A Search for Supernova-Remnant Masers Toward Unidentified EGRET Sources

Supernova remnants expanding into adjacent molecular clouds are believed to be sites of cosmic ray acceleration and sources of energetic gamma-rays. Under certain environmental conditions, such interactions also give rise to unusual OH masers in which the 1720 MHz satellite line dominates over the more common 1665/7 MHz emission. Motivated by the apparent coincidence of a handful of EGRET sources with OH(1720 MHz) maser-producing supernova remnants, we have carried out a search using the Very Large Array for new OH(1720 MHz) masers within the error regions of 11 unidentified EGRET sources at low Galactic latitude. While a previously known maser associated with an HII region was serendipitously detected, initial results indicate that no new masers were found down to a limiting flux of, typically, 50 mJy. We discuss the implications of this result on the nature of the unidentified Galactic EGRET sources.Comment: 5 pages, 1 figure. To appear in Proceedings, GAMMA2001 (Baltimore, MD, April 4-6, 2001), eds. N. Gehrels, C. Shrader, and S. Rit

The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability

In this paper we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken 2004 to 2009. We investigate the variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find Sgr A* is continuously emitting and continuously variable in the near-infrared, with some variability occurring on timescales as long as weeks. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low level emission above 5 mJy as flaring emission and of the low level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from August 5th, 2008) which reached an intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2, and 40% brighter than the next brightest flare ever observed from Sgr~A*.Comment: 14 pages, 6 figures, accepted for publication in Ap

Methanol Maser Emission from Galactic Center Sources with Excess 4.5 {\mu}m Emission

We present a study of signatures of on-going star formation in a sample of protostellar objects with enhanced 4.5 {\mu}m emission ('green' sources) near the Galactic center. To understand how star formation in the Galactic center region compares to that of the Galactic disk, we used the Expanded Very Large Array to observe radiatively excited Class II 6.7 GHz CH3OH masers and collisionally excited Class I 44 GHz CH3OH masers, both tracers of high-mass star formation, toward a sample of 34 Galactic center and foreground 'green' sources. We find that 33\pm15% of Galactic center sources are coincident with 6.7 GHz masers, and that 44\pm17% of foreground sources are coincident with 6.7 GHz masers. For 44 GHz masers, we find correlation rates of 27\pm13% and 25\pm13% for Galactic center green sources and foreground green sources, espectively. Based on these CH3OH maser detection rates, as well as correlations of green sources with other tracers of star formation, such as 24 {\mu}m emission and infrared dark clouds (IRDCs), we find no significant difference between the green sources in the Galactic center and those foreground to it. This suggests that once the star formation process has begun, the environmental differences between the Galactic center region and the Galactic disk have little effect on its observational signatures. We do find, however, some evidence that may support a recent episode of star formation in the Galactic center region.Comment: 73 pages, 34 figures, 5 tables. Accepted for publication in Ap

The magnetic environment in the central region of nearby galaxies

The central regions of galaxies harbor some of the most extreme physical phenomena, including dense stellar clusters, non-circular motions of molecular clouds and strong and pervasive magnetic field structures. In particular, radio observations have shown that the central few hundred parsecs of our Galaxy has a striking magnetic field configuration. It is not yet clear whether these magnetic structures are unique to our Milky Way or a common feature of all similar galaxies. Therefore, we report on (a) a new radio polarimetric survey of the central 200 pc of the Galaxy to better characterize the magnetic field structure and (b) a search for large-scale and organized magnetized structure in the nuclear regions of nearby galaxies using data from the Very Large Array (VLA) archive. The high angular resolution of the VLA allows us to study the central 1 kpc of the nearest galaxies to search for magnetized nuclear features similar to what is detected in our own Galactic center. Such magnetic features play a important role in the nuclear regions of galaxies in terms of gas transport and the physical conditions of the interstellar medium in this unusual region of galaxies.Comment: 8 pages; Proceedings for "The Universe under the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be published in Journal of Physics: Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.

