Lorant O. Sjouwerman

The VLBA Imaging and Polarimetry Survey at 5 GHz

We present the first results of the VLBA Imaging and Polarimetry Survey (VIPS), a 5 GHz VLBI survey of 1,127 sources with flat radio spectra. Through automated data reduction and imaging routines, we have produced publicly available I, Q, and U images and have detected polarized flux density from 37% of the sources. We have also developed an algorithm to use each sources I image to automatically classify it as a point-like source, a core-jet, a compact symmetric object (CSO) candidate, or a complex source. Using data from the Sloan Digital Sky Survey (SDSS), we have found no significant trend between optical flux and 5 GHz flux density for any of the source categories. Using the velocity width of the H{beta} emission line and the monochromatic luminosity at 5100 to estimate the central black hole mass, M{sub BH}, we have found a weak trend between M{sub BH} and 5 GHz luminosity density for objects with SDSS spectra. Ongoing optical follow-up for all VIPS sources will allow for more detailed explorations of these issues. The mean ratio of the polarized to total 5 GHz flux density for VIPS sources with detected polarized flux density ranges from 1% to 20% with a median value of about 5%. This ratio is a factor of {approx}3 larger if only the jet components of core-jet systems are considered and is noticeably higher for relatively large core-jet systems than for other source types, regardless of which components (i.e., core, jet, or both) are considered. We have also found significant evidence that the directions of the jets in core-jet systems tend to be perpendicular to the electric vector position angles (EVPAs). The data is consistent with a scenario in which {approx}24% of the polarized core-jets have EVPAs that are anti-aligned with the directions of their jet components and which have a substantial amount of Faraday rotation. Follow-up observations at multiple frequencies will address this issue in more detail. In addition to these initial results, plans for future follow-up observations are discussed.

Simultaneous NIR/sub-mm observation of flare emission from Sagittarius A*

Context. We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive (4×10 6 M⊙ ) black hole at the Galactic center. Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*. Methods. Our non-relativistic modeling is based on simultaneous observations that have been carried out on 03 June, 2008. We used the NACO adaptive optics (AO) instrument at the European Southern Observatory’s Very Large Telescope and the LABOCA bolometer at the Atacama Pathfinder Experiment (APEX). We emphasize the importance of a multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare model ing. Results. The observations reveal strong flare activity in the 0.87 mm ( 345 GHz) sub-mm domain and in the 3.8µ/2.2µm NIR. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5±0.5 hours. We explain the flare emission delay by an adiabatic expansion of the source components. The derived physical quantities that describe the flare emission give a source component expansion speed of vexp∼ 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of �=0.8 to 1.8, corresponding to particle spectral indices ∼2.6 to 4.6. At the start of the flare the spectra of these components peak at frequencies of a few THz. Conclusions. These parameters suggest that the adiabatically expanding source components either have a bulk motion greater than vexp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.

CHARACTERISTICS OF EGRET BLAZARS IN THE VLBA IMAGING AND POLARIMETRY SURVEY (VIPS)

We examine the radio properties of EGRET-detected blazars observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly an order of magnitude below the MOJAVE survey and most other samples that have been used to study the properties of EGRET blazars. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux density. We do find that the EGRET-detected blazars tend to have higher brightness temperatures, greater core fractions, and possibly larger than average jet opening angles. A weak correlation is also found with jet length and with polarization. All of the well-established trends can be explained by systematically larger Doppler factors in the gamma-ray-loud blazars, consistent with the measurements of higher apparent velocities found in monitoring programs carried out at radio frequencies above 10 GHz.

86 GHz SiO maser survey of late-type stars in the inner Galaxy : III. Interstellar extinction and colours of the SiO targets

We have determined extinction corrections for a sample of 441 late-type stars in the inner Galaxy, which we previously searched for SiO maser emission, using the 2MASS near-infrared photometry of the surrounding stars. From this, the near-infrared extinction law is found to be approximated by a power law A A λ -1.9±0.1 , Near- and mid-infrared colour-colour properties of known Mira stars are reviewed. From the distribution of the dereddened infrared colours of the SiO target stars we infer mass-loss rates between 10 -7 and 10 -5 M ○. yr -1 .

EXPANDED VERY LARGE ARRAY DETECTION OF 36.2 GHz CLASS I METHANOL MASERS IN SAGITTARIUS A

We report on the interferometric detection of 36.2 GHz Class I methanol emission with the new 27–40 GHz Ka-band receivers available on the Expanded Very Large Array (EVLA). The brightness temperatures of the interferometric 36 GHz detections unambiguously indicate for the first time that the emission is maser emission. The 36 GHz methanol masers are not co-spatial with 1720 MHz OH masers, indicating that the two species trace different shocks. The 36 GHz and 44 GHz methanol masers, which are collisionally pumped, do not necessarily co-exist and may trace different methanol gas. The methanol masers seem correlated with NH3(3,3) density peaks. We favor an explanation in which the 36 GHz Class I methanol masers outline regions of cloud–cloud collisions, perhaps just before the onset of the formation of individual massive stars. The transition of the Very Large Array (VLA) to the EVLA is well under way, and these detections demonstrate the bright future of this completely renewed instrument.

86 GHz SiO maser survey of late-type stars in the Inner Galaxy - I. Observational data

We present 86 GHz (v= 1; J= 2! 1) SiO maser line observations with the IRAM 30-m telescope of a sample of 441 late-type stars in the Inner Galaxy ( 4 < l<+30). These stars were selected on the basis of their infrared magnitudes and colours from the ISOGAL and MSX catalogues. SiO maser emission was detected in 271 sources, and their line-of-sight velocities indicate that the stars are located in the Inner Galaxy. These new detections double the number of line-of-sight velocities available from previous SiO and OH maser observations in the area covered by our survey and are, together with other samples of e.g. OH/IR stars, useful for kinematic studies of the central parts of the Galaxy.

