217 research outputs found
Wide-Field Chandra X-Ray Observations of AGN in Abell 85 & Abell 754
To better understand the mechanism or mechanisms that lead to AGN activity
today, we measure the X-ray AGN fraction in a new sample of nearby clusters and
examine how it varies with galaxy properties, projected cluster-centric radius,
and cluster velocity dispersion. We present new wide-field Chandra X-ray
Observatory observations of Abell 85, Abell 754 and the background cluster
Abell 89B out to their virial radii. Out of seventeen X-ray sources associated
with galaxies in these clusters, we classify seven as X-ray AGN with L_{X,B} >
10^{41} erg/s. Only two of these would be classified as AGN based on their
optical spectra. We combine these observations with archival data to create a
sample of X-ray AGN from six z < 0.08 clusters and find that 3.4+1.1/-0.8% of
M_R 10^{41} erg/s. We find that
more X-ray AGN are detected in more luminous galaxies and attribute this to
larger spheriods in more luminous galaxies and increased sensitivity to lower
Eddington-rate accretion from black holes in those spheroids. At a given X-ray
luminosity limit, more massive black holes can be accreting less efficiently,
yet still be detected. If interactions between galaxies are the principal
drivers of AGN activity, then the AGN fraction should be higher in lower
velocity dispersion clusters and the outskirts of clusters. However, the
tendency of the most massive and early-type galaxies to lie in the centers of
the richest clusters could dilute such trends. While we find no variation in
the AGN fraction with projected cluster-centric radius, we do find that the AGN
fraction increases significantly from 2.6+1.0/-0.8% in rich clusters to
10.0+6.2/-4.3% in those with lower velocity dispersions.Comment: Accepted by Astrophysical Journal, 17 pages using emulateapj.cls, 10
B & W Figures (degraded): Full resolution paper available at
http://www.astronomy.ohio-state.edu/~sivakoff/AGN/XAGN_A85_A754.pd
The Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115 (III): luminosity functions of LMXBs and dependence on stellar environments
We have studied the X-ray luminosity function (XLF) of low-mass X-ray
binaries (LMXBs) in the nearby lenticular galaxy NGC 3115, using the Megasecond
Chandra X-Ray Visionary Project Observation. With a total exposure time of ~1.1
Ms, we constructed the XLF down to a limiting luminosity of ~10^36 erg/s, much
deeper than typically reached for other early-type galaxies. We found
significant flattening of the overall LMXB XLF from dN/dL \propto
L^{-2.2\pm0.4} above 5.5x10^37 erg/s to dN/dL \propto L^{-1.0\pm0.1} below it,
though we could not rule out a fit with a higher break at ~1.6x10^38 erg/s. We
also found evidence that the XLF of LMXBs in globular clusters (GCs) is overall
flatter than that of field LMXBs. Thus our results for this galaxy do not
support the idea that all LMXBs are formed in GCs. The XLF of field LMXBs seems
to show spatial variation, with the XLF in the inner region of the galaxy being
flatter than that in the outer region, probably due to contamination of LMXBs
from undetected and/or disrupted GCs in the inner region. The XLF in the outer
region is probably the XLF of primordial field LMXBs, exhibiting dN/dL \propto
L^{-1.2\pm0.1} up to a break close to the Eddington limit of neutron star LMXBs
(~1.7x10^38 erg/s). The break of the GC LMXB XLF is lower, at ~1.1x10^37 erg/s.
We also confirm previous findings that the metal-rich/red GCs are more likely
to host LMXBs than the metal-poor/blue GCs, which is more significant for more
luminous LMXBs, and that more massive GCs are more likely to host LMXBs.Comment: 12 pages, 8 figures, accepted for publication in Ap
Metallicity Effect on LMXB Formation in Globular Clusters
We present comprehensive observational results of the metallicity effect on
the fraction of globular clusters (GC) that contain low-mass X-ray binaries
(LMXB), by utilizing all available data obtained with Chandra for LMXBs and HST
ACS for GCs. Our primary sample consists of old elliptical galaxies selected
from the ACS Virgo and Fornax surveys. To improve statistics at both the lowest
and highest X-ray luminosity, we also use previously reported results from
other galaxies. It is well known that the LMXB fraction is considerably higher
in red, metal-rich, than in blue, metal-poor GCs. In this paper, we test
whether this metallicity effect is X-ray luminosity-dependent, and find that
the effect holds uniformly in a wide luminosity range. This result is
statistically significant (at >= 3 sigma) in LMXBs with luminosities in the
range LX = 2 x 10^37 - 5 x 10^38 erg s-1, where the ratio of LMXB fractions in
metal-rich to metal-poor GCs is R = 3.4 +- 0.5. A similar ratio is also found
at lower (down to 10^36 erg s-1) and higher luminosities (up to the ULX
regime), but with less significance (~2 sigma confidence). Because different
types of LMXBs dominate in different luminosities, our finding requires a new
explanation for the metallicity effect in dynamically formed LMXBs. We confirm
that the metallicity effect is not affected by other factors such as stellar
age, GC mass, stellar encounter rate, and galacto-centric distance.Comment: 21 pages, 5 figures, accepted in Ap
The Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115 (II): properties of point sources
We have carried out an in-depth study of low-mass X-ray binaries (LMXBs)
detected in the nearby lenticular galaxy NGC 3115, using the Megasecond Chandra
X-Ray Visionary Project observation (total exposure time 1.1 Ms). In total we
found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above
2\sigma\ detection, with 0.3--8 keV luminosity L_X ~10^36-10^39 erg/s. Other
than 13 transient candidates, the sources overall have less long-term
variability at higher luminosity, at least at L_X > 2x10^37 erg/s. In order to
identify the nature and spectral state of our sources, we compared their
collective spectral properties based on single-component models (a simple power
law or a multicolor disk) with the spectral evolution seen in representative
Galactic LMXBs. We found that in the L_X versus photon index \Gamma_PL and L_X
versus disk temperature kT_MCD plots, most of our sources fall on a narrow
track in which the spectral shape hardens with increasing luminosity below
L_X~7x10^37 erg/s but is relatively constant (\Gamma_PL~1.5 or kT_MCD~1.5 keV)
above this luminosity, similar to the spectral evolution of Galactic neutron
star (NS) LMXBs in the soft state in the Chandra bandpass. Therefore we
identified the track as the NS LMXB soft-state track and suggested sources with
L_X7x10^37 erg/s
as Z sources. Ten other sources (five are transients) displayed significantly
softer spectra and are probably black hole X-ray binaries in the thermal state.
One of them (persistent) is in a metal-poor GC.Comment: 26 pages, 9 figures, four online tables, accepted for publication in
Ap
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