Kinematic and structural analysis of the Minispiral in the Galactic Center from BEAR spectro-imagery

Integral field spectroscopy of the inner region of the Galactic Center, over a field of roughly 40"x40" was obtained at 2.06 microns (He I) and 2.16 microns (Brackett-gamma) using BEAR, an imaging Fourier Transform Spectrometer, at spectral resolutions respectively of 52.9 km/s and 21.3 km/s, and a spatial resolution of ~0.5". The analysis of the data was focused on the kinematics of the gas flows, traditionally called the "Minispiral", concentrated in the neighborhood of the central black hole, Sgr A*. From the decomposition into several velocity components of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, and five additional, coherently-moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright, narrow North-South lane seen on intensity images, but it instead consists of a weak, continuous, triangular-shaped surface, drawn out into a narrow stream in the vicinity of Sgr A* where it shows a strong velocity gradient, and a bright western rim. The Eastern Arm is split into three components. We also report extinction of some interstellar structures by others, providing information on their relative position along the line of sight. A system of Keplerian orbits can be fitted to most of the Northern Arm, and the bright rim of this feature can be interpreted in terms of line-of-sight orbit crowding caused by the warping of the flowing surface at the western edge facing Sgr A*. The question of the origin of the ionized gas is addressed and a discussion of the lifetime of these features is presented.Comment: Accepted in A&A, 16 pages, 32 Postscript figures (v. 2: some more discussion about the individual structures, editorial changes

Multiwavelength VLBI observations of Sagittarius A*

The compact radio source Sgr\,A*, associated with the super massive black hole at the center of the Galaxy, has been studied with VLBA observations at 3 frequencies (22, 43, 86\,GHz) performed on 10 consecutive days in May 2007. The total VLBI flux density of Sgr\,A* varies from day to day. The variability is correlated at the 3 observing frequencies with higher variability amplitudes appearing at the higher frequencies. For the modulation indices, we find 8.4\,% at 22\,GHz, 9.3\,% at 43\,GHz, and 15.5\,% at 86\,GHz. The radio spectrum is inverted between 22 and 86\,GHz, suggesting inhomogeneous synchrotron self-absorption with a turnover frequency at or above 86\,GHz. The radio spectral index correlates with the flux density, which is harder (more inverted spectrum) when the source is brighter. The average source size does not appear to be variable over the 10-day observing interval. However, we see a tendency for the sizes of the minor axis to increase with increasing total flux, whereas the major axis remains constant. Towards higher frequencies, the position angle of the elliptical Gaussian increases, indicative of intrinsic structure, which begins to dominate the scatter broadening. At cm-wavelength, the source size varies with wavelength as $\lambda^{2.12\pm0.12}$, which is interpreted as the result of interstellar scatter broadening. After removal of this scatter broadening, the intrinsic source size varies as $\lambda^{1.4 ... 1.5}$. The VLBI closure phases at 22, 43, and 86\,GHz are zero within a few degrees, indicating a symmetric or point-like source structure. In the context of an expanding plasmon model, we obtain an upper limit of the expansion velocity of about 0.1\,c from the non-variable VLBI structure. This agrees with the velocity range derived from the radiation transport modeling of the flares from the radio to NIR wavelengths.}Comment: 14pages, 14 Figures, Accepted for publication in A&

The Mid-Infrared Colors of the ISM and Extended Sources at the Galactic Center

A mid-infrared (3.6-8 um) survey of the Galactic Center has been carried out with the IRAC instrument on the Spitzer Space Telescope. This survey covers the central 2x1.4 degree (~280x200 pc) of the Galaxy. At 3.6 and 4.5 um the emission is dominated by stellar sources, the fainter ones merging into an unresolved background. At 5.8 and 8 um the stellar sources are fainter, and large-scale diffuse emission from the ISM of the Galaxy's central molecular zone becomes prominent. The survey reveals that the 8 to 5.8 um color of the ISM emission is highly uniform across the surveyed region. This uniform color is consistent with a flat extinction law and emission from polycyclic aromatic hydrocarbons (PAHs). Models indicate that this broadband color should not be expected to change if the incident radiation field heating the dust and PAHs is <10^4 times that of the solar neighborhood. The few regions with unusually red emission are areas where the PAHs are underabundant and the radiation field is locally strong enough to heat large dust grains to produce significant 8 um emission. These red regions include compact H II regions, Sgr B1, and wider regions around the Arches and Quintuplet Clusters. In these regions the radiation field is >10^4 times that of the solar neighborhood. Other regions of very red emission indicate cases where thick dust clouds obscure deeply embedded objects or very early stages of star formation.Comment: 37 pages, 15 Postscript figures (low resolution). Accepted for publication in the Ap