Artikel zu "MWG — Central 200 Parsecs (v)"
MIV: The Galactic Center
Distance GC
MIV: Our Milky Way Galaxy
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Zentrum der Milchstraße
K1 
IRS 13E —
Zweites Schwarzes Loch im Zentrum der Milchstraße entdeckt
Kosmischer Moloch
Das Schwarze Loch im
Mittelpunkt der Milchstraße hat einen kleinen Bruder. Astronomen haben im
Zentrum unserer Galaxie Hinweise auf einen zweiten kosmischen Vielfraß gefunden,
der sieben Sterne gefangen hält.
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 Gemini Observatory
Mittelgroßes Schwarzes Loch im Zentrum der
Milchstraße: Kleiner Bruder von Sagittarius A |
Mit weiteren Daten der Weltraumteleskope Hubble und Chandra berechneten Wissenschaftler aus der einheitlichen Bewegung der Sterne, dass sie um ein mittelschweres Schwarzes Loch kreisen. Der neu entdeckte Vielfraß ist vergleichsweise klein: Er besitzt nur ein Zweitausendstel der Masse von Sagittarius A, des supermassiven Schwarzen Lochs im Mittelpunkt der Milchstraße, schreiben Jean-Pierre Maillard vom Pariser Institut für Astrophysik und seine Kollegen im Fachblatt "Astronomy and Astrophysics".
"Es ist das erste Schwarze Loch von mittlerer Masse, das in unserer Galaxie entdeckt wurde", sagte Maillard dem Online-Nachrichtendienst des Wissenschaftsmagazins "Nature". Der kosmische Moloch hat die 1300-fache Masse der Sonne und bewegt sich spiralförmig mit 280 Kilometern pro Sekunde auf seinen gewaltigen Verwandten Sagittarius A zu. Derzeit beträgt die Distanz zwischen ihnen rund drei Lichtjahre.
Wissenschaftler hatten bereits zuvor beobachtet, dass IRS 13 starke Röntgenstrahlen aussendet - ein verräterischer Hinweis auf ein Schwarzes Loch. Die sieben Sterne könnten die Reste eines großen Sternenhaufens sein, der von Sagittarius A in Stücke gerissen wurde, sagte Maillard. Die Beobachtung könnte auch ein neues Indiz dafür sein, dass große Schwarze Löcher ihre kleineren Verwandten auffressen und dabei selbst an Masse zulegen.
Erst vor drei Jahren hatten Forscher bestätigt, dass sich im Zentrum der Milchstraße mit Sagittarius A ein Schwarzes Loch befindet, das 2,6 Millionen Mal massereicher ist als die Sonne. Schwarze Löcher lassen sich nur indirekt beobachten, etwa anhand der Umlaufbahn von Sternen. Ihren Namen haben sie aufgrund ihrer gewaltigen Anziehungskraft: Sie ist so stark, dass sogar Licht geschluckt wird.
K1.2
Gemini Observes Remains of Massive Star Cluster Near Milky Way's Galactic Center
Image credit: Gemini Observatory
40” x 40” Gemini Hokupa’a/QUIRC adaptive optics image of region around Milky Way
galactic center. Expanded view shows IRS 13E and W from deconvolved Kp band
image. Coordinates are in arcseconds offset from SgrA*.
Image credit: Gemini Observatory
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The team also used data from Hubble Space Telescope, the Chandra X-Ray Observatory, the Canada-France-Hawai’i Telescope (CFHT), and the Very Large Array to complement the Gemini data. The Gemini observations consisted of deconvolved H and Kp band images that identified the existence of two formerly undetected sources within the eastern component of IRS 13 (called IRS 13E). When the Gemini data were used in conjunction with longer and shorter wavelength observations, it was possible to conclude that these sources were massive stars comparable to the stars of the same type which are exceptionally concentrated in the vicinity of the main black hole at the galactic center (which has a mass of about four million solar masses).
In all, seven individual stars of IRS 13E were seen within a diameter of about 0.5" (corresponding to 0.6 light-year across). These stars are co-moving westward in the plane of the sky with a similar velocity of about 280 kilometers per second.
