15,181 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
CAD-CAE methods to support restoration and museum exhibition of bronze statues: the “Principe Ellenistico”
Ancient bronze statues mainly require material integrity assessment and restoration.
Restoration may include also the update of the museum exhibition, defining new structural
frames and fragment re-composition to preserve the statue and improve the interpretation of the
original aspect. This paper proves how engineering methods (such as Finite Element Analysis,
Computer Aided Design modelling, Reverse Engineering) may assist cultural heritage experts
and restorers in these tasks. It presents the activities made together with the Museo Nazionale
Romano and the Istituto Superiore per la Conservazione e il Restauro, on the so-called “Principe
Ellenistico” (Hellenistic Prince). This bronze was found in pieces (body, left arm and right leg),
at the end of 19th century during an excavation made in Rome. No visual or reference sources
can say its origin and its final posture was defined by restorers at the end of the 19th century
according to their hypothesis and studies. In the 20th century, a further restoration was made on
the critical areas of the surface, together with some structural improvement of the inner frame.
Nowadays, after a review of its position inside the Museum, new experimental and numerical
analyses have been carried out to better understand surface weakness and correct left arm
positionin
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
The optical to gamma-ray emission of the Crab pulsar: a multicomponent model
We present a multicomponent model to explain the features of the pulsed
emission and spectrum of the Crab Pulsar, on the basis of X and gamma-ray
observations obtained with BeppoSAX, INTEGRAL and CGRO. This model explains the
evolution of the pulse shape and of the phase-resolved spectra, ranging from
the optical/UV to the GeV energy band, on the assumption that the observed
emission is due to more components. The first component, C_O, is assumed to
have the pulsed double-peaked profile observed at the optical frequencies,
while the second component, C_X, is dominant in the interpeak and second peak
phase regions. The spectra of these components are modelled with log-parabolic
laws and their spectral energy distributions have peak energies at 12.2 and 178
keV, respectively. To explain the properties of the pulsed emission in the
MeV-GeV band, we introduce two more components, C_Ogamma and C_Xgamma, with
phase distributions similar to those of C_O and C_X and log-parabolic spectra
with the same curvature but peak energies at about 300 MeV and 2 GeV. This
multicomponent model is able to reproduce both the broadband phase-resolved
spectral behaviour and the changes of the pulse shape with energy. We also
propose some possible physical interpretations in which C_O and C_X are emitted
by secondary pairs via a synchrotron mechanism while C_Ogamma and C_Xgamma can
originate either from Compton scattered or primary curvature photons.Comment: 14 pages, 16 figures; accepted by Astronomy and Astrophysic
X-ray absorption towards high-redshift sources: probing the intergalactic medium with blazars
The role played by the intergalactic medium (IGM) in the X-ray absorption
towards high-redshift sources has recently drawn more attention in spectral
analysis studies. Here, we study the X-ray absorption towards 15 flat-spectrum
radio quasars at , relying on high counting statistic (
photons) provided by XMM-Newton, with additional NuSTAR (and simultaneous
Swift-XRT) observations when available. Blazars can be confidently considered
to have negligible X-ray absorption along the line of sight within the host
galaxy, likely swept by the kpc-scale relativistic jet. This makes our sources
ideal for testing the absorption component along the IGM. Our new approach is
to revisit the origin of the soft X-ray spectral hardening observed in high-z
blazars in terms of X-ray absorption occurring along the IGM, with the help of
a low-z sample used as comparison. We verify that the presence of absorption in
excess of the Galactic value is the preferred explanation to explain the
observed hardening, while intrinsic energy breaks, predicted by blazars'
emission models, can easily occur out of the observing energy band in most
sources. First, we perform an indirect analysis comparing the inferred amount
of absorption in excess of the Galactic value with a simulated IGM absorption
contribution, that increases with redshift and includes both a minimum
component from diffuse IGM metals, and the additional contribution of discrete
denser intervening regions. Then, we directly investigate the warm-hot IGM with
a spectral model on the best candidates of our sample, obtaining an average IGM
density of cm and temperature of
. A more dedicated study is currently
beyond reach, but our results can be used as a stepping stone for future more
accurate analysis, involving Athena.Comment: 32 pages, 7 figures. Accepted, to be published in A&
Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field
Starting from the observation that kilohertz Quasi Period Oscillations (kHz
QPO) occur in a very narrow range of X-ray luminosities in neutron star low
mass X-ray binaries, we try to link the kHz QPO observability to variations of
the neutron star magnetospheric radius, in response to changing mass inflow
