2,815 research outputs found
Contribution of Unresolved Point Sources to the Diffuse X-ray Background below 1 keV
We present here the analysis of X-rays point sources detected in several
observations available in the XMM-Newton public archive. We focused, in
particular, on energies below 1 keV, which are of particular relevance to the
understanding of the Diffuse X-ray Background. The average field of all the
exposures is 0.09 deg^-2. We reached an average flux sensitivity of 5.8x10^-16
erg s^-1 cm^-2 in the soft band (0.5-2.0 keV) and 2.5x10^-16 erg s^-1 cm^-2 in
the very soft band (0.4-0.6 keV). In this paper we discuss the logN-logS
results, the contribution to the integrated X-ray sky flux, and the properties
of the cumulative spectrum from all sources. In particular, we found an excess
flux at around 0.5 keV in the composite spectrum of faint sources. The excess
seems to be a general property of all the fields observed suggesting an
additional class of weak sources is contributing to the X-ray emission at these
energies. Combining our results with previous investigations we have also
quantified the contribution of the individual components of the diffuse X-ray
Background in the 3/4 keV band.Comment: Accepted for publication in ApJ; 27 pages, 8 figure
Contemporaneous XMM-Newton investigation of a giant X-ray flare and quiescent state from a cool M-class dwarf in the local cavity
We report the serendipitous detection of a giant X-ray flare from the source
2XMM J043527.2-144301 during an XMM-Newton observation of the high latitude
molecular cloud MBM20. The source has not been previously studied at any
wavelength. The X-ray flux increases by a factor of more than 52 from quiescent
state to peak of flare. A 2MASS counterpart has been identified (2MASS
J04352724-1443017), and near-infrared colors reveal a spectral type of M8-M8.5
and a distance of (67\pm 13) pc, placing the source in front of MBM20. Spectral
analysis and source luminosity are also consistent with this conclusion. The
measured distance makes this object the most distant source (by about a factor
of 4) at this spectral type detected in X-rays. The X-ray flare was
characterized by peak X-ray luminosity of ~8.2E28 erg s-1 and integrated X-ray
energy of ~2.3E32 erg. The flare emission has been characterized with a
2-temperature model with temperatures of ~10 and 46 MK (0.82 and 3.97 keV), and
is dominated by the higher temperature component.Comment: 19 pages, 5 figures; Accepted for publication on Ap
X-Ray Emission from the Warm Hot Intergalactic Medium
The number of detected baryons in the Universe at z<0.5 is much smaller than
predicted by standard big bang nucleosynthesis and by the detailed observation
of the Lyman alpha forest at red-shift z=2. Hydrodynamical simulations indicate
that a large fraction of the baryons today is expected to be in a ``warm-hot''
(10^5-10^7K) filamentary gas, distributed in the intergalactic medium. This
gas, if it exists, should be observable only in the soft X-ray and UV bands.
Using the predictions of a particular hydrodynamic model, we simulated the
expected X-ray flux as a function of energy in the 0.1-2 keV band due to the
Warm-Hot Intergalactic Medium (WHIM), and compared it with the flux from local
and high red-shift diffuse components. Our results show that as much as 20% of
the total diffuse X-ray background (DXB) in the energy range 0.37-0.925keV
could be due to X-ray flux from the WHIM, 70% of which comes from filaments at
redshift z between 0.1 and 0.6. Simulations done using a FOV of 3', comparable
with that of Suzaku and Constellation-X, show that in more than 20% of the
observations we expect the WHIM flux to contribute to more than 20% of the DXB.
These simulations also show that in about 10% of all the observations a single
bright filament in the FOV accounts, alone, for more than 20% of the DXB flux.
Red-shifted oxygen lines should be clearly visible in these observations.Comment: 19 pages, 6 figure
Probing the anisotropy of the Milky Way gaseous halo: Sight-lines toward Mrk 421 and PKS2155-304
(Abridged) We recently found that the halo of the Milky Way contains a large
reservoir of warm-hot gas that contains a large fraction of the missing baryons
from the Galaxy. The average physical properties of this circumgalactic medium
(CGM) are determined by combining average absorption and emission measurements
along several extragalactic sightlines. However, there is a wide distribution
of both, the halo emission measure and the \ovii column density, suggesting
that the Galactic warm-hot gaseous halo is anisotropic. We present {\it Suzaku}
observations of fields close to two sightlines along which we have precise
\ovii absorption measurements with \chandran. The column densities along these
two sightlines are similar within errors, but we find that the emission
measures are different. Therefore the densities and pathlengths in the two
directions must be different, providing a suggestive evidence that the warm-hot
gas in the CGM of the Milky Way is not distributed uniformly. However, the
formal errors on derived parameters are too large to make such a claim. The
average density and pathlength of the two sightlines are similar to the global
averages, so the halo mass is still huge, over 10 billion solar masses. With
more such studies, we will be able to better characterize the CGM anisotropy
and measure its mass more accurately. We also show that the Galactic disk makes
insignificant contribution to the observed \ovii absorption; a similar
conclusion was also reached independently about the emission measure. We
further argue that any density inhomogeneity in the warm-hot gas, be it from
clumping, from the disk, or from a non-constant density gradient, would
strengthen our result in that the Galactic halo path-length and the mass would
become larger than what we estimate here. As such, our results are conservative
and robust.Comment: 27 pages, 5 figures, submitted to Ap
A huge reservoir of ionized gas around the Milky Way: Accounting for the Missing Mass?
