42 research outputs found
Small-Scale structure in the Galactic ISM: Implications for Galaxy Cluster Studies
Observations of extragalactic objects need to be corrected for Galactic
absorption and this is often accomplished by using the measured 21 cm HI
column. However, within the beam of the radio telescope there are variations in
the HI column that can have important effects in interpreting absorption line
studies and X-ray spectra at the softest energies. We examine the HI and
DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for
HI variations in of up to a factor of three in 1 degree fields. Column density
enhancements would preferentially absorb soft X-rays in spatially extended
objects and we find evidence for this effect in the ROSAT PSPC observations of
two bright clusters of galaxies, Abell 119 and Abell 2142.
For clusters of galaxies, the failure to include column density fluctuations
will lead to systematically incorrect fits to the X-ray data in the sense that
there will appear to be a very soft X-ray excess. This may be one cause of the
soft X-ray excess in clusters, since the magnitude of the effect is comparable
to the observed values.Comment: 16 pages, 9 figures, to appear in the Astrophysical Journal, vol. 597
(1 Nov 2003
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
TeX fil
The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center
We examine the X-ray spectra and variability of the sample of X-ray sources
with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the
Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We
find that the spectra of the point sources near the Galactic center are very
hard between 2--8 keV, even after accounting for absorption. When modeled as
power laws the median photon index is Gamma=0.7, while when modeled as thermal
plasma we can only obtain lower limits to the temperature of kT>8 keV. The
combined spectra of the point sources is similarly hard, with a photon index of
Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and
H-like Fe, both in the average spectra and in the brightest individual sources.
The line ratios of the highly-ionized Fe in the average spectra are consistent
with emission from a plasma in thermal equilibrium. This line emission is
observed whether average spectra are examined as a function of the count rate
from the source, or as a function of the hardness ratios of individual sources.
This suggests that the hardness of the spectra may in fact to due local
absorption that partially-covers the X-ray emitting regions in the Galactic
center systems. We suggest that most of these sources are intermediate polars,
which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely
exhibit either flares on short time scales or changes in their mean X-ray flux
on long time scales, and (3) are the most numerous hard X-ray sources with
comparable luminosities in the Galaxy.Comment: 27 pages, including 13 figures. To appear in ApJ, 1 October 2004,
v613 issue. An electronic version of table 2 is on
http://astro.ucla.edu/~mmuno/sgra/table2_electronic.txt and reduced data
files for each source are available on
http://www.astro.psu.edu/users/niel/galcen-xray-data/galcen-xray-data.htm
AGN in the XMM-Newton first-light image as probes for the interstellar medium in the LMC
The XMM-Newton first-light image revealed X-ray point sources which show
heavily absorbed power-law spectra. The spectral indices and the probable
identification of a radio counterpart for the brightest source suggest AGN
shining through the interstellar gas of the Large Magellanic Cloud (LMC). The
column densities derived from the X-ray spectra in combination with HI
measurements will allow to draw conclusions on HI to H_2 ratios in the LMC and
compare these with values found for the galactic plane.Comment: 4 pages, LaTex, 4 figures, Accepted for publication in A&A Letter
Maximum Entropy Reconstruction of the Interstellar Medium: I. Theory
We have developed a technique to map the three-dimensional structure of the
local interstellar medium using a maximum entropy reconstruction technique. A
set of column densities N to stars of known distance can in principle be used
to recover a three-dimensional density field n, since the two quantities are
related by simple geometry through the equation N = C n, where C is a matrix
characterizing the stellar spatial distribution. In practice, however, there is
an infinte number of solutions to this equation. We use a maximum entropy
reconstruction algorithm to find the density field containing the least
information which is consistent with the observations. The solution obtained
with this technique is, in some sense, the model containing the minimum
structure. We apply the algorithm to several simulated data sets to demonstrate
its feasibility and success at recovering ``real'' density contrasts.
This technique can be applied to any set of column densities whose end points
are specified. In a subsequent paper we shall describe the application of this
method to a set of stellar color excesses to derive a map of the dust
distribution, and to soft X-ray absorption columns to hot stars to derive a map
of the total density of the interstellar medium.Comment: 23 pages, 7 fig.; accepted for publication in the Ap.
