The ongoing monitoring of the Galactic center and Sgr A*, the central supermassive black hole, produces surprising and unexpected findings. This goes hand in hand with the technical evolution of ground- and space-based telescopes and instruments, but also with the progression of image filter techniques such as the Lucy Richardson algorithm. As we continue to trace the members of the S cluster close to Sgr A* on their expected trajectory around the supermassive black hole, we present the finding of a new stellar source, which we call S4716. The newly found star orbits SgrA* in about 4.0 yr and can be detected with NIRC2 (Keck), OSIRIS (Keck), SINFONI (VLT), NACO (VLT), and GRAVITY (VLTI). With a periapse distance of about 100 au, S4716 shows an equivalent distance toward Sgr A* as S4711. These fast-moving stars undergo a similar dynamical evolution, since S4711-S4716 share comparable orbital properties. We will furthermore draw a connection between the recent finding of a new faint star called S300 and the data presented here. Additionally, we observed a blend-star event with S4716 and another newly identified S star S148 in 2017.
We observe the S-cluster star S62 on its Keplerian orbit around the supermassive black hole in the center of our galaxy, SgrA*. The orbital time period of S62 around SgrA* is 9.9 years. We derive its mass to be around 2 M_solar which is consistent with other members of the S-cluster. From the Lucy-Richardson deconvolved images, we determine a K-band magnitude of 16.1 mag. We observe almost two complete orbits of the star with two different and independent telescopes and three different instruments. The close distance of S62 to SgrA* at its periapse of around 2 mas results in a gravitational pe…
Turbulence is a crucial factor in many models of planet formation, but it has only been directly constrained among a small number of planet forming disks. Building on the upper limits on turbulence placed in disks around HD 163296 and TW Hya, we present ALMA CO J=2-1 line observations at $\sim0.3"$ (20-50 au) resolution and 80 ms$^{-1}$ channel spacing of the disks around DM Tau, MWC 480, and V4046 Sgr. Using parametric models of disk structure, we robustly detect non-thermal gas motions around DM Tau of between 0.25 c$_s$ and 0.33 c$_s$, with the range dominated by systematic effects, making …
A review of modern VLTI observations of the orbital motion of closest stars to the relativistic compact object Sgr A* and its ability to test gravitation theories in the conditions of Post-Newtonian approximation. The observed orbital parameters, second order Doppler effect and gravitational redshift, measured for several S-stars, are compared with theoretical PN predictions.
After the amazing discoveries by the GRAVITY collaboration in the last few years on the star S2 orbiting the black hole Sgr A* in the center of the Milky Way, we present a detailed investigation of the impact of gravitational lensing on the reconstruction of stellar orbits around this massive black hole. We evaluate the lensing astrometric effects on the stars S2, S38 and S55 and how these systematically affect the derived orbital parameters. The effect is below current uncertainties, but not negligible. With the addition of more observations on these stars, it will be possible to let the astr…
We present high-pass filtered NACO and SINFONI images of the newly discovered stars S4711-S4715 between 2004 and 2016. Our deep H+K-band (SINFONI) and K-band (NACO) data show the S-cluster star S4711 on a highly eccentric trajectory around Sgr~A* with an orbital period of 7.6 years and a periapse distance of 144 AU to the super massive black hole (SMBH). S4711 is hereby the star with the shortest orbital period and the smallest mean distance to the SMBH during its orbit to date. The used high-pass filtered images are based on co-added data sets to improve the signal to noise. The spectroscopic…
Parametrized Post-Newtonian (PPN) equations of motion for several S-stars nearest to the Galactic Center compact relativistic object SgrA* are considered. The effect of the orbital periods difference between Newtonian and Post-Newtonian cases is taken into account. The best fit PN orbit of S2 has a period which is 16 days longer than Newtonian one. The PPN parameters $β_\text{PPN}$ and $γ_\text{PPN}$ are measured. Bayesian sampling is used to fit the trajectories of the PPN laws of motion. Posterior estimates of $β_\text{PPN}$ and $γ_\text{PPN}$ are $0.97^{+0.42}_{-0.65}$ and $0.81^{+0.46}_{-0…
The effects of the spin and quadrupole moment of SgrA* on the orbits of S stars
LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité（巴黎天文台，巴黎文理研究大学，法国国家科学研究中心，索邦大学，巴黎西岱大学）；Advanced Concepts Team, European Space Agency（欧洲空间局先进概念团队）
The center of the Milky Way hosts the closest supermassive black hole, SgrA$^*$. Decades of near-infrared observations of our Galactic Center have shown the presence of a small population of stars (the so called S-star cluster) orbiting SgrA$^*$, which were recently reported to be arranged in two orthogonal disks. In this case, the timescale for Lense-Thirring precession of S-stars should be longer than their age, implying a low spin for SgrA$^*$. In contrast, the recent results by the Event Horizon Telescope favor a highly-spinning SgrA$^*$, which seem to suggest that the S-stars could not be…
