148 research outputs found
The Birthplace of Low-Mass X-ray Binaries: Field Versus Globular Cluster Populations
Recent Chandra studies of low-mass X-ray binaries (LMXBs) within early-type
galaxies have found that LMXBs are commonly located within globular clusters of
the galaxies. However, whether all LMXBs are formed within globular clusters
has remained an open question. If all LMXBs formed within globular clusters,
the summed X-ray luminosity of the LMXBs in a galaxy should be directly
proportional to the number of globular clusters in the galaxy regardless of
where the LMXBs currently reside. We have compared these two quantities over
the same angular area for a sample of 12 elliptical and S0 galaxies observed
with Chandra and found that the correlation between the two quantities is
weaker than expected if all LMXBs formed within globular clusters. This
indicates that a significant number of the LMXBs were formed in the field, and
naturally accounts for the spread in field-to-cluster fractions of LMXBs from
galaxy to galaxy. We also find that the "pollution" of globular cluster LMXBs
into the field has been minimal within elliptical galaxies, but there is
evidence that roughly half of the LMXBs originally in the globular clusters of
S0 galaxies in our sample have escaped into the field. This is due to higher
globular cluster disruption rates in S0s resulting from stronger gravitational
shocks caused by the passage of globular clusters through the disks of S0
galaxies that are absent in elliptical galaxies.Comment: To appear in ApJ, 1 October 2005, v631 2 issue, 9 pages, 3 figures,
typos and a few minor issues correcte
Hard X-ray Emission from the M87 AGN Detected with NuSTAR
M87 hosts a 3-6 billion solar mass black hole with a remarkable relativistic
jet that has been regularly monitored in radio to TeV bands. However, hard
X-ray emission \gtrsim 10keV, which would be expected to primarily come from
the jet or the accretion flow, had never been detected from its unresolved
X-ray core. We report NuSTAR detection up to 40 keV from the the central
regions of M87. Together with simultaneous Chandra observations, we have
constrained the dominant hard X-ray emission to be from its unresolved X-ray
core, presumably in its quiescent state. The core spectrum is well fitted by a
power law with photon index Gamma=2.11 (+0.15 -0.11). The measured flux density
at 40 keV is consistent with a jet origin, although emission from the
advection-dominated accretion flow cannot be completely ruled out. The detected
hard X-ray emission is significantly lower than that predicted by synchrotron
self-Compton models introduced to explain emission above a GeV.Comment: 5 pages, 4 figures, updated to better match the published version in
the Astrophysical Journal Letters. A minor typo in the published version
(angular scale should be 1 arcsec = 78 pc instead, no result of the paper is
affected) is fixed her
The Search for Million Degree Gas Through The NVII Hyperfine Line
Gas in the million degree range occurs in a variety of astronomical
environments, and it may be the main component of the elusive missing baryons
at low redshift. The NVII ion is found in this material and it has a hyperfine
spin-flip transition with a rest frequency of 53.042 GHz, which can be observed
for z > 0.1, when it is shifted into a suitably transparent radio band. We used
the 42-48 GHz spectrometer on the Green Bank Telescope to search for both
emission and absorption from this NVII transmission. For absorption studies,
3C273, 3C 279, 3C 345, and 4C+39.25 were observed but no feature were seen
above the 5 sigma level. For emission line studies, we observed Abell 1835,
Abell 2390 and the star-forming galaxy PKS 1345+12, but no features were seen
exceeding 5 sigma. We examine whether the strongest emission feature, in Abell
2390 (3.7 sigma), and the strongest absorption feature, toward 4C+39.25 (3.8
sigma), might be expected from theoretical models. The emission feature would
require ~1E10 Msolar of 1E6 K gas, which is inconsistent with X-ray limits for
the O VII Kalpha line, so it is unlikely to be real. The NVII absorption
feature requires a NVII column of 6E16 cm^-2, higher than model predictions by
at least an order of magnitude, which makes it inconsistent with model
expectations. The individual observations were less than 1 hr in length, so for
lengthy observations, we show that NVII absorption line observations can begin
to be useful in in the search for hot intergalactic gas.Comment: 27 total pages; 16 figures; Accepted for publication in The
Astrophysical Journa
ROSAT X-ray Colors and Emission Mechanisms in Early-Type Galaxies
The X-ray colors and X-ray-to-optical luminosity ratios (L_X/L_B) of 61
early- type galaxies observed with the ROSAT PSPC are determined. The colors
indicate that the X-ray spectral properties of galaxies vary as a function of
L_X/L_B. The brightest X-ray galaxies have colors consistent with thermal
emission from hot gas with roughly the same metallicity of 50% solar. The
spatial variation of the colors indicates that the gas temperature in these
galaxies increases radially. Galaxies with medium L_X/L_B also have spectral
properties consistent with emission from hot gas. If a simple one-component
thermal model is assumed to describe the 0.1-2.0 keV X-ray emission in these
galaxies, then one possible explanation for the progressive decrease in L_X/L_B
among galaxies of this class could be the progressive decrease in metal
abundance of the X-ray emitting contained by the galaxies. Galaxies with the
lowest L_X/L_B values appear to be lacking a hot interstellar component. Their
X-ray colors are consistent with those derived from the bulges of the spiral
galaxies M31 and NGC1291. In M31 the X-ray emission is resolved into discrete
sources, and is apparently due primarily to low mass X-ray binaries (LMXBs). We
therefore suggest that the bulk of the X-ray emission in the faintest
ellipticals is also due to LMXBs. Previously, the X-ray spectra of X-ray faint
galaxies had been found to be described by a hard component which was
attributed to LMXB emission, and a very soft component of unknown origin. We
show that the very soft component also likely results from LMXBs, as a very
soft component is seen in the X-ray spectra of the nearby LMXB Her X-1 and
LMXBs in the bulge of M31. (Abridged)Comment: 49 pages, 17 embedded Postscript figures, uses aaspp4.sty,
Astrophysical Journal, volume 499, in pres
Chandra Observations of Low Mass X-ray Binaries and Diffuse Gas in the Early-Type Galaxies NGC 4365 and NGC 4382 (M85)
(Abridged) We used the Chandra X-ray Observatory ACIS S3 to image the X-ray
faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The
observations resolve much of the X-ray emission into 99 and 58 sources,
respectively, most of which are low-mass X-ray binaries (LMXBs) associated with
each of the galaxies. We identify 18 out of the 37 X-ray sources in a central
field in NGC 4365 with globular clusters. The luminosity functions of the
resolved sources for both galaxies are best fit with cutoff power-laws whose
cutoff luminosity is ergs s. These
luminosities are much larger than those previously measured for similar
galaxies; we do not find evidence for a break in the luminosity function at the
Eddington luminosity of a 1.4 neutron star. The spatial distributions
of the resolved sources for both galaxies are broader than the distribution of
optical stars. In both galaxies, a hard power-law model fits the summed
spectrum of all of the sources. The unresolved emission is best fit by the sum
of a soft mekal model representing emission from diffuse gas, and a hard
power-law, presumed to be from unresolved LMXBs. A standard beta model fits the
radial distribution of the diffuse gas in both galaxies. In the elliptical NGC
4365, the best-fit core radius is very small, while the S0 galaxy NGC 4382 has
a larger core radius. This may indicate that the gas in NGC 4382 is rotating
significantly.Comment: Astrophysical Journal, accepted: 38 pages with 20 embedded reduced
resolution Postscript figure
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