17 research outputs found
Galactic Center Minispiral: Interaction Modes of Neutron Stars
Streams of gas and dust in the inner parsec of the Galactic center form a
distinct feature known as the Minispiral, which has been studied in radio
waveband as well as in the infrared wavebands. A large fraction of the
Minispiral gas is ionized by radiation of OB stars present in the Nuclear Star
Cluster (NSC). Based on the inferred mass in the innermost parsec (
solar masses), over -- neutron stars should move in the
sphere of gravitational influence of the SMBH. We estimate that a fraction of
them propagate through the denser, ionized medium concentrated mainly along the
three arms of the Minispiral. Based on the properties of the gaseous medium, we
discuss different interaction regimes of magnetised neutron stars passing
through this region. Moreover, we sketch expected observational effects of
these regimes. The simulation results may be applied to other galactic nuclei
hosting NSC, where the expected distribution of the interaction regimes is
different across different galaxy types.Comment: 12 pages, 17 figures, published in Acta Polytechnic
Galactic Centre observations with local mm-telescope arrays
Radio interferometry observations are a powerful tool to study the central regions of the Galactic centre (GC). High angular resolution observations at millimetre wavelengths can disentangle the emission of Sgr A* from the thermal emission of the circum nuclear disk (CND) and the mini-spiral surrounding it. In this thesis I present the results of radio interferometric observations of the GC region with local millimetre-telescope arrays such as CARMA and ATCA, supplemented by observations in the infrared (IR) with ESO's VLT telescopes. My goal is to analyse the emission mechanisms present in the central few parsecs of the GC region, in particular the extended thermal emission of the ionized gas and dust of the mini-spiral region and the non-thermal emission of Sagittarius A* (Sgr A*), the radio source associated with the supermassive black hole (SMBH) at the GC. Observations were carried out in March and April 2009 at 1.3 and 3 mm with the mm telescope array CARMA in California, in June 2006 with the mid-infrared (MIR) instrument VISIR at ESO's VLT, and in the NIR Br_gamma in August 2009 with VLT NACO. I present high angular resolution continuum maps of the GC at 3 and 1.3 mm and the highest resolution spectral index map obtained at these wavelengths. I obtain a spectral index of 0.5 for Sgr A*, indicating an inverted synchrotron spectrum and a mixture of negative and positive values in the extended emission of the minispiral, including the thermal free-free emission from the ionized gas, and a possible contribution of dust at 1.3 mm. I infer the physical properties of the dust and gas in the mini-spiral by comparing the radio continuum maps to the MIR continuum map, and the Br_gamma line emission map in the NIR. An extinction map at NIR wavelengths for the mini-spiral region, showing extinctions ranging from 1.8-3 magnitudes, was also produced. To study the flaring activity of Sgr A*, global coordinated multiwavelength campaigns were carried out in 2007 and 2008 using ESO's VLT and the mm telescope arrays CARMA, ATCA, and the 30 m IRAM telescope in Spain, which detected four flares in the NIR, of which three were covered later by the mm data. I develop a new method for obtaining concatenated light curves of the compact mm-source Sgr A* from single dish telescopes and interferometers in the presence of significant flux density contributions from an extended and only partially resolved source, and model the observed flares in the NIR and mm using an adiabatic expansion model involving synchrotron source components. I derive physical quantities such as expansion velocities ranging from 0.005c-0.017c, source sizes of about one Schwarzschild radius, turnover frequencies of a few THz, flux densities of a few Janskys, and spectral indices of 0.6 to 1.3. These parameters suggest either a bulk motion of the adiabatically expanding source components greater than the expansion velocity or a confinement of expanding material within a corona or disk in the immediate surroundings of Sgr A*
Multiple accretion events as a trigger for Sgr A* activity
Gas clouds are present in the Galactic centre, where they orbit around the
supermassive black hole. Collisions between these clumps reduce their angular
momentum, and as a result some of the clumps are set on a plunging trajectory.
Constraints can be imposed on the nature of past accretion events based on the
currently observed X-ray reflection from the molecular clouds. We discuss
accretion of clouds in the context of enhanced activity of Sgr A* during the
past few hundred years. We put forward a scenario according to which gas clouds
bring material close to the horizon of the black hole on <~0.1 parsec scale. We
have modelled the source intrinsic luminosity assuming that multiple events
occur at various moments in time. These events are characterized by the amount
of accreted material and the distribution of angular momentum. We parameterized
the activity in the form of a sequence of discrete events, followed the viscous
evolution, and calculated the luminosity of the system from the time-dependent
accretion rate across the inner boundary. Accreting clumps settle near a
circularization radius, spread there during the viscous time, and subsequently
feed the black hole over a certain period. A significant enhancement (by factor
of ten) of the luminosity is only expected if the viscous timescale of the
inflow is very short. On the other hand, the increase in source activity is
expected to be much less prominent if the latter timescale is longer and a
considerable fraction of the material does not reach the centre. A solution is
obtained under two additional assumptions: (i) the radiative efficiency is a
decreasing function of the Eddington ratio; (ii) the viscous decay of the
luminosity proceeds somewhat faster than the canonical L(t)~t^{-5/3} profile.
