11,566 research outputs found
XMM-Newton observations of two transient millisecond X-ray pulsars in quiescence
We report on XMM-Newton observations of two X-ray transient millisecond
pulsars (XRTMSPs). We detected XTE J0929-314 with an unabsorbed luminosity of
\~7x10^{31} erg/s. (0.5-10 keV) at a fiducial distance of 10 kpc. The quiescent
spectrum is consistent with a simple power law spectrum. The upper limit on the
flux from a cooling neutron star atmosphere is about 20% of the total flux. XTE
J1807-294 instead was not detected. We can put an upper limit on the source
quiescent 0.5-10 keV unabsorbed luminosity <4x10^{31} erg/s at 8 kpc. These
observations strenghten the idea that XRTMSPs have quiescent luminosities
significantly lower than classical neutron star transients.Comment: 4 pages including 1 figures. Accepted for publication in A&A Letter
The optical counterpart of IGR J00291+5934 in quiescence
The recent (December 2004) discovery of the sixth accretion-powered
millisecond X-ray pulsar IGR J00291+5934 provides a very good chance to deepen
our knowledge of such systems. Although these systems are well studied at high
energies, poor informations are available for their optical/NIR counterparts
during quiescence. Up to now, only for SAX J1808.4-3658, the first discovered
system of this type, we have a secure multiband detection of its optical
counterpart in quiescence. Among the seven known system IGR J00291+5934 is the
one that resembles SAX J1808.4-3658 more closely. With the Italian 3.6 m TNG
telescope, we have performed deep optical and NIR photometry of the field of
IGR J00291+5934 during quiescence in order to look for the presence of a
variable counterpart. We present here the first multiband () detection
of the optical and NIR counterpart of IGR J00291+5934 in quiescence as well as
a deep upper limit in the band. We obtain an optical light curve that shows
variability consistent with a sinusoidal modulation at the known 2.46 hr
orbital period and present evidence for a strongly irradiated companion.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysic
Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence
We present the first optical and infrared polarimetric study of the low mass
transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed
to search for an intrinsic linear polarisation component in the system emitted
radiation that might be due, e.g., to synchrotron emission from a compact jet,
or to Thomson scattering with free electrons in an accretion disc. Multiband
(BVRI) optical polarimetric observations were obtained during two nights in
2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These
observations cover about the 30% of the 15.1 hours orbital period. J-band
observations were obtained in 2007 with the NICS (TNG) instrument at La Palma,
for a totality of 1 hour observation. We obtained 3-sigma upper limits to the
polarisation degree in all the optical bands, with the most constraining one
being in the I-band (P<0.5%). No phase-correlated variability has been noticed
in all the filters. The J-band observations provided a 6% upper limit on the
polarisation level. The constraining upper limits to the polarisation in the
optical allowed us to evaluate the contribution of the possible emission of a
relativistic particles jet to the total system radiation to be less then the
10%. This is in agreement with the observation of a spectral energy
distribution typical of a single black body of a K-spectral type main sequence
star irradiated from the compact object. Due to the low S/N ratio it was not
possible to investigate the possible dependency of the polarisation degree from
the wavelength, that could be suggestive of polarisation induced by Thomson
scattering of radiation with free electrons in the outer part of the accretion
disc. Observations with higher S/N ratio are required to examine in depth this
hypothesis, searching for significant phase-correlated variability.Comment: 7 pages, 9 figures, accepted for publication in section 7. Stellar
structure and evolution of Astronomy and Astrophysic
The quiescent X-ray emission of three transient X-ray pulsars
We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients
in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U
0115+63 and V 0332+53. These observations allowed us to study these transients
at the faintest flux levels thus far. Spectra are remarkably different from the
ones obtained at luminosities a factor >10 higher, testifying that the
quiescent emission mechanism is different. Pulsations were not detected in any
of the sources, indicating that accretion of matter down to the neutron star
surface has ceased. We conclude that the quiescent emission of the three X-ray
transients likely originates from accretion onto the magnetospheric boundary in
the propeller regime and/or from deep crustal heating resulting from
pycnonuclear reactions during the outbursts.Comment: Accepted for publication on ApJ (5 pages and 2 figures
