451 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
The X-ray Properties of the Nearby Star-Forming Galaxy IC 342: The XMM-Newton View
We present the X-ray properties of IC342 using XMM-Newton. Thirty-five
sources are detected coincident with the disk of IC342 (more than tripling the
number known), of which ~31 are likely to be intrinsic to IC342. This
population shows a range of spectral properties and has an X-ray luminosity
function slope and infrared luminosity comparable to that of starburst galaxies
such as M82 and the Antennae, while its relative lack of extended X-ray
emission is similar to the properties of quiescent spirals. We do detect
long-term variability between this observation and the 1991 ROSAT and 1993/2000
ASCA observations for five sources. Notably, the second most luminous source
IC342 X-2 is is found to be in its the lowest luminosity state observed for X-2
to date, although the slope of the spectrum is intermediate between the
previously observed low/hard and high/soft states. IC342 X-1, on the other
hand, is found to be in an identical state to that observed in 2000 with ASCA.
Assuming X-1 is in an anomalous very high (VH) state, then either (1) X-1 has
remained in this state between 2000 and 2002, and is therefore the longest
duration VH-state binary ever observed, or (2) it was simply caught in a VH
state by chance in both the 2000 ASCA and 2002 XMM-Newton observations. We have
also confirmed the ROSAT HRI result that the nucleus of IC342 is made up of
both point-like and extended emission. The relative fluxes of the two spectral
components suggest that the nucleus is complex, with a soft extended component
contributing approximately half of the total luminosity. (Abridged)Comment: AJ in press (December 2003), 9 pages, 7 figures, 2 tables,
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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
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
Spectroscopic follow-up of variability-selected active galactic nuclei in the Chandra Deep Field South
Luminous AGNs are usually selected by their non-stellar colours or their
X-ray emission. Colour selection cannot be used to select low-luminosity AGNs,
since their emission is dominated by the host galaxy. Objects with low X-ray to
optical ratio escape even the deepest X-ray surveys performed so far. In a
previous study we presented a sample of candidates selected through optical
variability in the Chandra Deep Field South, where repeated optical
observations were performed for the STRESS supernova survey. We obtained new
optical spectroscopy for a sample of variability selected candidates with the
ESO NTT telescope. We analysed the new spectra, together with those existing in
the literature and studied the distribution of the objects in U-B and B-V
colours, optical and X-ray luminosity, and variability amplitude. A large
fraction (17/27) of the observed candidates are broad-line luminous AGNs,
confirming the efficiency of variability in detecting quasars. We detect: i)
extended objects which would have escaped the colour selection and ii) objects
of very low X-ray to optical ratio. Several objects resulted to be
narrow-emission line galaxies where variability indicates nuclear activity,
while no emission lines were detected in others. Some of these galaxies have
variability and X-ray to optical ratio close to active galactic nuclei, while
others have much lower variability and X-ray to optical ratio. This result can
be explained by the dilution of the nuclear light due to the host galaxy. Our
results demonstrate the effectiveness of supernova search programmes to detect
large samples of low-luminosity AGNs. A sizable fraction of the AGN in our
variability sample had escaped X-ray detection (5/47) and/or colour selection
(9/48). Spectroscopic follow-up to fainter flux limits is strongly encouraged.Comment: 14 pages, 11 figures, to appear in A&
A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger
Neutron star-neutron star mergers are known to be associated with short
gamma-ray bursts. If the neutron star equation of state is sufficiently stiff,
at least some of such mergers will leave behind a supramassive or even a stable
neutron star that spins rapidly with a strong magnetic field (i.e., a
magnetar). Such a magnetar signature may have been observed as the X-ray
plateau following a good fraction (up to 50%) of short gamma-ray bursts, and it
has been expected that one may observe short gamma-ray burst-less X-ray
transients powered by double neutron star mergers. A fast X-ray transient
(CDF-S XT1) was recently found to be associated with a faint host galaxy whose
redshift is unknown. Its X-ray and host-galaxy properties allow several
possibleexplanations including a short gamma-ray burst seen off axis, a
low-luminosity gamma-ray burst at high redshift, or a tidal disruption event
involving an intermediate mass black hole and a white dwarf. Here we report a
second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift
z = 0.738. The light curve is fully consistent with being powered by a
millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its
star-forming host galaxy with a moderate offset from the galaxy center, as
short bursts often do. The estimated event rate density of similar X-ray
transients, when corrected to the local value, is consistent with the double
neutron star merger rate density inferred from the detection of GW170817.Comment: 29 pages, 4 figures, 3 tables, published in Nature on 11 April 201
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
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