The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission. (arXiv:1301.7307v1 [astro-ph.IM]):
The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 13
June 2012, is the first focusing high-energy X-ray telescope in orbit. NuSTAR
operates in the band from 3 -- 79 keV, extending the sensitivity of focusing
far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray
satellites. The inherently low-background associated with concentrating the
X-ray light enables NuSTAR to probe the hard X-ray sky with a more than
one-hundred-fold improvement in sensitivity over the collimated or coded-mask
instruments that have operated in this bandpass. Using its unprecedented
combination of sensitivity, spatial and spectral resolution, NuSTAR will pursue
five primary scientific objectives, and will also undertake a broad program of
targeted observations. The observatory consists of two co-aligned
grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis
stabilized spacecraft. Deployed into a 600 km, near-circular, 6degree
inclination orbit, the Observatory has now completed commissioning, and is
performing consistent with pre-launch expectations. NuSTAR is now executing its
primary science mission, and with an expected orbit lifetime of ten years, we
anticipate proposing a guest investigator program, to begin in Fall 2014.
Thursday, January 31, 2013
Ultra-fast outflows (aka UFOs) from AGNs and QSOs. (arXiv:1301.7199v1 [astro-ph.HE])
Ultra-fast outflows (aka UFOs) from AGNs and QSOs. (arXiv:1301.7199v1 [astro-ph.HE]):
During the last decade, strong observational evidence has been accumulated
for the existence of massive, high velocity winds/outflows (aka Ultra Fast
Outflows, UFOs) in nearby AGNs and in more distant quasars. Here we briefly
review some of the most recent developments in this field and discuss the
relevance of UFOs for both understanding the physics of accretion disk winds in
AGNs, and for quantifying the global amount of AGN feedback on the surrounding
medium.
During the last decade, strong observational evidence has been accumulated
for the existence of massive, high velocity winds/outflows (aka Ultra Fast
Outflows, UFOs) in nearby AGNs and in more distant quasars. Here we briefly
review some of the most recent developments in this field and discuss the
relevance of UFOs for both understanding the physics of accretion disk winds in
AGNs, and for quantifying the global amount of AGN feedback on the surrounding
medium.
A black-hole mass measurement from molecular gas kinematics in NGC4526. (arXiv:1301.7184v1 [astro-ph.CO])
A black-hole mass measurement from molecular gas kinematics in NGC4526. (arXiv:1301.7184v1 [astro-ph.CO]):
The masses of the supermassive black-holes found in galaxy bulges are
correlated with a multitude of galaxy properties, leading to suggestions that
galaxies and black-holes may evolve together. The number of reliably measured
black-hole masses is small, and the number of methods for measuring them is
limited, holding back attempts to understand this co-evolution. Directly
measuring black-hole masses is currently possible with stellar kinematics (in
early-type galaxies), ionised-gas kinematics (in some spiral and early-type
galaxies) and in rare objects which have central maser emission. Here we report
that by modelling the effect of a black-hole on the kinematics of molecular gas
it is possible to fit interferometric observations of CO emission and thereby
accurately estimate black hole masses. We study the dynamics of the gas in the
early-type galaxy NGC4526, and obtain a best fit which requires the presence of
a central dark-object of 4.5(+4.2-3.0)x10^8 Msun (3 sigma confidence limit).
With next generation mm-interferometers (e.g. ALMA) these observations could be
reproduced in galaxies out to 75 megaparsecs in less the 5 hours of observing
time. The use of molecular gas as a kinematic tracer should thus allow one to
estimate black-hole masses in hundreds of galaxies in the local universe, many
more than accessible with current techniques.
