461 research outputs found
The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. II: application to the HRI and first results
The wavelet detection algorithm (WDA) described in the accompanying paper by
Lazzati et al. is made suited for a fast and efficient analysis of images taken
with the High Resolution Imager (HRI) instrument on board the ROSAT satellite.
An extensive testing is carried out on the detection pipeline: HRI fields with
different exposure times are simulated and analysed in the same fashion as the
real data. Positions are recovered with few arcsecond errors, whereas fluxes
are within a factor of two from their input values in more than 90% of the
cases in the deepest images. At variance with the ``sliding-box'' detection
algorithms, the WDA provides also a reliable description of the source
extension, allowing for a complete search of e.g. supernova remnant or cluster
of galaxies in the HRI fields. A completeness analysis on simulated fields
shows that for the deepest exposures considered (~120 ks) a limiting flux of
\~3x10^{-15} erg/cm2/s can be reached over the entire field of view. We test
the algorithm on real HRI fields selected for their crowding and/or presence of
extended or bright sources (e.g. cluster of galaxies and of stars, supernova
remnants). We show that our algorithm compares favorably with other X-ray
detection algorithms such as XIMAGE and EXSAS. A complete catalog will result
from our analysis: it will consist of the Brera Multi-scale Wavelet Bright
Source Catalog (BMW-BSC) with sources detected with a significance >4.5 sigma
and of the Faint Source Catalog (BMW-FSC) with sources at >3.5 sigma. A
conservative estimate based on the extragalactic log(N)-log(S) indicates that
at least 16000 sources will be revealed in the complete analysis of the whole
HRI dataset.Comment: 6 pages, 11 PostScript figures, 1 gif figure, ApJ in pres
Flash ionization of the partially ionized wind of the progenitor of SN 1987A
The H II region created by the progenitor of SN 1987A was further heated and
ionized by the supernova flash. Prior to the flash, the temperature of the gas
was 4000 - 5000 K, and helium was neutral, while the post-flash temperature was
only slightly less than 10^5 K, with the gas being ionized to helium-like
ionization stages of C, N and O. We have followed the slow post-flash cooling
and recombination of the gas, as well as its line emission, and find that the
strongest lines are N V 1240 and O VI 1034. Both these lines are good probes
for the density of the gas, and suitable instruments to detect the lines are
STIS on HST and FUSE, respectively. Other lines which may be detectable are N
IV] 1486 and [O III] 5007, though they are expected to be substantially weaker.
The relative strength of the oxygen lines is found to be a good tracer of the
color temperature of the supernova flash. From previous observations, we put
limits on the hydrogen density, n_H, of the H II region. The early N V 1240
flux measured by IUE gives an upper limit which is n_H ~ 180 \eta^{-0.40}
cm^{-3}, where \eta is the filling factor of the gas. The recently reported
emission in [O III] 5007 at 2500 days requires n_H = (160\pm12) \eta^{-0.19}
cm^{-3}, for a supernova burst similar to that in the 500full1 model of Ensman
& Burrows (1992). For the more energetic 500full2 burst the density is n_H =
(215\pm15) \eta^{-0.19} cm^{-3}. These values are much higher than in models of
the X-ray emission from the supernova (n_H ~ 75 cm^{-3}), and it seems
plausible that the observed [O III] emission is produced primarily elsewhere
than in the H II region. We also discuss the type of progenitor consistent with
the H II region. In particular, it seems unlikely that its spectral type was
much earlier than B2 Ia.Comment: LaTeX, 23 pages including 4 figures. To appear in ApJ (Main Journal
The Interpretations For the Low and High Frequency QPO Correlations of X-ray Sources Among White Dwarfs, Neutron Stars and Black Holes
It is found that there exists an empirical linear relation between the high
frequency \nhigh and low frequency \nlow of quasi-periodic oscillations
(QPOs) for black hole candidate (BHC), neutron star (NS) and white dwarf (WD)
in the binary systems, which spans five orders of magnitude in frequency.