rate. At low luminosities, the drop off of kHz QPOs activity may be explained
by the onset of the centrifugal barrier, when the magnetospheric radius reaches
the corotation radius. At the opposite side, at higher luminosities, the
magnetospheric radius may reach the neutron star and the vanishing of the
magnetosphere may led to the stopping of the kHz QPOs activity. If we apply
these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1)
and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits
naturally apply in the framework of beat frequency models but can also work in
the case of general relativistic models.Comment: 4 pages (emulateapj macro), 1 figure. Accepted for publication in ApJ
Letter
Missing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472
4U 1630-472 is a recurrent X-ray transient classified as a black-hole
candidate from its spectral and timing properties. One of the peculiarities of
this source is the presence of regular outbursts with a recurrence period
between 600 and 730 d that has been observed since the discovery of the source
in 1969. We report on a comparative study on the spectral and timing behaviour
of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all
the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory
(INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years
of activity. We show that, in spite of having a similar spectral and timing
behaviour in the energy range between 3 and 30 keV, these three outbursts show
pronounced differences above 30 keV. In fact, the 2010 outburst extends at high
energies without any detectable cut-off until 150-200 keV, while the two
previous outbursts that occurred in 2006 and 2008 are not detected at all above
30 keV. Thus, in spite of a very similar accretion disk evolution, these three
outbursts exhibit totally different characteristics of the Compton electron
corona, showing a softening in their evolution rarely observed before in a low
mass X-ray binary hosting a black hole. We argue the possibility that the
unknown perturbation that causes the outbursts to be equally spaced in time
could be at the origin of this particular behaviour. Finally we describe
several possible scenarios that could explain the regularity of the outbursts,
identifying the most plausible, such as a third body orbiting around the binary
system.Comment: April 2015: accepted for publication in MNRAS. May 2015: in pres
Virtual prototyping of medieval weapons for historical reconstruction of siege scenarios starting from topography and archaeological investigations
Chronicles of sieges to castles or fortresses, using “machinae”, can often be found in historical sources. Moreover, archaeological excavations of castles or fortresses has brought to light rocks or projectiles whose carving suggests a military usage. Nevertheless, chronicles and discoveries alone, are seldom enough to propose a faithful reconstruction of these machines. Therefore, the aim of this research is the development of methodologies for reconstructing virtual scenarios of sieges, starting from the scarce information available. In order to achieve it, a procedure for the virtual reconstruction of the siege machine has been set up, focusing on typology and dimensions of the machines, also investigating possible fire positions according to topography. The entire procedure has been developed using the siege of Cervara di Roma’s Rocca as a case study. Late medieval chronicles (end of 13th Century) report the siege brought by the papal army in order to restore the jurisdiction on the Cervara’s stronghold, following the insurrection of a group of vassals headed by a monk named Pelagio. The discovery, in the area of the Rocca, of a stone that could have been used as a projectile confirms what reported. The proposed methodology is composed of two parts. The first one is connected to the study of the “internal ballistics”, to understand the performances and to build virtual models of siege machines. The second part is the study of the “external ballistics”, then to the positioning and shooting ability of possible machines, analysing the topography of the area. In this paper, we present the feasibility of this methodology through the preliminary results achieved correlating internal and external ballistics
Gamma Ray Bursts from the early Universe: predictions for present-day and future instruments
Long Gamma Ray Bursts (GRBs) constitute an important tool to study the
Universe near and beyond the epoch of reionization. We delineate here the
characteristics of an 'ideal' instrument for the search of GRBs at z>6-10. We
find that the detection of these objects requires soft band detectors with a
high sensitivity and moderately large FOV. In the light of these results, we
compare available and planned GRB missions, deriving conservative predictions
on the number of high-z GRBs detectable by these instruments along with the
maximum accessible redshift. We show that the Swift satellite will be able to
detect various GRBs at z>6, and likely at z>10 if the trigger threshold is
decreased by a factor of ~2. Furthermore, we find that INTEGRAL and GLAST are
not the best tool to detect bursts at z>6: the former being limited by the
small FOV, and the latter by its hard energy band and relatively low
sensitivity. Finally, future missions (SVOM, EDGE, but in particular EXIST)
will provide a good sample of GRBs at z>6 in a few years of operation.Comment: 6 pages, 2 figures, MNRAS in pres
- …