The masses of the supermassive black-holes found in galaxy bulges are
correlated with a multitude of galaxy properties, leading to suggestions that
galaxies and black-holes may evolve together. The number of reliably measured
black-hole masses is small, and the number of methods for measuring them is
limited, holding back attempts to understand this co-evolution. Directly
measuring black-hole masses is currently possible with stellar kinematics (in
early-type galaxies), ionised-gas kinematics (in some spiral and early-type
galaxies) and in rare objects which have central maser emission. Here we report
that by modelling the effect of a black-hole on the kinematics of molecular gas
it is possible to fit interferometric observations of CO emission and thereby
accurately estimate black hole masses. We study the dynamics of the gas in the
early-type galaxy NGC4526, and obtain a best fit which requires the presence of
a central dark-object of 4.5(+4.2-3.0)x10^8 Msun (3 sigma confidence limit).
With next generation mm-interferometers (e.g. ALMA) these observations could be
reproduced in galaxies out to 75 megaparsecs in less the 5 hours of observing
time. The use of molecular gas as a kinematic tracer should thus allow one to
estimate black-hole masses in hundreds of galaxies in the local universe, many
more than accessible with current techniques.
Tuesday, January 29, 2013
Probing the Dawn of Galaxies at z~9-12: New Constraints from HUDF12/XDF and CANDELS Data. (arXiv:1301.6162v1 [astro-ph.CO])
Probing the Dawn of Galaxies at z~9-12: New Constraints from HUDF12/XDF and CANDELS Data. (arXiv:1301.6162v1 [astro-ph.CO]):
We present a comprehensive analysis of z>8 galaxies based on ultra-deep
WFC3/IR data. We constrain the evolution of the UV luminosity function (LF) and
luminosity densities from z~11 to z~8 by exploiting all the WFC3/IR data over
the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in
addition to the HUDF09 parallel field data, as well as wider area WFC3/IR
imaging over GOODS-South. Galaxies are selected based on the Lyman Break
Technique in three samples centered around z~9, z~10 and z~11, with seven z~9
galaxy candidates, and one each at z~10 and z~11. We confirm a new z~10
candidate (with z=9.8+-0.6) that was not convincingly identified in our first
z~10 sample. The deeper data over the HUDF confirms all our previous z>~7.5
candidates as genuine high-redshift candidates, and extends our samples to
higher redshift and fainter limits (H_160~29.8 mag). We perform one of the
first estimates of the z~9 UV LF and improve our previous constraints at z~10.
Extrapolating the lower redshift UV LF evolution should have revealed 17 z~9
and 9 z~10 sources, i.e., a factor ~3x and 9x larger than observed. The
inferred star-formation rate density (SFRD) in galaxies above 0.7 M_sun/yr
decreases by 0.6+-0.2 dex from z~8 to z~9, in good agreement with previous
estimates. The low number of sources found at z>8 is consistent with a very
rapid build-up of galaxies across z~10 to z~8. From a combination of all
current measurements, we find a best estimate of a factor 10x decrease in the
SFRD from z~8 to z~10, following (1+z)^(-11.4+-3.1). Our measurements thus
confirm our previous finding of an accelerated evolution beyond z~8, and
signify a rapid build-up of galaxies with M_UV<-17.7 within only ~200 Myr from
z~10 to z~8, in the heart of cosmic reionization.
We present a comprehensive analysis of z>8 galaxies based on ultra-deep
WFC3/IR data. We constrain the evolution of the UV luminosity function (LF) and
luminosity densities from z~11 to z~8 by exploiting all the WFC3/IR data over
the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in
addition to the HUDF09 parallel field data, as well as wider area WFC3/IR
imaging over GOODS-South. Galaxies are selected based on the Lyman Break
Technique in three samples centered around z~9, z~10 and z~11, with seven z~9
galaxy candidates, and one each at z~10 and z~11. We confirm a new z~10
candidate (with z=9.8+-0.6) that was not convincingly identified in our first
z~10 sample. The deeper data over the HUDF confirms all our previous z>~7.5
candidates as genuine high-redshift candidates, and extends our samples to
higher redshift and fainter limits (H_160~29.8 mag). We perform one of the
first estimates of the z~9 UV LF and improve our previous constraints at z~10.