The compactness of the cluster and the common proper motion of the components suggest that they are kept together by a massive source, which is thought to be an intermediate mass stellar black hole at the center of IRS 13E. The size of the cluster allowed the team to infer a mean orbit radius for the cluster and radial velocities of individual stars. These velocities were derived from the BEAR Fourier Transform Spectrometer (CFHT) which revealed an estimated average orbital velocity of ± 30 kilometers per second. The authors then explored a range of orbital assumptions and were able to rather robustly constrain the mass of the holding black hole to about 1,300 solar masses.
The team also speculates that this cluster was once located farther from the galactic center, where the stars could form away from the extreme gravitational influence of the central supermassive black hole. IRS 13E seems to be the wreckage, or remnant core, of a once larger cluster of stars that is now spiraling towards Sgr A* at the galactic center.
This theory also explains the existence of other massive stars around the galactic center. It is thought that these stars were stripped from the cluster due to the gravitational environment around our galaxy’s central supermassive black hole.
K1.3 Kleiner Bruder für Sagittarius A*
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Zweites Schwarzes Loch, IRS 13E, in der Milchstraße entdeckt.
Ein französisch-amerikanisches Forscherteam hat in der Milchstraße Hinweise
auf ein zweites Schwarzes Loch entdeckt. Es ist zweitausendmal leichter
als Sagittarius A*, das bisher bekannte erste Schwarze Loch, um das unsere
Milchstraße kreist. Das Himmelsobjekt mit dem Namen IRS 13E ist offenbar
das Zentrum einer Gruppe von sieben Sternen, berichtet der Online-Dienst der Fachzeitschrift Nature.
Masse: 1.3·103M
Es wiegt das 1300fache der Sonne und bewegt sich spiralförmig mit 280 Kilometern pro Sekunde auf seinen gewaltigen Verwandten Sagittarius A* zu. Wissenschaftler hatten bereits beobachtet, dass IRS 13 starke Röntgenstrahlen aussendet – ein verräterischer Hinweis auf ein Schwarzes Loch. Da ist in der Galaxie mehrere weitere Quellen gibt, die Röntgenstrahlen aussenden, verstecken sich dahinter möglicherweise weitere kleine Schwarze Löcher. Diese Theorie ist bisher aber nicht bewiesen.
Erst vor drei Jahren hatten Forscher bestätigt, dass sich im Zentrum unserer Galaxie das supermassereiche Schwarze Loch Sagittarius A* befindet. Es ist 2,6 Millionen Mal schwerer als die Sonne. Schwarze Löcher lassen sich nur indirekt beobachten, etwa anhand der Umlaufbahn von Sternen. Ihren Namen haben sie aufgrund ihrer gewaltigen Anziehungskraft: Sie ist so stark, dass sogar Licht geschluckt wird.
K1.4 A BH in the Galactic Center Complex IRS 13E?
K2
The Radio Arc Bubble
Zum Thema: Galactic Center | |
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K3
Star Clusters
Zum Thema | |
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The Arches Cluster
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Authors: D.F. Figer, F. Najarro, D. Gilmore, M. Morris, S.S. Kim, E. Serabyn, I.S. McLean, A.M. Gilbert, J.R. Graham, J.E. Larkin, N.A. Levenson, H.I. Teplitz |
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Journal-ref: ApJ 581 (2002) 258-275 [astro-ph/0208145  ] |
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Title: Massive Stars in the Arches Cluster |
| Abstract:
We present and use new spectra and narrow-band images, along with previously
published broad-band images, of stars in the Arches cluster to extract
photometry, astrometry, equivalent width, and velocity information. The data
are interpreted with a wind/atmosphere code to determine stellar temperatures,
luminosities, mass-loss rates, and abundances.
