364 research outputs found
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
Evidence for a constant IMF in early-type galaxies based on their X-ray binary populations
A number of recent studies have proposed that the stellar initial mass
function (IMF) of early type galaxies varies systematically as a function of
galaxy mass, with higher mass galaxies having bottom heavy IMFs. These bottom
heavy IMFs have more low-mass stars relative to the number of high mass stars,
and therefore naturally result in proportionally fewer neutron stars and black
holes. In this paper, we specifically predict the variation in the number of
black holes and neutron stars based on the power-law IMF variation required to
reproduce the observed mass-to-light ratio trends with galaxy mass. We then
test whether such variations are observed by studying the field low-mass X-ray
binary populations (LMXBs) of nearby early-type galaxies. In these binaries, a
neutron star or black hole accretes matter from a low-mass donor star. Their
number is therefore expected to scale with the number of black holes and
neutron stars present in a galaxy. We find that the number of LMXBs per K-band
light is similar among the galaxies in our sample. These data therefore
demonstrate the uniformity of the slope of the IMF from massive stars down to
those now dominating the K-band light, and are consistent with an invariant
IMF. Our results are inconsistent with an IMF which varies from a
Kroupa/Chabrier like IMF for low mass galaxies to a steep power-law IMF (with
slope =2.8) for high mass galaxies. We discuss how these observations
constrain the possible forms of the IMF variations and how future Chandra
observations can enable sharper tests of the IMF.Comment: 12 pages, 5 figures, 2 tables, submitted to Ap
Pseudorandom Number Generators and the Square Site Percolation Threshold
A select collection of pseudorandom number generators is applied to a Monte
Carlo study of the two dimensional square site percolation model. A generator
suitable for high precision calculations is identified from an application
specific test of randomness. After extended computation and analysis, an
ostensibly reliable value of pc = 0.59274598(4) is obtained for the percolation
threshold.Comment: 11 pages, 6 figure
CamouflageFS: Increasing the Effective Key Length in Cryptographic Filesystems on the Cheap
One of the few quantitative metrics used to evaluate the security of a cryptographic file system is the key length of the encryption algorithm; larger key lengths correspond to higher resistance to brute force and other types of attacks. Since accepted cryptographic design principles dictate that larger key lengths also impose higher processing costs, increasing the security of a cryptographic file system also increases the overhead of the underlying cipher. We present a general approach to effectively extend the key length without imposing the concomitant processing overhead. Our scheme is to spread the ciphertext inside an artificially large file that is seemingly filled with random bits according to a key-driven spreading sequence. Our prototype implementation, CamouflageFS, offers improved performance relative to a cipher with a larger key-schedule, while providing the same security properties. We discuss our implementation (based on the Linux Ext2 file system) and present some preliminary performance results. While CamouflageFS is implemented as a stand-alone file system, its primary mechanisms can easily be integrated into existing cryptographic file systems
Discovery of the Most-Distant Double-Peaked Emitter at z=1.369
We report the discovery of the most-distant double-peaked emitter, CXOECDFS
J033115.0-275518, at z=1.369. A Keck/DEIMOS spectrum shows a clearly
double-peaked broad Mg II emission line, with FWHM 11000 km/s for
the line complex. The line profile can be well fit by an elliptical
relativistic Keplerian disk model. This is one of a handful of double-peaked
emitters known to be a luminous quasar, with excellent multiwavelength coverage
and a high-quality X-ray spectrum. CXOECDFS J033115.0-275518 is a radio-loud
quasar with two radio lobes (FR II morphology) and a radio loudness of f_{5
GHz}/f_{4400 \AA}~429. The X-ray spectrum can be modeled by a power law with
photon index 1.72 and no intrinsic absorption; the rest-frame 0.5-8.0 keV
luminosity is erg/s. The spectral energy distribution (SED)
of CXOECDFS J033115.0-275518 has a shape typical for radio-loud quasars and
double-peaked emitters at lower redshift. The local viscous energy released
from the line-emitting region of the accretion disk is probably insufficient to
power the observed line flux, and external illumination of the disk appears to
be required. The presence of a big blue bump in the SED along with the
unexceptional X-ray spectrum suggest that the illumination cannot arise from a
radiatively inefficient accretion flow.Comment: 6 pages, 5 figures, ApJ in pres
Testing the Universality of the Stellar IMF with Chandra and HST
The stellar initial mass function (IMF), which is often assumed to be
universal across unresolved stellar populations, has recently been suggested to
be "bottom-heavy" for massive ellipticals. In these galaxies, the prevalence of
gravity-sensitive absorption lines (e.g. Na I and Ca II) in their near-IR
spectra implies an excess of low-mass ( ) stars over that
expected from a canonical IMF observed in low-mass ellipticals. A direct
extrapolation of such a bottom-heavy IMF to high stellar masses (
) would lead to a corresponding deficit of neutron stars and black
holes, and therefore of low-mass X-ray binaries (LMXBs), per unit near-IR
luminosity in these galaxies. Peacock et al. (2014) searched for evidence of
this trend and found that the observed number of LMXBs per unit -band
luminosity () was nearly constant. We extend this work using new and
archival Chandra X-ray Observatory (Chandra) and Hubble Space Telescope (HST)
observations of seven low-mass ellipticals where is expected to be the
largest and compare these data with a variety of IMF models to test which are
consistent with the observed . We reproduce the result of Peacock et al.
