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Start Over Subject galactic center Content Level Research
31. Sgr A* 1.3mm VLBI observations with the EHT in 2013
ID:
ivo://CDS.VizieR/J/ApJ/859/60
Title:
Sgr A* 1.3mm VLBI observations with the EHT in 2013
Short Name:
J/ApJ/859/60
Date:
21 Oct 2021
Publisher:
CDS
Description:
We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3mm (230GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional uv coverage in the N-S direction, and leads to a spatial resolution of ~30 {mu}as (~3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of ~4%-13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of ~3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3mm VLBI observations with an expanded array and better sensitivity will allow more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.
32. Spitzer/IRS observations of the Galactic Center
ID:
ivo://CDS.VizieR/J/ApJ/857/59
Title:
Spitzer/IRS observations of the Galactic Center
Short Name:
J/ApJ/857/59
Date:
21 Oct 2021
Publisher:
CDS
Description:
It has long been shown that the extreme ultraviolet spectrum of the ionizing stars of HII regions can be estimated by comparing the observed line emission to detailed models. In the Galactic Center (GC), however, previous observations have shown that the ionizing spectral energy distribution (SED) of the local photon field is strange, producing both very low excitation ionized gas (indicative of ionization by late O stars) and also widespread diffuse emission from atoms too highly ionized to be found in normal HII regions. This paper describes the analysis of all GC spectra taken by Spitzer's Infrared Spectrograph and downloaded from the Spitzer Heritage Archive. In it, HII region densities and abundances are described, and serendipitously discovered candidate planetary nebulae, compact shocks, and candidate young stellar objects are tabulated. Models were computed with Cloudy, using SEDs from Starburst99 plus additional X-rays, and compared to the observed mid-infrared forbidden and recombination lines. The ages inferred from the model fits do not agree with recent proposed star formation sequences (star formation in the GC occurring along streams of gas with density enhancements caused by close encounters with the black hole, Sgr A*), with Sgr B1, Sgr C, and the Arches Cluster being all about the same age, around 4.5Myr old, with similar X-ray requirements. The fits for the Quintuplet Cluster appear to give a younger age, but that could be caused by higher-energy photons from shocks from stellar winds or from a supernova.
33. Spitzer, VLT and Keck NIR light curves of SgrA*
ID:
ivo://CDS.VizieR/J/ApJ/863/15
Title:
Spitzer, VLT and Keck NIR light curves of SgrA*
Short Name:
J/ApJ/863/15
Date:
21 Oct 2021
Publisher:
CDS
Description:
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We present an analysis of the most comprehensive NIR variability data set of Sgr A* to date: eight 24 hr epochs of continuous monitoring of Sgr A* at 4.5{mu}m with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18{mu}m data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12{mu}m data from the NIRC2 camera at the Keck Observatory, in total 94929 measurements. A new approximate Bayesian computation method for fitting the first-order structure function extracts information beyond current fast Fourier transformation (FFT) methods of power spectral density (PSD) estimation. With a combined fit of the data of all three observatories, the characteristic coherence timescale of Sgr A* is {tau}_b_=243_-57_^+82^min (90% credible interval). The PSD has no detectable features on timescales down to 8.5min (95% credible level), which is the ISCO orbital frequency for a dimensionless spin parameter a=0.92. One light curve measured simultaneously at 2.12 and 4.5{mu}m during a low flux-density phase gave a spectral index {alpha}_s_=1.6+/-0.1 (F_{nu}_{propto}{nu}^-{alpha}_s_^). This value implies that the Sgr A* NIR color becomes bluer during higher flux-density phases. The probability densities of flux densities of the combined data sets are best fit by log-normal distributions. Based on these distributions, the Sgr A* spectral energy distribution is consistent with synchrotron radiation from a non-thermal electron population from below 20GHz through the NIR.
34. Ultradeep catalog of X-ray point sources in the GC
ID:
ivo://CDS.VizieR/J/ApJS/235/26
Title:
Ultradeep catalog of X-ray point sources in the GC
Short Name:
J/ApJS/235/26
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present an updated catalog of X-ray point sources in the inner 500" (~20pc) of the Galactic center (GC), where the nuclear star cluster (NSC) stands, based on a total of ~4.5Ms of Chandra observations taken from 1999 September to 2013 April. This ultradeep data set offers unprecedented sensitivity for detecting X-ray sources in the GC, down to an intrinsic 2-10keV luminosity of 1.0x10^31^erg/s. A total of 3619 sources are detected in the 2-8keV band, among which ~3500 are probable GC sources and ~1300 are new identifications. The GC sources collectively account for ~20% of the total 2-8keV flux from the inner 250" region where detection sensitivity is the greatest. Taking advantage of this unprecedented sample of faint X-ray sources that primarily traces the old stellar populations in the NSC, we revisit global source properties, including long-term variability, cumulative spectra, luminosity function, and spatial distribution. Based on the equivalent width and relative strength of the iron lines, we suggest that in addition to the arguably predominant population of magnetic cataclysmic variables (CVs), nonmagnetic CVs contribute substantially to the detected sources, especially in the lower-luminosity group. On the other hand, the X-ray sources have a radial distribution closely following the stellar mass distribution in the NSC, but much flatter than that of the known X-ray transients, which are presumably low-mass X-ray binaries (LMXBs) caught in outburst. This, together with the very modest long-term variability of the detected sources, strongly suggests that quiescent LMXBs are a minor (less than a few percent) population.
