302 research outputs found
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
Inverse Compton Emission from Galactic Supernova Remnants: Effect of the Interstellar Radiation Field
The evidence for particle acceleration in supernova shells comes from
electrons whose synchrotron emission is observed in radio and X-rays. Recent
observations by the HESS instrument reveal that supernova remnants also emit
TeV gamma-rays; long awaited experimental evidence that supernova remnants can
accelerate cosmic rays up to the ``knee'' energies. Still, uncertainty exists
whether these gamma-rays are produced by electrons via inverse Compton
scattering or by protons via neutral pion decay. The multi-wavelength spectra
of supernova remnants can be fitted with both mechanisms, although a preference
is often given to neutral pion decay due to the spectral shape at very high
energies. A recent study of the interstellar radiation field indicates that its
energy density, especially in the inner Galaxy, is higher than previously
thought. In this paper we evaluate the effect of the interstellar radiation
field on the inverse Compton emission of electrons accelerated in a supernova
remnant located at different distances from the Galactic Centre. We show that
contribution of optical and infra-red photons to the inverse Compton emission
may exceed the contribution of cosmic microwave background and in some cases
broaden the resulted gamma-ray spectrum. Additionally, we show that if a
supernova remnant is located close to the Galactic Centre its gamma-ray
spectrum will exhibit a ``universal'' cutoff at very high energies due to the
Klein-Nishina effect and not due to the cut-off of the electron spectrum. As an
example, we apply our calculations to the supernova remnants RX J1713.7-3946
and G0.9+0.1 recently observed by HESS.Comment: 4 pages, 4 figures. Uses emulateapj.cls. Accepted by ApJ
Decaying Sterile Neutrinos as a Heating Source in the Milky Way Center
Recent Chandra and Newton observations indicate that there are
two-temperature components ( 8 keV, 0.8 keV) of the diffuse x-rays
emitted from deep inside the center of Milky Way. We show that this can be
explained by the existence of sterile neutrinos, which decay to emit photons
that can be bound-free absorbed by the isothermal hot gas particles in the
center of Milky Way. This model can account for the two-temperature components
naturally as well as the energy needed to maintain the 8 keV temperature
in the hot gas. The predicted sterile neutrino mass is between 16-18 keV.Comment: Accepted by MNRAS with minor correction
Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period
IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging
to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray
Transients. This system is one of the two members of this class, together with
IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are
known. We report on the first complete monitoring of the X-ray activity along
an entire orbital period of a Supergiant Fast X-ray Transient. These Swift
observations, lasting 28d, cover more than one entire orbital phase
consecutively. They are a unique data-set, which allows us to constrain the
different mechanisms proposed to explain the nature of this new class of X-ray
transients. We applied the new clumpy wind model for blue supergiants developed
by Ducci et al. (2009), to the observed X-ray light curve. Assuming an
eccentricity of e=0.4, the X-ray emission from this source can be explained in
terms of the accretion from a spherically symmetric clumpy wind, composed of
clumps with different masses, ranging from 10^{18}g to 5x 10^{21}g.Comment: Accepted for publication in MNRAS. 7 pages, 7 figures, 2 table
Monitoring Supergiant Fast X-ray Transients with Swift. Results from the first year
Swift has allowed the possibility to give Supergiant Fast X-ray Transients
(SFXTs), the new class of High Mass X-ray Binaries discovered by INTEGRAL, non
serendipitous attention throughout all phases of their life. We present our
results based on the first year of intense Swift monitoring of four SFXTs, IGR
J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the
first assessment of how long each source spends in each state using a
systematic monitoring with a sensitive instrument. The duty-cycle of inactivity
is 17, 28, 39, 55% (5% uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE
J1739-302, and IGR J17544-2619, respectively, so that true quiescence is a rare
state. This demonstrates that these transients accrete matter throughout their
life at different rates. AX J1841.0-0536 is the only source which has not
undergone a bright outburst during our campaign. Although individual sources
