L- and M-band imaging observations of the Galactic Center region

We present near-infrared H-, K-, L- and M-band photometry of the Galactic Center from images obtained at the ESO VLT in May and August 2002, using the NAOS/CONICA (H and K) and the ISAAC (L and M) instruments. The large field of view (70" x 70") of the ISAAC instrument and the large number of sources identified (L-M data for 541 sources) allows us to investigate colors, infrared excesses and extended dust emission. Our new L-band magnitude calibration reveals an offset to the traditionally used calibrations, which we attribute to the use of the variable star IRS7 as a flux calibrator. Together with new results on the extinction towards the Galactic Center (Scoville et al. 2003; Raab 2000), our magnitude calibration results in stellar color properties expected from standard stars and removes any necessity to modify the K-band extinction. The large number of sources for which we have obtained L-M colors allows us to measure the M-band extinction to A_M=(0.056+-0.006)A_V (approximately =A_L), a considerably higher value than what has so far been assumed. L-M color data has not been investigated previously, due to lack of useful M-band data. We find that this color is a useful diagnostic tool for the preliminary identification of stellar types, since hot and cool stars show a fairly clear L-M color separation. This is especially important if visual colors are not available, as in the Galactic Center. For one of the most prominent dust embedded sources, IRS3, we find extended L- and M-band continuum emission with a characteristic bow-shock shape. An explanation for this appearance is that IRS3 consists of a massive, hot, young mass-losing star surrounded by an optically thick, extended dust shell, which is pushed northwest by wind from the direction of the IRS16 cluster and SgrA*.Comment: 24 pages, 7 figures, 2 tables, accepted for publication in Astronomy & Astrophysic

Flares and variability from Sagittarius A*: five nights of simultaneous multi-wavelength observations

Aims. We report on simultaneous observations and modeling of mid-infrared (MIR), near-infrared (NIR), and submillimeter (submm) emission of the source Sgr A* associated with the supermassive black hole at the center of our Galaxy. Our goal was to monitor the activity of Sgr A* at different wavelengths in order to constrain the emitting processes and gain insight into the nature of the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007. Results. The observations reveal remarkable variability in the NIR and sub-mm during the five nights of observation. No source was detected in the MIR, but we derived the lowest upper limit for a flare at 8.59 microns (22.4 mJy with A_8.59mu = 1.6+/- 0.5). This observational constraint makes us discard the observed NIR emission as coming from a thermal component emitting at sub-mm frequencies. Moreover, comparison of the sub-mm and NIR variability shows that the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of our five-night campaign involving three flares. We explain this behavior by a loss of electrons to the system and/or by a decrease in the magnetic field, as might conceivably occur in scenarios involving fast outflows and/or magnetic reconnection.Comment: 10 pages, 7 figures, published in A&

An aperture masking mode for the MICADO instrument

MICADO is a near-IR camera for the Europea ELT, featuring an extended field (75" diameter) for imaging, and also spectrographic and high contrast imaging capabilities. It has been chosen by ESO as one of the two first-light instruments. Although it is ultimately aimed at being fed by the MCAO module called MAORY, MICADO will come with an internal SCAO system that will be complementary to it and will deliver a high performance on axis correction, suitable for coronagraphic and pupil masking applications. The basis of the pupil masking approach is to ensure the stability of the optical transfer function, even in the case of residual errors after AO correction (due to non common path errors and quasi-static aberrations). Preliminary designs of pupil masks are presented. Trade-offs and technical choices, especially regarding redundancy and pupil tracking, are explained.Comment: SPIE 2014 Proceeding -- Montrea

Discovery of X-ray eclipses from the transient source CXOGC J174540.0-290031 with XMM-Newton

We present the XMM-Newton observations obtained during four revolutions in Spring and Summer 2004 of CXOGC J174540.0-290031, a moderately bright transient X-ray source, located at only 2.9" from SgrA*. We report the discovery of sharp and deep X-ray eclipses, with a period of 27,961+/-5 s and a duration of about 1,100+/-100 s, observed during the two consecutive XMM revolutions from August 31 to September 2. No deep eclipses were present during the two consecutive XMM revolutions from March 28 to April 1, 2004. The spectra during all four observations are well described with an absorbed power law continuum. While our fits on the power law index over the four observations yield values that are consistent with Gamma=1.6-2.0, there appears to be a significant increase in the column density during the Summer 2004 observations, i.e. the period during which the eclipses are detected. The intrinsic luminosity in the 2-10 keV energy range is almost constant with 1.8-2.3 x 10^34 (d_8kpc)^2 erg/s over the four observations. In the framework of eclipsing semidetached binary systems, we show that the eclipse period constrains the mass of the assumed main-sequence secondary star to less than 1.0 M_odot. Therefore, we deduce that CXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover the eclipse duration constrains the mass of the compact object to less than about 60 M_odot, which is consistent with a stellar mass black hole or a neutron star. The absence of deep X-ray eclipses during the Spring 2004 observations could be explained if the centroid of the X-ray emitting region moves from a position on the orbital plane to a point above the compact object, possibly coincident with the base of the jet which was detected in radio at this epoch. [Abstract truncated].Comment: A&A, accepted for publication (10 pages, 8 figures, 2 Tables

