135 research outputs found
The large-scale bias of the hard X-ray background
Recent deep X-ray surveys combined with spectroscopic identification of the
sources have allowed the determination of the rest-frame 2-8 keV luminosity as
a function of redshift. In addition, an analysis of the HEAO1 A2 2-10 keV
full-sky map of the X-ray background (XRB) reveals clustering on the scale of
several degrees. Combining these two results in the context of the currently
favored Lambda-CDM cosmological model implies an average X-ray bias factor,
b_x, of b_x^2 = 1.12 +- 0.33, i.e., b_x = 1.06 +- 0.16. These error estimates
include only statistical error; the systematic error sources, while comparable,
appear to be sub-dominant. This result is in contrast to the large biases of
some previous estimates and is more in line with current estimates of the
optical bias of L* galaxies.Comment: 6 pages, 3 eps figures, accepted for ApJ, vol. 612, 10 September 200
The Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest: The X-ray Catalog
We present the X-ray catalog and basic results from our Chandra Large Area
Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field. Our 9
ACIS-I fields cover a contiguous solid angle of ~0.4 sq. deg. and reach fluxes
of 5E-16 cgs (0.4-2 keV) and 3E-15 cgs (2-8keV). Our survey bridges the gap
between ultradeep pencil-beam surveys, such as the Chandra Deep Fields (CDFs),
and shallower, large area surveys, allowing a better probe of the X-ray sources
that contribute most of the 2-10 keV cosmic X-ray background (CXB). We find a
total of 525 X-ray point sources and 4 extended sources. At ~10E-14 cgs 2-8
keV, our number counts are significantly higher than those of several
non-contiguous, large area surveys. On the other hand, the integrated flux from
the CLASXS field, combined with ASCA and Chandra ultradeep surveys, is
consistent with results from other large area surveys, within the variance of
the CXB. Spectral evolution is seen in the hardening of the sources at fluxes
below 1E-14 cgs Above 4E1-14 cgs(0.4-8 keV), ~60 of the sources are variable.
Four extended sources in CLASXS is consistent with the previously measured
LogN-LogS of galaxy clusters. We report the discovery of a gravitational
lensing arc associated with one of these sources. (Abridged)Comment: 67 pages, 26 figures, accepted for publication in the Astronomical
Journa
Absolute measurement of the unresolved cosmic X-ray background in the 0.5-8 keV band with Chandra
We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray
background (CXB) in the Chandra Deep Fields (CDFs) North and South, the longest
observations with Chandra (2 Ms and 1 Ms, respectively). We measure the
unresolved CXB intensity by extracting spectra of the sky, removing all point
and extended sources detected in the CDF. To model and subtract the
instrumental background, we use observations obtained with ACIS in stowed
position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band
is dominated by diffuse Galactic and local thermal-like emission. In the 1-8
keV band, the unresolved spectrum is adequately described by a power law with a
photon index 1.5. We find unresolved CXB intensities of (1.04+/-0.14)x10^-12
ergs cm^-2 s^-1 deg^-2 for the 1-2 keV band and (3.4+/-1.7)x10^-12 ergs cm^-2
s^-1 deg^-2 for the 2-8 keV band. Our detected unresolved intensities in these
bands significantly exceed the expected flux from sources below the CDF
detection limits, if one extrapolates the logN/logS curve to zero flux. Thus
these background intensities imply either a genuine diffuse component, or a
steepening of the logN/logS curve at low fluxes, most significantly for
energies <2 keV. Adding the unresolved intensity to the total contribution from
sources detected in these fields and wider-field surveys, we obtain a total
intensity of the extragalactic CXB of (4.6+/-0.3)x10^-12 ergs cm^-2 s^-1 deg^-2
for 1-2 keV and (1.7+/-0.2)x10^-11 ergs cm^-2 s^-1 deg^-2 for 2-8 keV. These
totals correspond to a CXB power law normalization (for photon index 1.4) of
10.9 photons cm^-2 s^-1 keV^-1 sr^-1 at 1 keV. This corresponds to resolved
fracations of 77+/-3% and 80+/-8% for 1-2 and 2-8 keV, respectively.Comment: 23 emulateapj pages, accepted for publication in ApJ. Minor
revisions, most notably a new summary of the error analysi
Dark Matter Search Using XMM-Newton Observations of Willman 1
We report the results of a search for an emission line from radiatively
decaying dark matter in the ultra-faint dwarf spheroidal galaxy Willman 1 based
on analysis of spectra extracted from XMM-Newton X-ray Observatory data. The
observation follows up our analysis of Chandra data of Willman 1 that resulted
in line flux upper limits over the Chandra bandpass and evidence of a 2.5 keV
feature at a significance below the 99% confidence threshold used to define the
limits. The higher effective area of the XMM-Newton detectors, combined with
application of recently developing methods for extended-source analysis, allow
us to derive improved constraints on the combination of mass and mixing angle
of the sterile neutrino dark matter candidate. We do not confirm the Chandra
evidence for a 2.5 keV emission line.Comment: 23 pages, including 17 figures; accepted for publication in Ap
Discovery of Strong Radiative Recombination Continua from The Supernova Remnant IC 443 with Suzaku
