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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 yr and a periapse distance of 144 au to the supermassive 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 coadded data sets to improve the signal to noise. The spectroscopic SINFONI data let us determine detailed stellar properties of S4711 like the mass and the rotational velocity. The faint S-cluster star candidates, S4712–S4715, can be observed in a projected distance to Sgr A* of at least temporarily ≤120 mas. From these stars, S4714 is the most prominent, with an orbital period of 12 yr and an eccentricity of 0.985. The stars S4712–S4715 show similar properties, with magnitudes and stellar masses comparable to those of S4711. The MCMC simulations determine confidently precise uncertainties for the orbital elements of S62 and S4711–S4715. The presence of S4711 in addition to S55, S62, and the also newly found star S4714 implies a population of faint stars that can be found at distances to Sgr A* that are comparable to the size of our solar system. These short orbital time period stars in the dense cluster around the SMBH in the center of our Galaxy are perfect candidates to observe gravitational effects such as the periapse shift.

We present the Keplerian orbit of S62 around the supermassive black hole Sagittarius A* (SgrA*) in the center of our Galaxy. We monitor this S-star cluster member over more than a full orbit around SgrA* using the Very Large Telescope with the near-infrared instruments Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) and NAOS+CONICA (NACO). For that, we are deriving positional information from deconvolved images. We apply the Lucy–Richardson algorithm to the data sets. The NACO observations cover data from 2002 to 2018, and the SINFONI data cover 2008–2012. S62 can be traced reliably in both data sets. Additionally, we adapt one KECK data point for 2019 that supports the reidentification of S62 after the pericenter passage of S2. With and a periapse velocity of approximately 10% of the speed of light, S62 has the shortest known stable orbit around the supermassive black hole in the center of our Galaxy to date. From the analysis, we also derive the enclosed mass from a maximum likelihood method to be 4.15 ± 0.6 × 106 M ⊙.

We investigate an infrared-excess source called G2 or Dusty S-cluster Object (DSO), which moves on a highly eccentric orbit around the Galaxy's central black hole, Sgr A*. We use, for the first time, near-infrared polarimetric imaging data to determine the nature and properties of the DSO and obtain an improved Ks-band identification of this source in median polarimetry images of different observing years. The source started to deviate from the stellar confusion in 2008, and it does not show any flux density variability over the years we analyzed it. We measured the polarization degree and angle of the DSO between 2008 and 2012 and conclude, based on the significance analysis on polarization parameters, that it is an intrinsically polarized source (> 20%) with a varying polarization angle as it approaches the position of Sgr A* . The DSO shows a near-infrared excess of Ks − L > 3 that remains compact close to the pericenter of its orbit. Its observed parameters and the significant polarization obtained in this work show that the DSO might be a dust-enshrouded young star, forming a bow shock as it approaches the super massive black hole. The significantly high measured polarization degree indicates that it has a non-spherical geometry, and it can be modeled as a combination of a bow shock with a bipolar wind of the star. We used a 3D radiative transfer model that can reproduce the observed properties of the source such as the total flux density and the polarization degree. We obtain that the change of the polarization angle can be due to an intrinsic change in the source structure. Accretion disk precession of the young star in the gravitational field of the black hole can lead to the change of the bipolar outflow and therefore the polarization angle variation. It might also be the result of the source interaction with the ambient medium.

We present a detailed analysis of the kinematics of 112 stars that mostly comprise the high-velocity S cluster and orbit the supermassive black hole Sgr A* at the center of the Milky Way. For 39 of them, orbital elements are known; for the remainder, we know proper motions. The distribution of the inclinations and the proper motion flight directions deviate significantly from a uniform distribution, which one expects if the orientation of the orbits are random. Across the central arcseconds, the S-cluster stars are arranged in two almost edge-on disks that are located at a position angle approximately ±45° with respect to the Galactic plane. The angular momentum vectors for stars in each disk point in both directions, i.e., the stars in a given disk rotate in opposite ways. The poles of this structure are located only about 25° from the line of sight. This structure may be the result of a resonance process that started with the formation of the young B-dwarf stars in the cluster about 6 Myr ago. Alternatively, it indicated the presence of a disturber at a distance from the center comparable to the distance of the compact stellar association IRS 13.

