6,018 research outputs found
Actions and symmetries of NSR superstrings and D-strings
We present all NSR superstring and super-D-string actions invariant under a
set of prescribed gauge transformations, and characterize completely their
global symmetries. In particular we obtain locally supersymmetric Born-Infeld
actions on general backgrounds in a formulation with extra target space
dimensions. The nontrivial global symmetries of the superstring actions
correspond to isometries of the background, whereas super-D-string actions can
have additional symmetries acting nontrivially also on the coordinates of the
extra dimensions.Comment: 4 pages, references added and errors correcte
Erratum for "Tropical superelliptic curves"
We correct two errors in our paper Tropical superelliptic curves published in Advances in Geometry 20 (2020), 527–551. These corrections do not change the main results of the paper
A Compton-thick AGN at z~5 in the 4 Ms Chandra Deep Field South
We report the discovery of a Compton-thick Active Galactic Nucleus (AGN) at
z=4.76 in the 4 Ms Chandra Deep Field South. This object was selected as a
V-band dropout in HST/ACS images and previously recognized as an AGN from
optical spectroscopy. The 4 Ms Chandra observations show a significant
(~4.2sigma) X-ray detection at the V-band dropout position. The X-ray source
displays a hardness ratio of HR=0.23+-0.24, which, for a source at z~5, is
highly suggestive of Compton-thick absorption. The source X-ray spectrum is
seen above the background level in the energy range of ~0.9-4 keV, i.e., in the
rest-frame energy range of ~5-23 keV. When fixing the photon index to
Gamma=1.8, the measured column density is N_H=1.4^{+0.9}_{-0.5} x 10^{24}
cm^{-2}, which is Compton-thick. To our knowledge, this is the most distant
heavily obscured AGN, confirmed by X-ray spectral analysis, discovered so far.
The intrinsic (de-absorbed), rest-frame luminosity in the 2-10 keV band is ~2.5
x 10^{44} erg s^{-1}, which places this object among type-2 quasars. The
Spectral Energy Distribution shows that massive star formation is associated
with obscured black hole accretion. This system may have then been caught
during a major co-eval episode of black hole and stellar mass assembly at early
times. The measure of the number density of heavily obscured AGN at high
redshifts will be crucial to reconstruct the BH/galaxy evolution history from
the beginning.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letter
Can the unresolved X-ray background be explained by emission from the optically-detected faint galaxies of the GOODS project?
The emission from individual X-ray sources in the Chandra Deep Fields and
XMM-Newton Lockman Hole shows that almost half of the hard X-ray background
above 6 keV is unresolved and implies the existence of a missing population of
heavily obscured active galactic nuclei (AGN). We have stacked the 0.5-8 keV
X-ray emission from optical sources in the Great Observatories Origins Deep
Survey (GOODS; which covers the Chandra Deep Fields) to determine whether these
galaxies, which are individually undetected in X-rays, are hosting the
hypothesised missing AGN. In the 0.5-6 keV energy range the stacked-source
emission corresponds to the remaining 10-20 per cent of the total background --
the fraction that has not been resolved by Chandra. The spectrum of the stacked
emission is consistent with starburst activity or weak AGN emission. In the 6-8
keV band, we find that upper limits to the stacked X-ray intensity from the
GOODS galaxies are consistent with the ~40 per cent of the total background
that remains unresolved, but further selection refinement is required to
identify the X-ray sources and confirm their contribution.Comment: 7 pages, 1 figure, accepted for publication in MNRA
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