1,437 research outputs found
Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?
The hard state of X-ray binaries (XRBs) is characterized by a power law
spectrum in the X-ray band, and a flat/inverted radio/IR spectrum associated
with occasionally imaged compact jets. It has generally been thought that the
hard X-rays result from Compton upscattering of thermal accretion disk photons
by a hot, coronal plasma whose properties are inferred via spectral fitting.
Interestingly, these properties-especially those from certain magnetized corona
models-are very similar to the derived plasma conditions at the jet footpoints.
Here we explore the question of whether the `corona' and `jet base' are in fact
related, starting by testing the strongest premise that they are synonymous. In
such models, the radio through the soft X-rays are dominated by synchrotron
emission, while the hard X-rays are dominated by inverse Compton at the jet
base - with both disk and synchrotron photons acting as seed photons. The
conditions at the jet base fix the conditions along the rest of the jet, thus
creating a direct link between the X-ray and radio emission. We also add to
this model a simple iron line and convolve the spectrum with neutral
reflection. After forward-folding the predicted spectra through the detector
response functions, we compare the results to simultaneous radio/X-ray data
obtained from the hard states of the Galactic XRBs GX339-4 and Cygnus X-1.
Results from simple Compton corona model fits are also presented for
comparison. We demonstrate that the jet model fits are statistically as good as
the single-component corona model X-ray fits, yet are also able to address the
simultaneous radio data.Comment: Accepted to the Astrophysical Journal. 14 pages, emulateapj.st
On the Absorption of X-rays in the Interstellar Medium
We present an improved model for the absorption of X-rays in the ISM intended
for use with data from future X-ray missions with larger effective areas and
increased energy resolution such as Chandra and XMM, in the energy range above
100eV. Compared to previous work, our formalism includes recent updates to the
photoionization cross section and revised abundances of the interstellar
medium, as well as a treatment of interstellar grains and the H2molecule. We
review the theoretical and observational motivations behind these updates and
provide a subroutine for the X-ray spectral analysis program XSPEC that
incorporates our model.Comment: ApJ, in press, for associated software see
http://astro.uni-tuebingen.de/nh
eROSITA Science Book: Mapping the Structure of the Energetic Universe
eROSITA is the primary instrument on the Russian SRG mission. In the first
four years of scientific operation after its launch, foreseen for 2014, it will
perform a deep survey of the entire X-ray sky. In the soft X-ray band (0.5-2
keV), this will be about 20 times more sensitive than the ROSAT all sky survey,
while in the hard band (2-10 keV) it will provide the first ever true imaging
survey of the sky at those energies. Such a sensitive all-sky survey will
revolutionize our view of the high-energy sky, and calls for major efforts in
synergic, multi-wavelength wide area surveys in order to fully exploit the
scientific potential of the X-ray data. The design-driving science of eROSITA
is the detection of very large samples (~10^5 objects) of galaxy clusters out
to redshifts z>1, in order to study the large scale structure in the Universe,
test and characterize cosmological models including Dark Energy. eROSITA is
also expected to yield a sample of around 3 millions Active Galactic Nuclei,
including both obscured and un-obscured objects, providing a unique view of the
evolution of supermassive black holes within the emerging cosmic structure. The
survey will also provide new insights into a wide range of astrophysical
phenomena, including accreting binaries, active stars and diffuse emission
within the Galaxy, as well as solar system bodies that emit X-rays via the
charge exchange process. Finally, such a deep imaging survey at high spectral
resolution, with its scanning strategy sensitive to a range of variability
timescales from tens of seconds to years, will undoubtedly open up a vast
discovery space for the study of rare, unpredicted, or unpredictable
high-energy astrophysical phenomena. In this living document we present a
comprehensive description of the main scientific goals of the mission, with
strong emphasis on the early survey phases.Comment: 84 Pages, 52 Figures. Published online as MPE document. Edited by S.
Allen. G. Hasinger and K. Nandra. Few minor corrections (typos) and updated
reference
The close T Tauri binary system V4046 Sgr: Rotationally modulated X-ray emission from accretion shocks
We report initial results from a quasi-simultaneous X-ray/optical observing
campaign targeting V4046 Sgr, a close, synchronous-rotating classical T Tauri
star (CTTS) binary in which both components are actively accreting. V4046 Sgr
is a strong X-ray source, with the X-rays mainly arising from high-density (n_e
~ 10^(11-12) cm^(-3)) plasma at temperatures of 3-4 MK. Our multiwavelength
campaign aims to simultaneously constrain the properties of this X-ray emitting
plasma, the large scale magnetic field, and the accretion geometry. In this
paper, we present key results obtained via time-resolved X-ray grating spectra,
gathered in a 360 ks XMM-Newton observation that covered 2.2 system rotations.
