Rapid Variability of Sgr A* across the Electromagnetic Spectrum

Einstein’s General theory of relativity (GR) successfully describes gravity. Although GR has been accurately tested in weak gravitational fields, it remains largely untested in the general strong field cases. One of the most fundamental predictions of GR is the existence of black holes (BHs). After the recent direct detection of gravitational waves by LIGO, there is now near conclusive evidence for the existence of stellar-mass BHs. In spite of this exciting discovery, there is not yet direct evidence of the existence of BHs using astronomical observations in the electromagnetic spectrum. Are BHs observable astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Milky Way, which hosts the closest and best-constrained supermassive BH candidate in the universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with new-generation instruments. The first experiment consists of making a standard astronomical image of the synchrotron emission from the relativistic plasma accreting onto Sgr A*. This emission forms a “shadow” around the event horizon cast against the background, whose predicted size ([Formula: see text]as) can now be resolved by upcoming very long baseline radio interferometry experiments at mm-waves such as the event horizon telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the next-generation near-infrared (NIR) interferometer GRAVITY at the very large telescope (VLT). The third experiment aims to detect and study a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama large millimeter array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and contributes directly to them at different levels. These efforts will eventually enable us to measure fundamental BH parameters (mass, spin, and quadrupole moment) with sufficiently high precision to provide fundamental tests of GR (e.g. testing the no-hair theorem) and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*. We conclude that the Galactic center provides a unique fundamental-physics laboratory for experimental tests of BH accretion and theories of gravity in their most extreme limits.

Download Full-text

The Nova V5584 Sgr: A Short Review

Acta Polytechnica CTU Proceedings ◽

10.14311/app.2015.02.0234 ◽

2015 ◽

Vol 2 (1) ◽

pp. 234-237

Author(s):

R. Poggiani

Keyword(s):

Short Review ◽

Electromagnetic Spectrum ◽

X Rays ◽

Infrared Observations ◽

X Ray ◽

Sgr A

The nova V5584 Sgr was discovered during 2009 October. It has been monitored in different domains of the electromagnetic spectrum: optical, infrared and X-rays. The optical and infrared observations suggest that V5584 Sgr is a Fe II nova that formed dust. No X-ray emission was observed around the time of maximum.

Download Full-text

Intraday Variability of Flat-Spectrum Radio Sources

International Astronomical Union Colloquium ◽

10.1017/s0252921100045474 ◽

1998 ◽

Vol 164 ◽

pp. 257-264

Author(s):

S. J. Wagner

Keyword(s):

High Frequency ◽

Large Fraction ◽

Small Scale ◽

Electromagnetic Spectrum ◽

Radio Sources ◽

Rapid Variability ◽

Spectrum Radio ◽

Interstellar Scattering ◽

Polarization Characteristics ◽

Intraday Variability

AbstractThe characteristics of rapid variability of flat-spectrum radio sources are reviewed. A large fraction of the blazar population is found to show variability on timescales shorter than one day throughout the entire electromagnetic spectrum. The spectral indices and polarization characteristics change equally fast.Structure functions of the well-monitored sources show pronounced breaks on scales of about 10 to 50 hours, with flatter slopes towards the fast end. This illustrates that Intraday Variability (IDV), i.e. the high frequency end of the power spectrum is qualitatively different and requires different mechanisms than slower variations.While intrinsic IDV provides direct clues on small-scale structure over fifteen decades in frequency, extrinsic contributions from interstellar scattering contributes at the lowest frequencies, and remains difficult to disentangle from the intrinsic effects.