X-ray and Radio Variability of M31*, The Andromeda Galaxy Nuclear Supermassive Black Hole

We confirm our earlier tentative detection of M31* in X-rays and measure its light curve and spectrum. Observations in 2004-2005 find M31* rather quiescent in the X-ray and radio. However, X-ray observations in 2006-2007 show M31* to be highly variable at times. A separate variable X-ray source is found near P1, the brighter of the two optical nuclei. The apparent angular Bondi radius of M31* is the largest of any black hole and large enough to be well resolved with Chandra. The diffuse emission within this Bondi radius is found to have an X-ray temperature ~0.3 keV and density 0.1 cm–3, indistinguishable from the hot gas in the surrounding regions of the bulge given the statistics allowed by the current observations. The X-ray source at the location of M31* is consistent with a point source and a power-law spectrum with energy slope 0.9 ± 0.2. Our identification of this X-ray source with M31* is based solely on positional coincidence.

VERY LARGE ARRAY OBSERVATIONS OF GALACTIC CENTER OH 1720 MHz MASERS IN SAGITTARIUS A EAST AND IN THE CIRCUMNUCLEAR DISK

We present Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. The majority of the newly found 1720 MHz OH masers are located to the northeast, independently indicating and confirming an area of shock interaction with the +50 km s−1 molecular cloud (M–0.02–0.07) on the far side of Sgr A East. The previously known bright masers in the southeast are suggested to be the result of the interaction between two supernova remnants, instead of between Sgr A East and the surrounding molecular clouds, as generally found elsewhere in the Galaxy. Together with masers north of the circumnuclear disk (CND) they outline an interaction on the near side of Sgr A East. In contrast to the interaction between the +50 km s−1 cloud and Sgr A East, OH absorption data do not support a direct interaction between the CND material and Sgr A East. We also present three new high-negative velocity masers, supporting a previous single detection. The location and velocities of the high-negative and high-positive velocity masers are consistent with being near the tangent points of, and physically located in, the CND. We argue that the high-velocity masers in the CND are pumped by dissipation between density clumps in the CND instead of a shock generated by the supernova remnant. That is, the CND masers are not coupled to the supernova remnant and are sustained independently.

18-cm VLA observations of OH towards the Galactic Centre Absorption and emission in the four ground-state OH lines ?

The Galactic Centre harbours the enigmatic central energy source of the Galaxy, which by theoretical and deductive methods now is identified as a massive black hole. Its interactions with the near surroundings, which embody a wide range of objects and phenomena – stars, giant molecular clouds, dust, star formation, star collisions, supernovae, etc. – are the tools to understand the processes going on in the boiling centre of the Galaxy and perhaps in other similar galaxies. The centre of the Galaxy, located at a mere 8 kpc from the Sun, is by far the nearest available nucleus of a galaxy, and a natural candidate for studies of galactic centres. However, the Galactic Centre is visually hidden behind massive obscuring dust clouds, mainly in the vicinity of the Sun, and hence observations are referred to radio-, (sub)millimeter-, IR-, Xrayand γ-ray wavelengths. Many atomic and molecular species have been identified in the Galactic Centre and their physical and kinematical properties can be investigated by spectral line observations – which are the subject of the present work. The Hydroxyl (OH) radical distribution in the Galactic Centre has been studied with the VLA, in its extended hybrid BnA and compact DnC configurations. The four transition lines, 1612-, 1665-, 1667and 1720-MHz, were observed in the BnA configuration, and the main lines, 1665and 1667-MHz, also with the DnC array. The total velocity coverage was about 550 kms –1 , with a velocity resolution of about 9 kms –1 , and angular resolutions of 3 × 4 and 22 × 24 for the BnA and DnC arrays, respectively. The BnA data cover a region of about 2 arcminutes from the Galactic Centre, while the DnC observations extend to a radius of about 20 arcminutes. The focus of this thesis is on the BnA observations, but the thesis also includes preliminary results of the DnC observations at 1665 and 1667 MHz. Both absorption and emission features have been studied. The Molecular Belt containing the +20 and +50 kms –1 clouds, the Circumnuclear Disk (CND), the High Negative Velocity Gas (HNVG), and a new feature which we call the OH-Streamer were identified in absorption at certain velocities. In addition, 45 point-like maser sources were identified in the four lines, 10 of the masers are new detections.

Expanded Very Large Array Detection of 44.1?GHz Class I Methanol Masers in Sagittarius A

We report on the detection of 44 GHz Class I methanol (CH3OH) maser emission in the Sagittarius A (Sgr A) complex with the Expanded Very Large Array (EVLA). These EVLA observations show that the Sgr A complex harbors at least four different tracers of shocked regions in the radio regime. The 44 GHz masers correlate with the positions and velocities of previously detected 36 GHz CH3OH masers, but less with 1720 MHz OH masers. Our detections agree with theoretical predictions that the densities and temperatures conducive for 1720 MHz OH masers may also produce 36 and 44 GHz CH3OH maser emission. However, many 44 GHz masers do not overlap with 36 GHz methanol masers, suggesting that 44 GHz masers also arise in regions too hot and too dense for 36 GHz masers to form. This agrees with the non-detection of 1720 MHz OH masers in the same area, which are thought to be excited under even cooler and less dense conditions. We speculate that the geometry of the 36 GHz masers outlines the current location of a shock front.