100 research outputs found
Shocked H2 and Fe+ Dynamics in the Orion Bullets
Observations of H2 velocity profiles in the two most clearly defined Orion
bullets are extremely difficult to reconcile with existing steady-state shock
models. We have observed [FeII] 1.644um velocity profiles of selected bullets
and H2 1-0 S(1) 2.122um velocity profiles for a series of positions along and
across the corresponding bow-shaped shock fronts driven into the surrounding
molecular cloud. Integrated [FeII] velocity profiles of the brightest bullets
are consistent with theoretical bow shock predictions. However, observations of
broad, singly-peaked H2 1-0 S(1) profiles at a range of positions within the
most clearly resolved bullet wakes are not consistent with molecular shock
modelling. A uniform, collisionally broadened background component which
pervades the region in both tracers is inconsistent with fluorescence due to
the ionizing radiation of the Trapezium stars alone.Comment: 20 pages including 18 figures, published in MNRA
The X-ray afterglows of gamma-ray bursts GRB 001025A and GRB 010220 observed with XMM-Newton
The X-ray afterglows of GRB 001025A and GRB 010220 were detected by
XMM-Newton with an average 0.2--10.0 keV flux of 4.4 and 3.3x10^{-14} erg
cm^{-2} s^{-1} respectively; the afterglow of GRB 001025A is observed to decay.
Afterglows at other wavelengths were not detected for either burst. A set of
broadened soft X-ray emission lines are detected in the afterglow of GRB
001025A, at 5.0 sigma significance above a Galactic-absorbed power-law
continuum. The spectra of both afterglows are significantly better fit by a
variable abundance thermal plasma model than by an absorbed power-law and are
consistent with the observations of GRB 011211, indicating that thermal
emission from light elements may be common in the early X-ray afterglows of
GRBs.Comment: 5 pages, 2 figures; accepted for publication in A&A letter
The soft X-ray blast in the apparently sub-luminous GRB 031203
GRB031203 was a very low apparent luminosity gamma-ray burst (GRB). It was
also the first GRB with a dust-scattered X-ray halo. The observation of the
halo allowed us to infer the presence of a large soft X-ray fluence in the
total burst output. It has, however, also been claimed that GRB031203 was
intrinsically sub-energetic, representative of a class of spectrally hard,
low-energy bursts quite different from other GRBs. Reanalysis of the available
data confirms our original finding that GRB031203 had a very large soft X-ray
component, the time of which can be constrained to within a few minutes after
the burst, implying that while GRB031203 did indeed have a very low apparent
luminosity, it was also very soft. Notions propagated in the literature
regarding the uncertainties in the determination of the soft X-ray fluence from
the halo data and on the available constraints from the hard X-ray data are
addressed: the properties of the scattering dust along the line of sight (grain
sizes, precise location and the geometry) are determined directly from the high
quality X-ray data so that there is little uncertainty about the scatterer;
constraints on the X-ray lightcurve from the Integral spacecraft at the time of
the soft X-ray blast are not complete because of a slew in the spacecraft
pointing shortly after the burst. Claims that GRB031203 was intrinsically
under-energetic and that it represents a deviation from the luminosity-peak
energy relation do not appear to be substantiated by the data, regardless of
whether the soft X-ray component is declared part of the prompt emission or the
afterglow. We conclude that the difference between the soft and hard X-ray
spectra from XMM-Newton and Integral indicate that a second soft pulse probably
occurred in this burst as has been observed in other GRBs, notably GRB050502B.Comment: 4 pages, 2 figures, emulateapj Accepted for publication in ApJ,
scheduled 10 January 2006 issu
X-ray-selected broad absorption line quasi-stellar objects
