Fulvio Melia

THE SUPERMASSIVE BLACK HOLE AT THE GALACTIC CENTER

▪ Abstract The inner few parsecs at the Galactic Center have come under intense scrutiny in recent years, in part due to the exciting broad-band observations of this region, but also because of the...

An accreting black hole model for Sagittarius A

Several observations, notably of broad He I, Br-alpha, and Br-gamma emission lines from the vicinity of IRS 16, indicate the presence of a strong circumnuclear wind near the dynamical center of the Galaxy. Sgr A, a hypothesized supermassive object situated about 0.06 pc to the west of IRS 16, should be accreting from this wind if it is not itself a source of gaseous outflow, for which there is currently no observational evidence. Here, the spectrum and flux of radiation resulting from this process are calculated, and it is shown that they are consistent with the data over at least 12 decades of frequency. Together with the kinematic studies of the stellar and gas distributions in this region, the model argues strongly in favor of Sgr A being a black hole with mass over a million solar masses.

A lower limit of 50 microgauss for the magnetic field near the Galactic Centre

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of ∼100 parsecs (pc), fields of ∼1,000 microgauss (μG; refs 1–3) have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas, the contested ‘very hot’ plasma, and the magnetic field) suggests fields of ∼100 μG over ∼400 pc size scales. Finally, assuming equipartition, fields of only ∼6 μG have been inferred from radio observations for 400 pc scales. Here we report a compilation of previous data that reveals a downward break in the regions non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least ∼50 μG on 400 pc scales, lest the synchrotron-emitting electrons produce too much γ-ray emission, given other existing constraints. Other considerations support a field of 100 μG, implying that over 10% of the Galaxys magnetic energy is contained in only ≲0.05% of its volume.

The cosmic horizon

The cosmological principle, promoting the view that the Universe is homogeneous and isotropic, is embodied within the mathematical structure of the Robertson‐Walker (RW) metric. The equations derived from an application of this metric to the Einstein Field Equations describe the expansion of the Universe in terms of comoving coordinates, from which physical distances may be derived using a time-dependent expansion factor. These coordinates, however, do not explicitly reveal the properties of the cosmic space‐time manifested in Birkhoff’s theorem and its corollary. In this paper, we compare two forms of the metric ‐ written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates ‐ first for the well-known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter. We show that Rindler’s event horizon ‐ evident in the comoving system ‐ coincides with what one might call the ‘curvature horizon’ appearing in the observer-dependent frame. The advantage of this dual prescription of the cosmic space‐time is that with the latest Wilkinson Microwave Anisotropy Probe results, we now have a much better determination of the Universe’s mass-energy content, which permits us to calculate this curvature with unprecedented accuracy. We use it here to demonstrate that our observations have probed the limit beyond which the cosmic curvature prevents any signal from having ever reached us. In the case of de Sitter, where the mass-energy density is a constant, this limit is fixed for all time. For a universe with a changing density, this horizon expands until de Sitter is reached asymptotically, and then it too ceases to change.

The Rh=ct universe

The backbone of standard cosmology is the Friedmann-Robertson-Walker solution to Einsteins equations of general relativity (GR). In recent years, observations have largely confirmed many of the properties of this model, which is based on a partitioning of the universes energy density into three primary constituents: matter, radiation, and a hypothesized dark energy which, in LambdaCDM, is assumed to be a cosmological constant Lambda. Yet with this progress, several unpalatable coincidences (perhaps even inconsistencies) have emerged along with the successful confirmation of expected features. One of these is the observed equality of our gravitational horizon R_h(t_0) with the distance ct_0 light has traveled since the big bang, in terms of the current age t_0 of the universe. This equality is very peculiar because it need not have occurred at all and, if it did, should only have happened once (right now) in the context of LambdaCDM. In this paper, we propose an explanation for why this equality may actually be required by GR, through the application of Birkhoffs theorem and the Weyl postulate, at least in the case of a flat spacetime. If this proposal is correct, R_h(t) should be equal to ct for all cosmic time t, not just its present value t_0. Therefore models such as LambdaCDM would be incomplete because they ascribe the cosmic expansion to variable conditions not consistent with this relativistic constraint. We show that this may be the reason why the observed galaxy correlation function is not consistent with the predictions of the standard model. We suggest that an R_h=ct universe is easily distinguishable from all other models at large redshift (i.e., in the early universe), where the latter all predict a rapid deceleration.