We applied our scheme to the case of G2 cloud in the Galactic centre to obtain
constraints on the core-less gaseous cloud model.Comment: Astronomy and Astrophysics accepte
OBLIQUE MAGNETIC FIELDS AND THE ROLE OF FRAME DRAGGING NEAR ROTATING BLACK HOLE
Magnetic null points can develop near the ergosphere boundary of a rotating black hole by the combined effects of strong gravitational field and the frame-dragging mechanism. The induced electric component does not vanish in the magnetic null and an efficient process of particle acceleration can occur in its immediate vicinity. Furthermore, the effect of imposed (weak) magnetic field can trigger an onset of chaos in the motion of electrically charged particles. The model set-up appears to be relevant for low-accretion-rate nuclei of some galaxies which exhibit episodic accretion events (such as the Milky Way's supermassive black hole) embedded in a large-scale magnetic field of external origin with respect to the central black hole. In this contribution we summarise recent results and we give an outlook for future work with the focus on the role of gravito-magnetic effects caused by rotation of the black hole
Molecular gas in the immediate vicinity of Sgr A* seen with ALMA
We report serendipitous detections of line emission with ALMA in band 3, 6,
and 7 in the central parsec of the Galactic center at an up to now highest
resolution (<0.7''). Among the highlights are the very first and highly
resolved images of sub-mm molecular emission of CS, H13CO+, HC3N, SiO, SO, C2H,
and CH3OH in the immediate vicinity (~1'' in projection) of Sgr A* and in the
circumnuclear disk (CND). The central association (CA) of molecular clouds
shows three times higher CS/X (X: any other observed molecule) luminosity
ratios than the CND suggesting a combination of higher excitation - by a
temperature gradient and/or IR-pumping - and abundance enhancement due to UV-
and/or X-ray emission. We conclude that the CA is closer to the center than the
CND is and could be an infalling clump consisting of denser cloud cores
embedded in diffuse gas. Moreover, we identified further regions in and outside
the CND that are ideally suited for future studies in the scope of hot/cold
core and extreme PDR/XDR chemistry and consequent star formation in the central
few parsecs
Data Combination: Interferometry and Single-dish Imaging in Radio Astronomy
Modern interferometers routinely provide radio-astronomical images down to
subarcsecond resolution. However, interferometers filter out spatial scales
larger than those sampled by the shortest baselines, which affects the
measurement of both spatial and spectral features. Complementary single-dish
data are vital for recovering the true flux distribution of spatially resolved
astronomical sources with such extended emission. In this work, we provide an
overview of the prominent available methods to combine single-dish and
interferometric observations. We test each of these methods in the framework of
the CASA data analysis software package on both synthetic continuum and
observed spectral data sets. We develop a set of new assessment tools that are
generally applicable to all radio-astronomical cases of data combination.
Applying these new assessment diagnostics, we evaluate the methods' performance
and demonstrate the significant improvement of the combined results in
comparison to purely interferometric reductions. We provide combination and
assessment scripts as add-on material. Our results highlight the advantage of
using data combination to ensure high-quality science images of spatially
resolved objects.Comment: 29 pages, 20 figures. Accepted for publication in PASP. Code
repository available at: github.com/teuben/DataCom
The ALMA Interferometric Pipeline Heuristics
We describe the calibration and imaging heuristics developed and deployed in
the ALMA interferometric data processing pipeline, as of ALMA Cycle 9. The
pipeline software framework is written in Python, with each data reduction
stage layered on top of tasks and toolkit functions provided by the Common
Astronomy Software Applications package. This framework supports a variety of
tasks for observatory operations, including science data quality assurance,
observing mode commissioning, and user reprocessing. It supports ALMA and VLA
interferometric data along with ALMA and NRO45m single dish data, via different
stages and heuristics. In addition to producing calibration tables, calibrated
measurement sets, and cleaned images, the pipeline creates a WebLog which
serves as the primary interface for verifying the data quality assurance by the
observatory and for examining the contents of the data by the user. Following
the adoption of the pipeline by ALMA Operations in 2014, the heuristics have
been refined through annual development cycles, culminating in a new pipeline
release aligned with the start of each ALMA Cycle of observations. Initial
development focused on basic calibration and flagging heuristics (Cycles 2-3),
followed by imaging heuristics (Cycles 4-5), refinement of the flagging and
imaging heuristics with parallel processing (Cycles 6-7), addition of the
moment difference analysis to improve continuum channel identification (2020
release), addition of a spectral renormalization stage (Cycle 8), and
improvement in low SNR calibration heuristics (Cycle 9). In the two most recent
Cycles, 97% of ALMA datasets were calibrated and imaged with the pipeline,
ensuring long-term automated reproducibility. We conclude with a brief
description of plans for future additions, including self-calibration,
multi-configuration imaging, and calibration and imaging of full polarization
data.Comment: accepted for publication by Publications of the Astronomical Society
of the Pacific, 65 pages, 20 figures, 10 tables, 2 appendice