(2014), strengthening the constraint that the slope of the IMF at
must be consistent with a Kroupa-like IMF. We construct an IMF model
that is a linear combination of a Milky Way-like IMF and a broken power-law
IMF, with a steep slope ( ) for stars < 0.5 (as
suggested by near-IR indices), and that flattens out ( ) for
stars > 0.5 , and discuss its wider ramifications and limitations.Comment: Accepted for publication in ApJ; 7 pages, 2 figures, 1 tabl
X-ray emission from star-forming galaxies - III. Calibration of the Lx-SFR relation up to redshift z1.3
We investigate the relation between total X-ray emission from star-forming
galaxies and their star formation activity. Using nearby late-type galaxies and
ULIRGs from Paper I and star-forming galaxies from Chandra Deep Fields, we
construct a sample of 66 galaxies spanning the redshift range z~0-1.3 and the
star-formation rate (SFR) range ~0.1-10^3 M_sun/yr. In agreement with previous
results, we find that the Lx-SFR relation is consistent with a linear law both
at z=0 and for the z=0.1-1.3 CDF galaxies, within the statistical accuracy of
~0.1 in the slope of the Lx-SFR relation. For the total sample, we find a
linear scaling relation Lx/SFR~(4.0\pm 0.4)x10^{39}(erg/s)/(Msun/yr), with a
scatter of ~0.4 dex. About ~2/3 of the 0.5-8 keV luminosity generated per unit
SFR is expected to be due to HMXBs. We find no statistically significant trends
in the mean Lx/SFR ratio with the redshift or star formation rate and constrain
the amplitude of its variations by <0.1-0.2 dex. These properties make X-ray
observations a powerful tool to measure the star formation rate in normal
star-forming galaxies that dominate the source counts at faint fluxes.Comment: 11 pages, 3 tables, 4 figures, accepted for publication by MNRAS.
Substantial changes since the last version, including the authors lis
Modeling the Redshift Evolution of the Normal Galaxy X-ray Luminosity Function
Emission from X-ray binaries (XRBs) is a major component of the total X-ray
luminosity of normal galaxies, so X-ray studies of high redshift galaxies allow
us to probe the formation and evolution of X-ray binaries on very long
timescales. In this paper, we present results from large-scale population
synthesis models of binary populations in galaxies from z = 0 to 20. We use as
input into our modeling the Millennium II Cosmological Simulation and the
updated semi-analytic galaxy catalog by Guo et al. (2011) to self-consistently
account for the star formation history (SFH) and metallicity evolution of each
galaxy. We run a grid of 192 models, varying all the parameters known from
previous studies to affect the evolution of XRBs. We use our models and
observationally derived prescriptions for hot gas emission to create
theoretical galaxy X-ray luminosity functions (XLFs) for several redshift bins.
Models with low CE efficiencies, a 50% twins mass ratio distribution, a steeper
IMF exponent, and high stellar wind mass loss rates best match observational
results from Tzanavaris & Georgantopoulos (2008), though they significantly
underproduce bright early-type and very bright (Lx > 10d41) late-type galaxies.
These discrepancies are likely caused by uncertainties in hot gas emission and
SFHs, AGN contamination, and a lack of dynamically formed Low-mass XRBs. In our
highest likelihood models, we find that hot gas emission dominates the emission
for most bright galaxies. We also find that the evolution of the normal galaxy
X-ray luminosity density out to z = 4 is driven largely by XRBs in galaxies
with X-ray luminosities between 10d40 and 10d41 erg/s.Comment: Accepted into ApJ, 17 pages, 3 tables, 7 figures. Text updated to
address referee's comment
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