The radiative deceleration of ultrarelativistic jets in active galactic nuclei

A detailed study of the dynamical interaction between a highly relativistic jet and the thermal radiation field from an AGN accretion disk is reported, and the Comptonized spectrum arising from this interaction is self-consistently determined. A simple model that captures the essential radiative and geometrical features of realistic disk configurations is presented, and the disk radiation field is calculated. The results confirm Phinneys (1987) suggestion that the thermal radiation field produced by accretion in an AGN could be very effective in decelerating ultrarelativistic jets that are accreted by electromagnetic or hydromagnetic forces closer to the central black hole. Terminal Lorentz factors are consistent with the values inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without additional acceleration in the interaction zone. A new interpretation of the hard X-ray component detected in BL Lac spectra is proposed. 55 refs.

Wild at Heart: The particle astrophysics of the Galactic Centre

We consider the high-energy astrophysics of the inner ∼200 pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to (i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the regions non-thermal radio and TeV γ-ray emission; (ii) drive a powerful wind that advects non-thermal particles out of the inner Galactic Centre; (iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm envelope H 2 detected throughout the Central Molecular Zone; (iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ∼150 pc scales above and below the plane (the Galactic Centre lobe); and (v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼ 10 kpc), generate the recently identified Wilkinson Microwave Anisotropy Probe (WMAP) haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/μG < 400 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the regions dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this starburst-like environment - act to substantially modify the conditions of star formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511 keV line emission. We present extensive appendices reviewing the environmental conditions in the Galactic Centre, deriving the star formation and supernova rates there, and setting out the extensive prior evidence that exists, supporting the notion of a fast outflow from the region.

Hydrodynamical Accretion onto Sagittarius A* from Distributed Point Sources

Spectral and kinematic studies suggest that the nonthermal radio source Sgr A*, located at the center of the Milky Way, is a supermassive compact object with a mass ~(2-3) × 106 M☉. Winds from nearby stars, located ≈ 0.06 pc to the east of Sgr A*, should, in the absence of any outflow from the putative black hole itself, be accreting onto this object. We report the results of the first three-dimensional Bondi-Hoyle hydrodynamical numerical simulations of this process under the assumption that the Galactic center wind is generated by several different point sources (here assumed to be 10 pseudorandomly placed stars). Our results show that the accretion rate onto the central object can be higher than in the case of a uniform flow since wind-wind shocks dissipate some of the bulk kinetic energy and lead to a higher capture rate for the gas. However, even for this highly nonuniform medium, most of the accreting gas carries with it a relatively low level of specific angular momentum, although large transient fluctuations can occur. Additionally, the post-bow shock focusing of the gas can be substantially different than that for a uniform flow, but it depends strongly on the stellar spatial distribution. We discuss how this affects the morphology of the gas in the inner 0.15 pc of the Galaxy and the consequences for accretion disk models of Sgr A*.

