Imaging optically-thin hotspots near the black hole horizon of Sgr A* at radio and near-infrared wavelengths

Submilliarcsecond astrometry and imaging of the black hole Sgr A* at the Galactic Centre may become possible in the near future at infrared and submillimetre wavelengths. Motivated by the observations of short-term infrared and X-ray variability of Sgr A*, in a previous paper, we computed the expected images and light curves, including polarization, associated with a compact emission region orbiting the central black hole. We extend this work, using a more realistic hotspot model and including the effects of opacity in the underlying accretion flow. We find that at infrared wavelengths, the qualitative features identified by our earlier work are present, namely it is possible to extract the black hole mass and spin from spot images and light curves of the observed flux and polarization. At radio wavelengths, disc opacity produces significant departures from the infrared behaviour, but there are still generic signatures of the black hole properties. Detailed comparison of these results with future data can be used to test general relativity and to improve existing models for the accretion flow in the immediate vicinity of the black hole.     

Injected spectrum for TeV-γ-ray emission from the galactic center

The detection of very high energy γ-ray emission from the Galactic center has been reported by four independent groups. One of these γ-ray sources, the 10TeV γ-ray radiation reported by HESS, has been suggested as having a hadronic origin when relativistic protons are injected into and interact with the dense ambient gas. Assuming that such relativistic protons required by the hadronic model come from the tidal disruption of a star by the massive black hole of Sgr A*, we explore the spectrum of the relativistic protons. In the calculations, we investigate cases where different types of stars are tidally disrupted by the black hole of Sgr A*, and we consider that different diffusion mechanisms are used for the propagation of protons. The initial energy distribution of the injected spectrum of protons is assumed to follow a power-law with an exponential cut-off, and we derive the different indices of the injected spectra for the tidal disruption of different types of stars. For the best fit to the spectrum of photons detected by HESS, the spectral index of the injected relativistic protons is about 2.05 when a red giant is tidally disrupted by the black hole of Sgr A* and the diffusion mechanism is the Effective Confinement of Protons.     

Sgr A*的结构和变化:银河中心的超大质量黑洞

银河中心为我们提供了一个唯一的天体物理实验室来用以研究各式各样的天体物理过程。在文中 ,我们总结和归纳了关于SgrA 观测的最新结果 ,主要涉及源的结构和在流量密度方面的变化。SgrA 现象代表着在低光度活动星系核中围绕一个超大质量黑洞的低辐射率的吸积盘外流的典型例子。从SgrA 观测得到的许多天体物理中悬而未决的问题对现存的天体物理理论是一个挑战。对SgrA 的最新理论模型也作了综述。     

The Galactic Center: a laboratory for AGN?

