226 research outputs found
EUV excess in the inner Virgo cluster
Observations with the Extreme UltraViolet Explorer (EUVE) satellite have
shown that the inner region of the Virgo cluster (centered in M87 galaxy) has a
strong Extreme UltraViolet (EUV) emission (up to ) in excess to
the low-energy tail expected from the hot, diffuse IntraCluster Medium (ICM).
Detailed observations of large scale radio emission and upper limits for hard,
non-thermal X-ray emission in the keV energy band have been also
reported. Here we show that all available observations can be accounted for by
the existence of two electron Populations (indicated as I and II) in the M87
Galaxy. The mildly relativistic Population I is responsible for the EUV excess
emission via IC scattering of CBR and starlight photons. Population II
electrons (with higher energy) are instead responsible for the radio emission
through synchrotron mechanism. The same electrons also give rise to hard
non-thermal X-ray emission (via IC scattering of CBR photons), but the
resulting power is always below the upper bounds placed by present
observations. The non-negligible energy budget of the two electron populations
with respect to that associated with thermal electrons indicates that the M87
galaxy is not today in a quiescent (relaxed) phase. Nuclear activity and
merging processes could have made available this energy budget that today is
released in the form of relativistic electrons.Comment: 7 pages, 1 Postscript figure, to be published in Astronomy and
Astrophysic
SENSE: A comparison of photon detection efficiency and optical crosstalk of various SiPM devices
This paper describes a comparison of photon detection efficiency and optical
crosstalk measurements performed by three partners: Geneva University, Catania
Observatory and Nagoya University. The measurements were compared for three
different SiPM devices with different active areas: from 9 up to 93.6
produced by Hamamatsu. The objective of this work is to establish the
measurements and analysis procedures for calculating the main SiPM parameters
and their precision. This work was done in the scope of SENSE project which
aims to build roadmap for the last developments in field of sensors for low
light level detection
The History and Future of the Local and Loop I Bubbles
The Local and Loop I superbubbles are the closest and best investigated
supernova (SN) generated bubbles and serve as test laboratories for
observations and theories of the interstellar medium. Since the morphology and
dynamical evolution of bubbles depend on the ambient density and pressure
distributions, a realistic modelling of the galactic environment is crucial for
a detailed comparison with observations. We have performed 3D high resolution
(down to 1.25 pc on a kpc-scale grid) hydrodynamic simulations of the Local
Bubble (LB) and the neighbouring Loop I (L1) superbubble in a realistically
evolving inhomogeneous background ISM, disturbed already by SN explosions at
the Galactic rate for 200 Myr before the LB and L1 are generated. The LB is the
result of 19 SNe occurring in a moving group, which passed through the present
day local HI cavity. We can reproduce (i) the OVI column density in absorption
within the LB in agreement with COPERNICUS and recent FUSE observations, giving
N(OVI) <2 10^{13} cm^-2 and N(OVI)<7 10^{12} cm^-2, respectively, (ii) the
observed sizes of the Local and Loop I superbubbles, (iii) the interaction
shell between LB and L1, discovered with ROSAT, (iv) constrain the age of the
LB to be 14.5+0.7/-0.4 Myr, (v) predict the merging of the two bubbles in about
3 Myr, when the interaction shell starts to fragment, (vi) the generation of
blobs like the Local Cloud as a consequence of a dynamical instability. We find
that evolving superbubbles strongly deviate from idealised self-similar
solutions due to ambient pressure and density gradients, as well as due to
turbulent mixing and mass loading. Hence, at later times the hot interior can
break through the surrounding shell, which may also help to explain the
puzzling energy "deficit" observed in LMC bubbles.Comment: Accepted for publication in Astronomy and Astrophysics Letters. The
paper contains 5 pages and 11 figures. Fig. 1a replaced by correct figur
On the local birth place of Geminga
Using estimates of the distance and proper motion of Geminga and the
constraints on its radial velocity posed by the shape of its bow shock, we
investigate its birth place by tracing its space motion backwards in time. Our
results exclude the lambda Ori association as the origin site because of the
large distance between both objects at any time. Our simulations place the
birth region at approximately 90-240 pc from the Sun, between 197 degrees and
199 degrees in Galactic longitude and -16 degrees and -8 degrees in latitude,
most probably inside the Cas-Tau OB association or the Ori OB1a association. We
discard the possibility of the progenitor being a massive field star. The
association of Geminga with either stellar association implies an upper limit
of M = 15 Msun for the mass of its progenitor. We also propose new members for
the Cas-Tau and Ori OB1 associations.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
XMM-Newton observations of Zeta Orionis (O9.7 Ib): A Collisional Ionization Equilibrium model
We present XMM-Newton observations of the O supergiant Zeta Orionis (O9.7
Ib). The spectra are rich in emission lines over a wide range of ionization
stages. The RGS-spectra show for the first time lines of low ion stages such as
C VI, N VI, N VII, and O VII. The line profiles are symmetric and rather broad
(FWHM approximately 1500 km/s) and show only a slight blue shift. With the
XMM-epic spectrometer several high ions are detected in this star for the first
time including Ar XVII and S XV. Simultaneous multi-temperature fits and
DEM-modeling were applied to the RGS and EPIC spectra to obtain emission
measures, elemental abundances and plasma temperatures. The calculations show
temperatures in the range of about 0.07-0.6 keV. According to the derived
models the intrinsic source X-ray luminosity at a distance of 251 pc
Lx=1.37(.03) times 10^{32} ergs/s, in the energy range 0.3-10 keV. In the best
multi-temperature model fit, the abundances of C, N, O, and Fe are near their
solar values, while the abundances of Ne, Mg, and Si appear somewhat enhanced.
