Dusty S-cluster Object Research Articles

Polarization: A Method to Reveal the True Nature of the Dusty S-Cluster Object (DSO/G2)

There have been different scenarios describing the nature of a dusty source, noted as Dusty S-cluster Object (DSO) or G2, orbiting around the Galactic centre super-massive black hole. Observing the polarized continuum emission of this source provides information on its nature and geometry. We show that this source is intrinsically polarized with polarization degree of 30%, implying that it has a non-spherical geometry, and a varying polarization angle in the ambient medium of the black hole. Its main observable properties can be well described and modeled with a pre-main-sequence star forming a bow shock as it approaches the Sgr A* position.

Sixteen years of X-ray monitoring of Sagittarius A*: Evidence for a decay of the faint flaring rate from 2013 August, 13 months before a rise in the bright flaring rate

Context.X-ray flaring activity from the closest supermassive black hole Sagittarius A* (Sgr A*) located at the center of our Galaxy has been observed since 2000 October 26 thanks to the current generation of X-ray facilities. In a study of X-ray flaring activity from Sgr A* usingChandraandXMM-Newtonpublic observations from 1999 to 2014 andSwiftmonitoring in 2014, it was argued that the “bright and very bright” flaring rate has increased from 2014 August 31.Aims.As a result of additional observations performed in 2015 withChandra,XMM-Newton, andSwift(total exposure of 482 ks), we seek to test the significance and persistence of this increase of flaring rate and to determine the threshold of unabsorbed flare flux or fluence leading to any change of flaring rate.Methods.We reprocessed theChandra,XMM-Newton, andSwiftdata from 1999 to 2015 November 2. From these data, we detected the X-ray flares via our two-step Bayesian blocks algorithm with a prior on the number of change points properly calibrated for each observation. We improved theSwiftdata analysis by correcting the effects of the target variable position on the detector and we detected the X-ray flares with a 3σthreshold on the binned light curves. The mean unabsorbed fluxes of the 107 detected flares were consistently computed from the extracted spectra and the corresponding calibration files, assuming the same spectral parameters. We constructed the observed distribution of flare fluxes and durations from theXMM-NewtonandChandradetections. We corrected this observed distribution from the detection biases to estimate the intrinsic distribution of flare fluxes and durations. From this intrinsic distribution, we determined the average flare detection efficiency for eachXMM-Newton,Chandra, andSwiftobservation. We finally applied the Bayesian blocks algorithm on the arrival times of the flares corrected from the corresponding efficiency.Results.We confirm a constant overall flaring rate from 1999 to 2015 and a rise in the flaring rate by a factor of three for the most luminous and most energetic flares from 2014 August 31, i.e., about four months after the pericenter passage of the Dusty S-cluster Object (DSO)/G2 close to Sgr A*. In addition, we identify a decay of the flaring rate for the less luminous and less energetic flares from 2013 August and November, respectively, i.e., about 10 and 7 months before the pericenter passage of the DSO/G2 and 13 and 10 months before the rise in the bright flaring rate.Conclusions.The decay of the faint flaring rate is difficult to explain in terms of the tidal disruption of a dusty cloud since it occurred well before the pericenter passage of the DSO/G2, whose stellar nature is now well established. Moreover, a mass transfer from the DSO/G2 to Sgr A* is not required to produce the rise in the bright flaring rate since the energy saved by the decay of the number of faint flares during a long period of time may be later released by several bright flares during a shorter period of time.

Nature of the Galactic centre NIR-excess sources

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 $K_{\rm s}$-band ($2.2\,{\rm \mu m}$) allows us to put further constraints on the geometry of the DSO. The purpose of this paper is to constrain the nature and the geometry of the DSO. 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. 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. 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 $(r \lesssim 0.04\,{\rm pc})$. 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.

Polarized near-infrared light of the Dusty S-cluster Object (DSO/G2) at the Galactic center

We investigate an infrared-excess source called G2 or Dusty S-cluster Object (DSO) moving 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 the properties of the DSO, and obtain an improved K_s-band identification of this source in median polarimetry images of different observing years. The source starts to deviate from the stellar confusion in 2008, and it does not show any flux density variability over the years we analyzed it. We measure 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 Sgr A* position. DSO shows a near-infrared excess of K_s-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 modelled as a combination of a bow shock with a bipolar wind of the star. We use 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 disc 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.