behave somewhat differently, common X-ray characteristics of this class are
emerging such as outburst lengths well in excess of hours, with a multiple
peaked structure. A high dynamic range (including bright outbursts) of 4 orders
of magnitude has been observed. We performed out-of-outburst intensity-based
spectroscopy. Spectral fits with an absorbed blackbody always result in
blackbody radii of a few hundred meters, consistent with being emitted from a
small portion of the neutron star surface, very likely the neutron star polar
caps. We also present the UVOT data of these sources. (Abridged)Comment: Accepted for publication in MNRAS. 20 pages, 9 figures, 8 table
XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient
IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of
supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching
luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been
ascribed to the combined effect of the intense magnetic field and rotation of
the compact object hosted in them and/or the presence of dense structures
("clumps") in the wind of their supergiant companion. IGR J18410-0535 was
observed for 45 ks by XMM-Newton as part of a program designed to study the
quiescent emission of supergiant fast X-ray transients and clarify the origin
of their peculiar X-ray variability. We carried out an in-depth spectral and
timing analysis of these XMM-Newton data. IGR J18410-0535 underwent a bright
X-ray flare that started about 5 ks after the beginning of the observation and
lasted for \sim15 ks. Thanks to the capabilities of the instruments on-board
XMM-Newton, the whole event could be followed in great detail. The results of
our analysis provide strong convincing evidence that the flare was produced by
the accretion of matter from a massive clump onto the compact object hosted in
this system. By assuming that the clump is spherical and moves at the same
velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl
\simeq1.4\times10^(22) g and Rcl \simeq8\times10^(11) cm. These are in
qualitative agreement with values expected from theoretical calculations. We
found no evidence of pulsations at \sim4.7 s after investigating coherent
modulations in the range 3.5 ms-100 s. A reanalysis of the archival ASCA and
Swift data of IGR J18410-0535, for which these pulsations were previously
detected, revealed that they were likely to be due to a statistical fluctuation
and an instrumental effect, respectively.Comment: Accepted for publication on A&A. V2: Inserted correct version of
Fig.1
X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature
SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly
between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was
the longest and brightest. We have analyzed X-ray data sets that focus on the
2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data
cover unabsorbed 2--10 keV fluxes between 0.1 and 5.3 X 10^-9 erg/s/cm^2. The
equivalent luminosity range is 6 X 10^35 to 2 X 10^37 erg/s. The 0.3--10 keV
spectrum is well described by a combination of a multi-temperature disk
blackbody, a hot Comptonization component and a narrow Fe-K emission line at
6.5 to 6.8 keV with an equivalent width of up to 285 eV. The hydrogen column
density in the line of sight is (8.8+/-0.5) X 10^22 cm^-2. The most conspicuous
spectral changes in this model are represented by variations of the temperature
and radius of the inner edge of the accretion disk, and a jump of the
equivalent width of the Fe-K line in one observation. Furthermore, 45 type-I
X-ray bursts were unambiguously detected between 1998 and 2001 which all
occurred during or close to outbursts. We derive a distance of 7.5+/-1.3 kpc
which is consistent with previous determinations. Our failure to detect bursts
for prolonged periods outside outbursts provides indirect evidence that the
source returns to quiescence between outbursts and is a true transient.Comment: accepted by A&A, minor adjustments incorporated from copy editing
proces
Flexural and fracture behaviour of a cement-based material reinforced with GO nanoplates
In the present research work, the mechanical properties of a cement-based material reinforced with Graphene Oxide (GO) nanoplates are experimentally investigated. In particular, a detail experimental campaign, consisting of three-point bending tests on both unnotched and edge-notched specimens, is performed in order to determine flexural strength and fracture toughness. More precisely, the flexural strength is computed as a function of the experimental values of the peak load according to UNI EN Recommendation, whereas the fracture toughness is analytically determined according to the Modified Two-Parameter Mode
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