555 research outputs found
XMM-Newton observation of MV Lyr and the sandwiched model confirmation
We present spectral and timing analyses of simultaneous X-ray and UV
observations of the VY Scl system MV Lyr taken by XMM-Newton, containing the
longest continuous X-ray+UV light curve and highest signal-to-noise X-ray
(EPIC) spectrum to date. The RGS spectrum displays emission lines plus
continuum, confirming model approaches to be based on thermal plasma models. We
test the sandwiched model based on fast variability that predicts a
geometrically thick corona that surrounds an inner geometrically thin disc. The
EPIC spectra are consistent with either a cooling flow model or a 2-T
collisional plasma plus Fe emission lines in which the hotter component may be
partially absorbed which would then originate in a central corona or a
partially obscured boundary layer, respectively. The cooling flow model yields
a lower mass accretion rate than expected during the bright state, suggesting
an evaporated plasma with a low density, thus consistent with a corona. Timing
analysis confirms the presence of a dominant break frequency around log(f/Hz) =
-3 in the X-ray Power Density Spectrum (PDS) as in the optical PDS. The complex
soft/hard X-ray light curve behaviour is consistent with a region close to the
white dwarf where the hot component is generated. The soft component can be
connected to an extended region. We find another break frequency around
log(f/Hz) = -3.4 that is also detected by Kepler. We compared flares at
different wavelengths and found that the peaks are simultaneous but the rise to
maximum is delayed in X-rays with respect to UV.Comment: 17 pages, 21 figures, 4 tables, Accepted for publication in MNRA
The puzzling symbiotic X-ray system 4U1700+24
Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems
consisting of a neutron star accreting material from a red giant donor star via
stellar wind or Roche lobe overflow. Only a few confirmed members are currently
known; 4U 1700+24 is a good candidate as it is a relatively bright X-ray
object, possibly associated with the late-type star V934 Her. We analysed the
archive {\it XMM}-Newton and Swift/XRT observations of 4U 1700+24 in order to
have a uniform high-energy ( keV) view of the source. We confirmed the
existence of a red-shifted O VIII Ly- transition (already observed in
the 2002 {\it XMM}-Newton data) in the high-resolution spectra collected via
the RGS instruments. The red-shift of the line is found in all the analysed
observations and, on average, it was estimated to be . We also
observed a modulation of the centroid energy of the line on short time scales
(a few days) and discuss the observations in the framework of different
scenarios. If the modulation is due to the gravitational red-shift of the
neutron star, it might arise from a sudden re-organization of the emitting
-ray matter on the scale of a few hundreds of km. Alternatively, we are
witnessing a uni-polar jet of matter (with typical velocity of km
s) possibly emitted by the neutron star in an almost face-on system. The
second possibility seems to be required by the apparent lack of any modulation
in the observed -ray light curve. We also note also that the low-resolution
spectra (both {\it XMM}-Newton and Swift/XRT in the keV band) show the
existence of a black body radiation emitted by a region (possibly associated
with the neutron star polar cap) with typical size from a few tens to hundreds
of meters. The size of this spot-like region reduces as the overall luminosity
of 4U 1700+24 decreases.Comment: In press on A&
Gravitational wave scintillation by a stellar cluster
The diffraction effects on gravitational waves propagating through a stellar
cluster are analyzed in the relevant approximation of Fresnel diffraction
limit. We find that a gravitational wave scintillation effect - similar to the
radio source scintillation effect - comes out naturally, implying that the
gravitational wave intensity changes in a characteristic way as the observer
moves.Comment: 9 pages, in press in IJMP
XMM observation of 1RXS J180431.1-273932: a new M-type X-ray binary with a 494 s-pulse period neutron star?
Low-mass X-ray binaries are binary systems composed of a compact object and a
low-mass star. Recently, a new class of these systems, known as symbiotic
-ray binaries (with a neutron star with a M-type giant companion), has been
discovered. Here, we present long-duration observations of the
source 1RXS J180431.1-273932. Temporal and spectral analysis of the source was
performed along with a search for an optical counterpart. We used a
Lomb-Scargle periodogram analysis for the period search and evaluated the
confidence level using Monte-Carlo simulations. The source is characterized by
regular pulses so that it is most likely a neutron star. A modulation of
s (3 error) was found with a confidence level of 99%.
