the blast wave by comparing electron thermal synchrotron radiation to available radio data at ∼ 3 days. Considering mildly relativistic outflows with bulk Lorentz factor Γ ≳ 2 (slower transrelativistic outflows are not treated), we find that the limited available energy does not allow for GRB 060218-like afterglows to be the main origin of UHECRs. This analysis independently constrains the prompt phase as a major UHECR source as well, given that the prompt energy budget is comparable to that of the afterglow kinetic energy. More generally, our study demonstrates that synchrotron emission from thermal electrons is a powerful diagnostic of the physics of mildly relativistic shocks.

Thermal synchrotron radiation from RRMHD simulations of the double tearing mode reconnection – application to the Crab flares

We study the magneto-hydrodynamic tearing instability occurring in a double current sheet configuration when a guide field is present. This is investigated by means of resistive relativistic magneto-hydrodynamic (RRMHD) simulations. Following the dynamics of the double tearing mode (DTM), we are able to compute synthetic synchrotron spectra in the explosive reconnection phase. The pulsar striped wind model represents a site where such current sheets are formed, including a guide field. The variability of the Crab nebula/pulsar system, seen as flares, can be therefore naturally explained by the DTM explosive phase in the striped wind. Our results indicate that the Crab GeV flare can be explained by the double tearing mode in the striped wind region if the magnetization parameter $\sigma$ is around $10^5$.

ON THE ROLE AND ORIGIN OF NONTHERMAL ELECTRONS IN HOT ACCRETION FLOWS

We study the X-ray spectra of tenuous, two-temperature accretion flows using a model involving an exact, Monte Carlo computation of the global Comptonization effect as well as general relativistic description of both the flow structure and radiative processes. In our previous work we found that in flows surrounding supermassive black holes, thermal synchrotron radiation is not capable of providing a sufficient seed photons flux to explain the X-ray spectral indices as well as the cut-off energies measured in several best-studied AGNs. In this work we complete the model by including seed photons provided by nonthermal synchrotron radiation and we find that it allows to reconcile the hot flow model with the AGN data. We take into account two possible sources of nonthermal electrons. First, we consider e+- produced by charged-pions decay, which should be always present in the innermost part of a two-temperature flow due to proton-proton interactions. We find that for a weak heating of thermal electrons (small delta) the synchrotron emission of pion-decay e+- is much stronger than the thermal synchrotron emission in the considered range of bolometric luminosities, L Ledd. The small-delta model including hadronic effects in general agrees with the AGN data, except for the case of a slowly rotating black hole and a thermal distribution of protons. For large-delta, the pion-decay e+- have a negligible effect and then in this model we consider nonthermal electrons produced by direct acceleration. We find an approximate agreement with the AGN data for the fraction of the heating power of electrons which is used for the nonthermal acceleration eta~0.1.

X-ray spectra of hot accretion flows

We study radiative properties of hot accretion flows in a general relativistic model with an exact treatment of global Comptonization, developed in our recent works. We note a strong dependence of electron temperature on the strength of magnetic field and we clarify that the underlying mechanism involves the change of the flow structure, with more strongly magnetised flows approaching the slab geometry more closely. We find that the model with thermal synchrotron radiation being the main source of seed photons agrees with the spectral index vs Eddington ratio relation observed in black hole transients below 1 per cent of the Eddington luminosity, LEdd, and models with a weak direct heating of electrons (small delta) are more consistent with observations. Models with large delta predict slightly too soft spectra, furthermore, they strongly overpredict electron temperatures at ~0.01 LEdd. The low-luminosity spectra, at <0.001 LEdd, deviate from a power-law shape in the soft X-ray range and we note that the first-scattering bump often resembles a thermal like component, with the temperature of a few hundred eV, superimposed on a power-law spectrum. The model with thermal Comptonization of thermal synchrotron radiation does not agree with well studied AGNs observed below ~0.01 LEdd, for which there is a substantial evidence for the lack of an inner cold disc. This indicates that the model of hot flows powering AGNs should be revised, possibly by taking into account an additional (but internal to the flow) source of seed photons.

Thermal synchrotron radiation and its Comptonization in compact X-ray sources

We investigate thermal synchrotron radiation at semi-relativistic and relativistic temperatures. We find an analytic expression for the angle-averaged emission coefficient, and show that it is significantly more accurate that those derived previously. We also present analytic approximations to the synchrotron turnover frequency. Then, we treat Comptonization of the synchrotron radiation, and give simple expressions for the spectral shape and the emitted power. We also consider modifications of the above results by bremsstrahlung. The importance of Comptonization of synchrotron radiation in compact X-ray sources is then studied. We consider emission from hot accretion flows and from active coronae above optically-thick accretion discs in black-hole binaries and AGNs. Synchrotron Comptonization is found to be, in general, negligible in luminous sources, except for those with hardest X-ray spectra and stellar masses. Increasing the black-hole mass results in a strong reduction of the maximum Eddington ratio possible from to this process. X-ray spectra of intermediate-luminosity sources, e.g., low-luminosity AGNs, can be explained by synchrotron Comptonization only in the case of hot accretion flows. Then, bremsstrahlung emission always dominates X-ray spectra of very weak sources. Finally, we consider weakly-magnetized neutron stars and find that synchrotron Comptonization can account for the power-law X-ray spectra observed in their low states.