For the NS Z (Atoll) sources,
and are identified as the lower kHz QPO frequency
and horizontal branch oscillations (HBOs) \nh (broad noise components); for
the black hole candidates and low-luminosity neutron stars, they are the QPOs
and broad noise components at frequencies between 1 and 10 Hz; for WDs, they
are the ``dwarf nova oscillations'' (DNOs) and QPOs of cataclysmic variables
(CVs). To interpret this relation, our model ascribes to the
Alfv\'en wave oscillation frequency at a preferred radius and to
the same mechanism at another radius. Then, we can obtain \nlow = 0.08
\nhigh and the relation between the upper kHz QPO frequency \nt and HBO to
be \nh \simeq 56 ({\rm Hz}) (\nt/{\rm kHz})^{2}, which are in accordance with
the observed empirical relations. Furthermore, some implications of model are
discussed, including why QPO frequencies of white dwarfs and neutron stars span
five orders of magnitude in frequency. \\Comment: 11 pages, 1 figure, accepted by PAS
Heating of the IGM
Using the cosmic virial theorem, Press-Schechter analysis and numerical
simulations, we compute the expected X-ray background (XRB) from the diffuse
IGM with the clumping factor expected from gravitational shock heating. The
predicted fluxes and temperatures are excluded from the observed XRB. The
predicted clumping can be reduced by entropy injection. The required energy is
computed from the two-point correlation function, as well as from
Press-Schechter formalisms. The minimal energy injection of 1 keV/nucleon
excludes radiative or gravitational heating as a primary energy source. We
argue that the intergalactic medium (IGM) must have been heated through violent
processes such as massive supernova bursts. If the heating proceeded through
supernova explosions, it likely proceeded in bursts which may be observable in
high redshift supernova searches. Within our model we reproduce the observed
cluster luminosity-temperature relation with energy injection of 1 keV/nucleon
if this injection is assumed to be uncorrelated with the local density. These
parameters predict that the diffuse IGM soft XRB has a temperature of ~1 keV
with a flux near 10 keV/cm^2 s str keV, which may be detectable in the near
future.Comment: to appear in ApJ Lett., 11 pages incl 1 figur
Gravitational Lensing of the X-Ray Background by Clusters of Galaxies
Gravitational lensing by clusters of galaxies affects the cosmic X-ray
background (XRB) by altering the observed density and flux distribution of
background X-ray sources. At faint detection flux thresholds, the resolved
X-ray sources appear brighter and diluted, while the unresolved component of
the XRB appears dimmer and more anisotropic, due to lensing. The diffuse X-ray
intensity in the outer halos of clusters might be lower than the sky-averaged
XRB, after the subtraction of resolved sources. Detection of the lensing signal
with a wide-field X-ray telescope could probe the mass distribution of a
cluster out to its virialization boundary. In particular, we show that the
lensing signature imprinted on the resolved component of the XRB by the cluster
A1689, should be difficult but possible to detect out to 8' at the 2-4 sigma
level, after 10^6 seconds of observation with the forthcoming AXAF satellite.