Sgr A East is the supernova remnant closest to the centre of the Milky Way. Its age has been estimated to be either very young, around 1-2 kyr, or about 10 kyr, and its exact origin remains unclear. We aspire to create a simple model of a supernova explosion that reproduces the shape, size, and location of Sgr A East. Using a simplified hydrodynamical code, we simulated the evolution of a supernova remnant in the medium around the Galactic centre. The latter consists of a nearby massive molecular cloud with which Sgr A East is known to be interacting and a wind from the nuclear star cluster. O…
In three previous papers I showed that the series of the midpoints of the times of all the X-ray flares of Sgr A* that have been detected so far harbor a statistical trend termed pacemaker regularity. This means that X-ray flares are detected more frequently around time points that are a subset of a periodic grid on the time axis of period P_X=0.1032 day=149 minutes. The series of the times of detection of the peaks of near IR (NIR) flares of the object are also regulated by a pacemaker with a period of P_IR=0.028 day=41 minutes. Here, I show that the series of the midpoints of the times of re…
We present a study of diffuse extended ionised gas toward three clouds located in the Galactic Centre (GC). One line of sight (LOS) is toward the 20 km s$^{-1}$ cloud (LOS$-$0.11) in the Sgr A region, another LOS is toward the 50 km s$^{-1}$ cloud (LOS$-$0.02), also in Sgr A, while the third is toward the Sgr B2 cloud (LOS+0.693). The emission from the ionised gas is detected from H$nα$ and H$mβ$ radio recombination lines (RRLs). He$nα$ and He$mβ$ RRL emission is detected with the same $n$ and $m$ as those from the hydrogen RRLs only toward LOS+0.693. RRLs probe gas with positive and negative …
Sagittarius A* (Sgr A*) is the central supermassive black hole with the mass $\sim 4\times 10^6 M_{\odot}$ in the Milky Way and stars are orbiting around it. In May 2018, one of the nearest stars to Sgr A* named S0-2/S2 experienced the pericenter passage. The redshift of photons from S0-2 had varied from $4000\>{\rm km\>s^{-1}}$ to $-2000\>{\rm km\>s^{-1}}$ during the pericenter passage, which is within $0.5\>{\rm yr}$. In this paper, we show that this steep variation of the redshift gives a strong constraint on a dark mass distribution inside the orbit of S0-2. By applying a simple $χ^2$ anal…
Probing dense environments around Sgr A* with S-stars dynamics
School of Natural Sciences, Institute for Advanced Study, Princeton（普林斯顿高等研究院自然科学学院）；Department of Theoretical Physics, CERN（欧洲核子研究中心理论物理部）；Dipartimento di Fisica, “Sapienza” Università di Roma & Sezione INFN Roma 1（罗马大学物理系与INFN罗马第一分部）；Department of Particle Physics and Astrophysics, Weizmann Institute of Science（魏茨曼科学研究所粒子物理与天体物理系）
High resolution imaging of supermassive black holes shadows is a direct way to verify the theory of general relativity at extreme gravity conditions. Very Long Baseline Interferometry (VLBI) observations at millimeter/sub-millimeter wavelengths can provide such angular resolution for supermassive black holes, located in Sgr A* and M87. Recent VLBI observations of M87 with the Event Horizon Telescope (EHT) has shown such capabilities. The maximum obtainable spatial resolution of EHT is limited by Earth diameter and atmospheric phase variations. In order to improve the image resolution longer ba…
Stars born on near-circular orbits in spiral galaxies can subsequently migrate to different orbits due to interactions with non-axisymmetric disturbances within the disc such as bars or spiral arms. This paper extends the study of migration to examine the role of external influences using the example of the interaction of the Sagittarius dwarf galaxy (Sgr) with the Milky Way (MW). We first make impulse approximation estimates to characterize the influence of Sgr disc passages. The tidal forcing from Sgr can produce changes in both guiding radius ($ΔR_g$) and orbital eccentricity, as quantified…
This work presents the results from extending the long-term monitoring program of stellar motions within the Galactic Center to include stars with separations of 2 - 7 arcseconds from the compact radio source, Sgr A*. In comparison to the well studied inner 2 arcsec, a longer baseline in time is required to study these stars. With 17 years of data, a sufficient number of positions along the orbits of these outer stars can now be measured. This was achieved by designing a source finder to track the positions of ~ 2000 stars in NACO/VLT adaptive-optics-assisted images of the Galactic Center from…