Summary. This paper reviews the physical state of stars and Interstellar Matter in the Galactic Bulge (radius kpc from the dynamical center of the Galaxy), in the Nuclear Bulge (kpc) and in the Sgr A Radio and GMC Complex, i.e. the central \,pc of our Galaxy. The Galactic Bulge is devoid of cold Interstellar Matter and consists mainly of old stars, while the Nuclear Bulge accounts for of the mass of all of the Interstellar Matter in the Galaxy. A similar ratio holds for the formation rate of medium and high mass stars in Bulge and Disk. The metal abundance of the Interstellar Matter in the Galactic Bulge is found to be . The H-to-CO conversion factors to be applied to molecular gas in the Central Region are by factors 3 (Arimoto et al. 1996) to 10 (Sodroski et al. 1995) lower than in the solar vicinity. Hence, most H masses derived for the Central Region appear to be considerably overestimated. The Nuclear Bulge is pervaded by a thermal plasma (K) which is responsible for the diffuse free-free emission. Lyman continuum photon and dust IR luminosity of the Nuclear Bulge again account for of the respective total luminosities of the Galaxy. Magnetic fields in the Nuclear Bulge are strong (up to mG) as compared with the Galactic Disk (a few tens of G). The field lines are oriented parallel to the galactic plane inside giant molecular clouds and perpendicular to the plane in the intercloud medium. The compact source Sgr A* is close to or at the dynamical center of the Galaxy. Its radio spectrum with a high frequency cut-off at GHz, a low frequency turnover at GHz and a flux density dependence in between can be explained by synchrotron emission from quasi-monoenergetic relativistic electrons. Due to an extinction between Sun and Galactic Center corresponding to , an intrinsic weakness of this source in the near infrared, and a strong background emission from warm dust there are only upper limits available for the flux density of Sgr A* in the far, mid and near infrared and X-ray regime. The size of Sgr A* in the radio regime is cm, its dereddened K-band flux density is mJy, its luminosity has upper limits of (if radiation comes from an Accretion Disk) and (if black-body radiation from an object with a single temperature of K is assumed). If anyone of the soft X-ray sources detected by ROSAT actually coincides with Sgr A*, its X-ray luminosity would be less than a few . With a dark mass of Sgr A* is the best candidate for a starving black hole, although there are no observational indications for the presence of a (Standard) Accretion Disk. While the radio/IR spectrum of Sgr A* is purely nonthermal, the spectrum integrated over the central parsec resembles that of a Seyfert galaxy. Sgr A* is embedded in the Hii region Sgr A West with part of the ionized gas forming a minispiral. Sgr A West is surrounded by the Circum Nuclear Disk, an irregular shaped assembly of molecular gas which extends from pc and rotates around the Galactic Center with an estimated dynamical time scale of \,yr. The total luminosity of of the central parsec is due to the radiation of early-type stars of which have now been directly identified as luminous blue supergiants. It is still debated, however, if these stars can also account for all of the ionization of Sgr A West. In addition, the central parsec contains red giants, AGB stars, and a few super giants of which the brightest are now identified by direct imaging. These stars – together with a few million low mass main sequence stars – account for the bulk of the 2.2\,m emission. The spatial distributions of the three stellar populations in the central pc are remarkably different. Sgr A* is – along the line-of-sight – presumably located close to the center of the Hii region Sgr A West, which in turn is located in front of the extended (pc) synchrotron source Sgr A East, which appears to be the remnant of a gigantic explosion (of the order of the energy of a single supernova explosion) which took place yr ago inside the GMC Sgr A East Core. X-ray observations show within pc a pervasive hot (keV) plasma of expansion age of yr. Both phenomena – as well as the formation of the Circum Nuclear Disk – may have the same origin. Influx of material is observed within the Nuclear Bulge on all distance scales. In the Nuclear Bulge (pc) as well as in the Circum Nuclear Disk (pc) inflow towards the Galactic Center occurs primarily in the galactic plane and amounts to a few . The accretion rate into the central Black Hole, deduced from the luminosity of Sgr A*, however, appears to be lower by at least five orders of magnitude (assuming standard disk accretion). But in an equilibrium state only part of the infalling mass which is not accreted by the Black Hole can be consumed by star formation. A mass inflow rate varying with time is a more natural explanation. Comparing the physical state of the Center of our Galaxy with that of Active Galactic Nuclei derived from observations and modelling, we find that most of the basic characteristics of an AGN are also present in the Galactic Center. Lacking are, however, both the evidence for a standard Accretion Disk and a hard UV spectrum with accompanying high excitation emission lines in the Galactic Center which are characteristic for AGN. The luminosity of the central parsec, , amounts to only of the total luminosity of the Galaxy of . Seen from a distance of M31 (kpc) with an angular resolution of (corresponding to a linear size of pc) the Center of our Galaxy would appear as a mildly active nucleus with some starburst activity and would probably be classified as a weak Seyfert galaxy. The synchrotron spectrum of Sgr A*, however, would be completely masked by reprocessed stellar light (i.e. free-free and dust emission). Received: October 21, 1996     

Hypervelocity collisions of binary stars at the Galactic Centre

Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N -body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. It is possible for a coalescing binary to have sufficient velocity to escape the galaxy. Furthermore, some of the massive S-stars near Sgr A* might be the merger remnants of binary systems, however this production method can not account for most of the S-stars.     

Dynamics and collisions of episodic jets from black holes and accretion disk systems

In many astrophysical black hole systems, episodic jets of plasma blobs have been observed, which are much faster and more powerful than continuous jets. A magnetohydrodynamical model was proposed by Yuan et al. to study the formation of episodic jets in Sgr A*. By taking Sgr A* and a stellar mass black hole as examples, we modify the model of Yuan et al. by including the effects of relativity, and further study the relativistic motion and expansion of episodic jets of plasma blobs. Then we study the collision between two consecutive ejections in the modified model, and calculate the magnetic energy released in the collision. Our results show two consecutive blobs can collide with each other, and the released magnetic energy is more than 1050 erg,which supports the idea that a gamma-ray burst is powered by the collision of episodic jets, as suggested by Yuan & Zhang.     