We present the Suzaku spectroscopic study of the Galactic middle-aged
supernova remnant (SNR) IC 443. The X-ray spectrum in the 1.75-6.0 keV band is
described by an optically-thin thermal plasma with the electron temperature of
0.6 keV and several additional Lyman lines. We robustly detect, for the first
time, strong radiative recombination continua (RRC) of H-like Si and S around
at 2.7 and 3.5 keV. The ionization temperatures of Si and S determined from the
intensity ratios of the RRC to He-like K-alpha line are 1.0 keV and 1.2 keV,
respectively. We thus find firm evidence for an extremely-overionized
(recombining) plasma. As the origin of the overionization, a thermal conduction
scenario argued in previous work is not favored in our new results. We propose
that the highly-ionized gas were made at the initial phase of the SNR evolution
in dense regions around a massive progenitor, and the low electron temperature
is due to a rapid cooling by an adiabatic expansion.Comment: 5 pages, 5 figures, accepted by ApJ Lette
The Evolution and Eddington Ratio Distribution of Compton Thick Active Galactic Nuclei
Previous studies of the active galactic nuclei (AGN) contribution to the
cosmic X-ray background (CXB) consider only observable parameters such as
luminosity and absorbing column. Here, for the first time, we extend the study
of the CXB to physical parameters including the Eddington ratio of the sources
and the black hole mass. In order to calculate the contribution to the CXB of
AGN accreting at various Eddington ratios, an evolving Eddington ratio space
density model is calculated. In particular, Compton thick (CT) AGN are modeled
as accreting at specific, physically motivated Eddington ratios instead of as a
simple extension of the Compton thin type 2 AGN population. Comparing against
the observed CT AGN space densities and log N-log S relation indicates that CT
AGN are likely a composite population of AGN made up of sources accreting
either at >90% or <1% of their Eddington rate.Comment: accepted ApJ
SUZAKU Observation of a New Merging Group of Galaxies at a Filamentary Junction
We report on a new merging group of galaxies, Suzaku J1552+2739 at z ~ 0.08
revealed by a SUZAKU observation. The group was found by observing a junction
of galaxy filaments optically identified in the Sloan Digital Sky Survey
spectroscopic data. Suzaku J1552+2739 exhibits an irregular morphology and
presents several peaks in its X-ray image. A bright elliptical galaxy,
observable in the central peak, allows the localization of the group at z =
0.083. We found a significant hot spot visible in the X-ray hardness map, close
to the second peak. The spectroscopic temperature is T = 1.6+0.4-0.1 keV within
R500 = 0.6 Mpc and T = 3 - 5 keV in the hot spot. We interpret those results as
Suzaku J1552+2739 being located in the center of a major merging process. The
observation of a galaxy group showing multiple X-ray peaks and a hot spot at
the same time is rare and we believe in particular that the study of Suzaku
J1552+2739 potentially presents a significant interest to better understand the
dynamical and thermal evolution of the intragroup and intracluster medium, as
well as its relation with surrounding environment.Comment: 6 pages, 4 figures 1 table, Accepted by ApJ Letter
Connecting Galaxy Evolution, Star Formation and the X-ray Background
As a result of deep hard X-ray observations by Chandra and XMM-Newton a
significant fraction of the cosmic X-ray background (CXRB) has been resolved
into individual sources. These objects are almost all active galactic nuclei
(AGN) and optical followup observations find that they are mostly obscured Type
2 AGN, have Seyfert-like X-ray luminosities (i.e., L_X ~ 10^{43-44} ergs
s^{-1}), and peak in redshift at z~0.7. Since this redshift is similar to the
peak in the cosmic star-formation rate, this paper proposes that the obscuring
material required for AGN unification is regulated by star-formation within the
host galaxy. We test this idea by computing CXRB synthesis models with a ratio
of Type 2/Type 1 AGN that is a function of both z and 2-10 keV X-ray
luminosity, L_X. The evolutionary models are constrained by parameterizing the
observed Type 1 AGN fractions from the recent work by Barger et al. The
parameterization which simultaneously best accounts for Barger's data, the CXRB
spectrum and the X-ray number counts has a local, low-L_X Type 2/Type 1 ratio
of 4, and predicts a Type 2 AGN fraction which evolves as (1+z)^{0.3}. Models
with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN
fractions. This particular evolution predicts a Type 2/Type 1 ratio of 1-2 for
log L_X > 44, and thus the deep X-ray surveys are missing about half the
obscured AGN with these luminosities. These objects are likely to be Compton
thick. Overall, these calculations show that the current data strongly supports
a change to the AGN unification scenario where the obscuration is connected
with star formation in the host galaxy rather than a molecular torus alone. The
evolution of the obscuration implies a close relationship between star
formation and AGN fueling, most likely due to minor mergers or interactions.Comment: 36 pages, 8 figures, ApJ in press. Minor changes to match published
versio
- …