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Register a new userL- and M-band imaging observations of the Galactic Center region
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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*.
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We present the largest currently existing subarcsecond 3-5 $mu$m atlas of 119 local ($z < 0.3$) active galactic nuclei (AGN). This atlas includes AGN of 5 subtypes: 22 are Seyfert 1; 5 are intermediate Seyferts; 46 are Seyfert 2; 26 are LINERs; and 20 are composites/starbursts. Each AGN was observed with VLT ISAAC in the $L$- and/or $M$-bands between 2000 and 2013. We detect at 3$sigma$ confidence 92 sources in the $L$-band and 83 sources in the $M$-band. We separate the flux into unresolved nuclear flux and resolved flux through two-Gaussian fitting. We report the nuclear flux, extended flux, apparent size, and position angle of each source, giving $3sigma$ upper-limits for sources which are undetected. Using WISE W1- and W2-band photometry we derive relations predicting the nuclear $L$ and $M$ fluxes for Sy1 and Sy2 AGN based on their W1-W2 color and WISE fluxes. Lastly, we compare the measured mid-infrared colors to those predicted by dusty torus models SKIRTOR, CLUMPY, CAT3D, and CAT3D-WIND, finding best agreement with the latter. We find that models including polar winds best reproduce the 3-5$mu$m colors, indicating that winds are an important component of dusty torus models. We find that several AGN are bluer than models predict. We discuss several explanations for this and find that it is most plausibly stellar light contamination within the ISAAC $L$-band nuclear fluxes.
We report on Galactic Center L-band observations made during NAOS/CONICA Science Verification. Colors of the inner 2 stars reveal an infrared excess of S2, the closest star to the black hole, that could sign the first thermal infrared detection of Sgr A*. A multi-wavelength maximum likelihood analysis has allowed us to eliminate all but two of the candidates for gravitational lensing proposed by Alexander (2001). Our observations of the thin and intersecting structures of the Northern Arm could trace several shocks heating the neighbooring dust rather than a stream of matter in orbit around the central mass as previously thought.
We present a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz. With a resolution of ~ 7 X 12 and an RMS noise of 1.6 mJy/beam, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency. The improved sensitivity has more than tripled the census of small diameter sources in the region, has resulted in the detection of two new Non Thermal Filaments (NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known extended sources in greater detail, and has resulted in the first detection of Sagittarius A* in this frequency range. A version of this paper containing full resolution images may be found at http://lwa.nrl.navy.mil/nord/AAAB.pdf.
The Galactic Center (GC) region hosts a variety of powerful astronomical sources and rare astrophysical processes that emit a large flux of non-thermal radiation. The inner 375 pc x 600 pc region, called the Central Molecular Zone, is home to the supermassive black hole Sagittarius A*, massive cloud complexes, and particle accelerators such as supernova remnants. We present the results of our improved analysis of the very-high-energy (VHE) gamma-ray emission above 2 TeV from the GC using 125 hours of data taken with the VERITAS imaging-atmospheric Cherenkov telescope between 2010 and 2018. The central source VER J1745-290, consistent with the position of Sagittarius A*, is detected at a significance of 38 standard deviations above the background level $(38sigma)$, and we report its spectrum and light curve. Its differential spectrum is consistent with a power law with exponential cutoff, with a spectral index of $2.12^{+0.22}_{-0.17}$, a flux normalization at 5.3 TeV of $1.27^{+0.22}_{-0.23}times 10^{-13}$ TeV-1 cm-2 s-1, and cutoff energy of $10.0^{+4.0}_{-2.0}$ TeV. We also present results on the diffuse emission near the GC, obtained by combining data from multiple regions along the GC ridge which yield a cumulative significance of $9.5sigma$. The diffuse GC ridge spectrum is best fit by a power law with a hard index of 2.19 $pm$ 0.20, showing no evidence of a cutoff up to 40 TeV. This strengthens the evidence for a potential accelerator of PeV cosmic rays being present in the GC. We also provide spectra of the other sources in our field of view with significant detections, composite supernova remnant G0.9+0.1 and HESS J1746-285.
We review the results obtained with the Galactic center campaigns of the BeppoSAX Wide Field X-ray Cameras (WFCs). This pertains to the study of luminous low-mass X-ray binaries (LMXBs). When pointed at the Galactic center, the WFC field of view contains more than half of the Galactic LMXB population. The results exemplify the excellent WFC capability to detect brief X-ray transients. Firstly, the WFCs expanded the known population of Galactic thermonuclear X-ray bursters by 50%. At least half of all LMXBs are now established to burst and, thus, to contain a neutron star as compact accretor rather than a black hole candidate. We provide a complete list of all 76 currently known bursters, including the new case 1RXS J170854.4-321857. Secondly, the WFCs have uncovered a population of weak transients with peak luminosities up to ~10^37 erg/s and durations from days to weeks. One is the first accretion-powered millisecond pulsar SAX J1808.4-3658. Thirdly, the WFCs contributed considerably towards establishing that nearly all (12 out of 13) luminous low-mass X-ray binaries in Galactic globular clusters contain neutron stars rather than black holes. Thus, the neutron star to black hole ratio in clusters differs from that in the Galactic disk at a marginal confidence level of 97%.
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