Detection of polarized continuum emission of the Dusty S-cluster Object (DSO/G2)

AbstractA peculiar source in the Galactic center known as the Dusty S-cluster Object (DSO/G2) moves on a highly eccentric orbit around the supermassive black hole with the pericenter passage in the spring of 2014. Its nature has been uncertain mainly because of the lack of any information about its intrinsic geometry. For the first time, we use near-infrared polarimetric imaging data to obtain constraints about the geometrical properties of the DSO. We find out that DSO is an intrinsically polarized source, based on the significance analysis of polarization parameters, with the degree of the polarization of ~30% and an alternating polarization angle as it approaches the position of Sgr A*. Since the DSO exhibits a near-infrared excess of Ks-L′ > 3 and remains rather compact in emission-line maps, its main characteristics may be explained with the model of a pre-main-sequence star embedded in a non-spherical dusty envelope.

Nature of the Dusty S-cluster Object (DSO/G2): Pre-main-sequence star with non-spherical dusty envelope

AbstractNear-infrared observations reveal several infrared-excess sources near the Galactic Centre with emission lines present in their spectra. One of these objects, DSO/G2, which moves around the supermassive black hole (Sgr A*) on a highly eccentric orbit, passed the pericentre at approximately 160 AU in 2014. It remained compact, which implies that at least in this case it is a star embedded in a dusty envelope. The spectral energy distribution and the detection of polarized continuum emission indicate that it is probably a pre-main-sequence star surrounded by a dense envelope with bipolar cavities. In addition, the star associated with DSO/G2 plausibly develops a bow shock due to its supersonic motion. The model of the star surrounded by the non-spherical dusty envelope can reproduce the main characteristics of the DSO/G2 source: 1. spectral energy distribution in near-infrared bands; 2. linear polarization in Ks band; and 3. the overall compact behaviour.

Monitoring the Galactic Centre with the Australia Telescope Compact Array

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity which is thought to arise from the innermost region of the accretion flow. We performed the monitoring observations of the Galactic Centre to study the flux-density variations at 3mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We obtain the light curves of Sgr A* by subtracting the contributions from the extended emission around it, and the elevation and time dependent gains of the telescope. We perform structure function analysis and the Bayesian blocks representation to detect flare events. The observations detect six instances of significant variability in the flux density of Sgr A* in three observations, with variations between 0.5 to 1.0 Jy, which last for 1.5 $-$ 3 hours. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, such as the source expansion speed $v_{\mathrm{exp}}$, source sizes, spectral indices, and the turnover frequency. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than $v_{\mathrm{exp}}$. No exceptional flux density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO/G2). This is consistent with its compactness and the absence of a large bow shock.

Effect of an isotropic outflow from the Galactic Centre on the bow-shock evolution along the orbit

Motivated by the observations of several infrared-excess bow-shock sources and proplyd-like objects near the Galactic centre, we analyse the effect of a potential outflow from the centre on bow shock properties. We show that due to the non-negligible isotropic central outflow the bow-shock evolution along the orbit becomes asymmetric between the pre-peribothron and post-peribothron phases. This is demonstrated by the calculation of the bow-shock size evolution, the velocity along the shocked layer, the surface density of the bow-shock, and by emission-measure maps close to the peribothron passage. Within the ambient velocity range of $\lesssim 2000\,{\rm km\, s^{-1}}$ the asymmetry is profound and the changes are considerable for different outflow velocities. As a case study we perform model calculations for the Dusty S-cluster Object (DSO/G2) as a potential young stellar object that is currently being monitored and has passed the pericentre at $\sim 2000$ Schwarzschild radii from the supermassive black hole (Sgr A*) in 2014. We show that the velocity field of the shocked layer can contribute to the observed increasing line width of the DSO source up to the peribothron. Subsequently, supposing that the line emission originates in the bow shock, a decrease of the line width is expected. Furthermore, the decline of the bow-shock emission measure in the post-peribothron phase could help to reveal the emission of the putative star. The dominant contribution of circumstellar matter (either inflow or outflow) is consistent with the observed stable luminosity and compactness of the DSO/G2 source during its pericentre passage.