We have doubled the number of known emission-line stars, and we have also made the first spectroscopic identification of the main sequence for any population in the Galactic Center. We conclude that the most massive stars are bona-fide Wolf-Rayet (WR) stars and are some of the most massive stars known, having Mini > 100 M , and
prodigious winds,
M' > 10-5 M yr-1,
that are enriched with helium
and nitrogen; with these identifications, the Arches cluster contains about 5%
of all known WR stars in the Galaxy.
We find an upper limit to the velocity dispersion of 22 km s-1, implying an upper limit to the cluster mass of 7 × 4 M within a radius of 0.23 pc;
we also estimate the bulk heliocentric
velocity of the cluster to be
vcluster, approximately +95 km s-1.
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K3.2
The H.E.S.S. survey of the inner Galactic plane
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Authors: S. Hoppe for the H.E.S.S. Collaboration |
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Journal-ref: ICRC (2007) [0710.3528 ] |
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Title: The H.E.S.S. survey of the inner Galactic plane |
| Abstract:
The High Energy Stereoscopic System (H.E.S.S.), located in the Khomas Highlands of
Namibia, is an array of four imaging atmospheric-Cherenkov telescopes designed
to detect gamma-rays in the very high energy (VHE; > 100 GeV) domain. Its
high sensitivity and large field-of-view (5 deg) make it an ideal instrument
to perform a survey within the Galactic plane for new VHE sources.
Previous observations in 2004/2005 resulted in numerous detections of VHE gamma-ray emitters in the region l = 330° - 30° Galactic longitude. Recently the survey was extended, covering the regions l = 280° - 330° and l = 30° - 60°, leading to the discovery of several previously unknown sources with high statistical significance. The current status of the survey will be presented. | |
K5.1
The H.E.S.S. View of the Central 200 Parsecs
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Authors: Jim Hinton (for the H.E.S.S. Collaboration) | ||||||||||
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Journal-ref: Journal of Physics: Conference Series 54 (2006) 140 [astro-ph/0607351 ] | ||||||||||
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Title: The H.E.S.S. View of the Central 200 Parsecs | ||||||||||
INTRODUCTION
The central 200 pc of our Galaxy is a unique region that harbours many remarkable objects
— including several potential sites of effective particle acceleration. Non-thermal emission
(particularly in the radio, X-ray and g-ray bands) can be used to trace the energetic particle
populations of this region. X-ray and radio observations of synchrotron emission provide
information on the product of the local magnetic field energy density and the density of
relativistic electrons. The flux of inverse Compton g-rays, on the other hand, is proportional
to the radiation field density (and the electron density). The combination of
-ray and X-ray
measurements therefore provides a powerful tool for probing both magnetic field strength and
energetic particle content. Moreover, g-rays provide an effective tracer for hadronic particles:
proton-proton interactions in the interstellar medium lead to the production and decay of pions
and hence g-ray-ray production. The combination of g-ray measurements with tracers of atomic
and molecular material may be the only way to effectively trace energetic hadrons in our galaxy.
The H.E.S.S. array was completed in late 2003 and 50 hours of full sensitivity observations of the GC took place in 2004. The upper panel of Fig. 1 shows a smoothed count map of the central four degrees of our galaxy from these 2004 data [2]. Two VHE g-ray sources are clearly apparent: one coincident with the SNR G0.9+0.1 and the unidentified source HESS J1745-290 in the Sgr A region. The central VHE gamma-ray source: HESS J1745-290
The updated photon index is G = 2.29 [10]. No significant variability is found on time-scales of days or hours in the 2004 dataset. Within the error circle of the H.E.S.S measurement lie three compelling candidates for the origin of the VHE emission: the shell-type supernova remnant (SNR) Sgr A East (see e.g. [11]), the newly discovered pulsar wind nebula G359.95-0.04 [12] and the supermassive black hole Sgr A* itself (see e.g. [13]). Several radiation mechanisms have been discussed for these three objects. Plausible mechanisms include inverse Compton scattering of energetic electrons, the decay of pions produced in the interactions of energetic hadrons with the interstellar medium or dense radiation fields and finally curvature radiation of ultra-high energy protons close to Sgr A*. For some production scenarios, correlated variability is expected in X-rays and g-rays. Simultaneous observations of the Sgr A region with H.E.S.S. and Chandra took place in July 2005 and may help to clarify this situation. A widely discussed alternative to these astrophysical origins is the annihilation of dark matter in the central cusp in the density profile of our galaxy. However, the hard power-law spectrum of the central source is hard to reconcile with a dark matter interpretation (see [14] for a detailed discussion). Diffuse emission from the Central Molecular Zone
The dense gas clouds of the central molecular zone provide effective targets for the production
of g-rays via the interactions of hadrons. Indeed, the recent H.E.S.S. observations
reveal TeV emission correlated with these clouds (see the lower panel of Fig. 1). This is the first case
in which such a correlation has been seen at these energies [2].