Equivalent width, shape and proper motion of the iron fluorescent line emission from molecular clouds as an indicator of the illuminating source X-ray flux history

Observations of the diffuse emission in the 8–22 keV energy range, elongated parallel to the Galactic plane, and detection of the strong 6.4-keV fluorescent line with ∼ 1 keV equivalent width from some giant molecular clouds (e.g. Sgr B2) in the Galactic Centre region suggest that the neutral matter of these clouds is (or was) illuminated by powerful X-ray radiation, which gave rise to the reprocessed radiation. The source of this radiation remains unknown. A transient source close to the Sgr B2 cloud, or a short outburst of the X-ray emission from a supermassive black hole at the Galactic Centre are the two prime candidates under consideration. We argue that a new generation of X-ray telescopes combining very high sensitivity and excellent energy and angular resolutions would be able to discriminate between these two possibilities when studying time-dependent changes of the morphology of the surface brightness distribution, the equivalent width and the shape of the fluorescent line in Sgr B2 and other molecular clouds in the region. We note also that detection of broad and complex structures near the 6.4-keV line in the spectra of distant AGNs, which are X-ray weak now, may prove the presence of violent activity in the central engines of these objects in the past. Accurate measurements of the line shape may provide information on the time elapsed since the outburst. Proper motion (super- or subluminal) of the fluorescent radiation wave front can give additional information on the location of the source. Observations of the described effects can provide unique information on the matter distribution inside Sgr B2 and other giant molecular clouds.     

Intra-Day Variability of Sagittarius A* at Multi-Wavelengths

This paper reviews the recent progress in the study of the intra-day variability (IDV) of Sagitarrius A* (Sgr A*), the best known supermassive black hole candidates with a dark mass concentration of 4 × 10\(^6 M_{\odot}\) at the center of our galaxy.     

Polarization of X-ray emission from the Sgr B2 cloud

The Sgr B2 giant molecular cloud is claimed to be an 'X-ray reflection nebula'– the reprocessing site of a powerful flare of the Sgr A* source, which occurred a few hundred years ago. The shape of the X-ray spectrum and the strength of the iron fluorescent line support this hypothesis. We argue that the cleanest test of the origin of X-rays from Sgr B2 would be a detection of polarized emission from this source.     

The influence of central black holes on gravitational lenses

Recent observations indicate that many if not all galaxies host massive central black holes. In this paper we explore the influence of black holes on the lensing properties. We model the lens as an isothermal ellipsoid with a finite core radius plus a central black hole. We show that the presence of the black hole substantially changes the critical curves and caustics. If the black hole mass is above a critical value, then it will completely suppress the central images for all source positions. Realistic central black holes are likely to have masses below this critical value. Even in such subcritical cases, the black hole can suppress the central image when the source is inside a zone of influence, which depends on the core radius and black hole mass. In the subcritical cases, an additional image may be created by the black hole in some regions, which for some radio lenses may be detectable with high-resolution and large dynamic range VLBI maps. The presence of central black holes should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses.     

Feast and Famine: regulation of black hole growth in low-redshift galaxies

We analyse the observed distribution of Eddington ratios  ( L / L _Edd)  as a function of supermassive black hole mass for a large sample of nearby galaxies drawn from the Sloan Digital Sky Survey. We demonstrate that there are two distinct regimes of black hole growth in nearby galaxies. The first is associated with galaxies with significant star formation [   M _*/star formation rate (SFR) ∼  a Hubble time] in their central kiloparsec regions, and is characterized by a broad lognormal distribution of accretion rates peaked at a few per cent of the Eddington limit. In this regime, the Eddington ratio distribution is independent of the mass of the black hole and shows little dependence on the central stellar population of the galaxy. The second regime is associated with galaxies with old central stellar populations (   M _*/SFR ≫  a Hubble time), and is characterized by a power-law distribution function of Eddington ratios. In this regime, the time-averaged mass accretion rate on to black holes is proportional to the mass of stars in the galaxy bulge, with a constant of proportionality that depends on the mean stellar age of the stars. This result is once again independent of black hole mass. We show that both the slope of the power law and the decrease in the accretion rate on to black holes in old galaxies are consistent with population synthesis model predictions of the decline in stellar mass loss rates as a function of mean stellar age. Our results lead to a very simple picture of black hole growth in the local Universe. If the supply of cold gas in a galaxy bulge is plentiful, the black hole regulates its own growth at a rate that does not further depend on the properties of the interstellar medium. Once the gas runs out, black hole growth is regulated by the rate at which evolved stars lose their mass.     

