A Transient Radio Source near the Center of the Milky Way Galaxy (original) (raw)
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The center of the galaxy: A summary of I.A.U. Symposium No. 136
Nuclear Physics B - Proceedings Supplements, 1989
The first international conference on the central region of our galaxy was held at UCLA during July 25-29, 1988. Radio, infrared, X-, and y-ray astronomers, benefitting from recent technological advances, confronted theorists, as well as each other, with impressive new imagery in each of these domains. Traditionally hidden by an impenetrable veil of dust, the galactic center is now largely transparent, particularly in the near-IR, where the extinction toward the galactic center is an order of magnitude less than at optical wavelengths. On the large scale, one observes a large reservoir of molecular gas, some of v, hich is exhibiting complex, noncircular motions. Some of the kinematical behavior is best explained in terms of energetic explosions occurring about 107 years ago. The radio continuum reveals numerous filaments of ionized gas, and strong, poloidal magnetic fields are implied, quite unlike the field anywhere else in the galaxy. On small scales, much effort is going into the attempt to identify a unique object at the nucleus that may or may not be a massive black hole (a mass of 3 x 106 M® has been defended). Candidates exist for a compact mass concentration, and for a strong central luminosity source, but they are not necessarily one arid the same, and there are problems identifying any of the candidates with a massive accreting object at the center. Surrounding the dynamical center of the galaxy is the prominent radio source Sgr A, which includes a warm, turbulent circumnuclear disk on 5 to 10 pc scales, streamers of ionized gas which may represent flows of material into the central potential well, and a background shell of nonthermal radio emission, probably a superimposed supernova remnant. This summary describes some of the current research activity on the galactic center, with particular attention to the implications of X-and y-ray observations.
GCRT J1742-3001: A New Radio Transient Toward the Galactic Center
The Astrophysical Journal, 2009
We report the detection of a new transient radio source, GCRT J1742−3001, located ∼1 • from the Galactic center. The source was detected ten times from late 2006 to 2007 May in our 235 MHz transient monitoring program with the Giant Metrewave Radio Telescope (GMRT). The radio emission brightened in about one month, reaching a peak observed flux density of ∼100 mJy on 2007 January 28, and decaying to ∼50 mJy by 2007 May when our last monitoring observation was made. Two additional faint, isolated 235 MHz detections were made in mid-2006, also with the GMRT. GCRT J1742−3001 is unresolved at each epoch, with typical resolutions of ∼20 ×10. No polarization information is available from the observations. Based on nondetections in observations obtained simultaneously at 610 MHz, we deduce that the spectrum of GCRT J1742−3001 is very steep, with a spectral index less than about-2. Follow-up radio observations in 2007 September at 330 MHz and 1.4 GHz, and in 2008 February at 235 MHz yielded no detections. No X-ray counterpart is detected in a serendipitous observation obtained with the X-ray telescope aboard the Swift satellite during the peak of the radio emission in early 2007. We consider the possibilities that GCRT J1742−3001 is either a new member of an existing class of radio transients, or is representative of a new class having no associated X-ray emission.
The Galactic Center: An Interacting System of Unusual Sources
Science, 2000
The region bounded by the inner tens of light years at the center of the Milky Way contains five principal components that coexist within the central deep gravitational potential well. These constituents are a black hole candidate (Sgr A*) with a mass equivalent to 2.6 ± 0.2 × 10 6 suns, a surrounding cluster of evolved stars, a complex of young stars, molecular and ionized gas clouds, and a powerful supernova-like remnant. The interaction of these components is responsible for many of the phenomena occurring in this complex and unique portion of the Galaxy. Developing a consistent picture of the primary interactions between the components at the Galactic Center will improve our understanding of the nature of galactic nuclei in general, and will provide with a better defined set of characteristics of black holes. For example, the accretion of stellar winds by Sgr A* appears to produce far less radiation than indicated by estimates based on models of galactic nuclei.
A new perspective on the radio active zone at the Galactic center – feedback from nuclear activities
Proceedings of the International Astronomical Union
Based on our deep image of Sgr A using broadband data observed with the VLA† at 6 cm, we present a new perspective of the radio bright zone at the Galactic center. We further show the radio detection of the X-ray Cannonball, a candidate neutron star associated with the Galactic center SNR Sgr A East. The radio image is compared with the Chandra X-ray image to show the detailed structure of the radio counterparts of the bipolar X-ray lobes. The bipolar lobes are likely produced by the winds from the activities within Sgr A West, which could be collimated by the inertia of gas in the CND, or by the momentum driving of Sgr A*; and the poloidal magnetic fields likely play an important role in the collimation. The less-collimated SE lobe, in comparison to the NW one, is perhaps due to the fact that the Sgr A East SN might have locally reconfigured the magnetic field toward negative galactic latitudes. In agreement with the X-ray observations, the time-scale of ∼1 × 104 yr estimated for t...