Context. The Dusty S-cluster Object (DSO/G2) orbiting the supermassive black hole (Sgr A*) in the Galactic centre has been monitored in both near-infrared continuum and line emission. There has been a dispute about the character and the compactness of the object: interpreting it as either a gas cloud or a dust-enshrouded star. A recent analysis of polarimetry data in K s -band (2.2 µm) allows us to put further constraints on the geometry of the DSO. Aims. The purpose of this paper is to constrain the nature and the geometry of the DSO. Methods. We compare 3D radiative transfer models of the DSO with the NIR continuum data including polarimetry. In the analysis, we use basic dust continuum radiative transfer theory implemented in the 3D Monte Carlo code Hyperion. Moreover, we implement analytical results of the two-body problem mechanics and the theory of non-thermal processes.Results. We present a composite model of the DSO -a dust-enshrouded star that consists of a stellar source, dusty, optically thick envelope, bipolar cavities, and a bow shock. This scheme can match the NIR total as well as polarized properties of the observed spectral energy distribution (SED). The SED may be also explained in theory by a young pulsar wind nebula that typically exhibits a large linear polarization degree due to magnetospheric synchrotron emission. Conclusions. The analysis of NIR polarimetry data combined with the radiative transfer modelling shows that the DSO is a peculiar source of compact nature in the S cluster (r 0.04 pc). It is most probably a young stellar object embedded in a non-spherical dusty envelope, whose components include optically thick dusty envelope, bipolar cavities, and a bow shock. Alternatively, the continuum emission could be of a non-thermal origin due to the presence of a young neutron star and its wind nebula. Although there has been so far no detection of X-ray and radio counterparts of the DSO, the analysis of the neutron star model shows that young, energetic neutron stars similar to the Crab pulsar could in principle be detected in the S cluster with current NIR facilities and they appear as apparent reddened, near-infrared-excess sources. The searches for pulsars in the NIR bands can thus complement standard radio searches, which can put further constraints on the unexplored pulsar population in the Galactic centre. Both thermal and non-thermal models are in accordance with the observed compactness, total as well polarized continuum emission of the DSO.

Context. The supermassive black hole named Sgr A* is located at the dynamical center of the Milky Way. This closest supermassive black hole is known to have a luminosity several orders of magnitude lower than the Eddington luminosity. Flares coming from the Sgr A* environment can be observed in infrared, X-ray, and submillimeter wavelengths, but their origins are still debated. Interestingly, the close passage of the Dusty S-cluster Object (DSO)/G2 near Sgr A* may increase the black hole flaring activity and could therefore help us to better constrain the radiation mechanisms from Sgr A*. Aims. Our aim is to study the X-ray, infrared, and radio flaring activity of Sgr A* close to the time of the DSO/G2 pericenter passage in order to constrain the physical properties and origin of the flares. Methods. Simultaneous observations were made with XMM-Newton and WFC3 onboard HST during the period Feb.-Apr. 2014, in addition to coordinated observations with SINFONI at ESO's VLT, VLA in its A-configuration, and CARMA. Results. We detected two X-ray flares on 2014 Mar. 10 and Apr. 2 with XMM-Newton, three near-infrared (NIR) flares with HST on 2014 Mar. 10 and Apr. 2, and two NIR flares on 2014 Apr. 3 and 4 with VLT. The X-ray flare on 2014 Mar. 10 is characterized by a long rise (∼7700 s) and a rapid decay (∼844 s). Its total duration is one of the longest detected so far in X-rays. Its NIR counterpart peaked well before (4320 s) the X-ray maximum, implying a dramatic change in the X-ray-to-NIR flux ratio during this event. This NIR/X-ray flare is interpreted as either a single flare where variation in the X-ray-to-NIR flux ratio is explained by the adiabatic compression of a plasmon, or two distinct flaring components separated by 1.2 h with simultaneous peaks in X-rays and NIR. We identified an increase in the rising radio flux density at 13.37 GHz on 2014 Mar. 10 with the VLA that could be the delayed radio emission from a NIR/X-ray flare that occurred before the start of our observation. The X-ray flare on 2014 Apr. 2 occurred for HST during the occultation of Sgr A* by the Earth, therefore we only observed the start of its NIR counterpart. With NIR synchrotron emission from accelerated electrons and assuming X-rays from synchrotron self-Compton emission, the region of this NIR/X-ray flare has a size of 0.03−7 times the Schwarzschild radius and an electron density of 10 8.5 -10 10.2 cm −3 , assuming a synchrotron spectral index of 0.3−1.5. When Sgr A* reappeared to the HST view, we observed the decay phase of a distinct bright NIR flare with no detectable counterpart in X-rays. On 2014 Apr. 3, two 3.2-mm flares were observed with CARMA, where the first may be the delayed (4.4 h) emission of a NIR flare observed with VLT. Conclusions. We observed a total of seven NIR flares, with three having a detected X-ray counterpart. The physical parameters of the flaring region are less constrained for the NIR flare without a detected X-ray counterpart, but none of the possible radiative processes (synchrotron, synchrotr...