Evidence of variability is also present, since the data show a rate of change
in the signal of counts s hr. A longer
observation will be necessary in order to determine if the source shows any
periodic behavior. The spectrum can be described by a power law with photon
index and a Gaussian line at 6.6 keV. The X-ray flux in the
0.2--10 keV energy band is erg s cm. The
identification of an optical counterpart (possibly an M6III red-giant star with
an apparent visual magnitude of ) allows a conservative distance
of kpc to be estimated. Other possibilities are also discussed. Once
the distance was estimated, we got an -ray luminosity of L_X\ut<6\times
10^{34} erg s, which is consistent with the typical -ray luminosity
of a symbiotic LMXB system.Comment: in press on A&
Messier 81's Planck view vs its halo mapping
This paper is a follow-up of a previous paper about the M82 galaxy and its
halo based on Planck observations. As in the case of M82, so also for the M81
galaxy a substantial North-South and East-West temperature asymmetry is found,
extending up to galactocentric distances of about . The temperature
asymmetry is almost frequency independent and can be interpreted as a
Doppler-induced effect related to the M81 halo rotation and/or triggered by the
gravitational interaction of the galaxies within the M81 Group. Along with the
analogous study of several nearby edge-on spiral galaxies, the CMB temperature
asymmetry method thus is shown to act as a direct tool to map the galactic
haloes and/or the intergalactic bridges, invisible in other bands or by other
methods.Comment: 5 pages, 3 figures, in press in Astronomy and Astrophysics, Main
Journa
Planck view of the M82 galaxy
Planck data towards the galaxy M82 are analyzed in the 70, 100 and 143 GHz
bands. A substantial north-south and East-West temperature asymmetry is found,
extending up to 1 degree from the galactic center. Being almost
frequency-independent, these temperature asymmetries are indicative of a
Doppler-induced effect regarding the line-of-sight dynamics on the halo scale,
the ejections from the galactic center and, possibly, even the tidal
interaction with M81 galaxy. The temperature asymmetry thus acts as a
model-independent tool to reveal the bulk dynamics in nearby edge-on spiral
galaxies, like the Sunyaev-Zeldovich effect for clusters of galaxies.Comment: 4 pages, 3 figures, in press on A&
Planck revealed bulk motion of Centaurus A lobes
Planck data towards the active galaxy Centaurus A are analyzed in the 70, 100
and 143 GHz bands. We find a temperature asymmetry of the northern radio lobe
with respect to the southern one that clearly extends at least up to 5 degrees
from the Cen A center and diminishes towards the outer regions of the lobes.
That transparent parameter - the temperature asymmetry - thus has to carry a
principal information, i.e. indication on the line-of-sight bulk motion of the
lobes, while the increase of that asymmetry at smaller radii reveals the
differential dynamics of the lobes as expected at ejections from the center.Comment: 4 pages, 3 figures, Astronomy & Astrophysics, Letter to the Editor
(in press
Apoastron Shift Constraints on Dark Matter Distribution at the Galactic Center
The existence of dark matter (DM) at scales of few pc down to pc around the centers of galaxies and in particular in the Galactic
Center region has been considered in the literature. Under the assumption that
such a DM clump, principally constituted by non-baryonic matter (like WIMPs)
does exist at the center of our galaxy, the study of the -ray emission
from the Galactic Center region allows us to constrain both the mass and the
size of this DM sphere. Further constraints on the DM distribution parameters
may be derived by observations of bright infrared stars around the Galactic
Center. Hall and Gondolo \cite{hallgondolo} used estimates of the enclosed mass
obtained in various ways and tabulated by Ghez et al.
\cite{Ghez_2003,Ghez_2005}. Moreover, if a DM cusp does exist around the
Galactic Center it could modify the trajectories of stars moving around it in a
sensible way depending on the DM mass distribution. Here, we discuss the
constraints that can be obtained with the orbit analysis of stars (as S2 and
S16) moving inside the DM concentration with present and next generations of
large telescopes. In particular, consideration of the S2 star apoastron shift
may allow improving limits on the DM mass and size.Comment: in press on Phys. Rev.
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