The Astrophysical Journal, 2020
We analyze the gas and dust emission in the immediate vicinity of the supermassive black hole Sgr A* at the Galactic center (GC) with the ESO VLT (Paranal/Chile) instruments SINFONI and VISIR. The SINFONI H+K data cubes show several emission lines with related line map counterparts. From these lines, the Brγ emission is the most prominent one and appears to be shaped as a bar extending along the North-South direction. With VISIR, we find a dusty counterpart to this filamentary emission. In this work, we present evidence that this feature can be most likely connected to the mini-spiral and potentially influenced by the winds of the massive stars in the central cluster or an accretion wind from Sgr A*. To this end, we co-add the SINFONI data between 2005 and 2015. The spectroscopic analysis reveals a range of Doppler-shifted emission lines. We also detect substructures in the shape of clumps that can be investigated in the channel maps of the Brγ-bar. In addition, we compare the detection of the near-infrared (NIR) Brγ feature to PAH1 mid-infrared (MIR) observations and published 226 GHz radio data. These clumps show a proper motion of about 320km/s that are consistent with other infrared continuum detected filaments in the Galactic center. Deriving a mass of 2.5 × 10 −5 M for the investigated Brγ-feature shows an agreement with former derived masses for similar objects. Besides the North-South Brγ-bar, we find a comparable additional East-West feature. Also, we identify several gas reservoirs that are located west of Sgr A* that may harbor dusty objects.
The Discovery of Radio Stars within 10 arcseconds of Sgr A* at 7mm
Very Large Array observations of the Galactic Center at 7 mm have produced an image of the 30 arcseconds surrounding Sgr A* with a resolution of 82x42 milliarcseconds (mas). A comparison with IR images taken simultaneously with the Very Large Telescope (VLT) identifies 41 radio sources with L-band (3.8 microns) stellar counterparts. The well-known young, massive stars in the central Sgr A* cluster (e.g., IRS 16C, IRS 16NE, IRS 16SE2, IRS 16NW, IRS 16SW, AF, AFNW, IRS 34W and IRS 33E) are detected with peak flux densities between 0.2 and 1.3 mJy. The origin of the stellar radio emission in the central cluster is discussed in terms of ionized stellar winds with mass-loss rates in the range 0.8-5x10^{-5} solar mass per year. Radio emission from eight massive stars is used as a tool for registration between the radio and infrared frames with mas precision within a few arcseconds of Sgr A*. This is similar to the established technique of aligning SiO masers and evolved stars except that ...
Galactic center research: manifestations of the central black hole
Research in Astronomy and Astrophysics, 2012
This review summarizes a few of the frontiers of Galactic center research that are currently the focus of considerable activity and attention. It is aimed at providing a necessarily incomplete sketch of some of the timely work being done on phenomena taking place in, or originating in, the central few parsecs of the Galaxy, with particular attention to topics related to the Galactic black hole (GBH). We have chosen to expand on the following exciting topics: 1) the characterization and the implications for the variability of emission from the GBH, 2) the strong evidence for a powerful X-ray flare in the Galactic center within the past few hundred years, and the likelihood that the GBH is implicated in that event, 3) the prospects for detecting the "shadow" of the GBH, 4) an overview of the current state of research on the central S-star cluster, and what has been learned from the stellar orbits within that cluster, and 5) the current hypotheses for the origin of the G2 dust cloud that is projected to make a close passage by the GBH in 2013.
High-energy spectrum of the Galactic center black hole Sgr A
Advances in Space Research, 1997
The massive blackhole candidate Sgr A* may be accreting from an ambient Galactic center wind at a rate x 1O22 g s-l. Dissipational processes within the large-scale quasi-spherical infall, from 50 rg to lo5 rg (where rg = 3 x 1O1l cm is the Schwarzschild radius for M = 10" Ma) can account well for the observed radio spectrum and flux (due to cyclotron/synchrotron emission) and its Xray/Gamma-ray luminosity (apparently due to Bremsstrahlung emission). Small scale instabilities associated with the stagnation region of the flow produce fluctuations in the accretion rate with an amplitude of up to 30% and a time scale of several months to over a year. This may account for the long term variability of the high-energy (and radio) luminosity observed from Sgr A*. Although the average accreted angular momentum is approximately zero, these instabilities also induce fluctuations in the specific angular momentum of the accreted gas that lead to a circularized flow at distances less than about 20 rg. Optically thick emission from this "disk" (roughly the size of Mercury's orbit) is the origin of the IR flux recently detected from the location of Sgr A*, but is not itself a significant source of high-energy emission.
Massive Star Formation of the SGR a East H II Regions Near the Galactic Center
The Astrophysical Journal, 2010
A group of four compact HII regions associated with the well-known 50 km s −1 molecular cloud is the closest site of ongoing star formation to the dynamical center of the Galaxy, at a projected distance of ∼6 pc. We present a study of ionized gas based on the [NeII] (12.8 µm) line, as well as multi-frequency radio continuum, HST Paα and Spitzer IRAC observations of the most compact member of the HII group, Sgr A East HII D. The radio continuum image at 6cm shows that this source breaks up into two equally bright ionized features, D1 and D2. The SED of the D source is consistent with it being due to a 25±3 M ⊙ star with a luminosity of 8 ± 3 × 10 4 L ⊙. The inferred mass, effective temperature of the UV source and the ionization rate are compatible with a young O9-B0 star. The ionized features D1 and D2 are considered to be ionized by UV radiation collimated by an accretion disk. We consider that the central massive star photoevaporates its circumstellar disk on a timescale of 3×10 4 years giving a mass flux ∼ 3 × 10 −5 M ⊙ yr −1 and producing the ionized material in D1 and D2 expanding in an inhomogeneous medium. The ionized gas kinematics, as traced by the [Ne II] emission, is difficult to interpret, but it could be explained by the interaction of a bipolar jet with surrounding gas along with what appears