Extrapolating the lower redshift UV LF evolution should have revealed 17 z~9
and 9 z~10 sources, i.e., a factor ~3x and 9x larger than observed. The
inferred star-formation rate density (SFRD) in galaxies above 0.7 M_sun/yr
decreases by 0.6+-0.2 dex from z~8 to z~9, in good agreement with previous
estimates. The low number of sources found at z>8 is consistent with a very
rapid build-up of galaxies across z~10 to z~8. From a combination of all
current measurements, we find a best estimate of a factor 10x decrease in the
SFRD from z~8 to z~10, following (1+z)^(-11.4+-3.1). Our measurements thus
confirm our previous finding of an accelerated evolution beyond z~8, and
signify a rapid build-up of galaxies with M_UV<-17.7 within only ~200 Myr from
z~10 to z~8, in the heart of cosmic reionization.
Wednesday, January 23, 2013
A link between measured neutron star masses and lattice QCD data. (arXiv:1212.5907v1 [astro-ph.SR])
A link between measured neutron star masses and lattice QCD data. (arXiv:1212.5907v1 [astro-ph.SR]):
We study the hadron-quark phase transition in neutron star matter and the
structural properties of hybrid stars using an equation of state (EOS) for the
quark phase derived with the Field Correlator Method (FCM). We make use of
measured neutron star masses, and particularly the mass of PSR J1614-2230, to
constrain the values of the gluon condensate $G_2$ which is one of the EOS
parameter within the FCM. We find that the values of $G_2$ extracted from the
mass measurement of PSR J1614-2230 are fully consistent with the values of the
same quantity derived, within the FCM, from recent lattice QCD calculations of
the deconfinement transition temperature at zero baryon chemical potential. The
FCM thus provides a powerful tool to link numerical calculations of QCD on a
space-time lattice with neutron stars physics.
We study the hadron-quark phase transition in neutron star matter and the
structural properties of hybrid stars using an equation of state (EOS) for the
quark phase derived with the Field Correlator Method (FCM). We make use of
measured neutron star masses, and particularly the mass of PSR J1614-2230, to
constrain the values of the gluon condensate $G_2$ which is one of the EOS
parameter within the FCM. We find that the values of $G_2$ extracted from the
mass measurement of PSR J1614-2230 are fully consistent with the values of the
same quantity derived, within the FCM, from recent lattice QCD calculations of
the deconfinement transition temperature at zero baryon chemical potential. The
FCM thus provides a powerful tool to link numerical calculations of QCD on a
space-time lattice with neutron stars physics.
What X-ray source counts can tell about the large scale matter distribution. (arXiv:1212.5891v1 [astro-ph.CO])
What X-ray source counts can tell about the large scale matter distribution. (arXiv:1212.5891v1 [astro-ph.CO]):
Sources generating most of the X-ray background (XRB) are dispersed over a
wide range of redshifts. Thus, statistical characteristics of the source
distribution carry the information on the matter distribution on very large
scales. We test the possibility to detect the variation of the X-ray source
number counts over the celestial sphere. A large number of Chandra pointings
spread over both galactic hemispheres is investigated. A search for all the
point-like sources in the soft band of 0.5 - 2 keV is performed, and
statistical assessment of the population of sources below the detection
threshold is carried out. A homogeneous sample of the number counts at fluxes
above ~10^{-16} erg/s/cm^2 for more than 300 ACIS fields was constructed. The
counts correlations between overlapping fields were used to assess the accuracy
of the computational methods used in the analysis. It is shown that the source
number counts vary between fields at the level only slightly larger than the
fluctuation amplitude expected for the random (Poissonian) distribution.
Nevertheless, small asymmetry between galactic hemispheres is present. The
average number of sources in the northern hemisphere is larger than in the
southern at the 2.75 sigma level. Also the autocorrelation function of the
source density in both hemispheres are substantially different. Possible
explanations for the observed anisotropies are considered. If the effect is
unrelated to the observational selection, a large scale inhomogeneities in the
distribution of X-ray sources are required. Correlations of the source number
counts observed in the southern hemisphere could be generated by a coherent
structure extending over 1200 Mpc.