An X-ray Investigation of Three Supernova Remnants in the Large Magellanic Cloud
We have investigated three SNRs in the LMC using multi-wavelength data. These
SNRs are generally fainter than the known sample and may represent a previously
missed population. One of our SNRs is the second LMC remnant analyzed which is
larger than any Galactic remnant for which a definite size has been
established. The analysis of such a large remnant contributes to the
understanding of the population of highly evolved SNRs. We have obtained X-ray
images and spectra of three of these recently identified SNRs using the
XMM-Newton observatory. These data, in conjunction with pre-existing optical
emission-line images and spectra, were used to determine the physical
conditions of the optical- and X-ray-emitting gas in the SNRs. We have compared
the morphologies of the SNRs in the different wavebands. The physical
properties of the warm ionized shell were determined from the H-alpha surface
brightness and the SNR expansion velocity. The X-ray spectra were fit with a
thermal plasma model and the physical conditions of the hot gas were derived
from the model fits. Finally, we have compared our observations with
simulations of SNR evolution
On the Internal Absorption of Galaxy Clusters
A study of the cores of galaxy clusters with the Einstein SSS indicated the
presence of absorbing material corresponding to 1E+12 Msun of cold cluster gas,
possibly resulting from cooling flows. Since this amount of cold gas is not
confirmed by observations at other wavelengths, we examined whether this excess
absorption is present in the ROSAT PSPC observations of 20 bright galaxy
clusters. For 3/4 of the clusters, successful spectral fits were obtained with
absorption due only to the Galaxy, and therefore no extra absorption is needed
within the clusters, in disagreement with the results from the Einstein SSS
data for some of the same clusters. For 1/4 of the clusters, none of our
spectral fits was acceptable, suggesting a more complicated cluster medium than
the two-temperature and cooling flow models considered here. However, even for
these clusters, substantial excess absorption is not indicated.Comment: accepted by the Astrophysical Journa
GRB 070311: a direct link between the prompt emission and the afterglow
We present prompt gamma-ray, early NIR/optical, late optical and X-ray
observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to
gain clues on the mechanisms responsible for the prompt gamma-ray pulse as well
as for the early and late multi-band afterglow of GRB 070311. We fitted with
empirical functions the gamma-ray and optical light curves and scaled the
result to the late time X-rays. The H-band light curve taken by REM shows two
pulses peaking 80 and 140 s after the peak of the gamma-ray burst and possibly
accompanied by a faint gamma-ray tail. Remarkably, the late optical and X-ray
afterglow underwent a major rebrightening between 3x10^4 and 2x10^5 s after the
burst with an X-ray fluence comparable with that of the prompt emission
extrapolated in the same band. Notably, the time profile of the late
rebrightening can be described as the combination of a time-rescaled version of
the prompt gamma-ray pulse and an underlying power law. This result supports a
common origin for both prompt and late X-ray/optical afterglow rebrightening of
GRB 070311 within the external shock scenario. The main fireball would be
responsible for the prompt emission, while a second shell would produce the
rebrightening when impacting the leading blastwave in a refreshed shock
(abridged).Comment: 14 pages, 11 figures, 4 tables, accepted to A&
Gravitational Lensing
Gravitational lensing has developed into one of the most powerful tools for
the analysis of the dark universe. This review summarises the theory of
gravitational lensing, its main current applications and representative results
achieved so far. It has two parts. In the first, starting from the equation of
geodesic deviation, the equations of thin and extended gravitational lensing
are derived. In the second, gravitational lensing by stars and planets,
galaxies, galaxy clusters and large-scale structures is discussed and
summarised.Comment: Invited review article to appear in Classical and Quantum Gravity, 85
pages, 15 figure
The XMM Cluster Survey: X-ray analysis methodology
The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters
using all publicly available data in the XMM-Newton Science Archive. Its main
aims are to measure cosmological parameters and trace the evolution of X-ray
scaling relations. In this paper we describe the data processing methodology
applied to the 5,776 XMM observations used to construct the current XCS source
catalogue. A total of 3,675 > 4-sigma cluster candidates with > 50
background-subtracted X-ray counts are extracted from a total non-overlapping
area suitable for cluster searching of 410 deg^2. Of these, 993 candidates are
detected with > 300 background-subtracted X-ray photon counts, and we
demonstrate that robust temperature measurements can be obtained down to this
count limit. We describe in detail the automated pipelines used to perform the
spectral and surface brightness fitting for these candidates, as well as to
estimate redshifts from the X-ray data alone. A total of 587 (122) X-ray
temperatures to a typical accuracy of < 40 (< 10) per cent have been measured
to date. We also present the methodology adopted for determining the selection
function of the survey, and show that the extended source detection algorithm
is robust to a range of cluster morphologies by inserting mock clusters derived
from hydrodynamical simulations into real XMM images. These tests show that the
simple isothermal beta-profiles is sufficient to capture the essential details
of the cluster population detected in the archival XMM observations. The
redshift follow-up of the XCS cluster sample is presented in a companion paper,
together with a first data release of 503 optically-confirmed clusters.Comment: MNRAS accepted, 45 pages, 38 figures. Our companion paper describing
our optical analysis methodology and presenting a first set of confirmed
clusters has now been submitted to MNRA