MONITORING THE DUSTY S-CLUSTER OBJECT (DSO/G2) ON ITS ORBIT TOWARD THE GALACTIC CENTER BLACK HOLE

We analyse and report in detail new near-infrared (1.45 - 2.45 microns) observations of the Dusty S-cluster Object (DSO/G2) during its approach to the black hole at the center of the Galaxy that were carried out with ESO VLT/SINFONI between February and September 2014. Before May 2014 we detect spatially compact Br-gamma and Pa-alpha line emission from the DSO at about 40mas east of SgrA*. The velocity of the source, measured from the red-shifted emission, is 2700+-60 km/s. No blue-shifted emission above the noise level is detected at the position of SgrA* or upstream the presumed orbit. After May we find spatially compact Br-gamma blue-shifted line emission from the DSO at about 30mas west of SgrA* at a velocity of -3320+-60 km/s and no indication for significant red-shifted emission. We do not detect any significant extension of velocity gradient across the source. We find a Br-gamma-line full width at half maximum of 50+-10 Angstroem before and 15+-10 Angstroem after the peribothron transit, i.e. no significant line broadening with respect to last year is observed. Br-gamma line maps show that the bulk of the line emission originates from a region of less than 20mas diameter. This is consistent with a very compact source on an elliptical orbit with a peribothron time passage in 2014.39+-0.14. For the moment, the flaring activity of the black hole in the near-infrared regime has not shown any statistically significant increment. Increased accretion activity of SgrA* may still be upcoming. We discuss details of a source model according to which the DSO is rather a young accreting star than a coreless gas and dust cloud.

Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei

Supermassive black holes reside in cores of galaxies, where they are often surrounded by a nuclear cluster and a clumpy torus of gas and dust. Mutual interactions can set some stars on a plunging trajectory towards the black hole. We model the pericentre passage of a dust-enshrouded star during which the dusty envelope becomes stretched by tidal forces and is affected by the interaction with the surrounding medium. In particular, we explore under which conditions these encounters can lead to periods of enhanced accretion activity. We discuss different scenarios for such a dusty source. To this end, we employed a modification of the Swift integration package. Elements of the cloud were modelled as numerical particles that represent the dust component that interacts with the optically thin gaseous environment. We determine the fraction of the total mass of the dust component that is diverted from the original path during the passages through the pericentre at $\sim 10^3$ Schwarzschild radii and find that the main part of the dust ($\gtrsim 90\%$ of its mass) is significantly affected upon the first crossing. The fraction of mass captured at the second passage generally decreases to very low values. As an example, we show predictions for the dusty source evolution assuming the current orbital parameters of the G2 cloud (also known as Dusty S-Cluster Object, DSO) in our Galactic centre. Encounter of a core-less cloud with a supermassive black hole is, most likely, a non-repeating event: the cloud is destroyed. However, in the case of a dust-enshrouded star, part of the envelope survives the pericentre passage. We discuss an offset of $\lesssim 0.3$ arcsec between the centre of mass of the diverted part and the star along the eccentric orbit. Finally, we examine an interesting possibility of a binary star embedded within a common wind envelope that becomes dispersed at the pericentre passage.

The infrared K-band identification of the DSO/G2 source from VLT and Keck data

AbstractA fast moving infrared excess source (G2) which is widely interpreted as a core-less gas and dust cloud approaches Sagittarius A* (Sgr A*) on a presumably elliptical orbit. VLT Ks-band and Keck K′-band data result in clear continuum identifications and proper motions of this ∼19m Dusty S-cluster Object (DSO). In 2002-2007 it is confused with the star S63, but free of confusion again since 2007. Its near-infrared (NIR) colors and a comparison to other sources in the field speak in favor of the DSO being an IR excess star with photospheric continuum emission at 2 microns than a core-less gas and dust cloud. We also find very compact L′-band emission (<0.1″) contrasted by the reported extended (0.03″ up to ∼0.2″ for the tail) Brγ emission. The presence of a star will change the expected accretion phenomena, since a stellar Roche lobe may retain a fraction of the material during and after the peri-bothron passage.

Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic center

There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature and origin is unclear. In addition to several isolated objects of this kind we find a small but dense cluster of co-moving sources (IRS13N) about 3" west of SgrA* just 0.5" north of the bright IRS13E cluster of WR and O-type stars. Based on their color and brightness, there are two main possibilities: (1) they may be dust embedded stars older than few Myr, or (2) extremely young, dusty stars with ages less than 1Myr. We present fist H- and Ks-band identifications or proper motions of the IRS13N members, the high velocity dusty S-cluster object (DSO), and other infrared excess sources in the central field. We also present results of NIR H- and Ks-band ESO-SINFONI integral field spectroscopy of ISR13N. We show that within the uncertainties, the proper motions of the IRS13N sources in Ks- and L'-band are identical. This indicates that the bright L'-band IRS13N sources are indeed dust enshrouded stars rather than core-less dust clouds. The proper motions show that the IRS13N sources are not strongly gravitationally bound to each other implying that they have been formed recently. We also present a first H- and Ks-band identification as well as proper motions and HKsL'-colors of a fast moving DSO which was recently found in the cluster of high speed S-stars that surround the super-massive black hole Sagittarius A* (SgrA*). Most of the compact L'-band excess emission sources have a compact H- or Ks-band counterpart and therefore are likely stars with dust shells or disks. Our new results and orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA* are very young dusty stars and that star formation at the GC is a continuously ongoing process.