We find that the emission lines produced by this high-density plasma display
periodic flux variations with a measured period, 1.22+/-0.01 d, that is
precisely half that of the binary star system (2.42 d). The observed rotational
modulation can be explained assuming that the high-density plasma occupies
small portions of the stellar surfaces, corotating with the stars, and that the
high-density plasma is not azimuthally symmetrically distributed with respect
to the rotational axis of each star. These results strongly support models in
which high-density, X-ray-emitting CTTS plasma is material heated in accretion
shocks, located at the base of accretion flows tied to the system by magnetic
field lines.Comment: paper accepted by Ap
INTEGRAL deep observations of the Small Magellanic Cloud
Deep observations of the Small Magellanic Cloud (SMC) and region were carried
out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field
of view of the instrument permitted simultaneous coverage of the entire SMC and
the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven
sources in the SMC and five in the Magellanic Bridge; the majority of the
sources were previously unknown systems. Several of the new sources were
detected undergoing bright X- ray outbursts and all the sources exhibited
transient behaviour except the supergiant system SMC X-1. They are all thought
to be High Mass X-ray Binary (HMXB) systems in which the compact object is a
neutron star.Comment: 7 pages, 10 figures Accepted for publication in MNRA
Synthetic simulations of the extragalactic sky seen by eROSITA I. Pre-launch selection functions from Monte-Carlo simulations
Context. Studies of galaxy clusters provide stringent constraints on models of structure formation. Provided that selection effects are under control, large X-ray surveys are well suited to derive cosmological parameters, in particular those governing the dark energy equation of state. Aims. We forecast the capabilities of the all-sky eROSITA (extended ROentgen Survey with an Imaging Telescope Array) survey to be achieved by the early 2020s. We bring special attention to modelling the entire chain from photon emission to source detection and cataloguing. Methods. The selection function of galaxy clusters for the upcoming eROSITA mission is investigated by means of extensive and dedicated Monte-Carlo simulations. Employing a combination of accurate instrument characterisation and a state-of-the-art source detection technique, we determine a cluster detection efficiency based on the cluster fluxes and sizes. Results. Using this eROSITA cluster selection function, we find that eROSITA will detect a total of approximately 10(5) clusters in the extra-galactic sky. This number of clusters will allow eROSITA to put stringent constraints on cosmological models. We show that incomplete assumptions on selection effects, such as neglecting the distribution of cluster sizes, induce a bias in the derived value of cosmological parameters. Conclusions. Synthetic simulations of the eROSITA sky capture the essential characteristics impacting the next-generation galaxy cluster surveys and they highlight parameters requiring tight monitoring in order to avoid biases in cosmological analyses.Peer reviewe
The TANAMI Program
TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond
Interferometry) is a monitoring program to study the parsec-scale structures
and dynamics of relativistic jets in active galactic nuclei (AGN) of the
Southern Hemisphere with the Long Baseline Array and associated telescopes.
Extragalactic jets south of -30 degrees declination are observed at 8.4 GHz and
22 GHz every two months at milliarcsecond resolution. The initial TANAMI sample
is a hybrid radio and gamma-ray selected sample since the combination of VLBI
and gamma-ray observations is crucial to understand the broadband emission
characteristics of AGN.Comment: Confernce Proceedings for "X-ray Astronomy 2009" (Bologna), 3 pages,
3 figures, needs cls-fil
Magnetic Fields of Accreting X-Ray Pulsars with the Rossi X-Ray Timing Explorer
Using a consistent set of models, we parameterized the X-ray spectra of all
accreting pulsars in the Rossi X-ray Timing Explorer database which exhibit
Cyclotron Resonance Scattering Features (CRSFs, or cyclotron lines). These
sources in our sample are Her X-1, 4U 0115+63, Cen X-3, 4U 1626-67, XTE
J1946-274, Vela X-1, 4U 1907+09, 4U 1538-52, GX 301-2, and 4U 0352+309 (X Per).
We searched for correlations among the spectral parameters, concentrating on
how the cyclotron line energy relates to the continuum and therefore how the
neutron star B-field influences the X-Ray emission. As expected, we found a
correlation between the CRSF energy and the spectral cutoff energy. However,
with our consistent set of fits we found that the relationship is more complex
than what has been reported previously. Also, we found that not only does the
width of the cyclotron line correlate with the energy (as suggested by theory),
but that the width scaled by the energy correlates with the depth of the
feature. We discuss the implications of these results, including the
possibility that accretion directly affects the relative alignment of the
neutron star spin and dipole axes. Lastly, we comment on the current state of
fitting phenomenological models to spectra in the RXTE/BeppoSAX era and the
need for better theoretical models of the X-Ray continua of accreting pulsars.Comment: 36 Pages, 9 Figures, 9 Tables, ApJ in pres
CXOM31 J004253.1+411422: The first ultra-luminous X-ray transient in M 31
We seek clarification of the nature of X-ray sources detected in M 31. Here
we focus on CXOM31 J004253.1+411422, the brightness of which suggests that it
belongs to the class of ultraluminous X-ray sources. We determine the X-ray
properties of sources detected in the XMM-Newton Chandra monitoring program. We
investigate spectral properties and search for periodic or quasi-periodic
oscillations. A multi-component model is applied to the spectra obtained from
XMM-Newton data to evaluate the relative contributions from thermal and
non-thermal emission. The time dependence of this ratio is evaluated over a
period of forty days. We simultaneously fit data from XMM-Newton EPIC-pn, MOS1
and MOS2 detectors with (non-thermal) powerlaw and (thermal) multicolored
blackbody. The X-ray spectrum is best fit by the combination of a thermal
component with kT ~ 1 keV and a powerlaw component with photon index
approximately 2.6. From combined analysis of Chandra, Swift and XMM-Newton
data, the unabsorbed total luminosity of this source decreases from ~ 3.8 x
10^{39} erg s^{-1} in the first observation to ~ 0.5 x 10^{39} ergs s^{-1} over
a period of three months. The decay closely follows an exponential decline with
a time constant of 32 days. The source spectrum evolves significantly,
exhibiting a faster decline of the thermal component. We do not find evidence
of any significant temporal features in the power density spectrum. The
presence of a thermal component at kT ~ 1 keV in conjunction with a non-thermal
high energy tail, is also consistent with spectral properties of other ULXs in
the "high state". Our analysis indicates that the underlying source of this
first ULX in M~31 is a black hole of mass, M > 13 M_{sun}, accreting near the
Eddington limit, that underwent a transient outburst followed by an exponential
decay reminiscent of transients associated with galactic X-ray novae.Comment: Accepted by Astronomy and Astrophysics. 6 pages, 8 figure
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