7 research outputs found
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
Millimeter Light Curves of Sagittarius A* Observed during the 2017 Event Horizon Telescope Campaign
The Event Horizon Telescope (EHT) observed the compact radio source, Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5â11 in the 1.3 mm wavelength band. At the same time, interferometric array data from the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array were collected, providing Sgr A* light curves simultaneous with the EHT observations. These data sets, complementing the EHT very long baseline interferometry, are characterized by a cadence and signal-to-noise ratio previously unattainable for Sgr A* at millimeter wavelengths, and they allow for the investigation of source variability on timescales as short as a minute. While most of the light curves correspond to a low variability state of Sgr A*, the April 11 observations follow an X-ray flare and exhibit strongly enhanced variability. All of the light curves are consistent with a red-noise process, with a power spectral density (PSD) slope measured to be between â2 and â3 on timescales between 1 minute and several hours. Our results indicate a steepening of the PSD slope for timescales shorter than 0.3 hr. The spectral energy distribution is flat at 220 GHz, and there are no time lags between the 213 and 229 GHz frequency bands, suggesting low optical depth for the event horizon scale source. We characterize Sgr A*'s variability, highlighting the different behavior observed just after the X-ray flare, and use Gaussian process modeling to extract a decorrelation timescale and a PSD slope. We also investigate the systematic calibration uncertainties by analyzing data from independent data reduction pipelines
Structural setting and origin of Proterozoic calcâsilicate megabreccias, Cloncurry region, northwestern Queensland
An Upper limit on the anomalous magnetic moment of the tau lepton
Using radiative Z^0 -> \tau^+ \tau^- \gamma events collected with the OPAL
detector at LEP at \sqrt{s}= M_Z during 1990-95, a direct study of the
electromagnetic current at the \tau\gamma vertex has been performed in terms of
the anomalous magnetic form factor F_2 of the \tau lepton. The analysis is
based on a data sample of 1429 e^+ e^- -> \tau^+ \tau^- \gamma events which are
examined for a deviation from the expectation with F_2 = 0. From the
non-observation of anomalous \tau^+ \tau^- \gamma production a limit of
-0.068 < F_2 < 0.065
is obtained. This can also be interpreted as a limit on the electric dipole
form factor F_3 as
-3.8 x 10^-16 e-cm < eF_3 < 3.6 x 10^-16 e-cm.
The above ranges are valid at the 95% confidence level.Comment: 20 pages, LaTeX, uses a4p.sty, 4 eps figures included, submitted to
Phys. Lett.