Context. We trace several dusty infrared sources on their orbits around Sgr A* with SINFONI and NACO mounted at the VLT/Chile. These sources show near-infrared excess and Doppler-shifted line emission. We investigate these sources in order to clarify their nature and compare their relationship to other observed NIR objects close to Sgr A*. Aims. By using SINFONI, we are able to determine the spectroscopic properties of the investigated dusty infrared sources. Furthermore, we extract spatial and velocity information of these objects. We are able to identify X7, X7.1, X8, G1, DSO/G2, D2, D23, D3, D3.1, D5, and D9 in the Doppler-shifted line maps of the SINFONI H+K data. From our K-and L -band NACO data, we derive the related magnitudes of the brightest sources located west of Sgr A*. Methods. For determining the line of sight velocity information and to investigate single emission lines, we use the near-infrared integral field spectrograph SINFONI data-sets between 2005 and 2015. For the kinematic analysis, we use NACO data-sets between 2002 and 2018. This study is done in the H, K s , and L band. From the 3D SINFONI data-cubes, we extract line-maps in order to derive positional information of the sources. In the NACO images, we identify the dusty counterpart of the objects. If possible, we determine Keplerian orbits and apply a photometric analysis.Results. The spectrum of the investigated objects show a Doppler-shifted Brγ and HeI line emission. For some objects west of Sgr A*, we find additionally [Fe III] line emission that can be clearly distinguished from the background. A one-component blackbody model fits the extracted near-infrared flux for the majority of the investigated objects, with the characteristic dust temperature of 500 K. The photometric derived H-and K S -band magnitudes are between mag H > 22.5 and mag K = 18.1 +0.3 −0.8 for the dusty sources. For the H-band magnitudes we can provide an upper limit. For the bright dusty sources D2, D23, and D3, the Keplerian orbits are elliptical with a semi-major axis of a D2 = (749 ± 13) mas, a D23 = (879 ± 13), and a D3 = (880 ± 13) mas. For the DSO/G2, a single-temperature and a two-component blackbody model is fitted to the H-, K-, L -, and M-band data, while the two-component model that consists of a star and an envelope fits its SED better than an originally proposed single-temperature dusty cloud. Conclusions. The spectroscopic analysis indicates, that the investigated objects could be dust embedded pre-main-sequence stars. The Doppler-shifted [Fe III] line can be spectroscopically identified in several sources that are located between 17:45:40.05 and 17:45:42.00 in DEC. However, the sources with a DEC less than 17:45:40.05 show no [Fe III] emission. Therefore, these two groups show different spectroscopic features that could be explained by the interaction with a non-spherical outflow that originates at the position of Sgr A*. Followed by this, the hot bubble around Sgr A* consists out of isolated sources with [Fe III] line emission that can partially ...