Broad-band X-ray/γ-ray spectra and binary parameters of GX 339–4 and their astrophysical implications

We present X-ray/γ-ray spectra of the binary GX 339–4 observed in the hard state simultaneously by Ginga and CGRO OSSE during an outburst in 1991 September. The Ginga X-ray spectra are well represented by a power law with a photon spectral index of Γ ≃ 1.75 and a Compton reflection component with a fluorescent Fe Kα line corresponding to a solid angle of an optically thick, ionized medium of ∼ 0.4 × 2 π. The OSSE data (≥ 50 keV) require a sharp high-energy cut-off in the power-law spectrum. The broad-band spectra are very well modelled by repeated Compton scattering in a thermal plasma with an optical depth of τ ∼ 1 and kT ≃ 50 keV. We also study the distance to the system and find it to be ≳ 3 kpc, ruling out earlier determinations of ∼ 1 kpc. Using this limit, the observed reddening and the orbital period, we find the allowed range of the mass of the primary is consistent with it being a black hole. We find the data are inconsistent with models of either homogenous or patchy coronae above the surface of an accretion disc. Rather, they are consistent with the presence of an inner hot disc with the viscosity parameter of α ∼ 1 accreting at a rate close to the maximum set by advection. The hot disc is surrounded by a cold outer disc, which gives rise to the reflection component and a soft X-ray excess, also present in the data. The seed photons for Comptonization are unlikely to be due to thermal synchrotron radiation. Rather, they are supplied by the outer cold disc and/or cold clouds within the hot disc. e± pair production is negligible if electrons are thermal. The hot disc model, for which scaled parameters are independent of the black hole mass, is supported by the similarity of the spectrum of GX 339–4 to those of other black hole binaries and Seyfert 1s. On the other hand, their spectra in the soft γ-ray regime are significantly harder than those of weakly magnetized neutron stars. Based on this difference, we propose that the presence of broad-band spectra corresponding to thermal Comptonization with kT ≳ 50 keV represents a black hole signature.

Strong observational constraints on advection-dominated accretion in the cores of elliptical galaxies

The growing evidence for supermassive black holes in the centres of nearby galaxies has brought into sharper focus the question of why elliptical galaxies, rich in hot gas, do not possess quasar-like luminosities. Recent studies suggest that advection-dominated accretion flows (ADAFs), with associated low radiative efficiency, might provide an explanation for the observed quiescence. Although ADAF models have been applied to a number of low-luminosity systems, compelling observational evidence for their existence is still required. Here, we examine new high-frequency radio observations of the giant, low-luminosity elliptical galaxies NGC 4649, NGC 4472 and NGC 4636 made with the Very Large Array (VLA) and the sub-millimetre common-user bolometer array (SCUBA) on the JCMT. At these frequencies the predictions are very precise and an ADAF is unequivocally characterised by a slowly rising spectrum with a sharp spectral cut-off produced by thermal synchrotron radiation. The new radio limits severely disagree with the canonical ADAF predictions which significantly overestimate the observed flux. While our observations do not rule out the presence of an ADAF in these objects, they place strong constraints on the model. If the accretion in these objects occurs in an advection-dominated mode then our radio limits imply that the emission from their central regions must be suppressed. We examine the possibility that the magnetic field in the flow is extremely low or that synchrotron emission is free-free absorbed by cold material in the accretion flow. We also discuss whether slow non-radiating accretion flows may drive winds/outflows to remove energy, angular momentum and mass so that the central densities, pressures and emissivities are much smaller than in a standard ADAF.

A Probable Origin and Radiation Mechanism for Gamma–Ray Bursts11The project supported by National Natural Science Foundation of China.

A model of the old and hot neutron stars (age , temperature ) with the low magnetic field as the sources of Gamma-ray bursts is presented. In the range of medium energy , the main radiation mechanism is still the thermal synchrotron radiation. The absorption features of the low frequency are resulted from the superposition of the blackbody spectrum emitted by the hot neutron star itself and the thermal synchrotron spectrum with self–absorption. And the spectrum for the high energy part (, including the emission features) is due to the annihilation emission from the unthermalized positrons and electrons. Using this model, we adopt the three component spectra to fit four representative spectra of Gamma–ray bursts. The results are satisfactory. Our model also avoids some difficulties existed in other models.

Red-shifted annihilation lines in thermal synchrotron model of gamma-ray bursts

The origins of the gravitationally-red-shifted annihilation lines of gamma-ray bursts in thermal synchrotron-radiation model are discussed in this paper. It is shown that the positrons produced by high-energy photons which are obtained by extrapolation from the continua through strong magnetic field could not be the main source of the annihilation lines, while that escaping from the hot thin region might account the emission features.

Electron-Positron Pair Equilibrium in a Mildly Relativistic Plasma: Effects of the Comptonization of Soft Photons

We investigate the equilibrium pair concentration in an optically thin mildly relativistic plasma when copious soft photons are supplied by thermal synchrotron radiation. The pair equilibrium condition restricts the attainable states of a hot plasma which is supposed to exist in active galactic nuclei. There exist the maximum values for the electron temperature, the Thomson scattering depth, the size of aplasma and the strength of magnetic fields. We show that magnetic fields strengthen these restrictions considerably through the synchrotron-Compton process. For a plasma of the electron temperature higher than 8 x 10 B K, the proton density of 10 11 cm3 and the magnetic field of 10 4 G, the maximum of the Thomson scattering depth is ~ 10 and luminosity is less than ~ lO 44 erg/ sec.