35. VLA observations of the Central Molecular Zone
ID:
ivo://CDS.VizieR/J/ApJS/244/35
Title:
VLA observations of the Central Molecular Zone
Short Name:
J/ApJS/244/35
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present new observations of the C-band continuum emission and masers to assess high-mass (>8M_{sun}_) star formation at early evolutionary phases in the inner 200pc of the Central Molecular Zone (CMZ) of the Galaxy. The continuum observation is complete to free-free emission from stars above 10-11M_{sun}_ in 91% of the covered area. We identify 104 compact sources in the continuum emission, among which five are confirmed ultracompact HII regions, 12 are candidates of ultracompact HII regions, and the remaining 87 sources are mostly massive stars in clusters, field stars, evolved stars, pulsars, extragalactic sources, or of unknown nature that is to be investigated. We detect class II CH_3_OH masers at 23 positions, among which six are new detections. We confirm six known H_2_CO masers in two high-mass star-forming regions and detect two new H_2_CO masers toward the Sgr C cloud, making it the ninth region in the Galaxy that contains masers of this type. In spite of these detections, we find that current high-mass star formation in the inner CMZ is only taking place in seven isolated clouds. The results suggest that star formation at early evolutionary phases in the CMZ is about 10 times less efficient than expected from the dense gas star formation relation, which is in line with previous studies that focus on more evolved phases of star formation. This means that if there will be any impending, next burst of star formation in the CMZ, it has not yet begun.
36. VVV survey microlensing events; -3.7<b{<}-3.9{deg}
ID:
ivo://CDS.VizieR/J/ApJ/889/56
Title:
VVV survey microlensing events; -3.7<b{<}-3.9{deg}
Short Name:
J/ApJ/889/56
Date:
17 Jan 2022 11:50:08
Publisher:
CDS
Description:
We search for microlensing events in fields along the Galactic minor axis, ranging from the Galactic center to -3.7{deg}<b<3.9{deg} using the VISTA Variables in the Via Lactea (VVV) survey near-IR photometry. The new search is made across VVV tiles b291, b305, b319, b347, b361, and b375, covering a total area of about 11.5deg^2^. We find a total of N=238 new microlensing events in this new area, N=74, which are classified as bulge red clump (RC) giant sources. Combining them with N=122 events that we had previously reported in the Galactic center (VVV tile b333), allows us to study the latitude distribution of the microlensing events reaching the Galactic plane at b=00 for the first time. We find a very strong dependence of the number of microlensing events with Galactic latitude, a number that increases rapidly toward the Galactic center by one order of magnitude from |b|=2{deg} to b=0{deg} with a much steeper gradient than with Galactic longitude. The microlensing event population shows a flattened distribution (axial ratio b/a~1.5). The final sample shows a shorter mean timescale distribution than the Galactic plane sample for both the complete population and RC stars.
37. 16yrs of radial velocity measurements of S0-2
ID:
ivo://CDS.VizieR/J/ApJ/854/12
Title:
16yrs of radial velocity measurements of S0-2
Short Name:
J/ApJ/854/12
Date:
21 Oct 2021
Publisher:
CDS
Description:
The star S0-2, which orbits the supermassive black hole (SMBH) in our Galaxy with a period of 16 years, provides the strongest constraint on both the mass of the SMBH and the distance to the Galactic center. S0-2 will soon provide the first measurement of relativistic effects near a SMBH. We report the first limits on the binarity of S0-2 from radial velocity (RV) monitoring, which has implications for both understanding its origin and robustness as a probe of the central gravitational field. With 87 RV measurements, which include 12 new observations that we present, we have the requisite data set to look for RV variations from S0-2's orbital model. Using a Lomb-Scargle analysis and orbit- fitting for potential binaries, we detect no RV variation beyond S0-2's orbital motion and do not find any significant periodic signal. The lack of a binary companion does not currently distinguish different formation scenarios for S0-2. The upper limit on the mass of a companion star (M_comp_) still allowed by our results has a median upper limit of M_comp_ sin i <=1.6M_{sun}_ for periods between 1 and 150 days, the longest period to avoid tidal break-up of the binary. We also investigate the impact of the remaining allowed binary system on the measurement of the relativistic redshift at S0-2's closest approach in 2018. While binary star systems are important to consider for this experiment, we find that plausible binaries for S0-2 will not alter a 5{sigma} detection of the relativistic redshift.