We study a sample of six X-ray-selected broad absorption line (BAL) quasi-stellar objects (QSOs) from the XMM–Newton Wide Angle Survey. All six objects are classified as BALQSOs using the classic balnicity index, and together they form the largest sample of X-ray-selected BALQSOs. We find evidence for absorption in the X-ray spectra of all six objects. An ionized absorption model applied to an X-ray spectral shape that would be typical for non-BAL QSOs (a power law with energy index α = 0.98) provides acceptable fits to the X-ray spectra of all six objects. The optical to X-ray spectral indices, αOX, of the X-ray-selected BALQSOs, have a mean value of 〈αOX〉 = 1.69 ± 0.05, which is similar to that found for X-ray-selected and optically selected non-BAL QSOs of a similar ultraviolet luminosity. In contrast, optically selected BALQSOs typically have much larger αOX and so are characterized as being X-ray weak. The results imply that X-ray selection yields intrinsically X-ray bright BALQSOs, but their X-ray spectra are absorbed by a similar degree to that seen in optically selected BALQSO samples; X-ray absorption appears to be ubiquitous in BALQSOs, but X-ray weakness is not. We argue that BALQSOs sit at one end of a spectrum of X-ray absorption properties in QSOs related to the degree of ultraviolet absorption in C iv 1550 Å
ESO Imaging Survey: Optical follow-up of 12 selected XMM-Newton fields
(Abridged) This paper presents the data recently released for the
XMM-Newton/WFI survey carried out as part of the ESO Imaging Survey (EIS)
project. The aim of this survey is to provide optical imaging follow-up data in
BVRI for identification of serendipitously detected X-ray sources in selected
XMM-Newton fields. In this paper, fully calibrated individual and stacked
images of 12 fields as well as science-grade catalogs for the 8 fields located
at high-galactic latitude are presented. The data covers an area of \sim 3
square degrees for each of the four passbands. The median limiting magnitudes
(AB system, 2" aperture, 5\sigma detection limit) are 25.20, 24.92, 24.66, and
24.39 mag for B-, V-, R-, and I-band, respectively. These survey products,
together with their logs, are available to the community for science
exploitation in conjunction with their X-ray counterparts. Preliminary results
from the X-ray/optical cross-correlation analysis show that about 61% of the
detected X-ray point sources in deep XMM-Newton exposures have at least one
optical counterpart within 2" radius down to R \simeq 25 mag, 50% of which are
so faint as to require VLT observations thereby meeting one of the top
requirements of the survey, namely to produce large samples for spectroscopic
follow-up with the VLT, whereas only 15% of the objects have counterparts down
to the DSS limiting magnitude.Comment: 24 pages, 10 figures, accepted for publication in Astronomy and
Astrophysics. Accompanying data releases available at
http://archive.eso.org/archive/public_datasets.html (WFI images),
http://www.eso.org/science/eis/surveys/release_65000025_XMM.html (optical
catalogs), http://www.aip.de/groups/xray/XMM_EIS/ (X-ray data). Full
resolution version available at
http://www.astro.uni-bonn.de/~dietrich/publications/3785.ps.g
Exploring the X-ray Sky with the XMM-Newton Bright Serendipitous Survey
ABRIDGED. We present "The XMM-Newton Bright Serendipitous Survey", two
flux-limited samples with flux limit fx ~7E-14 cgs in the 0.5-4.5 keV (BSS) and
4.5-7.5 keV (HBSS) energy band, respectively. After discussing the survey
strategy, we present the basic data on a complete sample of 400 X-ray sources
derived from the analysis of 237 XMM-Newton fields. The survey covers an area
of 28.10 (25.17 for the HBSS) sq. deg. Up to now ~71% (~90%) of the sources
have been spectroscopically identified making the BSS (HBSS) the sample with
the highest number of identified XMM-Newton sources published so far. We find
that: a) the optical counterpart in the majority (~90%) of cases has a
magnitude brighter than the POSS II limit; b) the majority of the objects
identified so far are broad line AGN. No obvious trend of the source spectra
(as deduced from the Hardness Ratios analysis) as a function of the count rate
is seen and the average spectra of the "extragalactic" population corresponds
to an X-ray energy spectral index of ~0.8 (~0.64) for the BSS (HBSS) sample.
