L. Meyer

Polarized NIR and X-ray flares from Sagittarius A

Context. Stellar dynamics indicate the presence of a supermassive 3−4 × 10 6 Mblack hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray source Sagittarius A* (SgrA*). Aims. The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). Results. We find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moder- ately bright flare event in the X-ray domain with an excess 2−8 keV luminosity of about 8 × 10 33 erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of ≤10 min. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. Conclusions. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of syn- chronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered X-rays from the compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained by spots on relativistic orbits around the central supermassive black hole (SMBH). The profile variations for the May 2007 flare can be interpreted as a variation of the spot structure due to differential rotation within the disk.

Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares

Context. Recent near-infrared polarization measurements of Sgr A* show that its emission is significantly polarized during flares and consists of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of ∼20 min in agreement with previous observations. Aims. By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or whether some more complex dependency of the intrinsic emissivity is required. Methods. We discuss the significance of the 20 min peak in the periodogram of a flare from 2003. We consider all general relativistic effects that imprint on the polarization degree and angle and fit the recent polarimetric data, assuming that the synchrotron mechanism is responsible for the intrinsic polarization and considering two different magnetic field configurations. Results. Within the quality of the available data, we think that the model of a single spot in addition to an underlying ring is favoured. In this model the broad near-infrared flares of Sgr A* are due to a sound wave that travels around the MBH once while the sub-flares, superimposed on the broad flare, are due to transiently heated and accelerated electrons which can be modeled as a plasma blob. Within this model it turns out that a strong statement about the spin parameter is difficult to achieve, while the inclination can be constrained to values ≥35° on a 3σ level.

K-band polarimetry of an Sgr A* flare with a clear sub-flare structure ⋆

Context. The supermassive black hole at the Galactic center, Sgr A*, shows frequent radiation outbursts, often called ’flares’. I n the near-infrared some of these flares were reported as showing i ntrinsic quasi-periodicities. The flux peaks associated wi th the quasi-periodic behavior were found to be highly polarized. Aims. The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation fr om Sgr A* and to again provide strong support for the quasi-periodicity of∼18± 3 min reported earlier. Methods. Observations were carried out at the European Southern Observatory’s Very Large Telescope on Paranal, Chile. We used th e NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetr ic lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray -tracing code in combination with a simple hot-spot/ring model. Results. We report on new polarization measurements of a K-band flare f rom the supermassive black hole at the Galactic center. The data provide very strong support for a quasi-periodicity of 15.5± 2 min. The mean polarization of the flare is consistent with th e direction of the electric field vector that was reported in previous obser vations. The data can be modeled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i> 20 ◦ on a 3σ level, and the dimensionless spin parameter of the black hole is derived to be a⋆> 0.5.

Multi-wavelength and polarimetric observations of Sagittarius A*

We summarize the results of some of the latest NIR/sub-millimeter/X-ray observing campaigns. Those include the latest simultaneous observations as well as the most recent results from VLT NACO observations of polarized NIR, flare emission of Sgr A*. We interpret the new NIR, polarimetry results using a model in which spots are on relativistic orbits around Sgr A*, which is associated with the massive 3.6 million solar mass black hole at the Galactic Center. In the NIR, the observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope. In the X-ray and radio domains we used the ACIS-I instrument aboard the Chandra X-ray Observatory and the Submillimeter Array on Mauna Kea, Hawaii, as well as the Very Large Array in New Mexico, respectively.

A two component hot spot/ring model for the NIR flares of Sagittarius A*

The supermassive black hole at the Galactic Center, Sgr A*, shows frequent radiation outbursts, so-called flares. In the near-infrared some of these flares were reported to show intrinsic quasi-periodicities of 18 ± 3min. In 2005, we have carried out polarimetric observations of these QPOs in the K-band. These observations allow for a detailed investigation of Sgr A* within the hot spot model. In this model, inhomogeneities in the accretion flow are represented as confined orbiting material. By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question whether these changes are consistent with the hot spot model, in which the interplay of relativistic effects plays the major role. We consider all general relativistic effects that imprint on the polarization lightcurves. As the synchrotron mechanism is most likely responsible for the intrinsic polarization, we consider two different magnetic field configurations as approximations to the complex structure of the magnetic field in the accretion flow. Considering the quality of the fit, we think that the spot model is favoured. Finally, our confidence contours give constraints to the spin parameter and the inclination of the supermassive black hole associated with Sgr A*.

Prospects for observing the Galactic Center: combining LBT LINC-NIRVANA observations in the near-infrared with observations in the mm/sub-mm wavelength domain

As a near-infrared (NIR) wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC-NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging the center of the Milky Way especially in conjunction with mm/sub-mm interferometers like CARMA, ATCA or, in the near future, ALMA. Sagittarius A* (Sgr A*) is the electromagnetc manifestation of the ~4×106M super-massive black hole (SMBH) at the Galactic Center. First results from a mult-wavelength campaign focused on Sgr A*, based on the VLT and on CARMA, ATCA, and the IRAM 30m-telescope, in May 2007 show that the NIR data are consistent with partially depolarized non-thermal emission from confined hot spots in relativistic orbits around SgrA*. A 3mm flare following a May 2007 NIR flare is consistent with SSC emission from adiabatically expanding plasma in a wind or jet. With the LBT and ALMA we will be able to study the spectral evolution of NIR/sub-mm/mm flare emission in order to constrain the emission mechanism, the jet/wind physics, and possibly determine the angular momentum of the SMBH. LINC/NIRVANA will also serve to investigate the stellar population and dynamics in the cluster surrounding Sgr A*. A particular emphasis will lie on examining dust embedded and young stars and to unravel the star formation history in the cluster. For the 0.3 parsec core radius central star cluster the investigation of will be investigated.

Variable and polarized emission from SgrA

The super-massive black hole in the Galactic Center (Sagittarius A*) is one of the most exciting targets in the sky. At a distance of ∼ 8 kpc it is about one hundred times closer than the second nearest nucleus of a similar galaxy, M31, and therefore the closest galactic nucleus that we can study. Here we report on the modeling of polarized near-infrared flare emission from SgrA* using a model in which a hot spot is moving on a relativistic orbit around the massive black hole. We also summarize the results from simultaneous radio/near-infrared/X-ray measurements of flare emission.

Interferometric observations of the galactic center: LBT and VLTI

Current and future opportunities for interferometric observations of the Galactic Center in the near- and mid-infrared (NIR/MIR) wavelength domain are highlighted. Main emphasis is being put on the Large Binocular Telescope (LBT) and the Very Large Telescope Interferometer (VLTI). The Galactic Center measurements of stellar orbits and strongly variable NIR and X-ray emission from Sagittarius A* (SgrA*) at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentration at this position is associated with a super massive black hole. Similar dark mass concentrations seen in many galactic nuclei are most likely super massive black holes as well. High angular resolution interferometric observations in the NIR/MIR will provide key information on the central massive black hole and the stellar cluster it is embedded in. These observations have already started: Recent results on the luminous dust enshrowded star IRS3 using MIDI at the VLTI are presented and future scientific possibilities in the GC using MIDI at the VLTI in the MIR and GRAVITY in the NIR are highlighted. As a NIR wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC/NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging galactic nuclei including the center of the Milky Way.