Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session K9: Invited Session: Astrophysical Black Holes on All Mass Scales |
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Sponsoring Units: DAP GGR Chair: Grzegorz Madejski, SLAC National Accelerator Laboratory Room: Key 5 |
Sunday, April 12, 2015 1:30PM - 2:06PM |
K9.00001: NuSTAR results on Ultra-Luminous X-ray sources: black holes or neutron stars? Invited Speaker: Felix Fuerst Ultraluminous X-ray sources (ULXs) are extremely bright, off-nuclear point sources in nearby galaxies. The only process known to power them is a very high accretion rate onto a compact object. If the compact object is similar to those observed in our own galaxy, i.e., a standard stellar remnant, the accretion rate has to exceed the Eddington rate by a factor of 10-100 in a so-called super-Eddington accretion regime. If on the other hand the compact were more massive, ULXs would be the only known evidence for intermediate mass black holes with masses of 100's or 1000's solar masses. Broadband spectral studies of a sample of ULXs, making full use of the hard X-ray sensitivity of the Nuclear Spectroscopic Telescope Array (NuSTAR), are suggestive of super-Eddington accretion. A definitive answer has, however, not yet been reached owing to continued difficulty constraining ULX masses. I will report on recent, multi-epoch NuSTAR observations, which allow us to examine the evolution of these enigmatic sources and their accretion process by studying their time variability and hard X-ray spectrum above 10keV. In a surprising discovery we have recently shown that the ULX M82 X-2 harbors a neutron star, the first evidence for a neutron star in a ULX. I will discuss possible modes of super-Eddington accretion on neutron stars and compare M82 X-2 to known accreting neutron stars in our galaxy. [Preview Abstract] |
Sunday, April 12, 2015 2:06PM - 2:42PM |
K9.00002: Emission from the Black Hole Sagittarius A* and its immediate environment Invited Speaker: Andreas Eckart New NIR data allows a new interpretation of the fast moving dusty S-cluster object (DSO/G2) which was earlier interpret as a core-less gas and dust could approaching SgrA* on an elliptical orbit. With the ESO VLT/SINFONI instrument we detect spatially compact Br$\gamma$ line emission from the DSO at all epochs before and after its peri-bothron passage implying that it is an embedded - presumably young - star rather than a freely expanding gas cloud. However, its outer shell may very well be subject to tidal disruptionr. Several polarized flares were observed with NACO at the ESO VLT during 2004 to 2012, allowing us to study the statistical properties of linearly polarized NIR 2.2 microns light from Sgr A*. The results constrain the physical conditions of the accretion process onto this super-massive black hole. Since the emission is due to optically thin synchrotron radiation, this preferred polarization angle is very likely coupled to the intrinsic orientation of the SgrA* system i.e. a disk or jet/wind scenario associated with the SMBH. If they are indeed linked to structural features of the source the data imply that SgrA* must be a very stable system - both in terms of geometrical orientation of a jet/wind or an accretion disk and in terms of the variability spectrum which must be linked to the accretion process. It may very well be that the close fly-by of the DSO has an effect on the accretion flow onto SgrA*. It cannot be excluded that the recent bright X-ray flares are linked to the DSO fly-by. Just looking at the statistics of NIR and X-ray flux density excursions, such bright X-ray flares may also be expected from a single state power law distribution of flare fluxes. Hence - polarization and variability measurements are the ideal tool to probe for any change in the apparently very stable system as a function of the DSO fly-by. [Preview Abstract] |
Sunday, April 12, 2015 2:42PM - 3:18PM |
K9.00003: Black holes on all scales: similarities and differences Invited Speaker: Chris Done I will review what we know about astrophysical black holes, from the stellar mass back holes formed from the death of massive stars, to the supermassive black holes in galaxy centres. Where material falls onto a black hole of any size, the enourmous gravitational energy released transforms these darkest objects in the Universe into the brightest. The luminous accretion flow lights up the regions of intensely curved spacetime, and its spectrum and variabilty carry the imprint of strong gravity as well as the geometry and dynamics of the emitting material. I will show how the stellar mass black holes form a homogeneous set, and how their large changes in mass accretion rate on easily observable timescales mean that they form a a template for how the spectrum and variability of the accretion flow, and its associated jet, change with mass accretion rate. They ubiquitously show a dramatic switch in both spectral, variability and jet properties as the mass accretion rate changes, probably associated with a change from a hot, geometrically thick flow to a cool, geometrically thin disc. Since the geometry and dynamics of the disc are well understood, these spectra give a clean test of Einstin's gravity in the strong field limit, with clear evidence for the existance of a last stable circular orbit. The hot flows are less well understood, but it is possible that the characteristic timescale for variabilty seen in these data is from Lens-Thirring (vertical) precession of the flow around the black hole. Scaling these models of a changing accretion flow up to the supermassive black holes can give an explanation for the multiple different types of unobscured AGN. However, as well as similarities, there are also some differences in the properties of the spectra, variability and particularly in the jet. A small subset of the most massive black holes have highly relativistic jets, with relativisitically emitting out to GeV or TeV energies. I show that the statistics of these jets may be pointing to their origin from the highest spin black holes formed in major merger events. [Preview Abstract] |
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