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Register a new userStellar rotation as a new observable to test general relativity in the Galactic Center
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Yun Fang
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S-stars in the Galactic Center are excellent testbeds of various general relativistic effects. While previous works focus on modeling their orbital motion around Sgr A*--the supermassive black hole in the Galactic Center--here we explore the possibility of using the rotation of S-stars to test the de Sitter precession predicted by general relativity. We show that by reorienting the rotation axes of S-stars, de Sitter precession will change the apparent width of the absorption lines in the stellar spectra. Our numerical simulations suggest that the newly discovered S4714 and S62 are best suited for such a test because of their small pericenter distances relative to Sgr A*. Depending on the initial inclination of the star, the line width would vary by as much as $20-76,{rm km,s^{-1}}$ within a period of $20-40$ years. Such a variation is comparable to the current detection limit. Since the precession rate is sensitive to the orbital eccentricity and stellar quadrupole structure, monitoring the rotation of S-stars could also help us better constrain the orbital elements of the S-stars and their internal structures.
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The main science driver for the coming generation of cosmological surveys is understanding dark energy which relies on testing General Relativity on the largest scales. Once we move beyond the simplest explanation for dark energy of a cosmological constant, the space of possible theories becomes both vast and extremely hard to compute realistic observables. A key discriminator of a cosmological constant, however, is that the growth of structure is scale-invariant on large scales. By carefully weighting observables derived from distributions of galaxies and a dipole pattern in their apparent sizes, we construct a null test which vanishes for any model of gravity or dark energy where the growth of structure is scale-independent. It relies only on very few assumptions about cosmology, and does not require any modelling of the growth of structure. We show that with a survey like DESI a scale-dependence of the order of 10-20 percent can be detected at 3 sigma with the null test, which will drop by a factor of 2 for a survey like the Square Kilometre Array. We also show that the null test is very insensitive to typical uncertainties in other cosmological parameters including massive neutrinos and scale-dependent bias, making this a key null test for dark energy.
A tidal disruption event (TDE) ensues when a star passes too close to the supermassive black hole (SMBH) in a galactic center and is ripped apart by the tidal field of the SMBH. The gaseous debris produced in a TDE can power a bright electromagnetic flare as it is accreted by the SMBH; so far, several dozen TDE candidates have been observed. For SMBHs with masses above $sim 10^7 M_odot$, the tidal disruption of solar-type stars occurs within ten gravitational radii of the SMBH, implying that general relativity (GR) is needed to describe gravity. Three promising signatures of GR in TDEs are: (1) a super-exponential cutoff in the volumetric TDE rate for SMBH masses above $sim 10^8 M_odot$ due to direct capture of tidal debris by the event horizon, (2) delays in accretion disk formation (and a consequent alteration of the early-time light curve) caused by the effects of relativistic precession on stream circularization, and (3) quasi-periodic modulation of X-ray emission due to global precession of misaligned accretion disks and the jets they launch. We review theoretical models and simulations of TDEs in Newtonian gravity, then describe how relativistic modifications give rise to these proposed observational signatures, as well as more speculative effects of GR. We conclude with a brief summary of TDE observations and the extent to which they show indications of these predicted relativistic signatures.
The S-Stars in the Galactic-center region are found to be on near-perfect Keplerian orbits around presumably a supermassive black hole, with periods of 15-50 yr. Since these stars reach a few percent of light speed at pericenter, various relativistic effects are expected, and have been discussed in the literature. We argue that an elegant test of the Einstein equivalence principle should be possible with existing instruments, through spectroscopic monitoring of an S-star concentrated during the months around pericenter, supplemented with an already-adequate astrometric determination of the inclination. In essence, the spectrum of an S-star can be considered a heterogeneous ensemble of clocks in a freely-falling frame, which near pericenter is moving at relativistic speeds.
The Galactic center offers us a unique opportunity to test General Relativity (GR) with the orbits of stars around a supermassive black hole. Observations of these stars have been one of the great successes of adaptive optics on 8-10 m telescopes, driving the need for the highest angular resolution and astrometric precision. New tests of gravitational physics in the strong gravity regime with stellar orbits will be made possible through the leap in angular resolution and sensitivity from the next generation of extremely large ground-based telescopes. We present new simulations of specific science cases such as the detection of the GR precession of stars, the measurement of extended dark mass, and the distance to the Galactic center. We use realistic models of the adaptive optics system for TMT and the IRIS instrument to simulate these science cases. In additions, the simulations include observational issues such as the impact of source confusion on astrometry and radial velocities in the dense environment of the Galactic center. We qualitatively show how improvements in sensitivity, astrometric and spectroscopic precision, and increasing the number of stars affect the science with orbits at the Galactic center. We developed a tool to determine the constraints on physical models using a joint fit of over 100 stars that are expected to be observable with TMT. These science cases require very high astrometric precision and stability, thus they provide some of the most stringent constraints on the planned instruments and adaptive optics systems.
In this paper we review and discuss some of the intriguing properties of the Galactic Center supermassive black hole candidate Sgr A*. Of all possible black hole sources, the event horizon of Sgr A*, subtends the largest angular scale on the sky. It is therefore a prime candidate to study and image plasma processes in strong gravity and it even allows imaging of the shadow cast by the event horizon. Recent mm-wave VLBI and radio timing observations as well as numerical GRMHD simulations now have provided several breakthroughs that put Sgr A* back into the focus. Firstly, VLBI observations have now measured the intrinsic size of Sgr A* at multiple frequencies, where the highest frequency measurements have approached the scale of the black hole shadow. Moreover, measurements of the radio variability show a clear time lag between 22 GHz and 43 GHz. The combination of size and timing measurements, allows one to actually measure the flow speed and direction of magnetized plasma at some tens of Schwarzschild radii. This data strongly support a moderately relativistic outflow, consistent with an accelerating jet model. This is compared to recent GRMHD simulation that show the presence of a moderately relativistic outflow coupled to an accretion flow Sgr A*. Further VLBI and timing observations coupled to simulations have the potential to map out the velocity profile from 5-40 Schwarzschild radii and to provide a first glimpse at the appearance of a jet-disk system near the event horizon. Future submm-VLBI experiments would even be able to directly image those processes in strong gravity and directly confirm the presence of an event horizon.
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