Iron line profiles and self-shadowing from relativistic thick accretion discs

We present Fe Kα line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray-illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and light bending. We find that the separation and the relative height between the blue and red peaks of the line profile diminish as the thickness of the disc increases. This code is also well suited to produce accretion disc images. We calculate the redshift and flux images of the accretion disc and find that the observed image of the disc strongly depends on the inclination angle. The self-shadowing effect appears remarkable for a high inclination angle, and leads to the black hole shadow being in this case, completely hidden by the disc itself.     

On the origin of the wide HI absorption line towards Sgr A*

We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of V_1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s⁻¹. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.     

Variable accretion and emission from the stellar winds in the Galactic Centre

We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic Centre, studying in particular the accretion of gas on to Sgr A*, the supermassive black hole. Unlike our previous work, here we use state-of-the-art observational data on orbits and wind properties of individual wind-producing stars. Since wind velocities were revised upwards and non-zero eccentricities were considered, our new simulations show fewer clumps of cold gas and no conspicuous disc-like structure. The accretion rate is dominated by a few close 'slow-wind stars' ( v _w≤ 750 km s⁻¹), and is consistent with the Bondi estimate, but variable on time-scales of tens to hundreds of years. This variability is due to the stochastic infall of cold clumps of gas, as in earlier simulations, and to the eccentric orbits of stars. The present models fail to explain the high luminosity of Sgr A* a few hundred years ago implied by Integral observations, but we argue that the accretion of a cold clump with a small impact parameter could have caused it. Finally, we show the possibility of constraining the total mass-loss rate of the 'slow-wind stars' using near infrared observations of gas in the central few arcseconds.     

On the formation and evolution of black hole binaries

We present the results of a systematic study of the formation and evolution of binaries containing black holes and normal-star companions with a wide range of masses. We first reexamine the standard formation scenario for close black hole binaries, where the progenitor system, a binary with at least one massive component, experienced a common-envelope phase and where the spiral-in of the companion in the envelope of the massive star caused the ejection of the envelope. We estimate the formation rates for different companion masses and different assumptions about the common-envelope structure and other model parameters. We find that black hole binaries with intermediate- and high-mass secondaries can form for a wide range of assumptions, while black hole binaries with low-mass secondaries can only form with apparently unrealistic assumptions (in agreement with previous studies).
We then present detailed binary evolution sequences for black hole binaries with secondaries of 2 to 17 M_⊙ and demonstrate that in these systems the black hole can accrete appreciably even if accretion is Eddington-limited (up to 7 M_⊙ for an initial black hole mass of 10 M_⊙) and that the black holes can be spun up significantly in the process. We discuss the implications of these calculations for well-studied black hole binaries (in particular GRS 1915+105) and ultraluminous X-ray sources of which GRS 1915+105 appears to represent a typical Galactic counterpart. We also present a detailed evolutionary model for Cygnus X-1, a massive black hole binary, which suggests that at present the system is most likely in a wind mass-transfer phase following an earlier Roche-lobe overflow phase. Finally, we discuss how some of the assumptions in the standard model could be relaxed to allow the formation of low-mass, short-period black hole binaries, which appear to be very abundant in nature.     