Download Full-text

Advances from Recent Multi-wavelength Campaigns on Sgr A*

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316012217 ◽

2016 ◽

Vol 11 (S322) ◽

pp. 1-10

Author(s):

Daryl Haggard

Keyword(s):

Gravitational Lensing ◽

Electromagnetic Spectrum ◽

Physical Processes ◽

Time Resolved ◽

X Ray ◽

Sagittarius A ◽

Near Ir ◽

The Status ◽

Multi Wavelength ◽

Sgr A

AbstractSagittarius A* is the closest example of a supermassive black hole and our proximity allows us to detect emission from its accretion flow in the radio, submillimeter, near IR, and X-ray regimes. Ambitious monitoring campaigns have yielded rich multi-wavelength, time-resolved data that have the power to probe the physical processes underlying Sgr A*’s quiescent and flare emission. Here, I review the status of Sgr A* X-ray monitoring campaigns from the Chandra X-ray Observatory (also XMM Newton, and Swift), and efforts to coordinate these with observations across the electromagnetic spectrum. I also discuss how these observations constrain models for Sgr A*’s variability, which range from tidal disruption of asteroids to gravitational lensing to collimated outflows to magnetic reconnection.

Download Full-text

Spectral responses of soybean leaflets crops to different water conditions and tillage systems

Agrometeoros ◽

10.31062/agrom.v26i1.26365 ◽

2018 ◽

Vol 26 (1) ◽

Author(s):

Tatiana Silva de Almeida ◽

Denise Cybis Fontana ◽

Homero Bergamaschi

Keyword(s):

Water Supply ◽

Rio Grande ◽

Incident Radiation ◽

Conventional Tillage ◽

Soil Tillage ◽

Integrating Sphere ◽

Electromagnetic Spectrum ◽

Rio Grande Do Sul ◽

Crop Cycle ◽

Spectral Responses

The state of Rio Grande do Sul is one of the greatest Brazilian soybean producers, which justifies the use of remote sensing techniques for monitoring areas occupied by this crop. The purpose of this work was to characterize throughout the crop cycle the variability of spectral responses of soybean leaflets, subjected to different conditions of soil tillage and water supply. The experiment was carried out in a 0.5 ha area, located in Eldorado do Sul, Rio Grande do Sul State, southern Brazil, in two systems of soil tillage (no-tillage and conventional tillage) and two levels of water supply (irrigated and non-irrigated). The cultivar Fepagro RS-10 was sown in a row spacing of 0.40 m and in a population of 400,000 plants per hectare. An integrating sphere of a spectroradiometer LI-COR, model LI-1800 was used for measuring the absorbance, reflectance, transmittance on soybean leaflets. The results showed that the pattern of the incident radiation partitioning in the reflectance, transmittance and absorbance components is influenced by the crop phenological stage and by the tillage system. Despite this, there is stability on the reflectance of soybean leaflets in the red and infrared portions of the electromagnetic spectrum, throughout the crop cycle. The inversely proportional relation between absorbance and reflectance of soybean leaflets revealed viability on the reflectance data, in monitoring of agricultural crops.

Download Full-text

Metrics and Motivations for Earth–Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope

The Astrophysical Journal ◽

10.3847/1538-4357/ab2bed ◽

2019 ◽

Vol 881 (1) ◽

pp. 62 ◽

Cited By ~ 7

Author(s):

Daniel C. M. Palumbo ◽

Sheperd S. Doeleman ◽

Michael D. Johnson ◽

Katherine L. Bouman ◽

Andrew A. Chael

Keyword(s):

Event Horizon ◽

Earth Space ◽

Sgr A

Download Full-text

Variable Radio Emission from X-Ray Sources

International Astronomical Union Colloquium ◽

10.1017/s0252921100032905 ◽

1971 ◽

Vol 15 ◽

pp. 173-175 ◽

Cited By ~ 1

Author(s):

L. L. E. Braes ◽

G. K. Miley

Keyword(s):

Radio Emission ◽

Aperture Synthesis ◽

Electromagnetic Spectrum ◽

Radio Telescopes ◽

Low Resolution ◽

X Ray ◽

Synthesis Technique ◽

Variable Radio

Dr. KELLOGG has just described some exciting new observations of X-ray sources made with the UHURU satellite. We shall now move some nine orders of magnitude in wavelength to the opposite end of the electromagnetic spectrum and report measurements of weak radio emission from some of the objects he mentioned. For the detection of weak sources most radio telescopes are not noise limited, but are confusion limited by their low resolution. The aperture synthesis technique minimizes this problem because it enables one to pinpoint the position of weak sources to the order of one second of arc.

Download Full-text