Sources generating most of the X-ray background (XRB) are dispersed over a
wide range of redshifts. Thus, statistical characteristics of the source
distribution carry the information on the matter distribution on very large
scales. We test the possibility to detect the variation of the X-ray source
number counts over the celestial sphere. A large number of Chandra pointings
spread over both galactic hemispheres is investigated. A search for all the
point-like sources in the soft band of 0.5 - 2 keV is performed, and
statistical assessment of the population of sources below the detection
threshold is carried out. A homogeneous sample of the number counts at fluxes
above ~10^{-16} erg/s/cm^2 for more than 300 ACIS fields was constructed. The
counts correlations between overlapping fields were used to assess the accuracy
of the computational methods used in the analysis. It is shown that the source
number counts vary between fields at the level only slightly larger than the
fluctuation amplitude expected for the random (Poissonian) distribution.
Nevertheless, small asymmetry between galactic hemispheres is present. The
average number of sources in the northern hemisphere is larger than in the
southern at the 2.75 sigma level. Also the autocorrelation function of the
source density in both hemispheres are substantially different. Possible
explanations for the observed anisotropies are considered. If the effect is
unrelated to the observational selection, a large scale inhomogeneities in the
distribution of X-ray sources are required. Correlations of the source number
counts observed in the southern hemisphere could be generated by a coherent
structure extending over 1200 Mpc.
Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc])
Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc]):
If astrophysical black hole candidates are the Kerr black holes predicted by
General Relativity, the value of their spin parameter must be subject to the
{\it theoretical bound} $|a_*| \le 1$. In this work, we consider the
possibility that these objects are either non-Kerr black holes in an
alternative theory of gravity or exotic compact objects in General Relativity.
Such a possibility is not in contradiction with current data and it can be
tested with future observational facilities. We study the accretion process
when their accretion disk is geometrically thick with a simple version of the
Polish doughnut model. The picture of the accretion process may be
qualitatively different from the one around a Kerr black hole. The inner edge
of the accretion disk may not have the typical cusp on the equatorial plane any
more, but there may be two cusps, respectively above and below the equatorial
plane. We discuss the evolution of the spin parameter as a consequence of the
accretion process and we estimate the maximum value of the spin parameter of
these objects as a function of their deformation. Lastly, we compare our
results with the current estimates of the mean radiative efficiency of AGNs. We
find the observational bound $|a_*| \lesssim 1.3$ for the spin parameter of the
super-massive black hole candidates at the centers of galaxies, which we argue
to be almost independent of the exact nature of these objects.
If astrophysical black hole candidates are the Kerr black holes predicted by
General Relativity, the value of their spin parameter must be subject to the
{\it theoretical bound} $|a_*| \le 1$. In this work, we consider the
possibility that these objects are either non-Kerr black holes in an
alternative theory of gravity or exotic compact objects in General Relativity.
Such a possibility is not in contradiction with current data and it can be
tested with future observational facilities. We study the accretion process
when their accretion disk is geometrically thick with a simple version of the
Polish doughnut model. The picture of the accretion process may be
qualitatively different from the one around a Kerr black hole. The inner edge
of the accretion disk may not have the typical cusp on the equatorial plane any
more, but there may be two cusps, respectively above and below the equatorial
plane. We discuss the evolution of the spin parameter as a consequence of the
accretion process and we estimate the maximum value of the spin parameter of
these objects as a function of their deformation. Lastly, we compare our
results with the current estimates of the mean radiative efficiency of AGNs. We
find the observational bound $|a_*| \lesssim 1.3$ for the spin parameter of the
super-massive black hole candidates at the centers of galaxies, which we argue
to be almost independent of the exact nature of these objects.
Subscribe to:
Posts (Atom)