Time-dependent Disk Models for the Microquasar GRS 1915+105

During the past three years, the Galactic black hole microquasar GRS 1915)105 has exhibited a bewildering diversity of large-amplitude, chaotic variability in X-rays. Although it is generally accepted that the variability in this source results from an accretion disk instability, the exact nature of the insta- bility remains unknown. Here we investigate diUerent accretion disk models and viscosity prescriptions in order to provide a basic explanation for some of the exotic temporal behavior in GRS 1915)105. We discuss a range of possible accretion —ow geometries. Any geometrically thick disk (e.g., an advection- dominated accretion —ow (ADAF) or a ii slim ˇˇ accretion disk) has trouble explaining the very long cycle times unless the a-parameter is exceedingly small (D10~4). In addition, the rise/fall timescales in GRS 1915)105 can be a factor of 100 shorter than the cycle times, whereas thick disks predict that these two timescales should be comparable. We thus concentrate on geometrically thin (though not necessarily standard) Shakura-Sunyaev type disks. We argue that X-ray observations clearly require a quasi-stable accretion disk solution at a high accretion rate at which radiation pressure begins to dominate, which excludes the standard a-viscosity prescription. We have therefore devised a simpli—ed model of a disk with a corona and a modi—ed viscosity law that has a quasi-stable upper branch, and we have developed a code to solve the time-dependent equations to study the evolution of this con—guration. Via numerical simulations, we show that the model does account for several gross observational features of GRS 1915)105, including its overall cyclic behavior on timescales of D100¨1000 s. On the other hand, the rise/fall timescales are not as short as those observed, no rapid oscillations on timescales s emerge (10 naturally from the model, and the computed cycle-time dependence on the average luminosity is stronger than is found in GRS 1915)105. We then consider, and numerically test, a more elaborate model that includes the ii cold ˇˇ disk, a corona, and plasma ejections from the inner disk region that occur when the luminosity of the source is near the Eddington luminosity. The inclusion of a jet allows us to reproduce several additional observed features of GRS 1915)105. We conclude that the most likely structure of the accretion —ow in this source is that of a cold disk with a modi—ed viscosity law, plus a corona that accounts for much of the X-ray emission and unsteady plasma ejections that occur when the luminosity of the source is high. The disk is geometrically thin (as required by the data) because most of the accre- tion power is drained by the corona and the jet. Subject headings: accretion, accretion disksblack hole physics ¨ quasars: individual (GRS 1915)105) ¨ X-rays: general

THE GAMMA-RAY BURST HUBBLE DIAGRAM AND ITS IMPLICATIONS FOR COSMOLOGY

In this paper, we continue to build support for the proposal to use gamma-ray bursts (GRBs) as standard candles in constructing the Hubble Diagram at redshifts beyond the current reach of Type Ia supernova observations. We confirm that correlations among certain spectral and lightcurve features can indeed be used as luminosity indicators, and demonstrate from the most up-to-date GRB sample appropriate for this work that the ΛCDM model optimized with these data is characterized by parameter values consistent with those in the concordance model. Specifically, we find that (Ωm,ΩΛ) ≈ (0.30, 0.70), versus (0.27, 0.73) obtained from the 5-yr WMAP data. We also carry out a comparative analysis between ΛCDM and the Rh = ct Universe and show that the latter is a better fit to the GRB data. We find that the optimal ΛCDM model fits the GRB Hubble Diagram with a reduced χdof ≈ 1.79, whereas the fit using Rh = ct results in a χdof ≈ 1.66. In both cases, about 20% of the events lie at least 2σ away from the best-fit curves, suggesting that either some contamination by non-standard GRB luminosities is unavoidable, or that the errors and intrinsic scatter associated with the data are being underestimated. Subject headings: cosmology: observations, redshift, theory; early universe; gamma-ray bursts: generalIn this paper, we continue to build support for the proposal to use gamma-ray bursts (GRBs) as standard candles in constructing the Hubble diagram at redshifts beyond the current reach of Type Ia supernova observations. We confirm that correlations among certain spectral and light-curve features can indeed be used as luminosity indicators, and demonstrate from the most up-to-date GRB sample appropriate for this work that the Lambda CDM model optimized with these data is characterized by parameter values consistent with those in the concordance model. Specifically, we find that (Omega(m), Omega(Lambda)) approximate to (0.25(-0.06)(+0.05), 0.75(-0.05)(+0.06)), which are consistent, to within 1 sigma, with (0.29, 0.71) obtained from the 9 yr Wilkinson Microwave Anisotropy Probe data. We also carry out a comparative analysis between Lambda CDM and the R-h = ct universe and find that the optimal Lambda CDM model fits the GRB Hubble diagram with a reduced chi(2)(dof) approximate to 2.26, whereas the fit using R-h = ct results in a chi(2)(dof) approximate to 2.14. In both cases, about 20% of the events lie at least 2 sigma away from the best-fit curves, suggesting that either some contamination by non-standard GRB luminosities is unavoidable or that the errors and intrinsic scatter associated with the data are being underestimated. With these optimized fits, we use three statistical tools-the Akaike information criterion, the Kullback information criterion, and the Bayes information criterion-to show that, based on the GRB Hubble diagram, the likelihood of R-h = ct being closer to the correct model is similar to 85%-96%, compared to similar to 4%-15% for Lambda CDM.