First proper motions of thin dust filaments at the Galactic Center

Context: L'-band (3.8 micron) images of the Galactic Center show a large number of thin filaments in the mini-spiral, located west of the mini-cavity and along the inner edge of the Northern Arm. One possible mechanism that could produce such structures is the interaction of a central wind with the mini-spiral. Additionally, we identify similar features that appear to be associated with stars. Aims: We present the first proper motion measurements of the thin dust filaments observed in the central parsec around SgrA* and investigate possible mechanisms that could be responsible for the observed motions. Methods: The observations have been carried out using the NACO adaptive optics system at the ESO VLT. The images have been transformed to a common coordinate system and features of interest were extracted. Then a cross-correlation technique could be performed in order to determine the offsets between the features with respect to their position in the reference epoch. Results: We derive the proper motions of a number of filaments and 2 cometary shaped dusty sources close (in projection) to SgrA*. We show that the shape and the motion of the filaments does not agree with a purely Keplerian motion of the gas in the potential of the supermassive black hole at the position of SgrA*. Therefore, additional mechanisms must be responsible for their formation and motion. We argue that the properties of the filaments are probably related to an outflow from the disk of young mass-losing stars around SgrA*. In part, the outflow may originate from the black hole itself. We also present some evidence and theoretical considerations that the outflow may be collimated.Comment: accepted for publication by A&

Model Order Reduction for Rotating Electrical Machines

The simulation of electric rotating machines is both computationally expensive and memory intensive. To overcome these costs, model order reduction techniques can be applied. The focus of this contribution is especially on machines that contain non-symmetric components. These are usually introduced during the mass production process and are modeled by small perturbations in the geometry (e.g., eccentricity) or the material parameters. While model order reduction for symmetric machines is clear and does not need special treatment, the non-symmetric setting adds additional challenges. An adaptive strategy based on proper orthogonal decomposition is developed to overcome these difficulties. Equipped with an a posteriori error estimator the obtained solution is certified. Numerical examples are presented to demonstrate the effectiveness of the proposed method

The mean infrared emission of SagittariusA*

(abridged) The massive black hole at the center of the Milky Way, SagittariusA* is, in relative terms, the weakest accreting black hole accessible to observations. At the moment, the mean SED of SgrA* is only known reliably in the radio to mm regimes. The goal of this paper is to provide constraints on the mean emission from SgrA* in the near-to-mid infrared. Excellent imaging quality was reached in the MIR by using speckle imaging combined with holographic image reconstruction, a novel technique for this kind of data. No counterpart of SgrA* is detected at 8.6 microns. At this wavelength, SgrA* is located atop a dust ridge, which considerably complicates the search for a potential point source. An observed 3 sigma upper limit of ~10 mJy is estimated for the emission of SgrA* at 8.6 microns, a tighter limit at this wavelength than in previous work. The de-reddened 3 sigma upper limit, including the uncertainty of the extinction correction, is ~84 mJy . Based on the available data, it is argued that, with currently available instruments, SgrA* cannot be detected in the MIR, not even during flares. At 4.8 and 3.8 microns, on the other hand, SgrA* is detected at all times, at least when considering timescales of a few up to 13 min. We derive well-defined time-averaged, de-reddened flux densities of 3.8+-1.3 mJy at 4.8 microns and 5.0+-0.6 mJy at 3.8 microns. Observations with NIRC2/Keck and NaCo/VLT from the literature provide good evidence that SgrA* also has a fairly well-defined de-reddened mean flux of 0.5-2.5 mJy at wavelengths of 2.1-2.2 microns. We present well-constrained anchor points for the SED of SgrA* on the high-frequency side of the Terahertz peak. The new data are in general agreement with published theoretical SEDs of the mean emission from SgrA*, but we expect them to have an appreciable impact on the model parameters in future theoretical work.Comment: accepted for publication by Astronomy & Astrophysics on 20 June 201

VLT/NACO infrared adaptive optics images of small scale structures in OMC1

International audienceNear-infrared observations of line emission from excited H 2 and in the continuum are reported in the direction of the Orion molecular cloud OMC1 , using the European Southern Observatory Very Large Telescope UT4 , equipped with the NAOS adaptive optics system on the CONICA infrared array camera. Spatial resolution has been achieved at close to the diffraction limit of the telescope (0. 08 −0. 12) and images show a wealth of morphological detail. Structure is not fractal but shows two preferred scale sizes of 2. (1100 AU) and 1. 2 (540 AU) , where the larger scale may be associated with star formation. Key words. ISM : individual objects : OMC1 – ISM : circumstellar matter – ISM : kinematics and dynamics – ISM : molecules – infrared : IS

New Galactic Wolf-Rayet stars, and candidates. An annex to The VIIth Catalogue of Galactic Wolf-Rayet Stars

This paper gathers, from the literature and private communication, 72 new Galactic Population I Wolf-Rayet stars and 17 candidate WCLd stars, recognized and/or discovered after the publication of The VIIth Catalogue of Galactic Wolf-Rayet Stars. This brings the total number of known Galactic Wolf-Rayet stars to 298, of which 24 (8%) are in open cluster Westerlund 1, and 60 (20%) are in open clusters near the Galactic Center.Comment: 10 pages. A&A Research Note, accepte