The sensitivity of the He-like forbidden and intercombination lines to Zeta
Ori's strong radiation field is used to derive the radial distances at which
lines are formed. Values of 34 R-star for N VI, 12.5 R-star for O VII, 4.8
R-star for Ne IX, and 3.9 R-star for Mg XI are obtained.Comment: content: 11 pages, 4 figure
Spectroscopic analysis of the B/Be visual binary HR 1847
We studied both components of a slightly overlooked visual binary HR 1847
spectroscopically to determine its basic physical and orbital parameters. Basic
stellar parameters were determined by comparing synthetic spectra to the
observed echelle spectra, which cover both the optical and near-IR regions. New
observations of this system used the Ond\v{r}ejov and Rozhen 2-m telescopes and
their coud\'e spectrographs. Radial velocities from individual spectra were
measured and then analysed with the code {\FOTEL} to determine orbital
parameters. The spectroscopic orbit of HR 1847A is presented for the first
time. It is a single-lined spectroscopic binary with a B-type primary, a period
of 719.79 days, and a highly eccentric orbit with e=0.7. We confirmed that HR
1847B is a Be star. Its H\alpha emission significantly decreased from 2003 to
2008. Both components have a spectral type B7-8 and luminosity class IV-V.Comment: Astronomy and Astrophysics, accepte
Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD\168112 (O5.5III(f^+))
We report the results of a multiwavelength study of the non-thermal radio
emitter HD168112 (O5.5III(f^+)). The detailed analysis of two
quasi-simultaneous XMM-Newton and VLA observations reveals strong variability
of this star both in the X-ray and radio ranges. The X-ray observations
separated by five months reveal a decrease of the X-ray flux of ~30%. The radio
emission on the other hand increases by a factor 5-7 between the two
observations obtained roughly simultaneously with the XMM-Newton pointings. The
X-ray data reveal a hard emission that is most likely produced by a thermal
plasma at kT ~2-3 keV while the VLA data confirm the non-thermal status of this
star in the radio waveband. Comparison with archive X-ray and radio data
confirms the variability of this source in both wavelength ranges over a yet
ill defined time scale. The properties of HD168112 in the X-ray and radio
domain point towards a binary system with a significant eccentricity and an
orbital period of a few years. However, our optical spectra reveal no
significant changes of the star's radial velocity suggesting that if HD168112
is indeed a binary, it must be seen under a fairly low inclination.Comment: 17 pages, 11 figures (10 postscript + 1 gif
The Search for Supernova-produced Radionuclides in Terrestrial Deep-sea Archives
An enhanced concentration of 60Fe was found in a deep ocean's crust in 2004
in a layer corresponding to an age of ~2 Myr. The confirmation of this signal
in terrestrial archives as supernova-induced and detection of other
supernova-produced radionuclides is of great interest. We have identified two
suitable marine sediment cores from the South Australian Basin and estimated
the intensity of a possible signal of the supernova-produced radionuclides
26Al, 53Mn, 60Fe and the pure r-process element 244Pu in these cores. A finding
of these radionuclides in a sediment core might allow to improve the time
resolution of the signal and thus to link the signal to a supernova event in
the solar vicinity ~2 Myr ago. Furthermore, it gives an insight on
nucleosynthesis scenarios in massive stars, the condensation into dust grains
and transport mechanisms from the supernova shell into the solar system
Global X-ray properties of the O and B stars in Carina
The key empirical property of the X-ray emission from O stars is a strong
correlation between the bolometric and X-ray luminosities. In the framework of
the Chandra Carina Complex Project, 129 O and B stars have been detected as
X-ray sources; 78 of those, all with spectral type earlier than B3, have enough
counts for at least a rough X-ray spectral characterization. This leads to an
estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging
to the same region and triples the number of Carina massive stars studied
spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single
objects, with a dispersion of only 0.21dex. Using the properties of hot massive
stars listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar for
bright and faint objects, as well as for stars of different luminosity classes
or spectral types. Binaries appear only slightly harder and slightly more
luminous in X-rays than single objects; the differences are not formally
significant (at the 1% level), except for the Lx/Lbol ratio in the medium
(1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities
but they display slightly softer spectra compared to "normal" O stars with the
same bolometric luminosity. Discarding three overluminous objects, we find a
very shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear to be
different overall (constant X-ray luminosities, harder spectra), hinting that
another mechanism for producing X-rays, besides wind shocks, might be at work.
However, it must be stressed that the earliest and X-ray brightest amongst
these few detected objects are similar to the latest O stars, suggesting a
possibly smooth transition between the two processes.Comment: 30 pages, 9 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
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