46 research outputs found
Unparticle contribution to the hydrogen atom ground state energy
In the present work we study the effect of unparticle modified static
potentials on the energy levels of the hydrogen atom. By using
Rayleigh-Schr\"odinger perturbation theory, we obtain the energy shift of the
ground state and we compare it with experimental data. Bounds on the unparticle
energy scale as a function of the scaling dimension
and the coupling constant are derived. We show that
there exists a parameter region where bounds on are
stringent, signalling that unparticles could be tested in atomic physics
experiments.Comment: 11 pages, 3 figure
Reply to "Comment on 'Gravitational Pair Production and Black Hole Evaporation'"
In a recent letter, the authors presented a unified derivation of the
electric Schwinger effect and Hawking radiation with an additional radiation
component. The approach discloses a radial profile of black hole pair
production and traces the emission back to local tidal forces which are
independent of the black hole event horizon. It uses an effective action valid
to second order in curvature and arbitrary order in proper time. A comment
supposed an inconsistency when applying the formula to other cases, i.e. the
Schwinger effect with magnetic fields and cosmological particle production.
This letter points out that these conclusions are drawn from using the formula
outside its range of applicability.Comment: 2 page
Gravitational Pair Production and Black Hole Evaporation
We present a new avenue to black hole evaporation using a heat-kernel
approach analogous as for the Schwinger effect. Applying this method to an
uncharged massless scalar field in a Schwarzschild spacetime, we show that
spacetime curvature takes a similar role as the electric field strength in the
Schwinger effect. We interpret our results as local pair production in a
gravitational field and derive a radial production profile. The resulting
emission peaks near the unstable photon orbit. Comparing the particle number
and energy flux to the Hawking case, we find both effects to be of similar
order. However, our pair production mechanism itself does not explicitly make
use of the presence of a black hole event horizon.Comment: 11 pages, 2 figures. To appear in Physical Review Letter
Probing Quadratic Gravity with the Event Horizon Telescope
Quadratic gravity constitutes a prototypical example of a perturbatively
renormalizable quantum theory of the gravitational interactions. In this work,
we construct the associated phase space of static, spherically symmetric, and
asymptotically flat spacetimes. It is found that the Schwarzschild geometry is
embedded in a rich solution space comprising horizonless, naked singularities
and wormhole solutions. Characteristically, the deformed solutions follow the
Schwarzschild solution up outside of the photon sphere while they differ
substantially close to the center of gravity. We then carry out an analytic
analysis of observable signatures accessible to the Event Horizon Telescope,
comprising the size of the black hole shadow as well as the radiation emitted
by infalling matter. On this basis, we argue that it is the brightness within
the shadow region which constrains the phase space of solutions. Our work
constitutes the first step towards bounding the phase space of black hole type
solutions with a clear quantum gravity interpretation based on observational
data.Comment: 18 pages, 6 figure
A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows
We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT
First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass
In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching ∼100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 ± 2.3 μas (68% credible intervals), with the ring thickness constrained to have an FWHM between ∼30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be 4.8−0.7+1.4 μas, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be 4.0−0.6+1.1×106 M ⊙
The Event Horizon Telescope Image of the Quasar NRAO 530
We report on the observations of the quasar NRAO 530 with the Event Horizon Telescope (EHT) on 2017 April 5−7, when NRAO 530 was used as a calibrator for the EHT observations of Sagittarius A*. At z = 0.902, this is the most distant object imaged by the EHT so far. We reconstruct the first images of the source at 230 GHz, at an unprecedented angular resolution of ∼20 μas, both in total intensity and in linear polarization (LP). We do not detect source variability, allowing us to represent the whole data set with static images. The images reveal a bright feature located on the southern end of the jet, which we associate with the core. The feature is linearly polarized, with a fractional polarization of ∼5%–8%, and it has a substructure consisting of two components. Their observed brightness temperature suggests that the energy density of the jet is dominated by the magnetic field. The jet extends over 60 μas along a position angle ∼ −28°. It includes two features with orthogonal directions of polarization (electric vector position angle), parallel and perpendicular to the jet axis, consistent with a helical structure of the magnetic field in the jet. The outermost feature has a particularly high degree of LP, suggestive of a nearly uniform magnetic field. Future EHT observations will probe the variability of the jet structure on microarcsecond scales, while simultaneous multiwavelength monitoring will provide insight into the high-energy emission origin