The lensing signal is fairly insensitive to the lens redshift in the range
0.1<z<0.6. The amplitude of the lensing signal is however sensitive to the
faint end slope of the number-flux relation for unresolved X-ray sources, and
can thus help constrain models of the XRB. A search for X-ray arcs or arclets
could identify the fraction of all faint sources which originate from extended
emission of distant galaxies. The probability for a 3 sigma detection of an
arclet which is stretched by a factor of about 3 after a 10^6 seconds
observation of A1689 with AXAF, is roughly comparable to the fraction of all
background X-ray sources that have an intrinsic size of order 1''.Comment: 41 LaTeX pages, 11 postscript figures, 1 table, in AASTeX v4.0
format. To appear in ApJ, April 1, 1997, Vol. 47
The X-ray surface brightness distribution from diffuse gas
We use simulations to predict the X-ray surface brightness distribution
arising from hot, cosmologically distributed diffuse gas. The distribution is
computed for two bands: 0.5-2 keV and 0.1-0.4 keV, using a
cosmological-constant dominated cosmology that fits many other observations. We
examine a number of numerical issues such as resolution, simulation volume and
pixel size and show that the predicted mean background is sensitive to
resolution such that higher resolution systematically increases the mean
predicted background. Although this means that we can compute only lower bounds
to the predicted level, these bounds are already quite restrictive. Since the
observed extra-galactic X-ray background is mostly accounted for by compact
sources, the amount of the observed background attributable to diffuse gas is
tightly constrained. We show that without physical processes in addition to
those included in the simulations (such as radiative cooling or
non-gravitational heating), both bands exceed observational limits. In order to
examine the effect of non-gravitational heating we explore a simple modeling of
energy injection and show that substantial amounts of heating are required
(i.e. 5 keV per particle when averaged over all baryons). Finally, we also
compute the distribution of surface brightness on the sky and show that it has
a well-resolved characteristic shape. This shape is substantially modified by
non-gravitational heating and can be used as a probe of such energy injection.Comment: 11 pages, 11 figures, submitted to Ap
High Resolution X-Ray Spectroscopy of SN 1987A: Monitoring with XMM-Newton
We report the results of our XMM-Newton monitoring of SN 1987A. The ongoing
propagation of the supernova blast wave through the inner circumstellar ring
caused a drastic increase in X-ray luminosity during the last years, enabling
detailed high resolution X-ray spectroscopy with the Reflection Grating
Spectrometer. The observations can be used to follow the detailed evolution of
the arising supernova remnant. The fluxes and broadening of the numerous
emission lines seen in the dispersed spectra provide information on the
evolution of the X-ray emitting plasma and its dynamics. These were analyzed in
combination with the EPIC-pn spectra, which allow a precise determination of
the higher temperature plasma. We modeled individual emission lines and fitted
plasma emission models. Especially from the observations between 2003 and 2007
we can see a significant evolution of the plasma parameters and a deceleration
of the radial velocity of the lower temperature plasma regions. We found an
indication (3-sigma-level) of an iron K feature in the co-added EPIC-pn
spectra. The comparison with Chandra grating observations in 2004 yields a
clear temporal coherence of the spectral evolution and the sudden deceleration
of the expansion velocity seen in X-ray images ~6100 days after the explosion.Comment: 10 pages, 8 Figures; accepted by A&
XMM-Newton observation of the Lockman Hole; I. The X-ray Data
We report on the first deep X-ray survey with the XMM-Newton observatory
during the performance verification phase. The field of the Lockman Hole, one
of the best studied sky areas over a very wide range of wavelengths, has been
observed. A total of ~100 ksec good exposure time has been accumulated.
Combining the images of the European Photon Imaging Camera (EPIC) detectors we
reach a flux limit of 0.31, 1.4 and 2.4 X 10^{-15} erg cm^{-2} s^{-1},
respectively in the 0.5-2, 2-10, and 5-10 keV band. Within an off-axis angle of
10 arcmin we detect 148, 112 and 61 sources, respectively. The log(N)-log(S)
relation in the three bands is compared with previous results. In particular in
the 5-10 keV band these observations present the deepest X-ray survey ever,
about a factor 20 more sensitive than the previous BeppoSAX observations. Using
X-ray spectral diagnostics and the set of previously known, spectroscopically
identified ROSAT sources in the field, the new sources can be classified.
XMM-Newton detects a significant number (~40%) of X-ray sources with hard,
probably intrinsically absorbed X-ray spectra, confirming a prediction of the
population synthesis models for the X-ray background.Comment: 6 pages, based on the new A&A style file (included), to be published
in A&A 365 (2001, special XMM issue), one page contains color image
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