Most of the baryons from galaxies have been "missing" and several studies
have attempted to map the circumgalactic medium (CGM) of galaxies in their
quest. Recent studies with the Hubble Space Telescope have shown that many
galaxies contain a large reservoir of ionized gas with temperatures of about
10^5 K. Here we report on X-ray observations made with the Chandra X-ray
Observatory probing an even hotter phase of the CGM of our Milky Way at about
10^6 K. We show that this phase of the CGM is massive, extending over a large
region around the Milky Way, with a radius of over 100 kpc. The mass content of
this phase is over ten billion solar masses, many times more than that in
cooler gas phases and comparable to the total baryonic mass in the disk of the
Galaxy. The missing mass of the Galaxy appears to be in this warm-hot gas
phase.Comment: 15 pages, 3 figures; http://stacks.iop.org/2041-8205/756/L
On the black hole from merging binary neutron stars: how fast can it spin?
The merger of two neutron stars will in general lead to the formation of a
torus surrounding a black hole whose rotational energy can be tapped to
potentially power a short gamma-ray burst. We have studied the merger of
equal-mass binaries with spins aligned with the orbital angular momentum to
determine the maximum spin the black hole can reach. Our initial data consists
of irrotational binaries to which we add various amounts of rotation to
increase the total angular momentum. Although the initial data violates the
constraint equations, the use of the constraint-damping CCZ4 formulation yields
evolutions with violations smaller than those with irrotational initial data
and standard formulations. Interestingly, we find that a limit of exists for the dimensionless spin and that any additional angular
momentum given to the binary ends up in the torus rather than in the black
hole, thus providing another nontrivial example supporting the cosmic
censorship hypothesis.Comment: 4 pages, 2 figures Version to appear in PRD Rapid Communication
A battery-operated, stabilized, high-energy pulsed electron gun for the production of rare gas excimers
We report on the design of a new type of electron gun to be used for
experiments of infrared emission spectroscopy of rare gas excimers. It is based
on a filament heated by means of a pack of rechargeable batteries floated atop
the high-voltage power supply. The filament current is controlled by a feedback
circuit including a superluminescent diode decoupled from the high voltage by
means of an optical fiber. Our experiment requires that the charge injection is
pulsed and constant and stable in time. This electron gun can deliver several
tens of nC per pulse of electrons of energy up to keV into the sample
cell. This new design eliminates ripples in the emission current and ensures up
to 12 hrs of stable performance.Comment: 1o pages, 8 figures, to be submitted to Review of Scientific
Instrument
Constituents of the soft X-ray background
The X-ray background is generated by various classes of objects and variety
of emission mechanisms. Relative contribution of individual components depends
on energy. The goal is to assess the integral emission of the major components
of the soft X-ray background (extragalactic discrete sources dominated by AGNs,
galactic plasma, and the Warm/Hot Intergalactic Medium), investigating the
angular structure of the background. Fluctuations of the background are
measured using the auto-correlation function of the XRB determined in 5 energy
bands between 0.3 and 4.5 keV. The investigation is based on the extensive
observational data set selected from the XMM-Newton archives. Amplitudes of the
auto-correlation functions calculated in three energy bands above ~1 keV are
consistent with the conjecture that the background fluctuations result solely
from clustering of sources which produce the background. At energies below 1
keV the relative fluctuation amplitude decreases indicating that a fraction of
the soft XRB is associated with a smooth plasma emission in the Galaxy. It is
shown, however, that the mean spectrum of extragalactic discrete sources
steepens in the soft X-rays and is not well represented by a single power law
in the energy range 0.3-4.5 keV. The WHIM contribution to the total background
fluctuations is small and consistent with the WHIM properties derived from the
cross-correlation of the XRB with galaxies.Comment: 7 pages, 2 figure
A Microcalorimeter and Bolometer Model
The standard non-equilibrium theory of noise in ideal bolometers and
microcalorimeters fails to predict the performance of real devices due to
additional effects that become important at low temperature. In this paper we
extend the theory to include the most important of these effects, and find that
the performance of microcalorimeters operating at 60 mK can be quantitatively
predicted. We give a simple method for doing the necessary calculations,
borrowing the block diagram formalism from electronic control theory.Comment: 20 pages, 15 figure
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