Missing cosmic metals revealed by X-ray absorption towards distant sources
The census of heavy elements (metals) produced by all stars through cosmic
times up to present-day is limited to ~50%; of these only half are still found
within their parent galaxy. The majority of metals is expelled from galaxies
into the circumgalactic (or even more distant, intergalactic) space by powerful
galactic winds, leaving unpleasant uncertainty on the amount, thermal
properties and distribution of these key chemical species. These dispersed
metals unavoidably absorb soft X-ray photons from distant sources. We show that
their integrated contribution can be detected in the form of increasing X-ray
absorption with distance, for all kinds of high-energy cosmic sources. Based on
extensive cosmological simulations, we assess that 10\% of all cosmic
metals reside in the intergalactic medium. Most of the X-ray absorption arises
instead from a few discrete structures along the line of sight. These extended
structures, possibly pin-pointing galaxy groups, contain million degree,
metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional
~10% of cosmic metals could reside in this phase.Comment: Accepted for publication in Astronomy & Astrophysics. 9 pages, 4
figures, 1 tabl
On the detection of very high redshift Gamma Ray Bursts with Swift
We compute the probability to detect long Gamma Ray Bursts (GRBs) at z>5 with
Swift, assuming that GRBs form preferentially in low-metallicity environments.
The model fits well both the observed BATSE and Swift GRB differential peak
flux distribution and is consistent with the number of z>2.5 detections in the
2-year Swift data. We find that the probability to observe a burst at z>5
becomes larger than 10% for photon fluxes P<1 ph s^{-1} cm^{-2}, consistent
with the number of confirmed detections. The corresponding fraction of z>5
bursts in the Swift catalog is ~10%-30% depending on the adopted metallicity
threshold for GRB formation. We propose to use the computed probability as a
tool to identify high redshift GRBs. By jointly considering promptly-available
information provided by Swift and model results, we can select reliable z>5
candidates in a few hours from the BAT detection. We test the procedure against
last year Swift data: only three bursts match all our requirements, two being
confirmed at z>5. Other three possible candidates are picked up by slightly
relaxing the adopted criteria. No low-z interloper is found among the six
candidates.Comment: 5 pages, 2 figures, MNRAS in pres
Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence
We present ESO-NTT low resolution spectroscopy of the neutron star X-ray
transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal
the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux
and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0
km s^{-1}. This, combined with a previous determination of the inclination
angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/-
1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus
undermassive for the inferred spectral type indicating it is probably evolved,
in agreement with previous studies. Doppler tomography of the H_alpha line
shows prominent emission located on the companion and a slightly asymmetric
accretion disc distribution similar to that seen in systems with precessing
eccentric discs. Strong H_alpha emission from the companion can be explained by
X-ray irradiation from the primary. No evidence is found for a hot spot in
H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This
can be interpreted as the hot spot and outer regions of the disc being at a
higher temperature than in other systems.Comment: 9 pages, 5 figures, accepted for publication in A&
Tecnologie GPS e Personal Data Assistant applicati all'archeologia dei paesaggi
The Department of Medieval Archaeology of the University of Siena has been engaged for several years in the testing of GPS survey application for landscape archaeology. In the first section of this paper we have summarised the GPS application developed for field-walking, aerial, geophysical and topographical surveys. In the second section we have discussed the fact that, since the second half of the 1990s, we have felt a progressive disjunction between work in the laboratory and work in the field. While the availability of advanced technologies has been rapidly growing, activities in the field have continued to make use of instruments and methodologies developed in the 1970s. A mobile GIS system managed through the merging of PDA and GPS technologies represents at the moment the best available solution for restoring the link. The conclusions reached in our experiments using these devices consistently go far beyond the increased fieldwork efficiency and finally make it possible to systematically apply strategies and methodologies developed in the past but rarely used up to now because they were too time consuming
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