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RE: NGC 7793
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Title: A Chandra Observation of the Nearby Sculptor Group Sd Galaxy NGC 7793
Authors: Thomas G. Pannuti, Eric M. Schlegel, Miroslav D. Filipovic, Jeffrey L. Payne, Robert Petre, Ilana M. Harrus, Wayne D. Staggs, Christina K. Lacey

We conducted a Chandra ACIS observation of the nearby Sculptor Group Sd galaxy NGC 7793. At the assumed distance to NGC 7793 of 3.91 Mpc, the limiting unabsorbed luminosity of the detected discrete X-ray sources (0.2-10.0 keV) is approximately 3x10^36 ergs s^-1. A total of 22 discrete sources were detected at the 3-sigma level or greater including one ultra-luminous X-ray source (ULX). Based on multiwavelength comparisons, we identify X-ray sources coincident with one SNR, the candidate microquasar N7793-S26, one HII region and two foreground Galactic stars. We also find that the X-ray counterpart to the candidate radio SNR R3 is time-variable in its X-ray emission: we therefore rule out the possibility that this source is a single SNR. A marked asymmetry is seen in the distribution of the discrete sources with the majority lying in the eastern half of this galaxy. All of the sources were analysed using quantiles to estimate spectral properties and spectra of the four brightest sources (including the ULX) were extracted and analysed. We searched for time-variability in the X-ray emission of the detected discrete sources using our measured fluxes along with fluxes measured from prior Einstein and ROSAT observations. From this study, three discrete X-ray sources are established to be significantly variable. A spectral analysis of the galaxy's diffuse emission is characterized by a temperature of kT = 0.19-0.25 keV. The luminosity function of the discrete sources shows a slope with an absolute value of Gamma = -0.650.11 if we exclude the ULX. If the ULX is included, the luminosity function has a long tail to high L_X with a poor-fitting slope of Gamma = -0.620.2. The ULX-less slope is comparable to the slopes measured for the distributions of NGC 6946 and NGC 2403 but much shallower than the slopes measured for the distributions of IC 5332 and M83.

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S26
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'Russian doll' galaxy reveals black holes' true power

Following a study of what is in effect a miniature galaxy buried inside a normal-sized one - like a Russian doll - astronomers using a CSIRO telescope have concluded that massive black holes are more powerful than we thought.
An international team of astronomers led by Dr Manfred Pakull at the University of Strasbourg in France has discovered a 'microquasar' - a small black hole, weighing only as much as a star, that shoots jets of radio-emitting particles into space.
Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13M light-years away in the Southern constellation of Sculptor.

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Posts: 131433
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Microquasar S26
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Black Holes may be a thousand times more powerful than anyone thought according to new findings from an international team of astronomers, and based partly on readings from the CSIRO Compact Array telescope near Narrabri in New South Wales.
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Posts: 131433
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RE: NGC 7793
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An international team of astronomers led by Dr Manfred Pakull at the University of Strasbourg in France has discovered a 'microquasar' - a small black hole, weighing only as much as a star, that shoots jets of radio-emitting particles into space.
Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13M light-years away in the Southern constellation of Sculptor.
Earlier this year Pakull and colleagues observed S26 with optical and X-ray telescopes (the European Southern Observatory's Very Large Telescope and NASA's Chandra space telescope).

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Microquasar S26
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Title: Radio lobes and X-ray hot spots in the microquasar S26
Authors: Roberto Soria, Manfred W. Pakull, Jess W. Broderick, Stephane Corbel, Christian Motch

We have studied the structure and energetics of the powerful microquasar/shock-ionised nebula S26 in NGC 7793, with particular focus on its radio and X-ray properties. Using the Australia Telescope Compact Array, we have resolved for the first time the radio lobe structure and mapped the spectral index of the radio cocoon. The steep spectral index of the radio lobes is consistent with optically-thin synchrotron emission; outside the lobes, the spectral index is flatter, suggesting an additional contribution from free-free emission, and perhaps ongoing ejections near the core. The radio core is not detected, while the X-ray core has a 0.3-8 keV luminosity ~6 x 10^{36} erg/s. The size of the radio cocoon matches that seen in the optical emission lines and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is ~2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about 3 times the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hot spots (combined 0.3-8 keV luminosity ~2 x 10^{37} erg/s) are located ~20 pc outwards of the radio hot spots (ie, downstream along the jet direction), consistent with a different physical origin of X-ray and radio emission (thermal-plasma and synchrotron, respectively). The total particle energy in the bubble is ~10^{53} erg: from the observed radio flux, we estimate that only about a few 10^{50} erg are stored in the relativistic electrons; the rest is in protons, nuclei and non-relativistic electrons. The X-ray-emitting component of the gas in the hot spots contains ~10^{51} erg, and ~10^{52} erg over the whole cocoon. We suggest that S26 provides a clue to understand how the ambient medium is heated by the mechanical power of a black hole near its Eddington accretion rate.

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NGC 7793
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Combining observations made with ESO's Very Large Telescope and NASA's Chandra X-ray telescope, astronomers have uncovered the most powerful pair of jets ever seen from a stellar black hole. This object, also known as a microquasar, blows a huge bubble of hot gas, 1000 light-years across, twice as large and tens of times more powerful than other known microquasars. The discovery is reported this week in the journal Nature.
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