Fuelling quasars with hot gas

We consider a model for quasar formation in which massive black holes are formed and fuelled largely by the accretion of hot gas during the process of galaxy formation. In standard hierarchical collapse models, objects about the size of normal galaxies and larger form a dense hot atmosphere when they collapse. We show that if such an atmosphere forms a nearly 'maximal' cooling flow, then a central black hole can accrete at close to its Eddington limit. This leads to exponential growth of a seed black hole, resulting in a quasar in some cases. In this model, the first quasars form soon after the first collapses to produce hot gas. The hot gas is depleted as time progresses, mostly by cooling, so that the accretion rate eventually falls below the threshold for advection-dominated accretion, at which stage radiative efficiency plummets and any quasar turns off. A simple implementation of this model, incorporated into a semi-analytical model for galaxy formation, overproduces quasars when compared with observed luminosity functions, but is consistent with models of the X-ray background, which indicate that most accretion is obscured. It produces few quasars at high redshift owing to the lack of time needed to grow massive black holes. Quasar fuelling by hot gas provides a minimum level, sufficient to power most quasars at redshifts between one and two, to which other sources of fuel can be added. The results are sensitive to feedback effects, such as might result from radio jets and other outflows.     

Quasi-stars: accreting black holes inside massive envelopes

We study the structure and evolution of 'quasi-stars', accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds. The accretion rate on to the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass,   M _*+ M _BH≈ M _*  . This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. We use analytic models and numerical stellar structure calculations to study the structure and evolution of quasi-stars. We show that the photospheric temperature of the envelope scales as   T _ph∝ M ^−2/5_BH M ^7/20_*  , and decreases with time while the black hole mass increases. Once   T _ph < 10⁴ K  , the photospheric opacity drops precipitously and T _ph hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hydrostatic solution for the convective envelope exists. For metal-free (Population III) opacities, this limiting temperature is approximately 4000 K. After a quasi-star reaches this limiting temperature, it is rapidly dispersed by radiation pressure. We find that black hole seeds with masses between 10³ and  10⁴ M_⊙  could form via this mechanism in less than a few Myr.     

Relation between radio core length and black hole mass for active galactic nuclei

We explore the relation between the linear length of radio core and the central black hole mass for a sample of radio-loud active galactic nuclei (AGNs). An empirical relation between the size of the broad line region (BLR) and optical luminosity is used to estimate the size of the BLR. The black hole mass is derived from H β linewidth and the radius of the BLR on the assumption that the clouds in BLRs are orbiting with Keplerian velocities. A significant intrinsic correlation is found between the linear length of the core and the black hole mass, which implies that the jet formation is closely related with the central black hole. We also find a strong correlation between the black hole mass and the core luminosity.     

The relationship between X-ray variability and the central black hole mass

We assembled a sample of Seyfert 1 galaxies, quasi-stellar objects (QSOs) and low-luminosity active galactic nuclei (LLAGNs) observed by ASCA , the central black hole masses of which have been measured. We found that the X-ray variability (which is quantified by the 'excess variance' σ _rms²) is significantly anti-correlated with the central black hole mass, and it is likely that a linear relationship of σ _rms²∝ M _bh⁻¹ exists. It can be interpreted that the short time-scale X-ray variability is caused by some global coherent variations in the X-ray emission region, which is scaled by the size of the central black hole. Hence the central black hole mass is the driving parameter of the previously established relation between X-ray variability and luminosity. Our findings favour the hypothesis that the narrow-line Seyfert 1 galaxies and QSOs harbour smaller black holes than the broad-line objects, and can also easily explain the observational fact that high-redshift QSOs have greater variability than local AGNs at a given luminosity. Further investigations are needed to confirm our findings, and a large sample X-ray variability investigation can give constraints on the physical mechanisms and evolution of AGNs.     

The obscured growth of massive black holes

The mass density of massive black holes observed locally is consistent with the hard X-ray background provided that most of the radiation produced during their growth was absorbed by surrounding gas. A simple model is proposed here for the formation of galaxy bulges and central black holes in which young spheroidal galaxies have a significant distributed component of cold dusty clouds, which accounts for the absorption. The central accreting black hole is assumed to emit both a quasar-like spectrum, which is absorbed by the surrounding gas, and a slow wind. The power in both is less than the Eddington limit for the black hole. The wind, however, exerts the most force on the gas and, as earlier suggested by Silk & Rees, when the black hole reaches a critical mass it is powerful enough to eject the cold gas from the galaxy, so terminating the growth of both black hole and galaxy. In the present model this point occurs when the Thomson depth in the surrounding gas has dropped to about unity and results in the mass of the black hole being proportional to the mass of the spheroid, with the normalization agreeing with that found for local galaxies by Magorrian et al. for reasonable wind parameters. The model predicts a new population of hard X-ray and submm sources at redshifts above 1, which are powered by black holes in their main growth phase.