The Pulsar Wind Nebula in G0.9+0.1
G0.9+0.1 is a composite radio SNR with a clear ‘core plus shell’ morphology (see for
example [17]). Observations with Chandra [18] and XMM [19] revealed hard X-ray emission from
the core and weak (likely thermal) emission from the SNR shell. The characteristic softening of
the X-ray spectrum with distance from the core indicates the likely plerionic nature of this source,
although no radio pulsar has been identified (likely due to the large dispersion associated with
the GC region). VHE g-ray emission coincident with G0.9+0.1 was discovered by
the H.E.S.S. collaboration in 2005 [20]. Fig. 4 shows the position of the H.E.S.S. source in comparison to
radio measurements. The VHE emission is unresolved by H.E.S.S. and an upper limit of 1.3'
on the rms size of the emission region has been derived.
Pulsar wind nebulae (PWN) are prominent members of the VHE g-ray source catalogue. Recent g-ray observations with H.E.S.S. of known X-ray PWN such as Vela-X [21] and MSH15-52 [22] show close correlations between X-ray and VHE g-ray emission. Combined X-ray/g-ray observations provide information on the magnetic field in the nebula and on the spatial distribution of electrons. For the distant PWN of G0.9+0.1 the emission is currently unresolvable in g-rays but the average magnetic field in the nebula can be estimated at ~ 6µG [20]. Unidentified Source HESS J1745-303
The H.E.S.S. survey of the inner galaxy [23], combined with deep exposures on targets such as
the GC and RXJ1713.7-3946 [24] has resulted in the detection of many
extended (~ 10'') g-ray
sources without clear counterparts at other wavelengths. One such source is HESS J1745-303,
located ~ 1° from Sgr A. Fig. 5 shows a count map of this source, illustrating the positions of
nearby supernova remnants and energetic pulsars. The most compelling potential counterpart
is the (also unidentified) lower energy g-ray source 3EG1744-3011.
A fit of a elliptical gaussian
source profile yields a position of l = 358.71±0.04°, b = -0.64±0.05° and an rms source size of
5'×13'. Deeper observations in all wavebands are clearly desirable to help identify this MeV-TeV
g-ray source.
References | |||||||||||
K5.2
Swift/XRT follow-up observations of TeV sources
| HESS J1614–518 — HESS J1640–465 — HESS J1804–216 — HESS J1813–178 — HESS J1834–087 — HESS J1837–069 | |
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Authors: R. Landi, L. Bassani, A. Malizia, N. Masetti, J.B. Stephen, A. Bazzano, P. Ubertini, A.J. Bird, A.J. Dean |
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Journal-ref: ApJ 651 (2006) 190 [astro-ph/0607354 ] |
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Title: Swift/XRT follow-up observations of TeV sources of the HESS Inner Galaxy survey |
| Abstract: In order to provide a firm identification of the newly discovered
Galactic TeV sources, a search for counterparts in a broad band from soft
X-ray to soft gamma-rays is crucial as data in these wavebands allow us to
distinguish between different types of suggested models (for example leptonic
versus hadronic) and, in turn, to disentangle their nature.