The "no-hair" theorem can, in principle, be tested at the center of the Milky Way by measuring the spin and the quadrupole moment of Sgr A$^*$ with the orbital precession of S-stars, measured over their full periods. Contrary to the original method, we show why it is possible to test the no-hair theorem using observations from only a single star, by measuring precession angles over a half-orbit. There are observational and theoretical reasons to expect S-stars to spin rapidly, and we have quantified the effect of stellar spin, via spin-curvature coupling (the leading-order manifestation of the…
After becoming ionized, low-density astrophysical plasmas will begin a process of slow recombination. Models for this still have significant uncertainties. The recombination cannot normally be observed in isolation, because the ionization follows the evolutionary time scale of the ionizing source. Laboratory experiments are unable to reach the appropriate conditions because of the required very long time scales. The extended nebula around the very late helium flash (VLTP) star V4334 Sgr provides a unique laboratory for this kind of study. The sudden loss of the ionizing UV radiation after the …
We report the detection of continuous positional and polarization changes of the compact source SgrA* in high states ('flares') of its variable near- infrared emission with the near-infrared GRAVITY-Very Large Telescope Interferometer (VLTI) beam-combining instrument. In three prominent bright flares, the position centroids exhibit clockwise looped motion on the sky, on scales of typically 150 micro-arcseconds over a few tens of minutes, corresponding to about 30% the speed of light. At the same time, the flares exhibit continuous rotation of the polarization angle, with about the same 45(+/-1…
A radio source at the Galactic center Sgr A* is a prime supermassive black hole candidate and therefore key to developing our understanding of them. Time variations in the 230 GHz band flux of Sgr A* have been found with the Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 5 observations. Measuring the flux density of Sgr A* in 1 min snapshots at 217.5, 219.5, and 234.0 GHz, we obtained light curves for ten 70 min periods. The light curves show variations at a few tens of minutes and hourly scales. The shorter timescale is similar to the orbital period of the innermost stable circular…
The Galactic Center black hole Sgr A* is the archetypical example of an underfed massive black hole. The extremely low accretion rate can be understood in radiatively inefficient accretion flow models. Testing those models has proven to be difficult due to the lack of suitable probes. Radio and submm polarization measurements constrain the flow very close to the event horizon. X-ray observations resolving the Bondi radius yield an estimate roughly four orders of magnitude further out. Here, we present a new, indirect measurement of the accretion flow density at intermediate radii. We use the d…
An unusual object, G2, had its pericenter passage around Sgr A*, the $4\times10^6$ M$_\odot$ supermassive black hole in the Galactic Centre, in Summer 2014. Several research teams have reported evidence that following G2's pericenter encounter the rate of Sgr A*'s bright X-ray flares increased significantly. Our analysis carefully treats varying flux contamination from a nearby magnetic neutron star and is free from complications induced by using data from multiple X-ray observatories with different spatial resolutions. We test the scenario of an increased bright X-ray flaring rate using a mas…
We trace several dusty infrared sources on their orbit around the supermassive black hole (SMBH) SgrA* in the center of our galaxy. We give an overview of known and unknown sources in the direct vicinity of our SMBH in a radius of around 0.04pc. For that, we are using NACO (K- and L'-band) and SINFONI (H+K-band) data (VLT, Chile/Paranal) between 2002 and 2018. Our spectroscopic analysis reveals a Doppler-shifted line emission of Br_gamma and HeI. Additionally, we report the detection of [FeIII] lines that are found exclusively in the investigated dusty sources west of SgrA*. We speculate, that…
Atacama Large Millimeter/Submillimeter Array (ALMA) is promising to be a powerful tool for precision astrometry of the area around Sagittarius A$^\ast$ (Sgr A$^\ast$) because it has the high angular resolution, high sensitivity, and wide field of view. We have observed the area including the Nuclear Star Cluster at 230 GHz with ALMA in October 2017. The angular resolution is ~0.03". We determined the relative positions to Sgr A$^\ast$ of 65 compact objects in the area with the accuracy of 0.001". We also analyzed the similar ALMA archival data obtained in June 2019 and determined the 64 relati…
In this work we characterize the expected gravitational wave signal detectable by the planned space-borne interferometer LISA and the proposed next generation space-borne interferometer $μ$Ares arising from a population of primordial black holes orbiting Sgr A*, the super-massive black hole at the Galactic center. Assuming that such objects indeed form the entire diffuse mass allowed by the observed orbit of S2 in the Galactic center, under the simplified assumption of circular orbits and monochromatic mass function, we assess the expected signal in gravitational waves, either from resolved an…
