F. Peissker

Monitoring the Dusty S-cluster Object (DSO/G2) on its Orbit toward the Galactic Center Black Hole

We analyze and report in detail new near-infrared (1.45-2.45 ?m) 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 the ESO Very Large Telescope/SINFONI between 2014 February and September. Before 2014 May we detect spatially compact Br? and Pa? line emission from the DSO at about 40 mas east of Sgr A*. The velocity of the source, measured from the redshifted emission, is 2700???60 km s?1. No blueshifted emission above the noise level is detected at the position of Sgr A* or upstream of the presumed orbit. After May we find spatially compact Br? blueshifted line emission from the DSO at about 30 mas west of Sgr A* at a velocity of ?3320???60 km s?1 and no indication for significant redshifted emission. We do not detect any significant extension of the velocity gradient across the source. We find a Br? line FWHM of 50???10 ? before and 15???10 ? after the peribothron transit, i.e., no significant line broadening with respect to last year is observed. Br? line maps show that the bulk of the line emission originates from a region of less than 20 mas 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 Sgr A* may still be upcoming. We discuss details of a source model according to which the DSO is a young accreting star rather than a coreless gas and dust cloud.

Multiwavelength study of the flaring activity of Sagittarius A in 2014 February−April

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, synchrotron self-Compton, or inverse Compton) can be ruled out for the production of the X-ray flares. The three X-ray flares were observed during the XMM-Newton total effective exposure of ~256 ks. This flaring rate is statistically consistent with those observed during the 2012 Chandra XVP campaign, implying that no increase in the flaring activity was triggered close to the pericenter passage of the DSO/G2. Moreover, higher flaring rates had already been observed with Chandra and XMM-Newton without any increase in the quiescent level, showing that there is no direct link between an increase in the flaring rate in X-rays and the change in the accretion rate.

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), 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 K s -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 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 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.

Nature of the Galactic centre NIR-excess sources - I. What can we learn from the continuum observations of the DSO/G2 source?

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

The Multifrequency Behavior of Sagittarius A

K_{\rm s}

The Center of the Milky Way from Radio to X-rays

-band (

Experimental Indicators of Accretion Processes in Active Galactic Nuclei

2.2\,{\rm \mu m}

Nuclear Activity and the Conditions of Star-formation at the Galactic Center

) 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

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

(r \lesssim 0.04\,{\rm pc})

Infrared-excess Source DSO/G2 Near the Galactic Center: Theory vs. Observations

. 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.