In this paper, we report the results of a set of follow-up observations performed by the Swift/X-Ray Telescope (XRT) on seven sources recently discovered by HESS, in the range from few hundred GeV to about 10 TeV, during the inner Galaxy survey (Aharonian et al. 2006). In all, but one case, we detect X-ray sources inside or close-by the extended TeV emitting region. All these putative X-ray counterparts have accurate arc-second location and are consistent with being point sources. The main result of our search is the discovery that three of them are located at the center of the diffuse radio emission of the supernova remnants, which have been spatially associated to these TeV objects. HESS J1640-465, HESS J1834-087 and HESS J1813-178 show this evidence, suggestive of a possible Pulsar Wind Nebula association. 1. Introduction
The HESS (High Energy Stereoscopic System) collaboration has recently reported results
of a first sensitive survey of the inner part of our galaxy in very high energy gamma-rays
(Aharonian et al. 2005, 2006).
This survey has revealed the existence of a population of
fourteen TeV objects, most of which were previously unknown. These findings have important
astrophysical implications for our understanding of cosmic particle accelerators via TeV
measurements. Various types of sources in the galaxy can act as cosmic accelerators: pulsars
and their pulsar wind nebulae (PWN), supernova remnants (SNR), star forming regions and
possibly binary systems containing a collapsed object such as a microquasar or a pulsar. Indeed,
four of these new TeV sources (HESS J1640–465, HESS J1713–381, HESS J1813–178
and HESS J1834–087) are spatially coincident with SNRs, which have been mapped and
studied at radio frequencies (Green et al. 1999; Green 2004; Brogan et al. 2005; Helfand et
al. 2005); in the first three cases X-ray radiation associated with the radio emission has also
been reported (Sugizaki et al. 2001; Aharonian et al. 2006; Ubertini et al. 2005), while in
the last case no X-ray data has so far been published. Three more sources, HESS J1616–509,
HESS J1804–216 and HESS J1825–137, could be associated with energetic nearby pulsars,
but their spatial offset from the TeV emitting region is too large to make the association
secure. Other three objects are consistent with the 95% positional error contour of unidentified
EGRET sources (HESS J1640–465, HESS J1745–303 and HESS J1825–137) (Hartman
et al. 1999), while two are located very close to a new class of highly absorbed hard X-ray
binaries recently detected by INTEGRAL (HESS J1632–478 and HESS J1634–472, Walter
et al. 2006; Lutovinov et al. 2006). Another source (HESS J1837–069) is related to X-ray
emission of unknown origin detected by ASCA and BeppoSAX/INTEGRAL (Malizia et al.
2005) and may either be associated to a SNR/PWN or belong to the class of absorbed hard
X-ray binaries. The three remaining objects (HESS J1614–518, HESS J1702–420 and HESS
J1708–410) have no plausible SNR, pulsar, or EGRET counterpart and therefore it has been
suggested in the literature that they belong to a new source type (Aharonian et al. 2005).
However, it should also be remembered that the probability of chance coincidence in the
crowded region of the galactic plane is quite high and therefore also for the other sources
the reported associations must be verified carefully.
References Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R. et al. 2006, ApJ 636, 777 | |
| Literatur zu "" | |||
| J.P. Maillard, T. Paumard, S.R. Stolovy, F. Rigaut | 2004 | A&A 423, 155-167 |
"The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared"
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| J. Hinton (H.E.S.S. Collaboration) | 2006 | JPhCS 54, 140 |
"The H.E.S.S. View of the Central 200 Parsecs"
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| R. Schoedel, A. Eckart, C. Iserlohe, R. Genzel, T. Ott | 2006 | ApJ 625, L111-L114 |
"A Black Hole in the Galactic Center Complex IRS 13E?"
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| R. Landi, L. Bassani, A. Malizia, et al. | 2006 | ApJ 651, 190 |
"Swift/XRT follow-up observations of TeV sources of the HESS Inner Galaxy survey"
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| J.P. Simpson, S.W.J. Colgan, A.S. Cotera, et al. | 2007 | ApJ 670, 111 |
"Spitzer IRS Observations of the Galactic Center"
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 | H. Heintzmann | ( Eintrag vom 4.1.2008) |
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