About 13% (40%) of the sources are described by an energy spectral index
flatter than that of the CXB. We measure a surface density of optically type 1
and type 2 AGN of 1.63+/-0.25 deg-2 and 0.83+/-0.18$ deg-2, respectively;
optically type 2 AGN represent 34+/-9% of the total AGN population. Finally, we
find a clear separation, in the hardness ratio diagram and in the (hardness
ratio) vs. (X-ray to optical flux ratio) diagram, between "coronal emitting"
stars and extragalactic sources.Comment: 42 pages, 7 figures, 8 Tables, Accepted for publication in A&
The discovery of an evolving dust scattered X-ray halo around GRB 031203
We report the first detection of a time-dependent, dust-scattered X-ray halo
around a gamma-ray burst. GRB 031203 was observed by XMM-Newton starting six
hours after the burst. The halo appeared as concentric ring-like structures
centered on the GRB location. The radii of these structures increased with time
as t^{1/2}, consistent with small-angle X-ray scattering caused by a large
column of dust along the line of sight to a cosmologically distant GRB. The
rings are due to dust concentrated in two distinct slabs in the Galaxy located
at distances of 880 and 1390 pc, consistent with known Galactic features. The
halo brightness implies an initial soft X-ray pulse consistent with the
observed GRB.Comment: 4 pages. 4 figures. Accepted for publication in ApJ Letter
A multi-wavelength survey of AGN in the XMM-LSS field: I. Quasar selection via the KX technique
AIMS: We present a sample of candidate quasars selected using the
KX-technique. The data cover 0.68 deg^2 of the X-ray Multi-Mirror (XMM)
Large-Scale Structure (LSS) survey area where overlapping multi-wavelength
imaging data permits an investigation of the physical nature of selected
sources. METHODS: The KX method identifies quasars on the basis of their
optical (R and z') to near-infrared (Ks) photometry and point-like morphology.
We combine these data with optical (u*,g'r',i',z') and mid-infrared (3.6-24
micron) wavebands to reconstruct the spectral energy distributions (SEDs) of
candidate quasars. RESULTS: Of 93 sources selected as candidate quasars by the
KX method, 25 are classified as quasars by the subsequent SED analysis.
Spectroscopic observations are available for 12/25 of these sources and confirm
the quasar hypothesis in each case. Even more, 90% of the SED-classified
quasars show X-ray emission, a property not shared by any of the false
candidates in the KX-selected sample. Applying a photometric redshift analysis
to the sources without spectroscopy indicates that the 25 sources classified as
quasars occupy the interval 0.7 < z < 2.5. The remaining 68/93 sources are
classified as stars and unresolved galaxies.Comment: 13 pages, 9 figures, A&A 494, p. 579-589. Replaced with published
version. Fig. 9 in first astro-ph submission has been update
The XMM-Newton Wide Angle Survey (XWAS): the X-ray spectrum of type-1 AGN
We discuss the broad band X-ray properties of one of the largest samples of
X-ray selected type-1 AGN to date (487 objects in total), drawn from the
XMM-Newton Wide Angle Survey. The objects cover 2-10 keV luminosities from
~10^{42}-10^{45} erg s^{-1} and are detected up to redshift ~4. We constrain
the overall properties of the broad band continuum, soft excess and X-ray
absorption, along with their dependence on the X-ray luminosity and redshift
and we discuss the implications for models of AGN emission. We constrained the
mean spectral index of the broad band X-ray continuum to =1.96+-0.02
with intrinsic dispersion sigma=0.27_{-0.02}^{+0.01}. The continuum becomes
harder at faint fluxes and at higher redshifts and luminosities. The dependence
of Gamma with flux is likely due to undetected absorption rather than to
spectral variation. We found a strong dependence of the detection efficiency of
objects on the spectral shape which can have a strong impact on the measured
mean continuum shapes of sources at different redshifts and luminosities. We
detected excess absorption in ~3% of our objects, with column densities ~a few
x10^{22} cm^{-2}. The apparent mismatch between the optical classification and
X-ray properties of these objects is a challenge for the standard AGN
unification model. We found that the fraction of objects with detected soft
excess is ~36%. Using a thermal model, we constrained the soft excess mean
temperature and intrinsic dispersion to ~100 eV and sigma~34 eV. The origin
of the soft excess as thermal emission from the accretion disk or Compton
scattered disk emission is ruled out on the basis of the temperatures detected
and the lack of correlation of the measured temperature with the X-ray
luminosity (abridged).Comment: 13 pages, 24 figures, Accepted for publication in Astronomy and
Astrophysic
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